![Programmer’s Model
2.4.3 The program counter, R15
Register R15 holds the Program Counter (PC). It can often be used in place of one of the general-purpose
registers R0 to R14, and is therefore considered one of the general-purpose registers. However, there are
also many instruction-specific restrictions or special cases about its use. These are noted in the individual
instruction descriptions. Usually, the instruction is UNPREDICTABLE if R15 is used in a manner that breaks
these restrictions.
The Program Counter is always used for a special purpose, as described in:
• Reading the program counter
• Writing the program counter on page A2-8.
Reading the program counter
When an instruction reads R15 without breaking any of the restrictions on its use, the value read is the
address of the instruction plus 8 bytes. As ARM instructions are always word-aligned, bits[1:0] of the
resulting value are always zero. (In T variants of the architecture, this behavior changes during Thumb state
execution - see Chapter A6 The Thumb Instruction Set for details.)
This way of reading the PC is primarily used for quick, position-independent addressing of nearby
instructions and data, including position-independent branching within a program.
An exception to the above rule occurs when an STR or STM instruction stores R15. Such instructions can
store either the address of the instruction plus 8 bytes, like other instructions that read R15, or the
instruction's own address plus 12 bytes. Whether the offset of 8 or the offset of 12 is used is
IMPLEMENTATION DEFINED. An implementation must use the same offset for all STR and STM instructions
that store R15. It cannot use 8 for some of them and 12 for others.
Because of this exception, it is usually best to avoid the use of STR and STM instructions that store R15. If
this is difficult, use a suitable instruction sequence in the program to ascertain which offset the
implementation uses. For example, if R0 points to an available word of memory, then the following
instructions put the offset of the implementation in R0:
SUB R1, PC, #4 ; R1 = address of following STR instruction
STR PC, [R0] ; Store address of STR instruction + offset,
LDR R0, [R0] ; then reload it
SUB R0, R0, R1 ; Calculate the offset as the difference
Note
The rules about how R15 is read apply only to reads by instructions. In particular, they do not necessarily
describe the values placed on a hardware address bus during instruction fetches. Like all other details of
hardware interfaces, such values are IMPLEMENTATION DEFINED.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-39-320.jpg)
![Programmer’s Model
Writing the program counter
When an instruction writes R15 without breaking any of the restrictions on its use, the normal result is that
the value written to R15 is treated as an instruction address and a branch occurs to that address.
Since ARM instructions are required to be word-aligned, values written to R15 are normally expected to
have bits[1:0] == 0b00. The precise rules for this depend on the architecture version:
• In ARM architecture versions 3 and below, bits[1:0] of a value written to R15 are ignored, so that the
actual destination address of the instruction is (value written to R15) AND 0xFFFFFFFC.
• In ARM architecture versions 4 and above, bits[1:0] of a value written to R15 in ARM state must be
0b00. If they are not, the results are UNPREDICTABLE.
Similarly, in T variants of ARM architecture versions 4 and above, Thumb instructions are required to be
halfword-aligned. Bit[0] of a value written to R15 in Thumb state is ignored, so that the actual destination
address of the instruction is (value written to R15) AND 0xFFFFFFFE.
Several instructions have their own rules for interpreting values written to R15. For example, BX and other
instructions designed to transfer between ARM and Thumb states use bit[0] of the value to select whether
to execute the code at the destination address in ARM state or Thumb state. Special rules of this type are
described on the individual instruction pages, and override the general rules in this section.
A2-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-40-320.jpg)
![Programmer’s Model
• For non-addition/subtractions that incorporate a shift operation, C is set to the last bit
shifted out of the value by the shifter.
• For other non-addition/subtractions, C is normally left unchanged (but see the
individual instruction descriptions for any special cases).
V Is set in one of two ways:
• For an addition or subtraction, V is set to 1 if signed overflow occurred, regarding the
operands and result as two's complement signed integers.
• For non-addition/subtractions, V is normally left unchanged (but see the individual
instruction descriptions for any special cases).
The flags can be modified in these additional ways:
• Execution of an MSR instruction, as part of its function of writing a new value to the CPSR or SPSR.
• Execution of MRC instructions with destination register R15. The purpose of such instructions is to
transfer coprocessor-generated condition code flag values to the ARM processor.
• Execution of some variants of the LDM instruction. These variants copy the SPSR to the CPSR, and
their main intended use is for returning from exceptions.
• Execution of flag-setting variants of arithmetic and logical instructions whose destination register is
R15. These also copy the SPSR to the CPSR, and are mainly intended for returning from exceptions.
The Q flag
In E variants of ARM architecture 5 and above, bit[27] of the CPSR is known as the Q flag and is used to
indicate whether overflow and/or saturation has occurred in some of the enhanced DSP instructions.
Similarly, bit[27] of each SPSR is a Q flag, and is used to preserve and restore the CPSR Q flag if an
exception occurs. For more details of the Q flag, see Chapter A10 Enhanced DSP Extension.
In architecture versions prior to version 5, and in non-E variants of architecture version 5 and above, bit[27]
of the CPSR and SPSRs should be treated as described in Other bits on page A2-12.
2.5.2 The control bits
The bottom eight bits of a Program Status Register (PSR), incorporating I, F, T and M[4:0], are known
collectively as the control bits. The control bits change when an exception arises and can be altered by
software only when the processor is in a privileged mode.
Interrupt disable bits
I and F are the interrupt disable bits:
I bit Disables IRQ interrupts when it is set.
F bit Disables FIQ interrupts when it is set.
A2-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-42-320.jpg)
![Programmer’s Model
The T bit
The T bit should be zero (SBZ) on ARM architecture version 3 and below, and on non-T variants of ARM
architecture version 4. No instructions exist in these architectures that can switch between ARM and Thumb
states.
On T variants of ARM architecture 4 and above, the T bit has the following meanings:
T = 0 Indicates ARM execution.
T = 1 Indicates Thumb execution.
Instructions that switch between ARM and Thumb states can be used freely on implementations of these
architectures.
On non-T variants of ARM architecture version 5 and above, the T bit has the following meanings:
T = 0 Indicates ARM execution.
T = 1 Forces the next instruction executed to cause an undefined instruction exception (see
Undefined Instruction exception on page A2-15).
Instructions that switch between ARM and Thumb states can be used on implementations of these
architectures, but only function correctly as long as the program remains in ARM state. If the program
attempts to switch to Thumb state, the first instruction executed after the attempted switch causes an
undefined instruction exception. Entry into that exception then switches back to ARM state. The exception
handler can detect that this was the cause of the exception from the fact that the T bit of SPSR_und is set.
Mode bits
M0, M1, M2, M3, and M4 (M[4:0]) are the mode bits, and these determine the mode in which the processor
operates. Their interpretation is shown in Table 2-2.
Table 2-2 The mode bits
M[4:0] Mode Accessible registers
0b10000 User PC, R14 to R0, CPSR
0b10001 FIQ PC, R14_fiq to R8_fiq, R7 to R0, CPSR, SPSR_fiq
0b10010 IRQ PC, R14_irq, R13_irq, R12 to R0, CPSR, SPSR_irq
0b10011 Supervisor PC, R14_svc, R13_svc, R12 to R0, CPSR, SPSR_svc
0b10111 Abort PC, R14_abt, R13_abt, R12 to R0, CPSR, SPSR_abt
0b11011 Undefined PC, R14_und, R13_und, R12 to R0, CPSR, SPSR_und
0b11111 System PC, R14 to R0, CPSR (ARM architecture v4 and above)
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-11](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-43-320.jpg)
![Programmer’s Model
Not all combinations of the mode bits define a valid processor mode. Only those combinations explicitly
described can be used. If any other value is programmed into the mode bits M[4:0], the result is
UNPREDICTABLE. See also Table 8-1 on page A8-9 for details of the mode bits in the 26-bit architectures.
2.5.3 Other bits
Other bits in the program status registers are reserved for future expansion. In general, programmers must
take care to write code in such a way that these bits are never modified. Failure to do this might result in
code which has unexpected side-effects on future versions of the architecture. See the usage notes for the
MSR instruction on page A4-62 for more details.
A2-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-44-320.jpg)
![Programmer’s Model
2.6 Exceptions
Exceptions are generated by internal and external sources to cause the processor to handle an event, such as
an externally generated interrupt or an attempt to execute an undefined instruction. The processor state just
before handling the exception must be preserved so that the original program can be resumed when the
exception routine has completed. More than one exception can arise at the same time.
ARM supports seven types of exception. Table 2-3 lists the types of exception and the processor mode that
is used to process that exception. When an exception occurs, execution is forced from a fixed memory
address corresponding to the type of exception. These fixed addresses are called the exception vectors.
Note
The normal vector at address 0x00000014 and the high vector at address 0xFFFF0014 are not normally
used and are reserved for future expansion. The reserved vector at address 0x00000014 was used for an
Address Exception vector in earlier versions of the ARM architecture which had a 26-bit address space. See
Chapter A8 The 26-bit Architectures for more information.
Table 2-3 Exception processing modes
Exception type Mode
Normal
address
High vector
address
Reset Supervisor 0x00000000 0xFFFF0000
Undefined instructions Undefined 0x00000004 0xFFFF0004
Software interrupt (SWI) Supervisor 0x00000008 0xFFFF0008
Prefetch Abort (instruction fetch memory abort) Abort 0x0000000C 0xFFFF000C
Data Abort (data access memory abort) Abort 0x00000010 0xFFFF0010
IRQ (interrupt) IRQ 0x00000018 0xFFFF0018
FIQ (fast interrupt) FIQ 0x0000001C 0xFFFF001C
When an exception occurs, the banked versions of R14 and the SPSR for the exception mode are used to
save state as follows:
R14_<exception_mode> = return link
SPSR_<exception_mode> = CPSR
CPSR[4:0] = exception mode number
CPSR[5] = 0 /* Execute in ARM state */
if <exception_mode> == Reset or FIQ then
CPSR[6] = 1 /* Disable fast interrupts */
/* else CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
PC = exception vector address
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-13](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-45-320.jpg)
![Programmer’s Model
To return after handling the exception, the SPSR is moved into the CPSR, and R14 is moved to the PC. This
can be done atomically in two ways:
• using a data-processing instruction with the S bit set, and the PC as the destination
• using the Load Multiple with Restore CPSR instruction, as described in LDM (3) on page A4-34.
The following sections show what happens automatically when the exception occurs, and also show the
recommended data-processing instruction to use to return from each exception. This instruction is always a
MOVS or SUBS instruction with the PC as its destination.
Note
When the recommended data-processing instruction is a SUBS and a Load Multiple with Restore CPSR
instruction is used to return from the exception handler, the subtraction must still be performed. This is
usually done at the start of the exception handler, before the return link is stored to memory.
For example, an interrupt handler that wishes to store its return link on the stack might use instructions of
the following form at its entry point:
SUB R14, R14, #4
STMFD SP!, {<other_registers>, R14}
and return using the instruction:
LDMFD SP!, {<other_registers>, PC}^
2.6.1 Reset
When the Reset input is asserted on the processor, the ARM processor immediately stops execution of the
current instruction. When Reset is de-asserted, the following actions are performed:
R14_svc = UNPREDICTABLE value
SPSR_svc = UNPREDICTABLE value
CPSR[4:0] = 0b10011 /* Enter Supervisor mode */
CPSR[5] = 0 /* Execute in ARM state */
CPSR[6] = 1 /* Disable fast interrupts */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF0000
else
PC = 0x00000000
After Reset, the ARM processor begins execution at address 0x00000000 or 0xFFFF0000 in Supervisor
mode with interrupts disabled. See About the MMU architecture on page B3-2 for more information on the
effects of Reset.
Note
There is no architecturally defined way of returning from a Reset.
A2-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-46-320.jpg)
![Programmer’s Model
2.6.2 Undefined Instruction exception
If the ARM processor executes a coprocessor instruction, it waits for any external coprocessor
to acknowledge that it can execute the instruction. If no coprocessor responds, an Undefined Instruction
exception occurs.
If an attempt is made to execute an instruction that is UNDEFINED, an Undefined Instruction exception occurs
(see Extending the instruction set on page A3-27).
The Undefined Instruction exception can be used for software emulation of a coprocessor in a system that
does not have the physical coprocessor (hardware), or for general-purpose instruction set extension by
software emulation.
When an Undefined Instruction exception occurs, the following actions are performed:
R14_und = address of next instruction after the undefined instruction
SPSR_und = CPSR
CPSR[4:0] = 0b11011 /* Enter Undefined mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF0004
else
PC = 0x00000004
To return after emulating the undefined instruction use:
MOVS PC,R14
This restores the PC (from R14_und) and CPSR (from SPSR_und) and returns to the instruction following
the undefined instruction.
In some coprocessor designs, an internal exceptional condition caused by one coprocessor instruction is
signaled imprecisely by refusing to respond to a later coprocessor instruction. In these circumstances, the
Undefined Instruction handler takes whatever action is necessary to clear the exceptional condition, then
returns to the second coprocessor instruction. To do this use:
SUBS PC,R14,#4
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-47-320.jpg)
![Programmer’s Model
2.6.3 Software Interrupt exception
The Software Interrupt instruction (SWI) enters Supervisor mode to request a particular supervisor
(operating system) function. When a SWI is executed, the following actions are performed:
R14_svc = address of next instruction after the SWI instruction
SPSR_svc = CPSR
CPSR[4:0] = 0b10011 /* Enter Supervisor mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF0008
else
PC = 0x00000008
To return after performing the SWI operation, use the following instruction to restore the PC
(from R14_svc) and CPSR (from SPSR_svc) and return to the instruction following the SWI:
MOVS PC,R14
2.6.4 Prefetch Abort (instruction fetch memory abort)
A memory abort is signaled by the memory system. Activating an abort in response to an instruction fetch
marks the fetched instruction as invalid. A Prefetch Abort exception is generated if the processor tries to
execute the invalid instruction. If the instruction is not executed (for example, as a result of a branch being
taken while it is in the pipeline), no Prefetch Abort occurs.
In ARM architecture version 5 and above, a Prefetch Abort exception can also be generated as the result of
executing a BKPT instruction. For details, see BKPT on page A4-14 (ARM instruction) and BKPT on
page A7-24 (Thumb instruction).
When an attempt is made to execute an aborted instruction, the following actions are performed:
R14_abt = address of the aborted instruction + 4
SPSR_abt = CPSR
CPSR[4:0] = 0b10111 /* Enter Abort mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF000C
else
PC = 0x0000000C
To return after fixing the reason for the abort, use:
SUBS PC,R14,#4
This restores both the PC (from R14_abt) and CPSR (from SPSR_abt), and returns to the aborted
instruction.
A2-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-48-320.jpg)
![Programmer’s Model
2.6.5 Data Abort (data access memory abort)
A memory abort is signaled by the memory system. Activating an abort in response to a data access (load
or store) marks the data as invalid. A Data Abort exception occurs before any following instructions or
exceptions have altered the state of the CPU. The following actions are performed:
R14_abt = address of the aborted instruction + 8
SPSR_abt = CPSR
CPSR[4:0] = 0b10111 /* Enter Abort mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF0010
else
PC = 0x00000010
To return after fixing the reason for the abort use:
SUBS PC,R14,#8
This restores both the PC (from R14_abt) and CPSR (from SPSR_abt), and returns to re-execute the aborted
instruction.
If the aborted instruction does not need to be re-executed use:
SUBS PC,R14,#4
Effects of data-aborted instructions
Instructions that access data memory can modify memory by storing one or more values. If a Data Abort
occurs in such an instruction, the value of each memory location that the instruction stores to is:
• unchanged if the memory system does not permit write access to the memory location
• UNPREDICTABLE otherwise.
Instructions that access data memory can modify registers in three ways:
• By loading values into one or more of the general-purpose registers, which can include the PC.
• By specifying base register writeback, in which the base register used in the address calculation has
a modified value written to it. All instructions that allow this to be specified have UNPREDICTABLE
results if base register writeback is specified and the base register is the PC, so only general-purpose
registers other than the PC can legitimately be modified in this way.
• By loading values into coprocessor registers.
If a Data Abort occurs, the values left in these registers are determined by the following rules:
1. The PC value on entry to the data abort handler is 0x00000010 or 0xFFFF0010, and the R14_abt
value is determined from the address of the aborted instruction. Neither is affected in any way by the
results of any PC load specified by the instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-17](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-49-320.jpg)
![Programmer’s Model
2.6.6 Interrupt request (IRQ) exception
The IRQ exception is generated externally by asserting the IRQ input on the processor. It has a lower
priority than FIQ (see Table 2-4 on page A2-20), and is masked out when an FIQ sequence is entered.
Interrupts are disabled when the I bit in the CPSR is set. If the I bit is clear, ARM checks for an IRQ at
instruction boundaries.
Note
The I bit can only be changed from a privileged mode.
When an IRQ is detected, the following actions are performed:
R14_irq = address of next instruction to be executed + 4
SPSR_irq = CPSR
CPSR[4:0] = 0b10010 /* Enter IRQ mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF0018
else
PC = 0x00000018
To return after servicing the interrupt, use:
SUBS PC,R14,#4
This restores both the PC (from R14_irq) and CPSR (from SPSR_irq), and resumes execution of the
interrupted code.
2.6.7 Fast interrupt request (FIQ) exception
The FIQ exception is generated externally by asserting the FIQ input on the processor. FIQ is designed to
support a data transfer or channel process, and has sufficient private registers to remove the need for register
saving in such applications, therefore minimizing the overhead of context switching.
Fast interrupts are disabled when the F bit in the CPSR is set. If the F bit is clear, ARM checks for an FIQ
at instruction boundaries.
Note
The F bit can only be changed from a privileged mode.
When an FIQ is detected, the following actions are performed:
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-19](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-51-320.jpg)
![Programmer’s Model
R14_fiq = address of next instruction to be executed + 4
SPSR_fiq = CPSR
CPSR[4:0] = 0b10001 /* Enter FIQ mode */
CPSR[5] = 0 /* Execute in ARM state */
CPSR[6] = 1 /* Disable fast interrupts */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF001C
else
PC = 0x0000001C
To return after servicing the interrupt, use:
SUBS PC, R14,#4
This restores both the PC (from R14_fiq) and CPSR (from SPSR_fiq), and resumes execution of the
interrupted code.
The FIQ vector is deliberately the last vector to allow the FIQ exception-handler software to be placed
directly at address 0x0000001C or 0xFFFF001C, without requiring a branch instruction from the vector.
2.6.8 Exception priorities
Table 2-4 shows the exception priorities:
Table 2-4 Exception priorities
Priority Exception
Highest 1 Reset
2 Data Abort
3 FIQ
4 IRQ
5 Prefetch Abort
Lowest 6 Undefined instruction
SWI
Undefined instruction and software interrupt cannot occur at the same time, as they each correspond to
particular (non-overlapping) decodings of the current instruction, and both must be lower priority than
prefetch abort, as a prefetch abort indicates that no valid instruction was fetched.
The priority of a Data Abort exception is higher than FIQ, which ensures that the data abort handler is
entered before the FIQ handler is entered (so that the Data Abort is resolved after the FIQ handler has
completed).
A2-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-52-320.jpg)
![Programmer’s Model
2.6.9 High vectors
Some ARM implementations allow the exception vector locations to be moved from their normal address
range 0x00000000-0x0000001C at the bottom of the 32-bit address space, to an alternative address
range 0xFFFF0000-0xFFFF001C near the top of the address space. These alternative locations are
known as the high vectors.
It is IMPLEMENTATION DEFINED whether the high vectors are supported. When they are, a hardware
configuration input selects whether the normal vectors or the high vectors are to be used.
The ARM instruction set does not contain any instructions which can directly change whether normal or
high vectors are configured. However, if the standard System Control coprocessor is attached to an ARM
processor which supports the high vectors, bit[13] of coprocessor 15 register 1 can be used to switch
between using the normal vectors and the high vectors (see Register 1: Control register on page B2-13).
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-21](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-53-320.jpg)
![Programmer’s Model
2.7 Memory and memory-mapped I/O
This section discusses memory and memory-mapped I/O, mainly with regard to the assumptions ARM
processor implementations make about how their memory systems behave and how programs should access
memory. As a result, it describes the memory system from the outside, specifying how it should behave
without going into much detail of how this behavior can or should be implemented. More details of how
some standard memory systems behave can be found in Part B: Memory and System Architectures.
The ARM architecture allows a wide variety of memory system designs, using the range of memory and I/O
devices which are available. This makes it difficult to specify absolute rules about how a memory system
should behave.
Many of the rules below can be broken if the hardware and software are designed appropriately. However,
breaking these rules is discouraged, for the following reasons:
• It might make implementing the memory system more difficult.
• It might cause difficulties in porting the system (hardware and/or software) to future ARM
processors.
• Standard software (such as compilers and other software toolkit components) might not work with
the rule-breaking system.
2.7.1 Address space
The ARM architecture uses a single, flat address space of 232 8-bit bytes. Byte addresses are treated as
unsigned numbers, running from 0 to 232 - 1.
This address space is regarded as consisting of 230 32-bit words, each of whose addresses is word-aligned,
which means that the address is divisible by 4. The word whose word-aligned address is A consists of the
four bytes with addresses A, A+1, A+2 and A+3.
In ARM architecture version 4 and above, the address space is also regarded as consisting of 231 16-bit
halfwords, each of whose addresses is halfword-aligned (divisible by 2). The halfword whose
halfword-aligned address is A consists of the two bytes with addresses A and A+1.
Note
Prior to ARM architecture version 3, the address space was only 226 bytes, with addresses running from 0
to 226 - 1. This address space was split into 224 words.
Some implementations of subsequent non-T variants of the ARM architecture include
backwards-compatibility features to allow execution of code written for this address space. These features
are described in Chapter A8 The 26-bit Architectures. Their use for any purpose other than executing old
code is strongly discouraged.
These backwards-compatibility features are not compatible with T variants of the architecture, due to
conflicting uses of bits[1:0] of R15.
A2-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-54-320.jpg)
![Programmer’s Model
The ARM instruction set does not contain any instructions that directly select the endianness. Instead, an
ARM implementation which supports both endiannesses has a hardware input to configure it to match the
endianness of the attached memory system. If a standard System Control coprocessor is attached to such an
ARM implementation, this configuration input can be changed by writing to bit[7] of register 1 of the
System Control coprocessor (see Register 1: Control register on page B2-13).
If an ARM implementation is configured for a memory system of one endianness but is actually attached to
a memory system of the opposite endianness, only word-sized instruction fetches, data loads and data stores
can be relied upon. Other memory accesses have UNPREDICTABLE results.
When the standard System Control coprocessor is attached to an ARM processor that supports both
endiannesses, bit[7] of the coprocessor’s register 1 is cleared on reset. This means that the ARM processor
is configured for a little-endian memory system immediately after reset. If it is attached to a big-endian
memory system, one of the first things the reset handler must do is switch the configured endianness to
big-endian, using an instruction sequence like:
MRC p15, 0, r0, c1, c0 ; r0 := CP15 register 1
ORR r0, r0, #0x80 ; Set bit[7] in r0
MCR p15, 0, r0, c1, c0 ; CP15 register 1 := r0
This must be done before there is any possibility of a byte or halfword data access occurring, or of a Thumb
instruction being executed.
Note
The rules on endianness imply that word loads and stores are not affected by the configured endianness.
Because of this, it is not possible to reverse the order of the bytes in a word by storing it, changing the
configured endianness, and reloading the stored word. Instead, use one of the code sequences in Swapping
endianness on page A9-4.
More generally, there is no point in changing the configured endianness of an ARM processor to be different
from that of the memory system it is attached to, because no additional architecturally defined operations
become available as a result of doing so. So normally, the only time the configured endianness is changed
is at reset to make it match the memory system endianness.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-25](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-57-320.jpg)
![Programmer’s Model
2.7.3 Unaligned memory accesses
The ARM architecture normally expects all memory accesses to be suitably aligned. In particular, the
address used for a word access should normally be word-aligned, and the address used for a halfword access
should normally be halfword-aligned. Memory accesses which are not aligned in this way are called
unaligned memory accesses.
Unaligned instruction fetches
If an address which is not word-aligned is written to R15 during ARM state execution, the result is normally
either UNPREDICTABLE or that bits[1:0] of the address are ignored. If an address which is not
halfword-aligned is written to R15 during Thumb state execution, bit[0] of the address is normally ignored.
See Writing the program counter on page A2-8 and individual instruction descriptions for more details. As
a result, a value which is read from R15 during execution of valid code always has bits[1:0] zero for ARM
state execution and bit[0] zero for Thumb state execution.
When it is specified that these bits are ignored, ARM implementations are not required to ensure that they
are cleared from the address sent to memory for the instruction fetch. They can instead send the value
written to R15 unchanged to memory, and require the memory system to ignore bits[1:0] of the address for
an ARM instruction fetch and bit[0] for a Thumb instruction fetch.
Unaligned data accesses
The architecturally defined behavior of a load/store instruction which generates an unaligned access is one
of the following:
• It is UNPREDICTABLE.
• It ignores the low-order address bits that make the access unaligned. This means it effectively uses
the formula (address AND 0xFFFFFFFE) for a halfword access, and uses the formula
(address AND 0xFFFFFFFC)for a word access.
• It ignores the low-order address bits that make the access unaligned for the memory access itself, but
then uses those low-order bits to control a rotation of the loaded data. (This behavior applies only to
the LDR and SWP instructions.)
Which of these three options applies to a load/store instruction depends on which instruction it is, and is
documented on the individual instruction pages.
ARM implementations are not required to ensure that the low-order address bits that make an access
unaligned are cleared from the address they send to memory. They can instead send the address as calculated
by the load/store instruction unchanged to memory, and require the memory system to ignore address[0] for
a halfword access and address[1:0] for a word access.
Note
When an instruction ignores the low-order address bits that make an access unaligned, the pseudo-code in
the instruction description does not mask them out explicitly. Instead, the Memory[<address>,<size>]
function used in the pseudo-code masks them out implicitly. This function is defined in the Glossary.
A2-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-58-320.jpg)
![The ARM Instruction Set
3.1 Instruction set encoding
Figure 3-1 shows the ARM instruction set encoding.
All other bit patterns are UNPREDICTABLE or UNDEFINED. See Extending the instruction set on page A3-27
for a description of the cases where instructions are UNDEFINED.
An entry in square brackets, for example [1], indicates that more information is given after the figure.
3 1 3 0 2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0 1 9 1 8 1 7 1 6 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0
Data processing immediate shift cond [1] 0 0 0 opcode S Rn Rd shift amount shift 0 Rm
Miscellaneous instructions:
x x x x x x x x x x x x x x x x x x x
See Figure 3-3 cond [1] 0 0 0 1 0 x x 0 0
Data processing register shift [2] cond [1] 0 0 0 opcode S Rn Rd Rs 0 shift 1 Rm
Miscellaneous instructions: x x x x x x x x x x x x 0 x x x x x x
cond [1] 0 0 0 1 0 x x 0 1
Multiplies, extra load/stores: x x x x x x x x x x x x 1 x x x x x x
cond [1] 0 0 0 x x x x x 1
See Figure 3-3
See Figure 3-2
Data processing immediate [2] cond [1] 0 0 1 opcode S Rn Rd rotate immediate
Undefined instruction [3] cond [1] 0 0 1 1 0 x 0 0 x x x x x x x x x x x x x x x x x x x x
Move immediate to status register cond [1] R Mask SBO rotate immediate
0 0 1 1 0 1 0
Load/store immediate offset cond [1] 0 1 0 P U B W L Rn Rd immediate
Load/store register offset cond [1] 0 1 1 P U B W L Rn Rd shift amount shift 0 Rm
Undefined instruction cond [1] 0 1 1 x x x x x x x x x x x x x x x x x x x x 1 x x x x
Undefined instruction [4,7] 0 x x x x x x x x x x x x x x x x x x x x x x x x x x x
1 1 1 1
Load/store multiple cond [1] 1 0 0 P U S W L Rn register list
Undefined instruction [4] 1 0 0 x x x x x x x x x x x x x x x x x x x x x x x x x
1 1 1 1
1 0 1 L
Branch and branch with link cond [1] 24-bit offset
Branch and branch with link
and change to Thumb [4] 1 1 1 1
1 0 1 H 24-bit offset
1 1 0 P
cond [5] U N W L Rn CRd cp_num 8-bit offset
Coprocessor data processing cond [5] opcode1 CRn CRd cp_num opcode2 0 CRm
1 1 1 0
Coprocessor register transfers cond [5] opcode1 L CRn Rd cp_num opcode2 1 CRm
1 1 1 0
1 1 1 1
Software interrupt cond [1] swi number
Undefined instruction [4] 1 1 1 1 x x x x x x x x x x x x x x x x x x x x x x x x
Figure 3-1 ARM instruction set summary
Coprocessor load/store and double
register transfers [6]
1 1 1 1
A3-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-68-320.jpg)
![The ARM Instruction Set
1. The cond field is not allowed to be 1111 in this line. Other lines deal with the cases where
bits[31:28] of the instruction are 1111.
2. If the opcode field is of the form 10xx and the S field is 0, one of the following lines applies instead.
3. UNPREDICTABLE prior to ARM architecture version 4.
4. UNPREDICTABLE prior to ARM architecture version 5.
5. If the cond field is 1111, this instruction is UNPREDICTABLE prior to ARM architecture version 5.
6. The coprocessor double register transfer instructions are described in Chapter A10 Enhanced DSP
Extension.
7. In E variants of architecture version 5 and above, the cache preload instruction PLD uses a small
number of these instruction encodings.
3.1.1 Multiplies and extra load/store instructions
Figure 3-2 shows extra multiply and load/store instructions. An entry in square brackets, for example [1],
indicates that more information is given below the figure.
3 1 3 0 2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0 1 9 1 8 1 7 1 6 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0
cond 0 0 0 0 0 0 A S Rd Rn Rs 1 0 0 1 Rm
cond 0 0 0 0 1 U A S RdHi RdLo Rs 1 0 0 1 Rm
cond 0 0 0 1 0 B 0 0 Rn Rd SBZ 1 0 0 1 Rm
cond 0 0 0 P U 0 W L Rn Rd SBZ 1 0 1 1 Rm
cond Rd Rn 0 0 0 1 U P L W HiOffset 1 0 1 1 LoOffset
cond 0 0 0 P U 0 W 0 Rn Rd SBZ 1 1 S 1 Rm
Load/store two words
register offset [2]
Load signed halfword/byte
register offset [1] cond 0 0 0 P U 0 W 1 Rn Rd SBZ 1 1 H 1 Rm
Load/store two words
immediate offset [2] cond 0 0 0 P U 1 W 0 Rn Rd HiOffset 1 1 S 1 LoOffset
immediate offset [1] cond 0 0 0 P U 1 W 1 Rn Rd HiOffset 1 1 H 1 LoOffset
Figure 3-2 Multiplies and extra load/store instructions
Multiply (accumulate)
Multiply (accumulate) long
Swap/swap byte
Load/store halfword
register offset [1]
Load/store halfword
immediate offset [1]
Load signed halfword/byte
1. UNPREDICTABLE prior to ARM architecture version 4.
2. These instructions are described in Chapter A10 Enhanced DSP Extension.
Note
Any instruction with bits[27:25] = 000, bit[7] = 1, bit[4] = 1, and cond not equal to 1111, and which is not
specified in Figure 3-2 or its notes, is an undefined instruction (or UNPREDICTABLE prior to ARM
architecture version 4).
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-3](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-69-320.jpg)
![The ARM Instruction Set
3.1.2 Miscellaneous instructions
Figure 3-3 shows the remaining ARM instruction encodings. An entry in square brackets, for example [1],
indicates that more information is given below the figure.
3 1 3 0 2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0 1 9 1 8 1 7 1 6 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0
Move status register to register cond 0 0 0 1 0 R 0 0 SBO Rd SBZ 0 0 0 0 SBZ
Move register to status register cond 0 0 0 1 0 R 1 0 mask SBO SBZ 0 0 0 0 Rm
Branch/exchange instruction set [1] cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 0 1 Rm
Count leading zeros [2] cond 0 0 0 1 0 1 1 0 SBO Rd SBO 0 0 0 1 Rm
Branch and link/exchange instruction set [2] cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 1 1 Rm
Enhanced DSP add/subtracts [4] cond 0 0 0 1 0 op 0
Rn Rd SBZ 0 1 0 1 Rm
Software breakpoint [2,3] cond 0 0 0 1 0 0 1 0 immed 0 1 1 1 immed
Enhanced DSP multiplies[4] cond 0 0 0 1 0 op 0 Rd Rn Rs 1 y x 0 Rm
Figure 3-3 Miscellaneous instructions
1. Defined in ARM architecture version 5 and above, and in T variants of ARM architecture version 4.
2. This is an undefined instruction is ARM architecture version 4, and is UNPREDICTABLE prior to ARM
architecture version 4.
3. If the cond field of this instruction is not 1110, it is UNPREDICTABLE.
4. The enhanced DSP instructions are described in Chapter A10 Enhanced DSP Extension.
Note
Any instruction with bits[27:23] = 00010, bit[20] = 0, bit[7] and bit[4] not both 1, and cond is not equal to
1111, and which is not specified in Figure 3-3 or its notes, is an undefined instruction (or UNPREDICTABLE
prior to architecture version 4).
A3-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-70-320.jpg)
![The ARM Instruction Set
3.2 The condition field
Almost all ARM instructions can be conditionally executed, which means that they only have their normal
effect on the programmer’s model state, memory and coprocessors if the N, Z, C and V flags in the CPSR
satisfy a condition specified in the instruction. If the flags do not satisfy this condition, the instruction acts
as a NOP: that is, execution advances to the next instruction as normal, including any relevant checks for
interrupts and prefetch aborts, but has no other effect.
Prior to ARM architecture version 5, all ARM instructions could be conditionally executed. A few
instructions have been introduced subsequently which can only be executed unconditionally.
Every instruction contains a 4-bit condition code field in bits 31 to 28:
31 28 27 0
cond
This field contains one of the 16 values described in Table 3-1 on page A3-6. Most instruction mnemonics
can be extended with the letters defined in the mnemonic extension field.
If the always (AL) condition is specified, the instruction is executed irrespective of the value of the
condition code flags. The absence of a condition code on an instruction mnemonic implies the AL condition
code.
3.2.1 Condition code 0b1111
As indicated in Table 3-1 on page A3-6, if the condition field is 0b1111, the behavior depends on the
architecture version:
• Prior to ARM architecture version 3, a condition field of 0b1111 meant that the instruction was never
executed. The mnemonic extension for this condition was NV.
Note
Use of this condition is now obsolete and unsupported.
• In ARM architecture version 3 and version 4, any instruction with a condition field of 0b1111 is
UNPREDICTABLE.
• In ARM architecture version 5 and above, a condition field of 0b1111 is used to encode various
additional instructions which can only be executed unconditionally. All instruction encoding
diagrams which show bits[31:28] as cond only match instructions in which these bits are not equal
to 0b1111, unless otherwise stated in the individual instruction description.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-71-320.jpg)
![The ARM Instruction Set
Table 3-1 Condition codes
Opcode
[31:28]
Mnemonic
extension Meaning Condition flag state
0000 EQ Equal Z set
0001 NE Not equal Z clear
0010 CS/HS Carry set/unsigned higher or same C set
0011 CC/LO Carry clear/unsigned lower C clear
0100 MI Minus/negative N set
0101 PL Plus/positive or zero N clear
0110 VS Overflow V set
0111 VC No overflow V clear
1000 HI Unsigned higher C set and Z clear
1001 LS Unsigned lower or same C clear or Z set
1010 GE Signed greater than or equal N set and V set, or
N clear and V clear (N == V)
1011 LT Signed less than N set and V clear, or
N clear and V set (N != V)
1100 GT Signed greater than Z clear, and either N set and V set, or
N clear and V clear (Z == 0,N == V)
1101 LE Signed less than or equal Z set, or N set and V clear, or
N clear and V set (Z == 1 or N != V)
1110 AL Always (unconditional) -
1111 (NV) See Condition code 0b1111 on page A3-5 -
A3-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-72-320.jpg)
![The ARM Instruction Set
3.3 Branch instructions
All ARM processors support a branch instruction that allows a conditional branch forwards or backwards
up to 32MB. As the PC is one of the general-purpose registers (R15), a branch or jump can also be generated
by writing a value to R15.
A subroutine call can be performed by a variant of the standard branch instruction. As well as allowing a
branch forward or backward up to 32MB, the Branch with Link (BL) instruction preserves the address of
the instruction after the branch (the return address) in the LR (R14).
In T variants of ARM architecture version 4, and in ARM architecture version 5 and above, the Branch and
Exchange (BX) instruction copies the contents of a general-purpose register Rm to the PC (like a MOV
PC,Rm instruction), with the additional functionality that if bit[0] of the transferred value is 1, the processor
shifts to Thumb state. Together with the corresponding Thumb instructions, this allows interworking
branches between ARM and Thumb code.
Interworking subroutine calls can be generated by combining BX with an instruction to write a suitable
return address to the LR, such as an immediately preceding MOV LR,PC instruction.
In ARM architecture version 5 and above, there are also two types of Branch with Link and Exchange (BLX)
instruction:
• One type takes a register operand Rm, like a BX instruction. This instruction behaves like a BX
instruction, and additionally writes the address of the next instruction into the LR. This provides a
more efficient interworking subroutine call than a sequence of MOV LR,PC followed by BX Rm.
• The other type behaves like a BL instruction, branching backwards or forwards by up to 32MB and
writing a return link to the LR, but shifts to Thumb state rather than staying in ARM state as BL does.
This provides a more efficient alternative to loading the subroutine address into Rm followed by a
BLX Rm instruction when it is known that a Thumb subroutine is being called and that the subroutine
lies within the 32MB range.
A load instruction provides a way to branch anywhere in the 4GB address space (known as a long branch).
A 32-bit value is loaded directly from memory into the PC, causing a branch. A long branch can be preceded
by MOV LR,PC or another instruction that writes the LR to generate a long subroutine call. In ARM
architecture version 5 and above, bit[0] of the value loaded by a long branch controls whether the subroutine
is executed in ARM state or Thumb state, just like bit[0] of the value moved to the PC by a BX instruction.
Prior to ARM architecture version 5, bits[1:0] of the value loaded into the PC are ignored, and a load into
the PC can only be used to call a subroutine in ARM state.
In non-T variants of ARM architecture version 5, the instructions described above can cause an entry into
Thumb state despite the fact that the Thumb instruction set is not present. This causes the instruction at the
branch target to enter the undefined instruction trap. See The control bits on page A2-10 for more details.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-73-320.jpg)
![The ARM Instruction Set
3.7 Status register access instructions
There are two instructions for moving the contents of a program status register to or from a general-purpose
register. Both the CPSR and SPSR can be accessed.
Each status register is split into four 8-bit fields that can be individually written:
Bits[31:24] The flags field.
Bits[23:16] The status field.
Bits[15:8] The extension field.
Bits[7:0] The control field.
To date, the ARM architecture does not use the status and extension fields, and three bits are unused in the
flags field. The four condition code flags occupy bits[31:28]. In E variants of architecture versions 5 and
above, the Q flag occupies bit[27]. See The Q flag on page A10-5 for more information on the Q flag. The
control field contains two interrupt disable bits, five processor mode bits, and the Thumb bit on ARM
architecture version 5 and above and on T variants of ARM architecture version 4 (see The T bit on
page A2-11).
The unused bits of the status registers might be used in future ARM architectures, and must not be modified
by software. Therefore, a read-modify-write strategy must be used to update the value of a status register to
ensure future compatibility.
The status registers are readable to allow the read part of the read-modify-write operation, and to allow all
processor state to be preserved (for instance, during process context switches).
The status registers are writable to allow the write part of the read-modify-write operation, and allow all
processor state to be restored.
3.7.1 CPSR value
Altering the value of the CPSR has three uses:
• sets the value of the condition code flags (and of the Q flag when it exists) to a known value
• enables or disable interrupts
• changes processor mode (for instance, to initialize stack pointers).
Note
The T bit must not be changed directly by writing to the CPSR, but only via the BX instruction, and in the
implicit SPSR to CPSR moves in instructions designed for exception return. Attempts to enter or leave
Thumb state by directly altering the T bit can have UNPREDICTABLE consequences.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-81-320.jpg)
![The ARM Instruction Set
H bit Distinguishes between a halfword (H==1) and a signed byte (H==0) access. If the S bit and
H bit are both zero, this instruction encodes a SWP or Multiply instruction.
Rn Specifies the base register used by the addressing mode.
Rd Specifies the register whose contents are to be loaded or stored.
3.8.4 Examples
LDR R1, [R0] ; Load R1 from the address in R0
LDR R8, [R3, #4] ; Load R8 from the address in R3 + 4
LDR R12, [R13, #-4] ; Load R12 from R13 - 4
STR R2, [R1, #0x100] ; Store R2 to the address in R1 + 0x100
LDRB R5, [R9] ; Load byte into R5 from R9
; (zero top 3 bytes)
LDRB R3, [R8, #3] ; Load byte to R3 from R8 + 3
; (zero top 3 bytes)
STRB R4, [R10, #0x200] ; Store byte from R4 to R10 + 0x200
LDR R11, [R1, R2] ; Load R11 from the address in R1 + R2
STRB R10, [R7, -R4] ; Store byte from R10 to addr in R7 - R4
LDR R11, [R3, R5, LSL #2] ; Load R11 from R3 + (R5 x 4)
LDR R1, [R0, #4]! ; Load R1 from R0 + 4, then R0 = R0 + 4
STRB R7, [R6, #-1]! ; Store byte from R7 to R6 - 1,
; then R6 = R6 - 1
LDR R3, [R9], #4 ; Load R3 from R9, then R9 = R9 + 4
STR R2, [R5], #8 ; Store R2 to R5, then R5 = R5 + 8
LDR R0, [PC, #40] ; Load R0 from PC + 0x40 (= address of
; the LDR instruction + 8 + 0x40)
LDR R0, [R1], R2 ; Load R0 from R1, then R1 = R1 + R2
LDRH R1, [R0] ; Load halfword to R1 from R0
; (zero top 2 bytes)
LDRH R8, [R3, #2] ; Load halfword into R8 from R3 + 2
LDRH R12, [R13, #-6] ; Load halfword into R12 from R13 - 6
STRH R2, [R1, #0x80] ; Store halfword from R2 to R1 + 0x80
LDRSH R5, [R9] ; Load signed halfword to R5 from R9
LDRSB R3, [R8, #3] ; Load signed byte to R3 from R8 + 3
LDRSB R4, [R10, #0xC1] ; Load signed byte to R4 from R10 + 0xC1
LDRH R11, [R1, R2] ; Load halfword into R11 from address
; in R1 + R2
STRH R10, [R7, -R4] ; Store halfword from R10 to R7 - R4
LDRSH R1, [R0, #2]! ; Load signed halfword R1 from R0 + 2,
; then R0 = R0 + 2
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-19](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-85-320.jpg)
![The ARM Instruction Set
LDRSB R7, [R6, #-1]! ; Load signed byte to R7 from R6 - 1,
; then R6 = R6 - 1
LDRH R3, [R9], #2 ; Load halfword to R3 from R9,
; then R9 = R9 + 2
STRH R2, [R5], #8 ; Store halfword from R2 to R5,
; then R5 = R5 + 8
3.8.5 List of load and store instructions
LDR Load Word. See LDR on page A4-37.
LDRB Load Byte. See LDRB on page A4-40.
LDRBT Load Byte with User Mode Privilege. See LDRBT on page A4-42.
LDRH Load Unsigned Halfword. See LDRH on page A4-44.
LDRSB Load Signed Byte. See LDRSB on page A4-46.
LDRSH Load Signed Halfword. See LDRSH on page A4-48.
LDRT Load Word with User Mode Privilege. See LDRT on page A4-50.
STR Store Word. See STR on page A4-88.
STRB Store Byte. See STRB on page A4-90.
STRBT Store Byte with User Mode Privilege. See STRBT on page A4-92.
STRH Store Halfword. See STRH on page A4-94.
STRT Store Word with User Mode Privilege. See STRT on page A4-96.
A3-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-86-320.jpg)
![The ARM Instruction Set
3.10 Semaphore instructions
The ARM instruction set has two semaphore instructions:
• Swap (S) WP
• Swap Byte (SWPB).
These instructions are provided for process synchronization. Both instructions generate an atomic load and
store operation, allowing a memory semaphore to be loaded and altered without interruption.
SWP and SWPB have a single addressing mode, whose address is the contents of a register. Separate registers
are used to specify the value to store and the destination of the load. If the same register is specified for both
of these, SWP exchanges the value in the register and the value in memory.
The semaphore instructions do not provide a compare and conditional write facility. If wanted, this must be
done explicitly.
3.10.1 Examples
SWP R12, R10, [R9] ; load R12 from address R9 and
; store R10 to address R9
SWPB R3, R4, [R8] ; load byte to R3 from address R8 and
; store byte from R4 to address R8
SWP R1, R1, [R2] ; Exchange value in R1 and address in R2
3.10.2 List of semaphore instructions
SWP Swap. See SWP on page A4-102.
SWPB Swap Byte. See SWPB on page A4-104.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-23](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-89-320.jpg)
![The ARM Instruction Set
LDC p6, CR1, [R4] ; Load from memory to coprocessor 6
; ARM register 4 contains the address
; Load to CP reg 1
LDC p6, CR4, [R2, #4] ; Load from memory to coprocessor 6
; ARM register R2 + 4 is the address
; Load to CP reg 4
STC p8, CR8, [R2, #4]! ; Store from coprocessor 8 to memory
; ARM register R2 + 4 is the address
; after the transfer R2 = R2 + 4
; Store from CP reg 8
STC p8, CR9, [R2], #-16 ; Store from coprocessor 8 to memory
; ARM register R2 holds the address
; after the transfer R2 = R2 - 16
; Store from CP reg 9
3.12.2 List of coprocessor instructions
CDP Coprocessor Data Operations. See CDP on page A4-20.
LDC Load Coprocessor Register. See LDC on page A4-28.
MCR Move to Coprocessor from ARM Register. See MCR on page A4-52.
MRC Move to ARM Register from Coprocessor. See MRC on page A4-58.
STC Store Coprocessor Register. See STC on page A4-82.
Note
Coprocessor instructions are not implemented in ARM architecture version 1.
A3-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-92-320.jpg)
![The ARM Instruction Set
3.13 Extending the instruction set
Successive versions of the ARM architecture have extended the instruction set in a number of areas. This
section describes the six areas where extensions have occurred, and where further extensions might occur
in the future:
• Undefined instruction space on page A3-28
• Arithmetic instruction extension space on page A3-29
• Control instruction extension space on page A3-30
• Load/store instruction extension space on page A3-32
• Coprocessor instruction extension space on page A3-33
• Unconditional instruction extension space on page A3-34.
Instructions in these areas which have not yet been allocated a meaning are either UNDEFINED or
UNPREDICTABLE. To determine which, use the following rules:
1. The decode bits of an instruction are defined to be bits[27:20] and bits[7:4].
In ARM architecture version 5 and above, the result of ANDing bits[31:28] together is also a decode
bit. This bit determines whether the condition field is 0b1111, which is used in ARM architecture
version 5 and above to encode various instructions which can only be executed unconditionally. See
Condition code 0b1111 on page A3-5 and Unconditional instruction extension space on page A3-34
for more information.
2. If the decode bits of an instruction are equal to those of a defined instruction, but the whole instruction
is not a defined instruction, then the instruction is UNPREDICTABLE.
For example, suppose an instruction has:
• bits[31:28] not equal to 0b1111
• bits[27:20] equal to 0b00010000
• bits[7:4] equal to 0b0000
but where:
• bit[11] of the instruction is 1.
Here, the instruction is in the control instruction extension space and has the same decode bits as an
MRS instruction, but is not a valid MRS instruction because bit[11] of an MRS instruction should be
zero. Using the above rule, this instruction is UNPREDICTABLE.
3. In ARM architecture version 4 and above, if the decode bits of an instruction are not equal to those
of any defined instruction, then the instruction is UNDEFINED.
4. In ARM architecture version 3 and below, if the decode bits of an instruction are not equal to those
of any defined instruction, then the instruction is:
• UNDEFINED if it is in the undefined instruction space
• UNPREDICTABLE if it is in any of the other five areas.
Each of rules 2 to 4 above applies separately to each ARM architecture version. As a result, the status of an
instruction might differ between architecture versions. Usually, this happens because an instruction which
was UNPREDICTABLE or UNDEFINED in an earlier architecture version becomes a defined instruction in a later
version.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-27](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-93-320.jpg)
![The ARM Instruction Set
3.13.1 Undefined instruction space
Instructions with the following opcodes are undefined instruction space:
opcode[27:25] = 0b011
opcode[4] = 1
31 28 27 26 25 24 5 4 3 0
cond 0 1 1 x x x x x x x x x x x x x x x x x x x x 1 x x x x
The meaning of instructions in the undefined instruction space is UNDEFINED on all versions of the ARM
architecture.
In general, undefined instructions might be used to extend the ARM instruction set in the future. However,
it is intended that instructions with the following encoding will not be used for this:
31 28 27 26 25 24 23 22 21 20 19 8 7 6 5 4 3 2 1 0
cond 0 1 1 1 1 1 1 1 x x x x x x x x x x x x 1 1 1 1 x x x x
If a programmer wants to use an undefined instruction for software purposes, with minimal risk that future
hardware will treat it as a defined instruction, one of the instructions with this encoding must be used.
A3-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-94-320.jpg)
![The ARM Instruction Set
3.13.2 Arithmetic instruction extension space
Instructions with the following opcodes are the arithmetic instruction extension space:
opcode[27:24] == 0b0000
opcode[7:4] == 0b1001
opcode[31:28] != 0b1111 /* Only required for version 5 and above */
The field names given are guidelines suggested to simplify implementation.
31 28 27 26 25 24 23 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 op1 Rn Rd Rs 1 0 0 1 Rm
Table 3-3 summarizes the instructions that have already been allocated in this area.
Table 3-3 Arithmetic instruction space
Instructions op1 Architecture versions
MUL, MULS 000S Version 2 and above
MLA, MLAS 001S Version 2 and above
UMULL, UMULLS 100S All M variants
UMLAL, UMLALS 101S All M variants
SMULL, SMULLS 110S All M variants
SMLAL, SMLALS 111S All M variants
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-29](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-95-320.jpg)
![The ARM Instruction Set
3.13.3 Control instruction extension space
Instructions with the following opcodes are the control instruction space.
opcode[27:26] == 0b00
opcode[24:23] == 0b10
opcode[20] == 0
opcode[31:28] != 0b1111 /* Only required for version 5 and above */
and not:
opcode[25] == 0
opcode[7] == 1
opcode[4] == 1
The field names given are guidelines suggested to simplify implementation.
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 op1 0 Rn Rd Rs op2 0 Rm
cond 0 0 0 1 0 op1 0 Rn Rd Rs 0 op2 1 Rm
cond 0 0 1 1 0 op1 0 Rn Rd rotate_imm immed_8
Table 3-4 summarizes the instructions that have already been allocated in this area.
Table 3-4 Control extension space instructions
Instruction Bit[25] Bits[7:4] op1 Architecture versions
MRS 0 0000 x0 Version 3 and above
MSR (register form) 0 0000 x1 Version 3 and above
BX 0 0001 01 Version 5 and above, plus T variants of
version 4
CLZ 0 0001 11 Version 5 and above
BLX (register form) 0 0011 01 Version 5 and above
QADD 0 0101 00 E variants of version 5 and above
QSUB 0 0101 01 E variants of version 5 and above
QDADD 0 0101 10 E variants of version 5 and above
QDSUB 0 0101 11 E variants of version 5 and above
A3-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-96-320.jpg)
![The ARM Instruction Set
Table 3-4 Control extension space instructions (continued)
Instruction Bit[25] Bits[7:4] op1 Architecture versions
BKPT 0 0111 01 Version 5 and above
SMLA<x><y> 0 1yx0 00 E variants of version 5 and above
SMLAW<y> 0 1y00 01 E variants of version 5 and above
SMULW<y> 0 1y10 01 E variants of version 5 and above
SMLAL<x><y> 0 1yx0 10 E variants of version 5 and above
SMUL<x><y> 0 1yx0 11 E variants of version 5 and above
MSR (immediate form) 1 xxxx x1 Version 3 and above
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-31](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-97-320.jpg)
![The ARM Instruction Set
3.13.4 Load/store instruction extension space
Instructions with the following opcodes are the load/store instruction extension space:
opcode[27:25] == 0b000
opcode[7] == 1
opcode[4] == 1
opcode[31:28] != 0b1111 /* Only required for version 5 and above */
and not:
opcode[24] == 0
opcode[6:5] == 0
The field names given are guidelines suggested to simplify implementation.
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 P U B W L Rn Rd Rs 1 op1 1 Rm
Table 3-5 summarizes the instructions that have already been allocated in this area.
Table 3-5 Load/store instructions
Instruction Bits[24:20] op1 Architecture versions
SWP/SWPB 1 0 B 0 0 0 0 Version 3 and above, plus ARMv2a
STRH P U I W 0 0 1 Version 4 and above
LDRD P U I W 0 1 0 E variants of version 5 and above,
except v5TExP
STRD P U I W 0 1 1 E variants of version 5 and above,
except v5TExP
LDRH P U I W 1 0 1 Version 4 and above
LDRSB P U I W 1 1 0 Version 4 and above
LDRSH P U I W 1 1 1 Version 4 and above
A3-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-98-320.jpg)
![The ARM Instruction Set
3.13.5 Coprocessor instruction extension space
Instructions with the following opcodes are the coprocessor instruction extension space:
opcode[27:23] == 0b11000
opcode[21] == 0
The field names given are guidelines suggested to simplify implementation.
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 0 0 x 0 x Rn CRd cp_num offset
In ARM architecture version 3 and below, all instructions in the coprocessor instruction extension space are
UNPREDICTABLE.
In all variants of architecture version 4, and in non-E variants of architecture 5, all instructions in the
coprocessor instruction extension space are UNDEFINED. It is IMPLEMENTATION DEFINED how an ARM
processor achieves this. The options are:
• The ARM processor might take the undefined instruction trap directly.
• The ARM processor might require attached coprocessors not to respond to such instructions. This
causes the undefined instruction trap to be taken (see Undefined Instruction exception on
page A2-15).
In E variants of architecture version 5, instructions in the coprocessor instruction extension space are treated
as follows:
• Instructions with bit[22] == 0 are UNDEFINED and are handled in precisely the same way as described
above for non-E variants.
• Instructions with bit[22] ==1 are the MCRR and MRRC instructions described in Chapter A10
Enhanced DSP Extension.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-33](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-99-320.jpg)
![The ARM Instruction Set
3.13.6 Unconditional instruction extension space
In ARM architecture version 5 and above, instructions with the following opcode are the unconditional
instruction space:
opcode[31:28] == 0b1111
31 30 29 28 27 20 19 8 7 4 3 0
1 1 1 1 opcode1 x x x x x x x x x x x x opcode2 x x x x
Table 3-6 summarizes the instructions that have already been allocated in this area.
Table 3-6 Unconditional instruction extension space
Instruction opcode1 opcode2 Architecture versions
PLD 0 1 I 1 U 1 0 1 x x x x
E variants of version 5 and
above, except v5TExP
BLX
(address form)
1 0 1 x x x x x x x x x Version 5 and above
STC2 1 1 0 x x x x 0 x x x x Version 5 and above
LDC2 1 1 0 x x x x 1 x x x x Version 5 and above
CDP2 1 1 1 0 x x x x x x x 0 Version 5 and above
MCR2 1 1 1 0 x x x 0 x x x 1 Version 5 and above
MRC2 1 1 1 0 x x x 1 x x x 1 Version 5 and above
A3-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-100-320.jpg)
![ARM Instructions
4.1.2 ADC
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 1 0 1 S Rn Rd shifter_operand
The ADC (Add with Carry) instruction adds the value of <shifter_operand> and the Carry flag to the
value of <Rn> and stores the result in <Rd>. The condition code flags are optionally updated, based on the
result.
Syntax
ADC{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the
instruction. Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the addition,
and the C and V flags are set according to whether the addition generated a carry
(unsigned overflow) and a signed overflow, respectively. The rest of the CPSR is
unchanged.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the addition.
<shifter_operand>
Specifies the second operand for the addition. The options for this operand are described in
Addressing Mode 1 - Data-processing operands on page A5-2, including how each option
causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not ADC.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-104-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn + shifter_operand + C Flag
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = CarryFrom(Rn + shifter_operand + C Flag)
V Flag = OverflowFrom(Rn + shifter_operand + C Flag)
Usage
ADC is used to synthesize multi-word addition. If register pairs R0, R1 and R2, R3 hold 64-bit values (where
R0 and R2 hold the least significant words) the following instructions leave the 64-bit sum in R4, R5:
ADDS R4,R0,R2
ADC R5,R1,R3
If the second instruction is changed from:
ADC R5,R1,R3
to:
ADCS R5,R1,R3
the resulting values of the flags indicate:
N The 64-bit addition produced a negative result.
C An unsigned overflow occurred.
V A signed overflow occurred.
Z The most significant 32 bits are all zero.
The following instruction produces a single-bit Rotate Left with Extend operation (33-bit rotate through the
Carry flag) on R0:
ADCS R0,R0,R0
See Data-processing operands - Rotate right with extend on page A5-17 for information on how to perform
a similar rotation to the right.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-105-320.jpg)
![ARM Instructions
4.1.3 ADD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 1 0 0 S Rn Rd shifter operand
The ADD instruction adds the value of <shifter_operand> to the value of register <Rn>, and stores
the result in the destination register <Rd>. The condition code flags are optionally updated, based on the
result.
Syntax
ADD{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The condition field on page A3-5.
If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the
instruction. Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the addition,
and the C and V flags are set according to whether the addition generated a carry
(unsigned overflow) and a signed overflow, respectively. The rest of the CPSR is
unchanged.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the addition.
<shifter_operand>
Specifies the second operand for the addition. The options for this operand are described in
Addressing Mode 1 - Data-processing operands on page A5-2, including how each option
causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not ADD.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-106-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn + shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = CarryFrom(Rn + shifter_operand)
V Flag = OverflowFrom(Rn + shifter_operand)
Usage
The ADD instruction is used to add two values together to produce a third.
To increment a register value in Rx use:
ADD Rx, Rx, #1
Constant multiplication of Rx by 2n+1 into Rd can be performed with:
ADD Rd, Rx, Rx, LSL #n
To form a PC-relative address use:
ADD Rs, PC, #offset
where the offset must be the difference between the required address and the address held in the PC, where
the PC is the address of the ADD instruction itself plus 8 bytes.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-107-320.jpg)
![ARM Instructions
4.1.4 AND
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 0 0 0 S Rn Rd shifter_operand
The AND instruction performs a bitwise AND of the value of register <Rn> with the value of
<shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags
are optionally updated, based on the result.
Syntax
AND{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the
instruction. Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the operation,
and the C flag is set to the carry output bit generated by the shifter (see Addressing
Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the
CPSR are unaffected.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the operation.
<shifter_operand>
Specifies the second operand for the operation. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not AND.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-108-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn AND shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
Usage
AND is most useful for extracting a field from a register, by ANDing the register with a mask value that has
1s in the field to be extracted, and 0s elsewhere.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-109-320.jpg)
![ARM Instructions
Usage
The BL instruction is used to perform a subroutine call. The return from subroutine is achieved by copying
the LR to the PC. Typically, this is done by one of the following methods:
• Executing a BX R14 instruction, on architecture versions that support that instruction.
• Executing a MOV PC,R14 instruction.
• Storing a group of registers and R14 to the stack on subroutine entry, using an instruction of the form:
STMFD R13!,{<registers>,R14}
and then restoring the register values and returning with an instruction of the form:
LDMFD R13!,{<registers>,PC}
To calculate the correct value of signed_immed_24, the assembler (or other toolkit component) needs to:
1. Form the base address for this branch instruction. This is the address of the instruction, plus 8. In
other words, this base address is equal to the PC value used by the instruction.
2. Subtract the base address from the target address to form a byte offset. This offset is always a multiple
of four, because all ARM instructions are word-aligned.
3. If the byte offset is outside the range -33554432 to +33554428, use an alternative code-generation
strategy or produce an error as appropriate.
4. Otherwise, set the signed_immed_24 field of the instruction to bits{25:2] of the byte offset.
Notes
Memory bounds Branching backwards past location zero and forwards over the end of the 32-bit
address space is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-11](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-111-320.jpg)
![ARM Instructions
4.1.6 BIC
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 1 1 0 S Rn Rd shifter_operand
The BIC (Bit Clear) instruction performs a bitwise AND of the value of register <Rn> with the complement
of the value of <shifter_operand>, and stores the result in the destination register <Rd>. The
condition code flags are optionally updated, based on the result.
Syntax
BIC{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the
instruction. Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the operation,
and the C flag is set to the carry output bit generated by the shifter (see Addressing
Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the
CPSR are unaffected.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the operation.
<shifter_operand>
Specifies the second operand for the operation. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not BIC.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-112-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn AND NOT shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
Usage
BIC can be used to clear selected bits in a register. For each bit, BIC with 1 clears the bit, and BIC with 0
leaves it unchanged.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-13](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-113-320.jpg)
![ARM Instructions
4.1.7 BKPT
31 28 27 26 25 24 23 22 21 20 19 8 7 4 3 0
1 1 1 0 0 0 0 1 0 0 1 0 immed 0 1 1 1 immed
The BKPT (Breakpoint) instruction causes a software breakpoint to occur. This breakpoint can be handled
by an exception handler installed on the prefetch abort vector. In implementations which also include debug
hardware, the hardware can optionally override this behavior and handle the breakpoint itself. When this
occurs, the prefetch abort vector is not entered.
Syntax
BKPT <immediate>
where:
<immediate> Is a 16-bit immediate value, the top 12 bits of which are placed in bits[19:8] of the
instruction, and the bottom 4 bits of which are placed in bits[3:0] of the instruction.
This value is ignored by the ARM hardware, but can be used by a debugger to store
additional information about the breakpoint.
Architecture version
Version 5 and above
Exceptions
Prefetch Abort
Operation
if (not overridden by debug hardware)
R14_abt = address of BKPT instruction + 4
SPSR_abt = CPSR
CPSR[4:0] = 0b10111 /* Enter Abort mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF000C
else
PC = 0x0000000C
A4-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-114-320.jpg)
![ARM Instructions
Usage
The exact usage of the BKPT instruction depends on the debug system being used. A debug system can use
the BKPT instruction in two ways:
• Debug hardware, if present, does not override the normal behavior of the BKPT instruction, and so
the prefetch abort vector is entered. If the system also allows real prefetch aborts to occur, the
prefetch abort handler determines, in a system-dependent manner, whether the vector entry occurred
as a result of a BKPT instruction or as a result of a real prefetch abort, and branches to debug code or
prefetch abort code accordingly. Otherwise, the prefetch abort handler just branches straight to debug
code.
When used in this manner, the BKPT instruction must be avoided within abort handlers, as it corrupts
R14_abt and SPSR_abt. For the same reason, it must also be avoided within FIQ handlers, since an
FIQ interrupt can occur within an abort handler.
• Debug hardware does override the normal behavior of the BKPT instruction and handles the software
breakpoint itself. When finished, it typically either resumes execution at the instruction following the
BKPT, or replaces the BKPT in memory with another instruction and resumes execution at that
instruction.
When BKPT is used in this manner, R14_abt and SPSR_abt are not corrupted, and so the above
restrictions about its use in abort and FIQ handlers do not apply.
Notes
Condition field The BKPT instruction must be unconditional. If bits[31:28] of the instruction
encode a valid condition other than the AL (always) condition, the instruction is
UNPREDICTABLE.
Hardware override Debug hardware in an implementation is specifically permitted to override the
normal behavior of the BKPT instruction. Because of this, software must not use this
instruction for purposes other than those documented by the debug system being
used (if any). In particular, software cannot rely on the Prefetch Abort exception
occurring, unless either there is guaranteed to be no debug hardware in the system
or the debug system specifies that it will occur.
For more information, consult the documentation for the debug system being used.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-115-320.jpg)
![ARM Instructions
4.1.8 BLX (1)
31 30 29 28 27 26 25 24 23 0
1 1 1 1 1 0 1 H signed_immed_24
This form of the BLX (Branch with Link and Exchange) instruction is used to call a Thumb subroutine from
the ARM instruction set at an address specified in the instruction. This instruction is unconditional (always
causing a change in program flow) and preserves the address of the instruction following the branch in the
link register (R14). Execution of Thumb instructions begins at the target address.
Syntax
BLX <target_addr>
where:
<target_addr> Specifies the address of the Thumb instruction to branch to. The branch target
address is calculated by:
1. Sign-extending the 24-bit signed (two’s complement) immediate to 32 bits
2. Shifting the result left two bits
3. Setting bit[1] of the result of step 2 to the H bit
4. Adding the result of step 3 to the contents of the PC, which contains the
address of the branch instruction plus 8.
The instruction can therefore specify a branch of approximately ±32MB.
Architecture version
Version 5 and above
Exceptions
None
Operation
LR = address of the instruction after the BLX instruction
T Flag = 1
PC = PC + (SignExtend(signed_immed_24) << 2) + (H << 1)
A4-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-116-320.jpg)
![ARM Instructions
Usage
To return from a Thumb subroutine called via BLX to the ARM caller, use the Thumb instruction:
BX R14
as described in BX on page A7-32, or use this instruction on subroutine entry:
PUSH {<registers>,R14}
and this instruction to return:
POP {<registers>,PC}
To calculate the correct value of signed_immed_24, the assembler (or other toolkit component) needs to:
1. Form the base address for this branch instruction. This is the address of the instruction, plus 8. In
other words, this base address is equal to the PC value used by the instruction.
2. Subtract the base address from the target address to form a byte offset. This offset is always even,
because all ARM instructions are word-aligned and all Thumb instructions are halfword-aligned.
3. If the byte offset is outside the range -33554432 to +33554430, use an alternative code-generation
strategy or produce an error as appropriate.
4. Otherwise, set the signed_immed_24 field of the instruction to bits{25:2] of the byte offset, and the
H bit of the instruction to bit[1] of the byte offset.
Notes
Condition Unlike most other ARM instructions, this instruction cannot be executed conditionally.
Bit[24] This bit is used as bit[1] of the target address.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-17](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-117-320.jpg)
![ARM Instructions
4.1.9 BLX (2)
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 1 1 Rm
This form of BLX is used to call an ARM or Thumb subroutine from the ARM instruction set, at an address
specified in a register. The branch target address is the value of register Rm, with its bit[0] forced to zero.
The instruction set to be used at the branch target is chosen by setting the CPSR T bit to bit[0] of Rm.
Register 14 is set to a return address. To return from the subroutine, use a BX R14 instruction, or store R14
on the stack and re-load the stored value into the PC.
Syntax
BLX{<cond>} <Rm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rm> Is the register containing the address of the target instruction. Bit[0] of Rm is 0 to select a
target ARM instruction, or 1 to select a target Thumb instruction. If R15 is specified for
<Rm>, the results are UNPREDICTABLE.
Architecture version
Version 5 and above
Exceptions
None
Operation
if ConditionPassed(cond) then
LR = address of instruction after the BLX instruction
T Flag = Rm[0]
PC = Rm AND 0xFFFFFFFE
Notes
ARM/Thumb state transfers
If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned
addresses are impossible in ARM state.
A4-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-118-320.jpg)
![ARM Instructions
4.1.10 BX
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 0 1 Rm
The BX (Branch and Exchange) instruction branches to an address held in a register Rm, with an optional
switch to Thumb execution. The branch target address is the value of register Rm, with its bit[0] forced to
zero. The instruction set to be used at the branch target is chosen by setting the CPSR T bit to bit[0] of Rm.
Syntax
BX{<cond>} <Rm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rm> Holds the value of the branch target address. Bit[0] of Rm is 0 to select a target ARM
instruction, or 1 to select a target Thumb instruction.
Architecture version
Version 5 and above, plus T variants of version 4
Exceptions
None
Operation
if ConditionPassed(cond) then
T Flag = Rm[0]
PC = Rm AND 0xFFFFFFFE
Notes
ARM/Thumb state transfers
If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned
addresses are impossible in ARM state.
Use of R15 Register 15 can be specified for <Rm>, but doing so is discouraged.
In a BX R15 instruction, R15 is read as normal for ARM code, that is, it is the address of
the BX instruction itself plus 8. The result is to branch to the second following word,
executing in ARM state. This is precisely the same effect that would have been obtained if
a B instruction with an offset field of 0 had been executed, or an ADD PC,PC,#0 or MOV
PC,PC instruction. In new code, use these instructions in preference to the more complex
BX PC instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-19](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-119-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
Coprocessor[cp_num]-dependent operation
Usage
CDP is used to initiate coprocessor instructions that do not operate on values in ARM registers or in main
memory. An example is a floating-point multiply instruction for a floating-point coprocessor.
Notes
Coprocessor fields Only instruction bits[31:24], bits[11:8], and bit[4] are architecturally defined. The
remaining fields are recommendations, for compatibility with ARM Development
Systems.
Unimplemented coprocessor instructions
Hardware coprocessor support is optional, regardless of the architecture version.
An implementation can choose to implement a subset of the coprocessor
instructions, or no coprocessor instructions at all. Any coprocessor instructions that
are not implemented instead cause an undefined instruction trap.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-21](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-121-320.jpg)
![ARM Instructions
4.1.12 CLZ
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 1 1 0 SBO Rd SBO 0 0 0 1 Rm
The CLZ (Count Leading Zeros) instruction returns the number of binary zero bits before the first binary
one bit in a register value. The source register is scanned from the most significant bit (bit[31]) towards the
least significant bit (bit[0]). The result value is 32 if no bits are set in the source register, and zero if bit[31]
is set.
This instruction does not update the condition code flags.
Syntax
CLZ{<cond>} <Rd>, <Rm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the operation. If R15 is specified for <Rd>, the result
is UNPREDICTABLE.
<Rm> Specifies the source register for this operation. If R15 is specified for <Rm>, the result is
UNPREDICTABLE.
Architecture version
Version 5 and above
Exceptions
None
Operation
if Rm == 0
Rd = 32
else
Rd = 31 - (bit position of most significant ’1’ in Rm)
Usage
To normalize the value of register Rm, use CLZ followed by a left shift of Rm by the resulting Rd value. This
shifts Rm so that its most significant 1 bit is in bit[31]. Using MOVS rather than MOV sets the Z flag in the
special case that Rm is zero and so does not have a most significant 1 bit:
CLZ Rd, Rm
MOVS Rm, Rm, LSL Rd
A4-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-122-320.jpg)
![ARM Instructions
4.1.13 CMN
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 0 1 1 1 Rn SBZ shifter_operand
The CMN (Compare Negative) instruction compares a register value with the negative of another arithmetic
value. The condition flags are updated, based on the result of adding the second arithmetic value to the
register value, so that subsequent instructions can be conditionally executed.
Syntax
CMN{<cond>} <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rn> Specifies the register that contains the first operand for the operation.
<shifter_operand>
Specifies the second operand for the operation. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not CMN.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
Exceptions
None
Operation
if ConditionPassed(cond) then
alu_out = Rn + shifter_operand
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = CarryFrom(Rn + shifter_operand)
V Flag = OverflowFrom(Rn + shifter_operand)
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-23](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-123-320.jpg)
![ARM Instructions
4.1.14 CMP
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 0 1 0 1 Rn SBZ shifter_operand
The CMP (Compare) instruction compares a register value with another arithmetic value. The condition flags
are updated, based on the result of subtracting the second arithmetic value from the register value, so that
subsequent instructions can be conditionally executed.
Syntax
CMP{<cond>} <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rn> Specifies the register that contains the first operand for the operation.
<shifter_operand>
Specifies the second operand for the operation. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not CMP.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
Exceptions
None
Operation
if ConditionPassed(cond) then
alu_out = Rn - shifter_operand
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - shifter_operand)
V Flag = OverflowFrom(Rn - shifter_operand)
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-25](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-125-320.jpg)
![ARM Instructions
4.1.15 EOR
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 0 0 1 S Rn Rd shifter_operand
The EOR (Exclusive OR) instruction performs a bitwise Exclusive-OR of the value of register <Rn> with
the value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition
code flags are optionally updated, based on the result.
Syntax
EOR{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the operation,
and the C flag is set to the carry output bit generated by the shifter (see Addressing
Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the
CPSR are unaffected.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the operation.
<shifter_operand>
Specifies the second operand for the operation. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not EOR.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-126-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn EOR shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
Usage
EOR can be used to invert selected bits in a register. For each bit, EOR with 1 inverts that bit, and EOR with
0 leaves it unchanged.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-27](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-127-320.jpg)
![ARM Instructions
4.1.16 LDC
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 P U N W 1 Rn CRd cp_num 8_bit_word_offset
The LDC (Load Coprocessor) instruction loads memory data from the sequence of consecutive memory
addresses calculated by <addressing_mode> to the coprocessor whose number is cp_num. If no
coprocessors indicate that they can execute the instruction, an Undefined Instruction exception is generated.
Syntax
LDC{<cond>}{L} <coproc>, <CRd>, <addressing_mode>
LDC2{L} <coproc>, <CRd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
LDC2 Causes the condition field of the instruction to be set to 0b1111. This provides additional
opcode space for coprocessor designers. The resulting instructions can only be executed
unconditionally.
L Sets the N bit (bit[22]) in the instruction to 1 and specifies a long load (for example,
double-precision instead of single-precision data transfer). If L is omitted, the N bit is 0 and
the instruction specifies a short load.
<coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to
be placed in the cp_num field of the instruction. The standard generic coprocessor names
are p0, p1, ..., p15.
<CRd> Specifies the coprocessor destination register of the instruction.
<addressing_mode>
Is described in Addressing Mode 5 - Load and Store Coprocessor on page A5-56. It
determines the P, U, Rn, W and 8_bit_word_offset bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
LDC is in Version 2 and above.
LDC2 is in Version 5 and above.
A4-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-128-320.jpg)
![ARM Instructions
Exceptions
Undefined Instruction, Data Abort
Operation
if ConditionPassed(cond) then
address = start_address
load Memory[address,4] for Coprocessor[cp_num]
while (NotFinished(Coprocessor[cp_num]))
address = address + 4
load Memory[address,4] for Coprocessor[cp_num]
assert address == end_address
Usage
LDC is useful for loading coprocessor data from memory.
Notes
Coprocessor fields Only instruction bits[31:23], bits[21:16], and bits[11:0] are ARM
architecture-defined. The remaining fields (bit[22] and bits[15:12]) are
recommendations, for compatibility with ARM Development Systems.
In the case of the Unindexed addressing mode (P==0, U==1, W==0), instruction
bits[7:0] are also not defined by the ARM architecture, and can be used to specify
additional coprocessor options.
Data abort For details of the effects of the instruction if a Data Abort occurs, see Effects of
data-aborted instructions on page A2-17.
Non word-aligned addresses
Load coprocessor register instructions ignore the least significant two bits of
address.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The
System Control Coprocessor), and alignment checking is enabled, an address with
bits[1:0] != 0b00 causes an alignment exception.
Unimplemented coprocessor instructions
Hardware coprocessor support is optional, regardless of the architecture version.
An implementation can choose to implement a subset of the coprocessor
instructions, or no coprocessor instructions at all. Any coprocessor instructions that
are not implemented instead cause an undefined instruction trap.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-29](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-129-320.jpg)
![ARM Instructions
4.1.17 LDM (1)
31 28 27 26 25 24 23 22 21 20 19 16 15 0
cond 1 0 0 P U 0 W 1 Rn register_list
This form of the LDM (Load Multiple) instruction is useful for block loads, stack operations and procedure
exit sequences. It loads a non-empty subset, or possibly all, of the general-purpose registers from sequential
memory locations.
The general-purpose registers loaded can include the PC. If they do, the word loaded for the PC is treated
as an address and a branch occurs to that address. In ARM architecture version 5 and above, bit[0] of the
loaded value determines whether execution continues after this branch in ARM state or in Thumb state, as
though a BX (loaded_value) instruction had been executed. In earlier versions of the architecture,
bits[1:0] of the loaded value are ignored and execution continues in ARM state, as though the instruction
MOV PC,(loaded_value) had been executed.
Syntax
LDM{<cond>}<addressing_mode> <Rn>{!}, <registers>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<addressing_mode>
Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines
the P, U, and W bits of the instruction.
<Rn> Specifies the base register used by <addressing_mode>. Using R15 as the base register
<Rn> gives an UNPREDICTABLE result.
! Sets the W bit, causing the instruction to write a modified value back to its base register Rn
as specified in Addressing Mode 4 - Load and Store Multiple on page A5-48. If ! is omitted,
the W bit is 0 and the instruction does not change its base register in this way. (However, if
the base register is included in <registers>, it changes when a value is loaded into it.)
<registers>
Is a list of registers, separated by commas and surrounded by { and }. It specifies the set of
registers to be loaded by the LDM instruction.
The registers are loaded in sequence, the lowest-numbered register from the lowest memory
address (start_address), through to the highest-numbered register from the highest
memory address (end_address). If the PC is specified in the register list (opcode bit[15]
is set), the instruction causes a branch to the address (data) loaded into the PC.
For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list
and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE.
A4-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-130-320.jpg)
![ARM Instructions
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to 14
if register_list[i] == 1 then
Ri = Memory[address,4]
address = address + 4
if register_list[15] == 1 then
value = Memory[address,4]
if (architecture version 5 or above) then
pc = value AND 0xFFFFFFFE
T Bit = value[0]
else
pc = value AND 0xFFFFFFFC
address = address + 4
assert end_address = address - 4
Notes
Operand restrictions
If the base register <Rn> is specified in <registers>, and base register writeback is
specified, the final value of <Rn> is UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non word-aligned addresses
Load Multiple instructions ignore the least significant two bits of address (the words are
not rotated as for Load Word).
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
ARM/Thumb state transfers (ARM architecture version 5 and above)
If bits[1:0] of a value loaded for R15 are 0b10, the result is UNPREDICTABLE, as branches to
non word-aligned addresses are impossible in ARM state.
Time order The time order of the accesses to individual words of memory generated by this instruction
is only defined in some circumstances. See Data accesses to memory-mapped I/O on
page A2-32 for details.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-31](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-131-320.jpg)
![ARM Instructions
4.1.18 LDM (2)
31 28 27 26 25 24 23 22 21 20 19 16 15 14 0
cond 1 0 0 P U 1 0 1 Rn 0 register_list
This form of LDM loads User mode registers when the processor is in a privileged mode (useful when
performing process swaps, and in instruction emulators). The instruction loads a non-empty subset of
the User mode general-purpose registers from sequential memory locations.
Syntax
LDM{<cond>}<addressing_mode> <Rn>, <registers_without_pc>^
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<addressing_mode>
Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines
the P and U bits of the instruction. Only the forms of this addressing mode with W == 0 are
available for this form of the LDM instruction.
<Rn> Specifies the base register used by <addressing_mode>. Using R15 as <Rn> gives an
UNPREDICTABLE result.
<registers_without_pc>
Is a list of registers, separated by commas and surrounded by { and }. This list must not
include the PC, and specifies the set of registers to be loaded by the LDM instruction.
The registers are loaded in sequence, the lowest-numbered register from the lowest memory
address (start_address), through to the highest-numbered register from the highest
memory address (end_address).
For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list
and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE.
^ For an LDM instruction that does not load the PC, this indicates that User mode registers are
to be loaded.
Architecture version
All
Exceptions
Data Abort
A4-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-132-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to 14
if register_list[i] == 1
Ri_usr = Memory[address,4]
address = address + 4
assert end_address == address - 4
Notes
Banked registers This form of LDM must not be followed by an instruction which accesses banked
registers (a following NOP is a good way to ensure this).
Writeback Setting bit[21] (the W bit) has UNPREDICTABLE results.
User and System mode
This form of LDM is UNPREDICTABLE in User mode or System mode.
Base register mode The base register is read from the current processor mode registers, not the User
mode registers.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of
data-aborted instructions on page A2-17.
Non word-aligned addresses
LDM instructions ignore the least significant two bits of address (words are not
rotated as for Load Word).
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The
System Control Coprocessor), and alignment checking is enabled, an address with
bits[1:0] != 0b00 causes an alignment exception.
Time order The time order of the accesses to individual words of memory generated by this
instruction is only defined in some circumstances. See Data accesses to
memory-mapped I/O on page A2-32 for details.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-33](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-133-320.jpg)
![ARM Instructions
4.1.19 LDM (3)
31 28 27 26 25 24 23 22 21 20 19 16 15 14 0
cond 1 0 0 P U 1 W 1 Rn 1 register_list
This form of is useful for returning from an exception. It loads a subset (or possibly all) of the
general-purpose registers and the PC from sequential memory locations. Also, the SPSR of the current
mode is copied to the CPSR.
The value loaded for the PC is treated as an address and a branch occurs to that address. In ARM architecture
version 5 and above, and in T variants of version 4, the value copied from the SPSR T bit to the CPSR T bit
determines whether execution continues after the branch in ARM state or in Thumb state. In earlier
architecture versions, it continues after the branch in ARM state (the only possibility in those architecture
versions).
Syntax
LDM{<cond>}<addressing_mode> <Rn>{!}, <registers_and_pc>^
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<addressing_mode>
Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines
the P, U, and W bits of the instruction.
<Rn> Specifies the base register used by <addressing_mode>. Using R15 as <Rn> gives an
UNPREDICTABLE result.
! Sets the W bit, and the instruction writes a modified value back to its base register Rn (see
Addressing Mode 4 - Load and Store Multiple on page A5-48). If ! is omitted, the W bit is
0 and the instruction does not change its base register in this way. (However, if the base
register is included in <registers>, it changes when a value is loaded into it.)
<registers_and_pc>
Is a list of registers, separated by commas and surrounded by { and }. This list must include
the PC, and specifies the set of registers to be loaded by the LDM instruction.
The registers are loaded in sequence, the lowest-numbered register from the lowest memory
address (start_address), through to the highest-numbered register from the highest
memory address (end_address).
For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list
and 0 otherwise.
^ For an LDM instruction that loads the PC, this indicates that the SPSR of the current mode
is copied to the CPSR.
A4-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-134-320.jpg)
![ARM Instructions
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to 14
if register_list[i] == 1 then
Ri = Memory[address,4]
address = address + 4
CPSR = SPSR
value = Memory[address,4]
if (architecture version 4T, 5 or above) and (T Bit == 1) then
pc = value AND 0xFFFFFFFE
else
pc = value AND 0xFFFFFFFC
address = address + 4
assert end_address = address - 4
Notes
User and System mode
This instruction is UNPREDICTABLE in User or System mode.
Operand restrictions
If the base register <Rn> is specified in <registers_and_pc>, and base register
writeback is specified, the final value of <Rn> is UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non word-aligned addresses
Load Multiple instructions ignore the least significant two bits of address (the words are
not rotated as for Load Word).
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-35](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-135-320.jpg)
![ARM Instructions
ARM/Thumb state transfers (ARM architecture versions 4T, 5 and above)
If the SPSR T bit is 0 and bit[1] of the value loaded into the PC is 1, the results are
UNPREDICTABLE because it is not possible to branch to an ARM instruction at a non
word-aligned address. Note that no special precautions against this are needed on normal
exception returns, because exception entries always either set the T bit of the SPSR to 1 or
bit[1] of the return link value in R14 to 0.
Time order The time order of the accesses to individual words of memory generated by this instruction
is not defined. See Data accesses to memory-mapped I/O on page A2-32 for details.
A4-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-136-320.jpg)
![ARM Instructions
4.1.20 LDR
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I P U 0 W 1 Rn Rd addr_mode
The LDR (Load Register) instruction loads a word from the memory address calculated by
<addressing_mode> and writes it to register <Rd>. If the address is not word-aligned, the loaded value
is rotated right by 8 times the value of bits[1:0] of the address. For a little-endian memory system, this
rotation causes the addressed byte to occupy the least significant byte of the register. For a big-endian
memory system, it causes the addressed byte to occupy bits[31:24] or bits[15:8] of the register, depending
on whether bit[0] of the address is 0 or 1 respectively.
If the PC is specified as register <Rd>, the instruction loads a data word which it treats as an address, then
branches to that address. In ARM architecture version 5 and above, bit[0] of the loaded value determines
whether execution continues after this branch in ARM state or in Thumb state, as though a BX
(loaded_value) instruction had been executed. In earlier versions of the architecture, bits[1:0] of the
loaded value are ignored and execution continues in ARM state, as though a MOV PC,(loaded_value)
instruction had been executed.
Syntax
LDR{<cond>} <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the loaded value.
<addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, P, U, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
All
Exceptions
Data Abort
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-37](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-137-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
if address[1:0] == 0b00 then
value = Memory[address,4]
else if address[1:0] == 0b01 then
value = Memory[address,4] Rotate_Right 8
else if address[1:0] == 0b10 then
value = Memory[address,4] Rotate_Right 16
else /* address[1:0] == 0b11 */
value = Memory[address,4] Rotate_Right 24
if (Rd is R15) then
if (architecture version 5 or above) then
PC = value AND 0xFFFFFFFE
T Bit = value[0]
else
PC = value AND 0xFFFFFFFC
else
Rd = value
Usage
Using the PC as the base register allows PC-relative addressing, which facilitates position-independent
code. Combined with a suitable addressing mode, LDR allows 32-bit memory data to be loaded into a
general-purpose register where its value can be manipulated. If the destination register is the PC, this
instruction loads a 32-bit address from memory and branches to that address.
To synthesize a Branch with Link, precede the LDR instruction with MOV LR, PC.
A4-38 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-138-320.jpg)
![ARM Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Operand restrictions
If <addressing_mode> specifies base register writeback, and the same register is
specified for <Rd> and <Rn>, the results are UNPREDICTABLE.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
Use of R15 If register 15 is specified for <Rd>, address[1:0] must be 0b00. If not, the result is
UNPREDICTABLE.
ARM/Thumb state transfers (ARM architecture version 5 and above)
If bits[1:0] of the loaded value are 0b10, the result is UNPREDICTABLE, as branches to non
word-aligned addresses are not possible in ARM state.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-39](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-139-320.jpg)
![ARM Instructions
4.1.21 LDRB
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I P U 1 W 1 Rn Rd addr_mode
The LDRB (Load Register Byte) instruction loads a byte from the memory address calculated by
<addressing_mode>, zero-extends the byte to a 32-bit word, and writes the word to register <Rd>.
Syntax
LDR{<cond>}B <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the loaded value. If register 15 is specified for <Rd>,
the result is UNPREDICTABLE.
<addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, P, U, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
Rd = Memory[address,1]
Usage
Combined with a suitable addressing mode, LDRB allows 8-bit memory data to be loaded into a
general-purpose register where it can be manipulated.
Using the PC as the base register allows PC-relative addressing, to facilitate position-independent code.
A4-40 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-140-320.jpg)
![ARM Instructions
4.1.22 LDRBT
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I 0 U 1 1 1 Rn Rd addr_mode
The LDRBT (Load Register Byte with Translation) instruction loads a byte from the memory address
calculated by <post_indexed_addressing_mode>, zero-extends the byte to a 32-bit word, and
writes the word to register <Rd>.
If the instruction is executed when the processor is in a privileged mode, the memory system is signaled to
treat the access as if the processor were in User mode.
Syntax
LDR{<cond>}BT <Rd>, <post_indexed_addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the
result is UNPREDICTABLE.
<post_indexed_addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms
of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W ==
0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W
== 1 instead, but the addressing mode is the same in all other respects.
The syntax of all forms of <post_indexed_addressing_mode> includes a base
register <Rn>. All forms also specify that the instruction modifies the base register value
(this is known as base register writeback).
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
Rd = Memory[address,1]
A4-42 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-142-320.jpg)
![ARM Instructions
4.1.23 LDRH
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 P U I W 1 Rn Rd addr_mode 1 0 1 1 addr_mode
The LDRH (Load Register Halfword) instruction loads a halfword from the memory address calculated by
<addressing_mode>, zero-extends the halfword to a 32-bit word, and writes the word to
register <Rd>. If the address is not halfword-aligned, the result is UNPREDICTABLE.
Syntax
LDR{<cond>}H <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the
result is UNPREDICTABLE.
<addressing_mode>
Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It
determines the P, U, I, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
Version 4 and above
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
if address[0] == 0
data = Memory[address,2]
else /* address[0] == 1 */
data = UNPREDICTABLE
Rd = data
A4-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-144-320.jpg)
![ARM Instructions
Usage
Used with a suitable addressing mode, LDRH allows 16-bit memory data to be loaded into a general-purpose
register where its value can be manipulated.
Using the PC as the base register allows PC-relative addressing to facilitate position-independent code.
Notes
Operand restrictions
If <addressing_mode> specifies base register writeback, and the same register is
specified for <Rd> and <Rn>, the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non halfword-aligned addresses
If the load address is not halfword-aligned, the loaded value is UNPREDICTABLE.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bit[0] != 0
causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-45](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-145-320.jpg)
![ARM Instructions
4.1.24 LDRSB
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 P U I W 1 Rn Rd addr_mode 1 1 0 1 addr_mode
The LDRSB (Load Register Signed Byte) instruction loads a byte from the memory address calculated by
<addressing_mode>, sign-extends the byte to a 32-bit word, and writes the word to register <Rd>.
Syntax
LDR{<cond>}SB <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the
result is UNPREDICTABLE.
<addressing_mode>
Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It
determines the P, U, I, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
Version 4 and above
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
data = Memory[address,1]
Rd = SignExtend(data)
Usage
Used with a suitable addressing mode, LDRSB allows 8-bit signed memory data to be loaded into a
general-purpose register where it can be manipulated. Using the PC as the base register allows PC-relative
addressing, which facilitates position-independent code.
A4-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-146-320.jpg)
![ARM Instructions
4.1.25 LDRSH
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 P U I W 1 Rn Rd addr_mode 1 1 1 1 addr_mode
The LDRSH (Load Register Signed Halfword) instruction loads a halfword from the memory address
calculated by <addressing_mode>, sign-extends the halfword to a 32-bit word, and writes the word to
register <Rd>. If the address is not halfword-aligned, the result is UNPREDICTABLE.
Syntax
LDR{<cond>}SH <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the
result is UNPREDICTABLE.
<addressing_mode>
Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It
determines the P, U, I, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
Version 4 and above
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
if address[0] == 0
data = Memory[address,2]
else /* address[0] == 1 */
data = UNPREDICTABLE
Rd = SignExtend(data)
A4-48 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-148-320.jpg)
![ARM Instructions
Usage
Used with a suitable addressing mode, LDRSH allows 16-bit signed memory data to be loaded into
a general-purpose register where its value can be manipulated.
Using the PC as the base register allows PC-relative addressing, which facilitates position-independent
code.
Notes
Operand restrictions
If <addressing_mode> specifies base register writeback, and the same register is
specified for <Rd> and <Rn>, the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non halfword-aligned addresses
If the load address is not halfword-aligned, the loaded value is UNPREDICTABLE.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bit[0] != 0
causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-49](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-149-320.jpg)
![ARM Instructions
4.1.26 LDRT
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I 0 U 0 1 1 Rn Rd addr_mode
The LDRT (Load Register with Translation) instruction loads a word from the memory address calculated
by <addressing_mode> and writes it to register <Rd>. If the address is not word-aligned, the loaded
data is rotated as for the LDR instruction (see LDR on page A4-37).
If the instruction is executed when the processor is in a privileged mode, the memory system is signaled to
treat the access as if the processor were in User mode.
Syntax
LDR{<cond>}T <Rd>, <post_indexed_addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the
result is UNPREDICTABLE.
<post_indexed_addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms
of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W ==
0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W
== 1 instead, but the addressing mode is the same in all other respects.
The syntax of all forms of <post_indexed_addressing_mode> includes a base
register <Rn>. All forms also specify that the instruction modifies the base register value
(this is known as base register writeback).
Architecture version
All
Exceptions
Data Abort
A4-50 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-150-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
if address[1:0] == 0b00
Rd = Memory[address,4]
else if address[1:0] == 0b01
Rd = Memory[address,4] Rotate_Right 8
else if address[1:0] == 0b10
Rd = Memory[address,4] Rotate_Right 16
else /* address[1:0] == 0b11 */
Rd = Memory[address,4] Rotate_Right 24
Usage
LDRT can be used by a (privileged) exception handler that is emulating a memory access instruction that
would normally execute in User mode. The access is restricted as if it had User mode privilege.
Notes
User mode If this instruction is executed in User mode, an ordinary User mode access is performed.
Operand restrictions
If the same register is specified for <Rd> and <Rn> the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If an implementation includes a System Control coprocessor (See Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-51](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-151-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
send Rd value to Coprocessor[cp_num]
Usage
MCR is used to initiate coprocessor instructions that operate on values in ARM registers. An example is
a fixed-point to floating-point conversion instruction for a floating-point coprocessor.
Notes
Coprocessor fields Only instruction bits[31:24], bit[20], bits[15:8], and bit[4] are defined by the ARM
architecture. The remaining fields are recommendations, for compatibility with
ARM Development Systems.
Unimplemented coprocessor instructions
Hardware coprocessor support is optional, regardless of the architecture version.
An implementation can choose to implement a subset of the coprocessor
instructions, or no coprocessor instructions at all. Any coprocessor instructions that
are not implemented instead cause an undefined instruction trap.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-53](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-153-320.jpg)
![ARM Instructions
4.1.28 MLA
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 0 0 1 S Rd Rn Rs 1 0 0 1 Rm
The MLA (Multiply Accumulate) multiplies signed or unsigned operands to produce a 32-bit result, which
is then added to a third operand, and written to the destination register. The condition code flags are
optionally updated, based on the result.
Syntax
MLA{<cond>}{S} <Rd>, <Rm>, <Rs>, <Rn>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR by setting the N and Z flags according to the result of the
multiply-accumulate. If S is omitted, the S bit of the instruction is set to 0 and the entire
CPSR is unaffected by the instruction.
<Rd> Specifies the destination register of the instruction.
<Rm> Holds the value to be multiplied with the value of <Rs>.
<Rs> Holds the value to be multiplied with the value of <Rm>.
<Rn> Contains the value that is added to the product of <Rs> and <Rm>.
Architecture version
Version 2 and above
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = (Rm * Rs + Rn)[31:0]
if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected /* See "C flag" note */
V Flag = unaffected
A4-54 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-154-320.jpg)
![ARM Instructions
4.1.29 MOV
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 1 0 1 S SBZ Rd shifter_operand
The MOV (Move) instruction moves the value of <shifter_operand> to the destination register <Rd>.
The condition code flags are optionally updated, based on the result.
Syntax
MOV{<cond>}{S} <Rd>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the value moved (post-shift
if a shift is specified), and the C flag is set to the carry output bit generated by the
shifter (see Addressing Mode 1 - Data-processing operands on page A5-2). The V
flag and the rest of the CPSR are unaffected.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<shifter_operand>
Specifies the operand for the operation. The options for this operand are described in
Addressing Mode 1 - Data-processing operands on page A5-2, including how each option
causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not MOV.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
Exceptions
None
A4-56 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-156-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
Rd = shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
Usage
MOV is used to:
• Move a value from one register to another.
• Put a constant value into a register.
• Perform a shift without any other arithmetic or logical operation. A left shift by n can be used to
multiply by 2n.
• When the PC is the destination of the instruction, a branch occurs. The instruction:
MOV PC, LR
can therefore be used to return from a subroutine (see instructions B, BL on page A4-10). In T
variants of architecture 4 and in architecture 5 and above, the instruction BX LR must be used in place
of MOV PC, LR, as the BX instruction automatically switches back to Thumb state if appropriate.
• When the PC is the destination of the instruction and the S bit is set, a branch occurs and the SPSR
of the current mode is copied to the CPSR. This means that a MOVS PC, LR instruction can be used
to return from some types of exception (see Exceptions on page A2-13).
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-57](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-157-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
data = value from Coprocessor[cp_num]
if Rd is R15 then
N flag = data[31]
Z flag = data[30]
C flag = data[29]
V flag = data[28]
else /* Rd is not R15 */
Rd = data
Usage
MRC has two uses:
1. If <Rd> specifies R15, the condition code flags bits are updated from the top four bits of the value
from the coprocessor specified by <coproc> (to allow conditional branching on the status of a
coprocessor) and the other 28 bits are ignored.
An example of this use would be to transfer the result of a comparison performed by a floating-point
coprocessor to the ARM’s condition flags.
2. Otherwise the instruction writes into register <Rd> a value from the coprocessor specified by
<coproc>.
An example of this use is a floating-point to integer conversion instruction in a floating-point
coprocessor.
Notes
Coprocessor fields Only instruction bits[31:24], bit[20], bits[15:8] and bit[4] are defined by the ARM
architecture. The remaining fields are recommendations, for compatibility with
ARM Development Systems.
Unimplemented coprocessor instructions
Hardware coprocessor support is optional, regardless of the architecture version.
An implementation can choose to implement a subset of the coprocessor
instructions, or no coprocessor instructions at all. Any coprocessor instructions that
are not implemented instead cause an undefined instruction trap.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-59](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-159-320.jpg)
![ARM Instructions
Usage
The MRS instruction is commonly used for three purposes:
• As part of a read/modify/write sequence for updating a PSR. For more details, see MSR on
page A4-62.
• When an exception occurs and there is a possibility of a nested exception of the same type occurring,
the SPSR of the exception mode is in danger of being corrupted. To deal with this, the SPSR value
must be saved before the nested exception can occur, and later restored in preparation for the
exception return. The saving is normally done by using an MRS instruction followed by a store
instruction. Restoring the SPSR uses the reverse sequence of a load instruction followed by an MSR
instruction. For an example of this usage, see Interrupt prioritization on page A9-15.
• In process swap code, the programmer's model state of the process being swapped out needs to be
saved, including relevant PSR contents, and similar state of the process being swapped in needs to be
restored. Again, this involves the use of MRS/store and load/MSR instruction sequences. For an
example of this usage, see Context switch on page A9-16.
Notes
Opcode [11:0] Execution of MRS instructions with opcode[11:0] != 0x000 is UNPREDICTABLE.
Opcode [19:16] Execution of MRS instructions with opcode[19:16] != 0b1111 is UNPREDICTABLE.
User mode SPSR Accessing the SPSR when in User mode or System mode is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-61](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-161-320.jpg)
![ARM Instructions
Architecture version
Version 3 and above
Exceptions
None
Operation
if ConditionPassed(cond) then
if opcode[25] == 1
operand = 8_bit_immediate Rotate_Right (rotate_imm * 2)
else /* opcode[25] == 0 */
operand = Rm
if R == 0 then
if field_mask[0] == 1 and InAPrivilegedMode() then
CPSR[7:0] = operand[7:0]
if field_mask[1] == 1 and InAPrivilegedMode() then
CPSR[15:8] = operand[15:8]
if field_mask[2] == 1 and InAPrivilegedMode() then
CPSR[23:16] = operand[23:16]
if field_mask[3] == 1 then
CPSR[31:24] = operand[31:24]
else /* R == 1 */
if field_mask[0] == 1 and CurrentModeHasSPSR() then
SPSR[7:0] = operand[7:0]
if field_mask[1] == 1 and CurrentModeHasSPSR() then
SPSR[15:8] = operand[15:8]
if field_mask[2] == 1 and CurrentModeHasSPSR() then
SPSR[23:16] = operand[23:16]
if field_mask[3] == 1 and CurrentModeHasSPSR() then
SPSR[31:24] = operand[31:24]
Usage
This instruction is used to update the value of the condition code flags, interrupt enables, or the processor
mode.
The value of a PSR should normally be updated by moving the PSR to a general-purpose register (using the
MRS instruction), modifying the relevant bits of the general-purpose register, and restoring the updated
general-purpose register value back into the PSR (using the MSR instruction). For example, a good way to
switch the ARM to Supervisor mode from another privileged mode is:
MRS R0,CPSR ; Read CPSR
BIC R0,R0,#0x1F ; Modify by removing current mode
ORR R0,R0,#0x13 ; and substituting Supervisor mode
MSR CPSR_c,R0 ; Write the result back to CPSR
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-63](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-163-320.jpg)
![ARM Instructions
For maximum efficiency, MSR instructions should only write to those fields that they can potentially change.
For example, the last instruction in the above code can only change the CPSR control field, as all bits in the
other fields are unchanged since they were read from the CPSR by the first instruction. So it writes to
CPSR_c, not CPSR_fsxc or some other combination of fields.
However, if the only reason that an MSR instruction cannot change a field is that no bits are currently
allocated to the field, then the field must be written, to ensure future compatibility. For example, when the
process swap code in Context switch on page A9-16 restores the new process’s CPSR value, it writes to
SPSR_fsxc (which is later copied to the CPSR by the LDM instruction which restarts the process). There are
no bits allocated to the state or extension fields at present, so writing to SPSR_fc would work just as well
in current architecture versions (5 and below). However, writing to SPSR_fsxc will continue to work
correctly in future versions of the architecture that do have bits allocated in the state or extension fields,
while writing to SPSR_fc will not.
Note
Due to deficiencies in the handling of the state and extension fields in versions 2.50 and below of the ARM
Software Development Toolkit, only specify the flags and control fields when using these toolkit versions.
In cases where the above guidelines mean that the state and extension fields should have been written for
future compatibility, it is recommended that you keep a record of the need to change the set of fields
specified when it becomes possible to do so.
The immediate form of this instruction can be used to set any of the fields of a PSR, but you must take care
to adhere to the read-modify-write technique described above. The immediate form of the instruction is
equivalent to reading the PSR concerned, replacing all the bits in the fields concerned by the corresponding
bits of the immediate constant and writing the result back to the PSR. The immediate form must therefore
only be used when the intention is to modify all the bits in the specified fields and, in particular, must not
be used if the specified fields include any as-yet-unallocated bits. Failure to observe this rule might result
in code which has unanticipated side-effects on future versions of the ARM architecture.
As an exception to the above rule, it is legitimate to use the immediate form of the instruction to modify the
flags byte, despite the fact that bits[26:24] of the PSRs have no allocated function at present. For example,
this instruction can be used to set all four flags (and clear the Q flag if the processor implements the
Enhanced DSP extension):
MSR CPSR_f,#0xF0000000
Any functionality allocated to bits[26:24] in a future version of the ARM architecture will be designed so
that such code does not have unexpected side effects.
A4-64 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-164-320.jpg)
![ARM Instructions
Notes
The R bit Bit[22] of the instruction is 0 if the CPSR is to be written and 1 if the SPSR is to be written.
User mode CPSR
Any writes to CPSR[23:0] in User mode are ignored (so that User mode programs cannot
change to a privileged mode).
User mode SPSR
Accessing the SPSR when in User mode is UNPREDICTABLE.
System mode SPSR
Accessing the SPSR when in System mode is UNPREDICTABLE.
Obsolete field specification
The CPSR, CPSR_flg, CPSR_ctl, CPSR_all, SPSR, SPSR_flg, SPSR_ctl and
SPSR_all forms of PSR field specification have been superseded by the csxf format
shown on page A4-62.
CPSR, SPSR, CPSR_all and SPSR_all produce a field mask of 0b1001.
CPSR_flg and SPSR_flg produce a field mask of 0b1000.
CPSR_ctl and SPSR_ctl produce a field mask of 0b0001.
The T bit The MSR instruction must not be used to alter the T bit in the CPSR. If such an attempt is
made, the results are UNPREDICTABLE.
Addressing modes
The immediate and register forms are specified in precisely the same way as the immediate
and unshifted register forms of Addressing Mode 1 (see Addressing Mode 1 -
Data-processing operands on page A5-2). All other forms of Addressing Mode 1 yield
UNPREDICTABLE results.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-65](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-165-320.jpg)
![ARM Instructions
4.1.33 MUL
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 0 0 0 S Rd SBZ Rs 1 0 0 1 Rm
The MUL (Multiply) instruction is used to multiply signed or unsigned variables to produce a 32-bit result.
The condition code flags are optionally updated, based on the result.
Syntax
MUL{<cond>}{S} <Rd>, <Rm>, <Rs>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR by setting the N and Z flags according to the result of the multiplication.
If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by
the instruction.
<Rd> Specifies the destination register for the instruction.
<Rm> Specifies the register that contains the first value to be multiplied.
<Rs> Holds the value to be multiplied with the value of <Rm>.
Architecture version
Version 2 and above
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = (Rm * Rs)[31:0]
if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected /* See "C flag" note */
V Flag = unaffected
A4-66 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-166-320.jpg)
![ARM Instructions
4.1.34 MVN
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 1 1 1 S SBZ Rd shifter_operand
The MVN (Move Negative) instruction moves the logical one’s complement of the value of
<shifter_operand> to the destination register <Rd>. The condition code flags are optionally updated,
based on the result.
Syntax
MVN{<cond>}{S} <Rd>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the operation,
and the C flag is set to the carry output bit generated by the shifter (see Addressing
Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the
CPSR are unaffected.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<shifter_operand>
Specifies the operand for the operation. The options for this operand are described in
Addressing Mode 1 - Data-processing operands on page A5-2, including how each option
causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not MVN.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
Exceptions
None
A4-68 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-168-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
Rd = NOT shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
Usage
MVN is used to:
• write a negative value into a register
• form a bit mask
• take the one’s complement of a value.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-69](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-169-320.jpg)
![ARM Instructions
4.1.35 ORR
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 1 0 0 S Rn Rd shifter_operand
The ORR (Logical OR) instruction performs a bitwise (inclusive) OR of the value of register <Rn> with the
value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition
code flags are optionally updated, based on the result.
Syntax
ORR{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the operation,
and the C flag is set to the carry output bit generated by the shifter (see Addressing
Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the
CPSR are unaffected.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the operation.
<shifter_operand>
Specifies the second operand for the operation. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not ORR.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-70 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-170-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn OR shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
Usage
ORR can be used to set selected bits in a register. For each bit, OR with 1 sets the bit, and OR with 0 leaves
it unchanged.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-71](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-171-320.jpg)
![ARM Instructions
4.1.36 RSB
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 0 1 1 S Rn Rd shifter_operand
The RSB (Reverse Subtract) instruction subtracts the value of register <Rn> from the value of
<shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags
are optionally updated, based on the result.
Syntax
RSB{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction,
and the C and V flags are set according to whether the subtraction generated a borrow
(unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is
unchanged.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the second operand for the subtraction.
<shifter_operand>
Specifies the first operand for the subtraction. The options for this operand are described in
Addressing Mode 1 - Data-processing operands on page A5-2, including how each option
causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not RSB.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-72 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-172-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = shifter_operand - Rn
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(shifter_operand - Rn)
V Flag = OverflowFrom(shifter_operand - Rn)
Usage
The following instruction stores the negation (two’s complement) of Rx in Rd:
RSB Rd, Rx, #0
Constant multiplication (of Rx) by 2n–1 (into Rd) can be performed with:
RSB Rd, Rx, Rx, LSL #n
Notes
C flag If S is specified, the C flag is set to:
1 if no borrow occurs
0 if a borrow does occur.
In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow
condition is usually compensated for by subsequent instructions. For example:
• The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing
a normal subtraction if C == 1 and subtracting one more than usual if C == 0.
• The HS (unsigned higher or same) and LO (unsigned lower) conditions are
equivalent to CS (carry set) and CC (carry clear) respectively.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-73](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-173-320.jpg)
![ARM Instructions
4.1.37 RSC
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 1 1 1 S Rn Rd shifter_operand
The RSC (Reverse Subtract with Carry) instruction subtracts the value of register <Rn> and the value of
NOT(Carry flag) from the value of <shifter_operand>, and stores the result in the destination register
<Rd>. The condition code flags are optionally updated, based on the result.
Syntax
RSC{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction,
and the C and V flags are set according to whether the subtraction generated a borrow
(unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is
unchanged.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the second operand for the subtraction.
<shifter_operand>
Specifies the first operand for the subtraction. The options for this operand are described in
Addressing Mode 1 - Data-processing operands on page A5-2, including how each option
causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not RSC.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-74 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-174-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = shifter_operand - Rn - NOT(C Flag)
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(shifter_operand - Rn - NOT(C Flag))
V Flag = OverflowFrom(shifter_operand - Rn - NOT(C Flag))
Usage
To negate the 64-bit value in R0,R1, use the following sequence (R0 holds the least significant word) which
stores the result in R2,R3:
RSBS R2,R0,#0
RSC R3,R1,#0
Notes
C flag If S is specified, the C flag is set to:
1 if no borrow occurs
0 if a borrow does occur.
In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow
condition is usually compensated for by subsequent instructions. For example:
• The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing
a normal subtraction if C == 1 and subtracting one more than usual if C == 0.
• The HS (unsigned higher or same) and LO (unsigned lower) conditions are
equivalent to CS (carry set) and CC (carry clear) respectively.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-75](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-175-320.jpg)
![ARM Instructions
4.1.38 SBC
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 1 1 0 S Rn Rd shifter_operand
The SBC (Subtract with Carry) instruction is used to synthesize multi-word subtraction. SBC subtracts the
value of <shifter_operand> and the value of NOT(Carry flag) from the value of register <Rn>, and
stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on
the result.
Syntax
SBC{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction,
and the C and V flags are set according to whether the subtraction generated a borrow
(unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is
unchanged.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the subtraction.
<shifter_operand>
Specifies the second operand for the subtraction. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not SBC.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-76 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-176-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn - shifter_operand - NOT(C Flag)
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - shifter_operand - NOT(C Flag))
V Flag = OverflowFrom(Rn - shifter_operand - NOT(C Flag))
Usage
If register pairs R0,R1 and R2,R3 hold 64-bit values (R0 and R2 hold the least significant words), the
following instructions leave the 64-bit difference in R4,R5:
SUBS R4,R0,R2
SBC R5,R1,R3
Notes
C flag If S is specified, the C flag is set to:
1 if no borrow occurs
0 if a borrow does occur.
In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow
condition is usually compensated for by subsequent instructions. For example:
• The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing
a normal subtraction if C == 1 and subtracting one more than usual if C == 0.
• The HS (unsigned higher or same) and LO (unsigned lower) conditions are
equivalent to CS (carry set) and CC (carry clear) respectively.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-77](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-177-320.jpg)
![ARM Instructions
4.1.39 SMLAL
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 1 1 1 S RdHi RdLo Rs 1 0 0 1 Rm
The SMLAL (Signed Multiply Accumulate Long) instruction multiplies the signed value of register <Rm>
with the signed value of register <Rs> to produce a 64-bit product. This product is added to the 64-bit value
held in <RdHi> and <RdLo>, and the sum is written back to <RdHi> and <RdLo>. The condition code
flags are optionally updated, based on the result.
Syntax
SMLAL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR by setting the N and Z flags according to the result of the
multiply-accumulate. If S is omitted, the S bit of the instruction is set to 0 and the entire
CPSR is unaffected by the instruction.
<RdLo> Supplies the lower 32 bits of the value to be added to the product of <Rm> and <Rs>, and
is the destination register for the lower 32 bits of the result.
<RdHi> Supplies the upper 32 bits of the value to be added to the product of <Rm> and <Rs>, and
is the destination register for the upper 32 bits of the result.
<Rm> Holds the signed value to be multiplied with the value of <Rs>.
<Rs> Holds the signed value to be multiplied with the value of <Rm>.
Architecture version
All M variants
Exceptions
None
A4-78 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-178-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
RdLo = (Rm * Rs)[31:0] + RdLo /* Signed multiplication */
RdHi = (Rm * Rs)[63:32] + RdHi + CarryFrom((Rm * Rs)[31:0] + RdLo)
if S == 1 then
N Flag = RdHi[31]
Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0
C Flag = unaffected /* See "C and V flags" note */
V Flag = unaffected /* See "C and V flags" note */
Usage
SMLAL multiplies signed variables to produce a 64-bit result, which is added to the 64-bit value in the two
destination general-purpose registers. The result is written back to the two destination general-purpose
registers.
Notes
Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has
UNPREDICTABLE results.
Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are
UNPREDICTABLE.
Early termination If the multiplier implementation supports early termination, it must be implemented
on the value of the <Rs> operand. The type of early termination used (signed or
unsigned) is IMPLEMENTATION DEFINED.
C and V flags The SMLALS instruction is defined to leave the C and V flags unchanged in ARM
architecture version 5 and above. In earlier versions of the architecture, the values
of the C and V flags were UNPREDICTABLE after an SMLALS instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-79](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-179-320.jpg)
![ARM Instructions
4.1.40 SMULL
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 1 1 0 S RdHi RdLo Rs 1 0 0 1 Rm
The SMULL (Signed Multiply Long) instruction multiplies the signed value of register <Rm> with the signed
value of register <Rs> to produce a 64-bit result. The upper 32 bits of the result are stored in <RdHi>.
The lower 32 bits are stored in <RdLo>. The condition code flags are optionally updated, based on the
64-bit result.
Syntax
SMULL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR by setting the N and Z flags according to the result of the multiplication.
If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by
the instruction.
<RdLo> Stores the lower 32 bits of the result.
<RdHi> Stores the upper 32 bits of the result.
<Rm> Holds the signed value to be multiplied with the value of <Rs>.
<Rs> Holds the signed value to be multiplied with the value of <Rm>.
Architecture version
All M variants
Exceptions
None
A4-80 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-180-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
RdHi = (Rm * Rs)[63:32] /* Signed multiplication */
RdLo = (Rm * Rs)[31:0]
if S == 1 then
N Flag = RdHi[31]
Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0
C Flag = unaffected /* See "C and V flags" note */
V Flag = unaffected /* See "C and V flags" note */
Usage
SMULL multiplies signed variables to produce a 64-bit result in two general-purpose registers.
Notes
Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has
UNPREDICTABLE results.
Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are
UNPREDICTABLE.
Early termination If the multiplier implementation supports early termination, it must be implemented
on the value of the <Rs> operand. The type of early termination used (signed or
unsigned) is IMPLEMENTATION DEFINED.
C and V flags The SMULLS instruction is defined to leave the C and V flags unchanged in ARM
architecture version 5 and above. In earlier versions of the architecture, the values
of the C and V flags were UNPREDICTABLE after an SMULLS instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-81](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-181-320.jpg)
![ARM Instructions
4.1.41 STC
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 P U N W 0 Rn CRd cp_num 8_bit_word_offset
The STC (Store Coprocessor) instruction stores data from the coprocessor whose name is cp_num to the
sequence of consecutive memory addresses calculated by <addressing_mode>. If no coprocessors
indicate that they can execute the instruction, an Undefined Instruction exception is generated.
Syntax
STC{<cond>}{L} <coproc>, <CRd>, <addressing_mode>
STC2{L} <coproc>, <CRd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
STC2 Causes the condition field of the instruction to be set to 0b1111. This provides additional
opcode space for coprocessor designers. The resulting instructions can only be executed
unconditionally.
L Sets the N bit (bit[22]) in the instruction to 1 and specifies a long store (for example,
double-precision instead of single-precision data transfer). If L is omitted, the N bit is 0 and
the instruction specifies a short store.
<coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to
be placed in the cp_num field of the instruction. The standard generic coprocessor names
are p0, p1, ..., p15.
<CRd> Specifies the coprocessor source register of the instruction.
<addressing_mode>
Is described in Addressing Mode 5 - Load and Store Coprocessor on page A5-56. It
determines the P, U, Rn, W and 8_bit_word_offset bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
STC is in version 2 and above.
STC2 is in version 5 and above.
A4-82 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-182-320.jpg)
![ARM Instructions
Exceptions
Undefined Instruction, Data Abort
Operation
if ConditionPassed(cond) then
address = start_address
Memory[address,4] = value from Coprocessor[cp_num]
while (NotFinished(coprocessor[cp_num]))
address = address + 4
Memory[address,4] = value from Coprocessor[cp_num]
assert address == end_address
Usage
STC is useful for storing coprocessor data to memory. The L (long) option controls the N bit and could be
used to distinguish between a single- and double-precision transfer for a floating-point store instruction.
Notes
Coprocessor fields Only instruction bits[31:23], bits[21:16} and bits[11:0] are defined by the ARM
architecture. The remaining fields (bit[22] and bits[15:12]) are recommendations,
for compatibility with ARM Development Systems.
In the case of the Unindexed addressing mode (P==0, U==1, W==0), instruction
bits[7:0] are also not ARM architecture-defined, and can be used to specify
additional coprocessor options.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of
data-aborted instructions on page A2-17.
Non word-aligned addresses
Store coprocessor register instructions ignore the least significant two bits of
address.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The
System Control Coprocessor), and alignment checking is enabled, an address with
bits[1:0] != 0b00 causes an alignment exception.
Unimplemented coprocessor instructions
Hardware coprocessor support is optional, regardless of the architecture version.
An implementation can choose to implement a subset of the coprocessor
instructions, or no coprocessor instructions at all. Any coprocessor instructions that
are not implemented instead cause an undefined instruction trap.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-83](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-183-320.jpg)
![ARM Instructions
4.1.42 STM(1)
31 28 27 26 25 24 23 22 21 20 19 16 15 0
cond 1 0 0 P U 0 W 0 Rn register_list
This form of the STM (Store Multiple) instruction stores a non-empty subset (or possibly all) of the
general-purpose registers to sequential memory locations.
Syntax
STM{<cond>}<addressing_mode> <Rn>{!}, <registers>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined
in The condition field on page A3-5. If <cond> is omitted, the AL (always)
condition is used.
<addressing_mode>
Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It
determines the P, U, and W bits of the instruction.
<Rn> Specifies the base register used by <addressing_mode>. If R15 is specified as
<Rn>, the result is UNPREDICTABLE.
! Sets the W bit, causing the instruction to write a modified value back to its base
register Rn as specified in Addressing Mode 4 - Load and Store Multiple on
page A5-48. If ! is omitted, the W bit is 0 and the instruction does not change its
base register in this way.
<registers> Is a list of registers, separated by commas and surrounded by { and }. It specifies
the set of registers to be stored by the STM instruction.
The registers are stored in sequence, the lowest-numbered register to the lowest
memory address (start_address), through to the highest-numbered register to
the highest memory address (end_address).
For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in
the list and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE.
If R15 is specified in <registers>, the value stored is IMPLEMENTATION
DEFINED. For more details, see Reading the program counter on page A2-7.
Architecture version
All
Exceptions
Data Abort
A4-84 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-184-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to 15
if register_list[i] == 1
Memory[address,4] = Ri
address = address + 4
assert end_address == address - 4
Usage
STM is useful as a block store instruction (combined with LDM it allows efficient block copy) and for stack
operations. A single STM used in the sequence of a procedure can push the return address and
general-purpose register values on to the stack, updating the stack pointer in the process.
Notes
Operand restrictions
If <Rn> is specified as <registers> and base register writeback is specified:
• If <Rn> is the lowest-numbered register specified in <register_list>, the
original value of <Rn> is stored.
• Otherwise, the stored value of <Rn> is UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non word-aligned addresses
STM instructions ignore the least significant two bits of address.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
Time order The time order of the accesses to individual words of memory generated by this instruction
is only defined in some circumstances. See Data accesses to memory-mapped I/O on
page A2-32 for details.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-85](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-185-320.jpg)
![ARM Instructions
4.1.43 STM (2)
31 28 27 26 25 24 23 22 21 20 19 16 15 0
cond 1 0 0 P U 1 0 0 Rn register_list
This form of STM stores a subset (or possibly all) of the User mode general-purpose registers to sequential
memory locations.
Syntax
STM{<cond>}<addressing_mode> <Rn>, <registers>^
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined
in The condition field on page A3-5. If <cond> is omitted, the AL (always)
condition is used.
<addressing_mode>
Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It
determines the P and U bits of the instruction. Only the forms of this addressing
mode with W == 0 are available for this form of the STM instruction.
<Rn> Specifies the base register used by <addressing_mode>. If R15 is specified as
the base register <Rn>, the result is UNPREDICTABLE.
<registers> Is a list of registers, separated by commas and surrounded by { and }. It specifies
the set of registers to be stored by the STM instruction.
The registers are stored in sequence, the lowest-numbered register to the lowest
memory address (start_address), through to the highest-numbered register to
the highest memory address (end_address).
For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in
the list and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE.
If R15 is specified in <registers> the value stored is IMPLEMENTATION
DEFINED. For more details, see Reading the program counter on page A2-7.
^ For an STM instruction, indicates that User mode registers are to be stored.
Architecture version
All
Exceptions
Data Abort
A4-86 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-186-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to 15
if register_list[i] == 1
Memory[address,4] = Ri_usr
address = address + 4
assert end_address == address - 4
Usage
STM is used to store the User mode registers when the processor is in a privileged mode (useful when
performing process swaps, and in instruction emulators).
Notes
Banked registers This instruction must not be followed by an instruction which accesses banked
registers (a following NOP is a good way to ensure this).
Writeback Setting bit 21 (the W bit) has UNPREDICTABLE results.
User and System mode
This instruction is UNPREDICTABLE in User or System mode.
Base register mode For the purpose of address calculation, the base register is read from the current
processor mode registers, not the User mode registers.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of
data-aborted instructions on page A2-17.
Non word-aligned addresses
STM instructions ignore the least significant two bits of address.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The
System Control Coprocessor), and alignment checking is enabled, an address with
bits[1:0] != 0b00 causes an alignment exception.
Time order The time order of the accesses to individual words of memory generated by this
instruction is only defined in some circumstances. See Data accesses to
memory-mapped I/O on page A2-32 for details.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-87](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-187-320.jpg)
![ARM Instructions
4.1.44 STR
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I P U 0 W 0 Rn Rd addr_mode
The STR (Store Register) instruction stores a word from register <Rd> to the memory address calculated
by <addressing_mode>.
Syntax
STR{<cond>} <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the value stored
is IMPLEMENTATION DEFINED. For more details, see Reading the program counter on
page A2-7.
<addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, P, U, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
Memory[address,4] = Rd
Usage
Combined with a suitable addressing mode, STR stores 32-bit data from a general-purpose register into
memory. Using the PC as the base register allows PC-relative addressing, which facilitates
position-independent code.
A4-88 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-188-320.jpg)
![ARM Instructions
Notes
Operand restrictions
If <addressing_mode> specifies base register writeback, and the same register is
specified for <Rd> and <Rn>, the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non word-aligned addresses
STR instructions ignore the least significant two bits of address. So if these bits are not
0b00, the effects of STR are not precisely opposite to those of LDR.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-89](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-189-320.jpg)
![ARM Instructions
4.1.45 STRB
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I P U 1 W 0 Rn Rd addr_mode
The STRB (Store Register Byte) instruction stores a byte from the least significant byte of register <Rd> to
the memory address calculated by <addressing_mode>.
Syntax
STR{<cond>}B <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the result is
UNPREDICTABLE.
<addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, P, U, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
Memory[address,1] = Rd[7:0]
Usage
Combined with a suitable addressing mode, STRB writes the least significant byte of a general-purpose
register to memory. Using the PC as the base register allows PC-relative addressing, which facilitates
position-independent code.
A4-90 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-190-320.jpg)
![ARM Instructions
4.1.46 STRBT
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I 0 U 1 1 0 Rn Rd addr_mode
The STRBT (Store Register Byte with Translation) instruction stores a byte from the least significant byte
of register <Rd> to the memory address calculated by <post_indexed_addressing_mode>. If the
instruction is executed when the processor is in a privileged mode, the memory system is signaled to treat
the access as if the processor were in User mode.
Syntax
STR{<cond>}BT <Rd>, <post_indexed_addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the result is
UNPREDICTABLE.
<post_indexed_addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms
of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W ==
0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W
== 1 instead, but the addressing mode is the same in all other respects.
The syntax of all forms of <post_indexed_addressing_mode> includes a base
register <Rn>. All forms also specify that the instruction modifies the base register value
(this is known as base register writeback).
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
Memory[address,1] = Rd[7:0]
A4-92 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-192-320.jpg)
![ARM Instructions
4.1.47 STRH
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 P U I W 0 Rn Rd addr_mode 1 0 1 1 addr_mode
The STRH (Store Register Halfword) instruction stores a halfword from the least significant halfword of
register <Rd> to the memory address calculated by <addressing_mode>. If the address is not
halfword-aligned, the result is UNPREDICTABLE.
Syntax
STR{<cond>}H <Rd>, <addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the result is
UNPREDICTABLE.
<addressing_mode>
Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It
determines the P, U, I, W, Rn and addr_mode bits of the instruction.
The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some
forms also specify that the instruction modifies the base register value (this is known as base
register writeback).
Architecture version
Version 4 and above
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
if address[0] == 0
data = Rd[15:0]
else /* address[0] == 1 */
data = UNPREDICTABLE
Memory[address,2] = data
A4-94 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-194-320.jpg)
![ARM Instructions
Usage
Combined with a suitable addressing mode, STRH allows 16-bit data from a general-purpose register to be
stored to memory. Using the PC as the base register allows PC-relative addressing, to facilitate
position-independent code.
Notes
Operand restrictions If <addressing_mode> specifies base register writeback, and the same register
is specified for <Rd> and <Rn>, the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of
data-aborted instructions on page A2-17.
Non halfword-aligned addresses
If the store address is not halfword-aligned, the stored value is UNPREDICTABLE.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The
System Control Coprocessor), and alignment checking is enabled, an address with
bit[0] != 0 causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-95](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-195-320.jpg)
![ARM Instructions
4.1.48 STRT
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 I 0 U 0 1 0 Rn Rd addr_mode
The STRT (Store Register with Translation) instruction stores a word from register <Rd> to the memory
address calculated by <post_indexed_addressing_mode>. If the instruction is executed when
the processor is in a privileged mode, the memory system is signaled to treat the access as if the processor
was in User mode.
Syntax
STR{<cond>}T <Rd>, <post_indexed_addressing_mode>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the value stored
is IMPLEMENTATION DEFINED. For more details, see Reading the program counter on
page A2-7.
<post_indexed_addressing_mode>
Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18.
It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms
of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W ==
0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W
== 1 instead, but the addressing mode is the same in all other respects.
The syntax of all forms of <post_indexed_addressing_mode> includes a base
register <Rn>. All forms also specify that the instruction modifies the base register value
(this is known as base register writeback).
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
Memory[address,4] = Rd
A4-96 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-196-320.jpg)
![ARM Instructions
Usage
STRT can be used by a (privileged) exception handler that is emulating a memory access instruction that
would normally execute in User mode. The access is restricted as if it had User mode privilege.
Notes
User mode If this instruction is executed in User mode, an ordinary User mode access is performed.
Operand restrictions
If the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-97](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-197-320.jpg)
![ARM Instructions
4.1.49 SUB
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 0 0 1 0 S Rn Rd shifter_operand
The SUB (Subtract) instruction subtracts the value of <shifter_operand> from the value of register
<Rn>, and stores the result in the destination register <Rd>. The condition code flags are optionally
updated, based on the result.
Syntax
SUB{<cond>}{S} <Rd>, <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the
CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction.
Two types of CPSR update can occur when S is specified:
• If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction,
and the C and V flags are set according to whether the subtraction generated a borrow
(unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is
unchanged.
• If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of
the instruction is UNPREDICTABLE if executed in User mode or System mode, because
these modes do not have an SPSR.
<Rd> Specifies the destination register of the instruction.
<Rn> Specifies the register that contains the first operand for the subtraction.
<shifter_operand>
Specifies the second operand for the subtraction. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not SUB.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
A4-98 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-198-320.jpg)
![ARM Instructions
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Rn - shifter_operand
if S == 1 and Rd == R15 then
CPSR = SPSR
else if S == 1 then
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - shifter_operand)
V Flag = OverflowFrom(Rn - shifter_operand)
Usage
SUB is used to subtract one value from another to produce a third. To decrement a register value (in Rx) use:
SUBS Ri, Ri, #1
SUBS is useful as a loop counter decrement, as the loop branch can test the flags for the appropriate
termination condition, without the need for a compare instruction:
CMP Rx, #0
This both decrements the loop counter in Ri and checks whether it has reached zero.
The form of this instruction with the PC as its destination register and the S bit set can be used to return from
interrupts and various other types of exception. See Exceptions on page A2-13 for more details.
Notes
C flag If S is specified, the C flag is set to:
1 if no borrow occurs
0 if a borrow does occur.
In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow
condition is usually compensated for by subsequent instructions. For example:
• The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing
a normal subtraction if C == 1 and subtracting one more than usual if C == 0.
• The HS (unsigned higher or same) and LO (unsigned lower) conditions are
equivalent to CS (carry set) and CC (carry clear) respectively.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-99](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-199-320.jpg)
![ARM Instructions
4.1.50 SWI
31 28 27 26 25 24 23 0
cond 1 1 1 1 immed_24
The SWI (Software Interrupt) instruction causes a SWI exception (see Exceptions on page A2-13).
Syntax
SWI{<cond>} <immed_24>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<immed_24> Is a 24-bit immediate value that is put into bits[23:0] of the instruction. This value is ignored
by the ARM processor, but can be used by an operating system SWI exception handler to
determine what operating system service is being requested (see Usage on page A4-101
below for more details).
Architecture version
All
Exceptions
Software interrupt
Operation
if ConditionPassed(cond) then
R14_svc = address of next instruction after the SWI instruction
SPSR_svc = CPSR
CPSR[4:0] = 0b10011 /* Enter Supervisor mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF0008
else
PC = 0x00000008
A4-100 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-200-320.jpg)
![ARM Instructions
4.1.51 SWP
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 0 0 0 Rn Rd SBZ 1 0 0 1 Rm
The SWP (Swap) instruction swaps a word between registers and memory. SWP loads a word from the
memory address given by the value of register <Rn>. The value of register <Rm> is then stored to the
memory address given by the value of <Rn>, and the original loaded value is written to register <Rd>. If
the same register is specified for <Rd> and <Rm>, this instruction swaps the value of the register and
the value at the memory address.
Syntax
SWP{<cond>} <Rd>, <Rm>, [<Rn>]
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the instruction.
<Rm> Contains the value that is stored to memory.
<Rn> Contains the memory address to load from.
Architecture version
Version 3 and above, plus version 2a
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
if Rn[1:0] == 0b00 then
temp = Memory[Rn,4]
else if Rn[1:0] == 0b01 then
temp = Memory[Rn,4] Rotate_Right 8
else if Rn[1:0] == 0b10 then
temp = Memory[Rn,4] Rotate_Right 16
else /* Rn[1:0] == 0b11 */
temp = Memory[Rn,4] Rotate_Right 24
Memory[Rn,4] = Rm
Rd = temp
A4-102 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-202-320.jpg)
![ARM Instructions
Usage
The SWP instruction can be used to implement semaphores. For sample code, see Semaphore instructions
on page A9-11.
Notes
Non word-aligned addresses
If the address is not word-aligned, the loaded value is rotated right by 8 times the
value of Rn[1:0]. The stored value is not rotated.
Use of R15 If R15 is specified for <Rd>, <Rn>, or <Rm>, the result is UNPREDICTABLE.
Operand restrictions If the same register is specified as <Rn> and <Rm>, or <Rn> and <Rd>, the result
is UNPREDICTABLE.
Data abort If a data abort is signaled on either the load access or the store access, the loaded
value is not written to <Rd>. If a data abort is signaled on the load access, the store
access does not occur.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The
System Control Coprocessor), and alignment checking is enabled, an address with
bits[1:0] != 0b00 causes an alignment exception.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-103](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-203-320.jpg)
![ARM Instructions
4.1.52 SWPB
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 1 0 0 Rn Rd SBZ 1 0 0 1 Rm
The SWPB (Swap Byte) instruction swaps a byte between registers and memory. SWPB loads a byte from
the memory address given by the value of register <Rn>. The value of the least significant byte of register
<Rm> is stored to the memory address given by <Rn>, the original loaded value is zero-extended to a 32-bit
word, and the word is written to register <Rd>. If the same register is specified for <Rd> and <Rm>, this
instruction swaps the value of the least significant byte of the register and the byte value at the memory
address.
Syntax
SWP{<cond>}B <Rd>, <Rm>, [<Rn>]
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register for the instruction.
<Rm> Contains the value that is stored to memory.
<Rn> Contains the memory address to load from.
Architecture version
Version 3 and above, plus version 2a
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
temp = Memory[Rn,1]
Memory[Rn,1] = Rm[7:0]
Rd = temp
Usage
The SWPB instruction can be used to implement semaphores, in a similar manner to that shown for SWP
instructions in Semaphore instructions on page A9-11.
A4-104 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-204-320.jpg)
![ARM Instructions
4.1.53 TEQ
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 0 0 1 1 Rn SBZ shifter_operand
The TEQ (Test Equivalence) instruction compares a register value with another arithmetic value. The
condition flags are updated, based on the result of logically exclusive-ORing the two values, so that
subsequent instructions can be conditionally executed.
Syntax
TEQ{<cond>} <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rn> Specifies the register that contains the first operand for the comparison.
<shifter_operand>
Specifies the second operand for the comparison. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option sets the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) in the instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not TEQ.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
Exceptions
None
Operation
if ConditionPassed(cond) then
alu_out = Rn EOR shifter_operand
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
Usage
TEQ is used to test if two values are equal, without affecting the V flag (as CMP does). The C flag is also
unaffected in many cases. TEQ is also useful for testing whether two values have the same sign. After the
comparison, the N flag is the logical Exclusive OR of the sign bits of the two operands.
A4-106 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-206-320.jpg)
![ARM Instructions
4.1.54 TST
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 0 0 0 1 Rn SBZ shifter_operand
The TST (Test) instruction compares a register value with another arithmetic value. The condition flags are
updated, based on the result of logically ANDing the two values, so that subsequent instructions can be
conditionally executed.
Syntax
TST{<cond>} <Rn>, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rn> Specifies the register that contains the first operand for the comparison.
<shifter_operand>
Specifies the second operand for the comparison. The options for this operand are described
in Addressing Mode 1 - Data-processing operands on page A5-2, including how each
option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the
instruction.
If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not TST.
Instead, see Extending the instruction set on page A3-27 to determine which instruction it is.
Architecture version
All
Exceptions
None
Operation
if ConditionPassed(cond) then
alu_out = Rn AND shifter_operand
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = shifter_carry_out
V Flag = unaffected
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-107](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-207-320.jpg)
![ARM Instructions
4.1.55 UMLAL
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 1 0 1 S RdHi RdLo Rs 1 0 0 1 Rm
The UMLAL (Unsigned Multiply Accumulate Long) instruction multiplies the unsigned value of register
<Rm> with the unsigned value of register <Rs> to produce a 64-bit product. This product is added to the
64-bit value held in <RdHi> and <RdLo>, and the sum is written back to <RdHi> and <RdLo>. The
condition code flags are optionally updated, based on the result.
Syntax
UMLAL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR by setting the N and Z flags according to the result of the
multiply-accumulate. If S is omitted, the S bit of the instruction is set to 0 and the entire
CPSR is unaffected by the instruction.
<RdLo> Supplies the lower 32 bits of the value to be added to the product of <Rm> and <Rs>, and
is the destination register for the lower 32 bits of the result.
<RdHi> Supplies the upper 32 bits of the value to be added to the product of <Rm> and <Rs>, and
is the destination register for the upper 32 bits of the result.
<Rm> Holds the signed value to be multiplied with the value of <Rs>.
<Rs> Holds the signed value to be multiplied with the value of <Rm>.
Architecture version
All M variants
Exceptions
None
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-109](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-209-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
RdLo = (Rm * Rs)[31:0] + RdLo /* Unsigned multiplication */
RdHi = (Rm * Rs)[63:32] + RdHi + CarryFrom((Rm * Rs)[31:0] + RdLo)
if S == 1 then
N Flag = RdHi[31]
Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0
C Flag = unaffected /* See "C and V flags" note */
V Flag = unaffected /* See "C and V flags" note */
Usage
UMLAL multiplies unsigned variables to produce a 64-bit result, which is added to the 64-bit value in the
two destination general-purpose registers. The result is written back to the two destination general-purpose
registers.
Notes
Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has
UNPREDICTABLE results.
Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are
UNPREDICTABLE.
Early termination If the multiplier implementation supports early termination, it must be implemented
on the value of the <Rs> operand. The type of early termination used (signed or
unsigned) is IMPLEMENTATION DEFINED.
C and V flags The UMLALS instruction is defined to leave the C and V flags unchanged in ARM
architecture version 5 and above. In earlier versions of the architecture, the values
of the C and V flags were UNPREDICTABLE after a UMLALS instruction.
A4-110 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-210-320.jpg)
![ARM Instructions
4.1.56 UMULL
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 1 0 0 S RdHi RdLo Rs 1 0 0 1 Rm
The UMULL (Unsigned Multiply Long) instruction multiplies the unsigned value of register <Rm> with the
unsigned value of register <Rs> to produce a 64-bit result. The upper 32 bits of the result are stored in
<RdHi>. The lower 32 bits are stored in <RdLo>. The condition code flags are optionally updated, based
on the 64-bit result.
Syntax
UMULL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction
updates the CPSR by setting the N and Z flags according to the result of the multiplication.
If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by
the instruction.
<RdLo> Stores the lower 32 bits of the result.
<RdHi> Stores the upper 32 bits of the result.
<Rm> Holds the signed value to be multiplied with the value of <Rs>.
<Rs> Holds the signed value to be multiplied with the value of <Rm>.
Architecture version
All M variants
Exceptions
None
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-111](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-211-320.jpg)
![ARM Instructions
Operation
if ConditionPassed(cond) then
RdHi = (Rm * Rs)[63:32] /* Unsigned multiplication */
RdLo = (Rm * Rs)[31:0]
if S == 1 then
N Flag = RdHi[31]
Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0
C Flag = unaffected /* See "C and V flags" note */
V Flag = unaffected /* See "C and V flags" note */
Usage
UMULL multiplies unsigned variables to produce a 64-bit result in two general-purpose registers.
Notes
Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has
UNPREDICTABLE results.
Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are
UNPREDICTABLE.
Early termination If the multiplier implementation supports early termination, it must be implemented
on the value of the <Rs> operand. The type of early termination used (signed or
unsigned) is IMPLEMENTATION DEFINED.
C and V flags The UMULLS instruction is defined to leave the C and V flags unchanged in ARM
architecture version 5 and above. In earlier versions of the architecture, the values
of the C and V flags were UNPREDICTABLE after a UMULLS instruction.
A4-112 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-212-320.jpg)
![ARM Addressing Modes
5.1.1 Encoding
The following diagrams show the encodings for this addressing mode:
32-bit immediate
31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 0
cond 0 0 1 opcode S Rn Rd rotate_imm immed_8
Immediate shifts
31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd shift_imm shift 0 Rm
Register shifts
31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd Rs 0 shift 1 Rm
opcode Specifies the operation of the instruction.
S bit Indicates that the instruction updates the condition codes.
Rd Specifies the destination register.
Rn Specifies the first source operand register.
Bits[11:0] The fields within bits[11:0] are collectively called a shifter operand. This is described in The
shifter operand on page A5-4.
Bit[25] Is referred to as the I bit, and is used to distinguish between an immediate shifter operand
and a register-based shifter operand.
If all three of the following bits have the values shown, the instruction is not a data-processing instruction,
but lies in the arithmetic or Load/Store instruction extension space:
bit[25] == 0
bit[4] == 1
bit[7] == 1
See Extending the instruction set on page A3-27 for more information.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-3](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-219-320.jpg)
![ARM Addressing Modes
5.1.3 Data-processing operands - Immediate
31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 0
cond 0 0 1 opcode S Rn Rd rotate_imm immed_8
This data-processing operand provides a constant (defined in the instruction) operand to a data-processing
instruction.
The shifter_operand value is formed by rotating (to the right) an 8-bit immediate value to any even
bit position in a 32-bit word. If the rotate immediate is zero, the carry-out from the shifter is the value of the
C flag, otherwise, it is set to bit[31] of the value of <shifter_operand>.
Syntax
#<immediate>
where:
<immediate> Specifies the immediate constant wanted. It is encoded in the instruction as an 8-bit
immediate (immed_8) and a 4-bit immediate (rotate_imm), so that <immediate>
is equal to the result of rotating immed_8 right by (2 * rotate_imm) bits.
Architecture version
All
Operation
shifter_operand = immed_8 Rotate_Right (rotate_imm * 2)
if rotate_imm == 0 then
shifter_carry_out = C flag
else /* rotate_imm != 0 */
shifter_carry_out = shifter_operand[31]
Notes
Legitimate immediates
Not all 32-bit immediates are legitimate. Only those that can be formed by rotating an 8-bit
immediate right by an even amount are valid 32-bit immediates for this format.
Encoding Some values of <immediate> have more than one possible encoding. For example, a
value of 0x3F0 could be encoded as:
immed_8 == 0x3F, rotate_imm == 0xE
or as:
immed_8 == 0xFC, rotate_imm == 0xF
When more than one encoding is available, an assembler needs to choose the correct one to
use, as follows:
A5-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-222-320.jpg)
![ARM Addressing Modes
5.1.5 Data-processing operands - Logical shift left by immediate
31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd shift_imm 0 0 0 Rm
This data-processing operand is used to provide either the value of a register directly (lone register operand,
as described in Data-processing operands - Register on page A5-8), or the value of a register shifted left
(multiplied by a constant power of two).
This instruction operand is the value of register Rm, logically shifted left by an immediate value in the range
0 to 31. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last bit shifted
out, or the C flag if no shift is specified.
Syntax
<Rm>, LSL #<shift_imm>
where:
<Rm> Specifies the register whose value is to be shifted.
LSL Indicates a logical shift left.
<shift_imm> Specifies the shift. This is a value between 0 and 31.
Architecture version
All
Operation
if shift_imm == 0 then /* Register Operand */
shifter_operand = Rm
shifter_carry_out = C Flag
else /* shift_imm > 0 */
shifter_operand = Rm Logical_Shift_Left shift_imm
shifter_carry_out = Rm[32 - shift_imm]
Notes
Default shift If the value of <shift_imm> == 0, the operand can be written as just <Rm> (see
Data-processing operands - Register on page A5-8).
Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current
instruction plus 8.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-225-320.jpg)
![ARM Addressing Modes
5.1.6 Data-processing operands - Logical shift left by register
31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd Rs 0 0 0 1 Rm
This data-processing operand is used to provide the value of a register multiplied by a variable power of two.
This instruction operand is the value of register Rm, logically shifted left by the value in the least significant
byte of register Rs. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last
bit shifted out, which is zero if the shift amount is more than 32, or the C flag if the shift amount is zero.
Syntax
<Rm>, LSL <Rs>
where:
<Rm> Specifies the register whose value is to be shifted.
LSL Indicates a logical shift left.
<Rs> Is the register containing the value of the shift.
Architecture version
All
Operation
if Rs[7:0] == 0 then
shifter_operand = Rm
shifter_carry_out = C Flag
else if Rs[7:0] < 32 then
shifter_operand = Rm Logical_Shift_Left Rs[7:0]
shifter_carry_out = Rm[32 - Rs[7:0]]
else if Rs[7:0] == 32 then
shifter_operand = 0
shifter_carry_out = Rm[0]
else /* Rs[7:0] > 32 */
shifter_operand = 0
shifter_carry_out = 0
Notes
Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE
results.
A5-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-226-320.jpg)
![ARM Addressing Modes
5.1.7 Data-processing operands - Logical shift right by immediate
31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd shift_imm 0 1 0 Rm
This data-processing operand is used to provide the unsigned value of a register shifted right (divided by a
constant power of two).
This instruction operand is the value of register Rm, logically shifted right by an immediate value in the
range 1 to 32. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last bit
shifted out.
Syntax
<Rm>, LSR #<shift_imm>
where:
<Rm> Specifies the register whose value is to be shifted.
LSR Indicates a logical shift right.
<shift_imm> Specifies the shift. This is an immediate value between 1 and 32. (A shift by 32 is
encoded by shift_imm == 0.)
Architecture version
All
Operation
if shift_imm == 0 then
shifter_operand = 0
shifter_carry_out = Rm[31]
else /* shift_imm > 0 */
shifter_operand = Rm Logical_Shift_Right shift_imm
shifter_carry_out = Rm[shift_imm - 1]
Notes
Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current
instruction plus 8.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-11](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-227-320.jpg)
![ARM Addressing Modes
5.1.8 Data-processing operands - Logical shift right by register
31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd Rs 0 0 1 1 Rm
This data-processing operand is used to provide the unsigned value of a register shifted right (divided by a
variable power of two).
It is produced by the value of register Rm, logically shifted right by the value in the least significant byte of
register Rs. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last bit
shifted out, which is zero if the shift amount is more than 32, or the C flag if the shift amount is zero.
Syntax
<Rm>, LSR <Rs>
where:
<Rm> Specifies the register whose value is to be shifted.
LSR Indicates a logical shift right.
<Rs> Is the register containing the value of the shift.
Architecture version
All
Operation
if Rs[7:0] == 0 then
shifter_operand = Rm
shifter_carry_out = C Flag
else if Rs[7:0] < 32 then
shifter_operand = Rm Logical_Shift_Right Rs[7:0]
shifter_carry_out = Rm[Rs[7:0] - 1]
else if Rs[7:0] == 32 then
shifter_operand = 0
shifter_carry_out = Rm[31]
else /* Rs[7:0] > 32 */
shifter_operand = 0
shifter_carry_out = 0
Notes
Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE
results.
A5-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-228-320.jpg)
![ARM Addressing Modes
5.1.9 Data-processing operands - Arithmetic shift right by immediate
31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd shift_imm 1 0 0 Rm
This data-processing operand is used to provide the signed value of a register arithmetically shifted right
(divided by a constant power of two).
This instruction operand is the value of register Rm, arithmetically shifted right by an immediate value in
the range 1 to 32. The sign bit of Rm (Rm[31]) is inserted into the vacated bit positions. The carry-out from
the shifter is the last bit shifted out.
Syntax
<Rm>, ASR #<shift_imm>
where:
<Rm> Specifies the register whose value is to be shifted.
ASR Indicates an arithmetic shift right.
<shift_imm> Specifies the shift. This is an immediate value between 1 and 32. (A shift by 32 is
encoded by shift_imm == 0.)
Architecture version
All
Operation
if shift_imm == 0 then
if Rm[31] == 0 then
shifter_operand = 0
shifter_carry_out = Rm[31]
else /* Rm[31] == 1 */
shifter_operand = 0xFFFFFFFF
shifter_carry_out = Rm[31]
else /* shift_imm > 0 */
shifter_operand = Rm Arithmetic_Shift_Right <shift_imm>
shifter_carry_out = Rm[shift_imm - 1]
Notes
Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current
instruction plus 8.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-13](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-229-320.jpg)
![ARM Addressing Modes
5.1.10 Data-processing operands - Arithmetic shift right by register
31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd Rs 0 1 0 1 Rm
This data-processing operand is used to provide the signed value of a register arithmetically shifted right
(divided by a variable power of two).
This instruction operand is the value of register Rm arithmetically shifted right by the value in the least
significant byte of register Rs. The sign bit of Rm (Rm[31]) is inserted into the vacated bit positions. The
carry-out from the shifter is the last bit shifted out, which is the sign bit of Rm if the shift amount is more
than 32, or the C flag if the shift amount is zero.
Syntax
<Rm>, ASR <Rs>
where:
<Rm> Specifies the register whose value is to be shifted.
ASR Indicates an arithmetic shift right.
<Rs> Is the register containing the value of the shift.
Architecture version
All
Operation
if Rs[7:0] == 0 then
shifter_operand = Rm
shifter_carry_out = C Flag
else if Rs[7:0] < 32 then
shifter_operand = Rm Arithmetic_Shift_Right Rs[7:0]
shifter_carry_out = Rm[Rs[7:0] - 1]
else /* Rs[7:0] >= 32 */
if Rm[31] == 0 then
shifter_operand = 0
shifter_carry_out = Rm[31]
else /* Rm[31] == 1 */
shifter_operand = 0xFFFFFFFF
shifter_carry_out = Rm[31]
Notes
Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE
results.
A5-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-230-320.jpg)
![ARM Addressing Modes
5.1.11 Data-processing operands - Rotate right by immediate
31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd shift_imm 1 1 0 Rm
This data-processing operand is used to provide the value of a register rotated by a constant value.
This instruction operand is the value of register Rm rotated right by an immediate value in the range 1 to
31. As bits are rotated off the right end, they are inserted into the vacated bit positions on the left. The
carry-out from the shifter is the last bit rotated off the right end.
Syntax
<Rm>, ROR #<shift_imm>
where:
<Rm> Specifies the register whose value is to be rotated.
ROR Indicates a rotate right.
<shift_imm> Specifies the rotation. This is an immediate value between 1 and 31. When
shift_imm == 0, an RRX operation (rotate right with extend) is performed. This is
described in Data-processing operands - Rotate right with extend on page A5-17.
Architecture version
All
Operation
if shift_imm == 0 then
See “Data-processing operands - Rotate right with extend” on page A5-17
else /* shift_imm > 0 */
shifter_operand = Rm Rotate_Right shift_imm
shifter_carry_out = Rm[shift_imm - 1]
Notes
Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current
instruction plus 8.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-231-320.jpg)
![ARM Addressing Modes
5.1.12 Data-processing operands - Rotate right by register
31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd Rs 0 1 1 1 Rm
This data-processing operand is used to provide the value of a register rotated by a variable value.
This instruction operand is produced by the value of register Rm rotated right by the value in the least
significant byte of register Rs. As bits are rotated off the right end, they are inserted into the vacated bit
positions on the left. The carry-out from the shifter is the last bit rotated off the right end, or the C flag if the
shift amount is zero.
Syntax
<Rm>, ROR <Rs>
where:
<Rm> Specifies the register whose value is to be rotated.
ROR Indicates a rotate right.
<Rs> Is the register containing the value of the rotation.
Architecture version
All
Operation
if Rs[7:0] == 0 then
shifter_operand = Rm
shifter_carry_out = C Flag
else if Rs[4:0] == 0 then
shifter_operand = Rm
shifter_carry_out = Rm[31]
else /* Rs[4:0] > 0 */
shifter_operand = Rm Rotate_Right Rs[4:0]
shifter_carry_out = Rm[Rs[4:0] - 1]
Notes
Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE
results.
A5-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-232-320.jpg)
![ARM Addressing Modes
5.1.13 Data-processing operands - Rotate right with extend
31 28 27 26 25 24 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 0 0 0 opcode S Rn Rd 0 0 0 0 0 1 1 0 Rm
This data-processing operand can be used to perform a 33-bit rotate right using the Carry Flag as the 33rd
bit.
This instruction operand is the value of register Rm shifted right by one bit, with the Carry Flag replacing
the vacated bit position. The carry-out from the shifter is the bit shifted off the right end.
Syntax
<Rm>, RRX
where:
<Rm> Specifies the register whose value is shifted right by one bit.
RRX Indicates a rotate right with extend.
Architecture version
All
Operation
shifter_operand = (C Flag Logical_Shift_Left 31) OR (Rm Logical_Shift_Right 1)
shifter_carry_out = Rm[0]
Notes
Encoding The instruction encoding is in the space that would be used for ROR #0.
Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current
instruction plus 8.
ADC instruction A rotate left with extend can be performed with an ADC instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-17](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-233-320.jpg)
![ARM Addressing Modes
5.2 Addressing Mode 2 - Load and Store Word or Unsigned Byte
There are nine addressing modes used to calculate the address for a Load and Store Word or Unsigned Byte
instruction. The general instruction syntax is:
LDR|STR{<cond>}{B}{T} <Rd>, <addressing_mode>
where <addressing_mode> is one of the nine options listed below.
All nine of the following options are available for LDR, LDRB, STR and STRB. For LDRBT, LDRT, STRBT
and STRBT, only the post-indexed options (the last three in the list) are available. For the PLD instruction
described in PLD on page A10-14, only the offset options (the first three in the list) are available.
1. [<Rn>, #+/-<offset_12>]
See Load and Store Word or Unsigned Byte - Immediate offset on page A5-20.
2. [<Rn>, +/-<Rm>]
See Load and Store Word or Unsigned Byte - Register offset on page A5-21.
3. [<Rn>, +/-<Rm>, <shift> #<shift_imm>]
See Load and Store Word or Unsigned Byte - Scaled register offset on page A5-22.
4. [<Rn>, #+/-<offset_12>]!
See Load and Store Word or Unsigned Byte - Immediate pre-indexed on page A5-24.
5. [<Rn>, +/-<Rm>]!
See Load and Store Word or Unsigned Byte - Register pre-indexed on page A5-25.
6. [<Rn>, +/-<Rm>, <shift> #<shift_imm>]!
See Load and Store Word or Unsigned Byte - Scaled register pre-indexed on page A5-26.
7. [<Rn>], #+/-<offset_12>
See Load and Store Word or Unsigned Byte - Immediate post-indexed on page A5-28.
8. [<Rn>], +/-<Rm>
See Load and Store Word or Unsigned Byte - Register post-indexed on page A5-30.
9. [<Rn>], +/-<Rm>, <shift> #<shift_imm>
See Load and Store Word or Unsigned Byte - Scaled register post-indexed on page A5-32.
A5-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-234-320.jpg)
![ARM Addressing Modes
5.2.2 Load and Store Word or Unsigned Byte - Immediate offset
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 0 1 U B 0 L Rn Rd offset_12
This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or
from the value of the base register Rn.
Syntax
[<Rn>, #+/-<offset_12>]
where:
<Rn> Specifies the register containing the base address.
<offset_12> Specifies the immediate offset used with the value of Rn to form the address.
Architecture version
All
Operation
if U == 1 then
address = Rn + offset_12
else /* U == 0 */
address = Rn - offset_12
Usage
This addressing mode is useful for accessing structure (record) fields, and accessing parameters and local
variables in a stack frame. With an offset of zero, the address produced is the unaltered value of the base
register Rn.
Notes
Offset of zero The syntax [<Rn>] is treated as an abbreviation for [<Rn>, #0], unless the instruction
is one that only allows post-indexed addressing modes (LDRBT, LDRT, STRBT or STRT).
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8.
A5-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-236-320.jpg)
![ARM Addressing Modes
5.2.3 Load and Store Word or Unsigned Byte - Register offset
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 0 1 1 1 U B 0 L Rn Rd 0 0 0 0 0 0 0 0 Rm
This addressing mode calculates an address by adding or subtracting the value of the index register Rm to
or from the value of the base register Rn.
Syntax
[<Rn>, +/-<Rm>]
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the value to add to or subtract from Rn.
Architecture version
All
Operation
if U == 1 then
address = Rn + Rm
else /* U == 0 */
address = Rn - Rm
Usage
This addressing mode is used for pointer plus offset arithmetic, and accessing a single element of an array
of bytes.
Notes
Encoding This addressing mode is encoded as an LSL scaled register offset, scaled by zero.
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8.
Specifying R15 as register Rm has UNPREDICTABLE results.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-21](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-237-320.jpg)
![ARM Addressing Modes
5.2.4 Load and Store Word or Unsigned Byte - Scaled register offset
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 1 1 1 U B 0 L Rn Rd shift_imm shift 0 Rm
These five addressing modes calculate an address by adding or subtracting the shifted or rotated value of
the index register Rm to or from the value of the base register Rn.
Syntax
One of:
[<Rn>, +/-<Rm>, LSL #<shift_imm>]
[<Rn>, +/-<Rm>, LSR #<shift_imm>]
[<Rn>, +/-<Rm>, ASR #<shift_imm>]
[<Rn>, +/-<Rm>, ROR #<shift_imm>]
[<Rn>, +/-<Rm>, RRX]
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
LSL Specifies a logical shift left.
LSR Specifies a logical shift right.
ASR Specifies an arithmetic shift right.
ROR Specifies a rotate right.
RRX Specifies a rotate right with extend.
<shift_imm> Specifies the shift or rotation.
LSL 0 to 31, encoded directly in the shift_imm field.
LSR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift
amounts are encoded directly.
ASR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift
amounts are encoded directly.
ROR 1 to 31, encoded directly in the shift_imm field. (The shift_imm == 0
encoding is used to specify the RRX option.)
Architecture version
All
A5-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-238-320.jpg)
![ARM Addressing Modes
Operation
case shift of
0b00 /* LSL */
index = Rm Logical_Shift_Left shift_imm
0b01 /* LSR */
if shift_imm == 0 then /* LSR #32 */
index = 0
else
index = Rm Logical_Shift_Right shift_imm
0b10 /* ASR */
if shift_imm == 0 then /* ASR #32 */
if Rm[31] == 1 then
index = 0xFFFFFFFF
else
index = 0
else
index = Rm Arithmetic_Shift_Right shift_imm
0b11 /* ROR or RRX */
if shift_imm == 0 then /* RRX */
index = (C Flag Logical_Shift_Left 31) OR
(Rm Logical_Shift_Right 1)
else /* ROR */
index = Rm Rotate_Right shift_imm
endcase
if U == 1 then
address = Rn + index
else /* U == 0 */
address = Rn - index
Usage
These addressing modes are used for accessing a single element of an array of values larger than a byte.
Notes
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8.
Specifying R15 as register Rm has UNPREDICTABLE results.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-23](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-239-320.jpg)
![ARM Addressing Modes
5.2.5 Load and Store Word or Unsigned Byte - Immediate pre-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 0 1 U B 1 L Rn Rd offset_12
This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or
from the value of the base register Rn.
If the condition specified in the instruction matches the condition code status, the calculated address is
written back to the base register Rn. The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>, #+/-<offset_12>]!
where:
<Rn> Specifies the register containing the base address.
<offset_12> Specifies the immediate offset used with the value of Rn to form the address.
! Sets the W bit, causing base register update.
Architecture version
All
Operation
if U == 1 then
address = Rn + offset_12
else /* if U == 0 */
address = Rn - offset_12
if ConditionPassed(cond) then
Rn = address
Usage
This addressing mode is used for pointer access to arrays with automatic update of the pointer value.
Notes
Offset of zero The syntax [<Rn>] must never be treated as an abbreviation for [<Rn>, #0]!.
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results.
A5-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-240-320.jpg)
![ARM Addressing Modes
5.2.6 Load and Store Word or Unsigned Byte - Register pre-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 0 1 1 1 U B 1 L Rn Rd 0 0 0 0 0 0 0 0 Rm
This addressing mode calculates an address by adding or subtracting the value of an index register Rm to or
from the value of the base register Rn.
If the condition specified in the instruction matches the condition code status, the calculated address is
written back to the base register Rn. The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>, +/-<Rm>]!
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
! Sets the W bit, causing base register update.
Architecture version
All
Operation
if U == 1 then
address = Rn + Rm
else /* U == 0 */
address = Rn - Rm
if ConditionPassed(cond) then
Rn = address
Notes
Encoding This addressing mode is encoded as an LSL scaled register offset, scaled by zero.
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results.
Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-25](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-241-320.jpg)
![ARM Addressing Modes
5.2.7 Load and Store Word or Unsigned Byte - Scaled register pre-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 1 1 1 U B 1 L Rn Rd shift_imm shift 0 Rm
These five addressing modes calculate an address by adding or subtracting the shifted or rotated value of
the index register Rm to or from the value of the base register Rn.
If the condition specified in the instruction matches the condition code status, the calculated address is
written back to the base register Rn. The conditions are defined in The condition field on page A3-5.
Syntax
One of:
[<Rn>, +/-<Rm>, LSL #<shift_imm>]!
[<Rn>, +/-<Rm>, LSR #<shift_imm>]!
[<Rn>, +/-<Rm>, ASR #<shift_imm>]!
[<Rn>, +/-<Rm>, ROR #<shift_imm>]!
[<Rn>, +/-<Rm>, RRX]!
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
LSL Specifies a logical shift left.
LSR Specifies a logical shift right.
ASR Specifies an arithmetic shift right.
ROR Specifies a rotate right.
RRX Specifies a rotate right with extend.
<shift_imm> Specifies the shift or rotation.
LSL 0 to 31, encoded directly in the shift_imm field.
LSR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift
amounts are encoded directly.
ASR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift
amounts are encoded directly.
ROR 1 to 31, encoded directly in the shift_imm field. (The shift_imm == 0
encoding is used to specify the RRX option.)
! Sets the W bit, causing base register update.
A5-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-242-320.jpg)
![ARM Addressing Modes
Architecture version
All
Operation
case shift of
0b00 /* LSL */
index = Rm Logical_Shift_Left shift_imm
0b01 /* LSR */
if shift_imm == 0 then /* LSR #32 */
index = 0
else
index = Rm Logical_Shift_Right shift_imm
0b10 /* ASR */
if shift_imm == 0 then /* ASR #32 */
if Rm[31] == 1 then
index = 0xFFFFFFFF
else
index = 0
else
index = Rm Arithmetic_Shift_Right shift_imm
0b11 /* ROR or RRX */
if shift_imm == 0 then /* RRX */
index = (C Flag Logical_Shift_Left 31) OR
(Rm Logical_Shift_Right 1)
else /* ROR */
index = Rm Rotate_Right shift_imm
endcase
if U == 1 then
address = Rn + index
else /* U == 0 */
address = Rn - index
if ConditionPassed(cond) then
Rn = address
Notes
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results.
Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-27](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-243-320.jpg)
![ARM Addressing Modes
5.2.8 Load and Store Word or Unsigned Byte - Immediate post-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 1 0 0 U B 0 L Rn Rd offset_12
This addressing mode uses the value of the base register Rn as the address for the memory access.
If the condition specified in the instruction matches the condition code status, the value of the immediate
offset is added to or subtracted from the value of the base register Rn and written back to the base register
Rn. The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>], #+/-<offset_12>
where:
<Rn> Specifies the register containing the base address.
<offset_12> Specifies the immediate offset used with the value of Rn to form the address.
Architecture version
All
Operation
address = Rn
if ConditionPassed(cond) then
if U == 1 then
Rn = Rn + offset_12
else /* U == 0 */
Rn = Rn - offset_12
Usage
This addressing mode is used for pointer access to arrays with automatic update of the pointer value.
A5-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-244-320.jpg)
![ARM Addressing Modes
Notes
Post-indexed addressing modes
LDRBT, LDRT, STRBT, and STRT only support post-indexed addressing modes. They use
a minor modification of the above bit pattern, where bit[21] (the W bit) is 1, not 0 as shown.
Offset of zero The syntax [<Rn>] is treated as an abbreviation for [<Rn>],#0 for instructions that only
support post-indexed addressing modes (LDRBT, LDRT, STRBT, STRT), but not for other
instructions.
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-29](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-245-320.jpg)
![ARM Addressing Modes
5.2.9 Load and Store Word or Unsigned Byte - Register post-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 0 1 1 0 U B 0 L Rn Rd 0 0 0 0 0 0 0 0 Rm
This addressing mode uses the value of the base register Rn as the address for the memory access.
If the condition specified in the instruction matches the condition code status, the value of the index register
Rm is added to or subtracted from the value of the base register Rn and written back to the base register Rn.
The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>], +/-<Rm>
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
Architecture version
All
Operation
address = Rn
if ConditionPassed(cond) then
if U == 1 then
Rn = Rn + Rm
else /* U == 0 */
Rn = Rn - Rm
A5-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-246-320.jpg)
![ARM Addressing Modes
Notes
Encoding This addressing mode is encoded as an LSL scaled register offset, scaled by zero.
Post-indexed addressing modes
LDRBT, LDRT, STRBT, and STRT only support post-indexed addressing modes.
They use a minor modification of the above bit pattern, where bit[21] (the W bit) is
1, not 0 as shown.
The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
Use of R15 Specifying R15 as register Rn or Rm has UNPREDICTABLE results.
Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-31](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-247-320.jpg)
![ARM Addressing Modes
5.2.10 Load and Store Word or Unsigned Byte - Scaled register post-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0
cond 0 1 1 0 U B 0 L Rn Rd shift_imm shift 0 Rm
This addressing mode uses the value of the base register Rn as the address for the memory access.
If the condition specified in the instruction matches the condition code status, the shifted or rotated value of
index register Rm is added to or subtracted from the value of the base register Rn and written back to the
base register Rn. The conditions are defined in The condition field on page A3-5.
Syntax
One of:
[<Rn>], +/-<Rm>, LSL #<shift_imm>
[<Rn>], +/-<Rm>, LSR #<shift_imm>
[<Rn>], +/-<Rm>, ASR #<shift_imm>
[<Rn>], +/-<Rm>, ROR #<shift_imm>
[<Rn>], +/-<Rm>, RRX
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
LSL Specifies a logical shift left.
LSR Specifies a logical shift right.
ASR Specifies an arithmetic shift right.
ROR Specifies a rotate right.
RRX Specifies a rotate right with extend.
<shift_imm> Specifies the shift or rotation.
LSL 0 to 31, encoded directly in the shift_imm field.
LSR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift
amounts are encoded directly.
ASR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift
amounts are encoded directly.
ROR 1 to 31, encoded directly in the shift_imm field. (The shift_imm == 0
encoding is used to specify the RRX option.)
A5-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-248-320.jpg)
![ARM Addressing Modes
Architecture version
All
Operation
address = Rn
case shift of
0b00 /* LSL */
index = Rm Logical_Shift_Left shift_imm
0b01 /* LSR */
if shift_imm == 0 then /* LSR #32 */
index = 0
else
index = Rm Logical_Shift_Right shift_imm
0b10 /* ASR */
if shift_imm == 0 then /* ASR #32 */
if Rm[31] == 1 then
index = 0xFFFFFFFF
else
index = 0
else
index = Rm Arithmetic_Shift_Right shift_imm
0b11 /* ROR or RRX */
if shift_imm == 0 then /* RRX */
index = (C Flag Logical_Shift_Left 31) OR
(Rm Logical_Shift_Right 1)
else /* ROR */
index = Rm Rotate_Right shift_imm
endcase
if ConditionPassed(cond) then
if U == 1 then
Rn = Rn + index
else /* U == 0 */
Rn = Rn - index
Notes
Post-indexed addressing modes
LDRBT, LDRT, STRBT, and STRT only support post-indexed addressing modes.
They use a minor modification of the above bit pattern, where bit[21] (the W bit) is
1, not 0 as shown.
The B bit This bit distinguishes between an unsigned byte (B == 1) and a word (B == 0)
access.
The L bit This bit distinguishes between a Load (L == 1) and a Store (L == 0) instruction.
Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results.
Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-33](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-249-320.jpg)
![ARM Addressing Modes
5.3 Addressing Mode 3 - Miscellaneous Loads and Stores
There are six addressing modes used to calculate the address for load and store (signed or unsigned)
halfword, load signed byte, or load and store doubleword instructions. The general instruction syntax is:
LDR|STR{<cond>}H|SH|SB|D <Rd>, <addressing_mode>
where <addressing_mode> is one of the following six options:
1. [<Rn>, #+/-<offset_8>]
See Miscellaneous Loads and Stores - Immediate offset on page A5-36.
2. [<Rn>, +/-<Rm>]
See Miscellaneous Loads and Stores - Register offset on page A5-38.
3. [<Rn>, #+/-<offset_8>]!
See Miscellaneous Loads and Stores - Immediate pre-indexed on page A5-40.
4. [<Rn>, +/-<Rm>]!
See Miscellaneous Loads and Stores - Register pre-indexed on page A5-42.
5. [<Rn>], #+/-<offset_8>
See Miscellaneous Loads and Stores - Immediate post-indexed on page A5-44.
6. [<Rn>], +/-<Rm>
See Miscellaneous Loads and Stores - Register post-indexed on page A5-46.
5.3.1 Encoding
The following diagrams show the encodings for this addressing mode:
Immediate offset/index
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 P U 1 W L Rn Rd immedH 1 S H 1 ImmedL
Register offset/index
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 P U 0 W L Rn Rd SBZ 1 S H 1 Rm
A5-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-250-320.jpg)
![ARM Addressing Modes
5.3.2 Miscellaneous Loads and Stores - Immediate offset
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 U 1 0 L Rn Rd immedH 1 S H 1 immedL
This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or
from the value of the base register Rn.
Syntax
[<Rn>, #+/-<offset_8>]
where:
<Rn> Specifies the register containing the base address.
<offset_8> Specifies the immediate offset used with the value of Rn to form the address. The
offset is encoded in immedH (top 4 bits) and immedL (bottom 4 bits).
Architecture version
Version 4 and above
Operation
offset_8 = (immedH << 4) OR immedL
if U == 1 then
address = Rn + offset_8
else /* U == 0 */
address = Rn - offset_8
Usage
This addressing mode is used for accessing structure (record) fields, and accessing parameters and locals
variable in a stack frame. With an offset of zero, the address produced is the unaltered value of the base
register Rn.
Notes
Zero offset The syntax [<Rn>] is treated as an abbreviation for [<Rn>,#0].
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword access.
The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access.
Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8.
A5-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-252-320.jpg)
![ARM Addressing Modes
5.3.3 Miscellaneous Loads and Stores - Register offset
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 U 0 0 L Rn Rd SBZ 1 S H 1 Rm
This addressing mode calculates an address by adding or subtracting the value of the index register Rm to
or from the value of the base register Rn.
Syntax
[<Rn>, +/-<Rm>]
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
Architecture version
Version 4 and above
Operation
if U == 1 then
address = Rn + Rm
else /* U == 0 */
address = Rn - Rm
Usage
This addressing mode is useful for pointer plus offset arithmetic and for accessing a single element of an
array.
Notes
The L bit Distinguishes between a Load (L==1) and a Store (L==0) instruction.
The S bit Distinguishes between a signed (S==1) and an unsigned (S==0) halfword access.
The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access.
Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not
one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or
SWPB instruction, or an unallocated instruction in the arithmetic or load/store
instruction extension space (see Extending the instruction set on page A3-27).
Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions,
which use addressing mode 2 rather than addressing mode 3.
A5-38 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-254-320.jpg)
![ARM Addressing Modes
5.3.4 Miscellaneous Loads and Stores - Immediate pre-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 U 1 1 L Rn Rd immedH 1 S H 1 ImmedL
This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or
from the value of the base register Rn.
If the condition specified in the instruction matches the condition code status, the calculated address is
written back to the base register Rn. The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>, #+/-<offset_8>]!
where:
<Rn> Specifies the register containing the base address.
<offset_8> Specifies the immediate offset used with the value of Rn to form the address. The
offset is encoded in immedH (top 4 bits) and immedL (bottom 4 bits).
! Sets the W bit, causing base register update.
Architecture version
Version 4 and above
Operation
offset_8 = (immedH << 4) OR immedL
if U == 1 then
address = Rn + offset_8
else /* U == 0 */
address = Rn - offset_8
if ConditionPassed(cond) then
Rn = address
Usage
This addressing mode gives pointer access to arrays, with automatic update of the pointer value.
A5-40 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-256-320.jpg)
![ARM Addressing Modes
Notes
Offset of zero The syntax [<Rn>] must not be treated as an abbreviation for [<Rn>,#0]!.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword
access.
The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access.
Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not
one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or
SWPB instruction, or an unallocated instruction in the arithmetic or load/store
instruction extension space (see Extending the instruction set on page A3-27).
Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions,
which use addressing mode 2 rather than addressing mode 3.
Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction
is an unallocated instruction in the load/store extension space (see Extending the
instruction set on page A3-27).
Signed bytes and halfwords can be stored with the same STRB and STRH
instructions as are used for unsigned quantities, so no separate signed store
instructions are provided.
Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-41](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-257-320.jpg)
![ARM Addressing Modes
5.3.5 Miscellaneous Loads and Stores - Register pre-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 U 0 1 L Rn Rd SBZ 1 S H 1 Rm
This addressing mode calculates an address by adding or subtracting the value of the index register Rm to
or from the value of the base register Rn.
If the condition specified in the instruction matches the condition code status, the calculated address is
written back to the base register Rn. The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>, +/-<Rm>]!
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
! Sets the W bit, causing base register update.
Architecture version
Version 4 and above
Operation
if U == 1 then
address = Rn + Rm
else /* U == 0 */
address = Rn - Rm
if ConditionPassed(cond) then
Rn = address
A5-42 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-258-320.jpg)
![ARM Addressing Modes
5.3.6 Miscellaneous Loads and Stores - Immediate post-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 U 1 0 L Rn Rd immedH 1 S H 1 ImmedL
This addressing mode uses the value of the base register Rn as the address for the memory access.
If the condition specified in the instruction matches the condition code status, the value of the immediate
offset is added to or subtracted from the value of the base register Rn and written back to the base
register Rn. The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>], #+/-<offset_8>
where:
<Rn> Specifies the register containing the base address.
<offset_8> Specifies the immediate offset used with the value of Rn to form the address. The
offset is encoded in immedH (top 4 bits) and immedL (bottom 4 bits).
Architecture version
Version 4 and above
Operation
address = Rn
offset_8 = (immedH << 4) OR immedL
if ConditionPassed(cond) then
if U == 1 then
Rn = Rn + offset_8
else /* U == 0 */
Rn = Rn - offset_8
Usage
This addressing mode gives pointer access to arrays, with automatic update of the pointer value.
A5-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-260-320.jpg)
![ARM Addressing Modes
Notes
Offset of zero The syntax [<Rn>] must not be treated as an abbreviation for [<Rn>],#0.
The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction.
The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword
access.
The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access.
Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not
one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or
SWPB instruction, or an unallocated instruction in the arithmetic or load/store
instruction extension space (see Extending the instruction set on page A3-27).
Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions,
which use addressing mode 2 rather than addressing mode 3.
Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction
is an unallocated instruction in the load/store extension space (see Extending the
instruction set on page A3-27).
Signed bytes and halfwords can be stored with the same STRB and STRH
instructions as are used for unsigned quantities, so no separate signed store
instructions are provided.
Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-45](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-261-320.jpg)
![ARM Addressing Modes
5.3.7 Miscellaneous Loads and Stores - Register post-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 0 U 0 0 L Rn Rd SBZ 1 S H 1 Rm
This addressing mode uses the value of the base register Rn as the address for the memory access.
If the condition specified in the instruction matches the condition code status, the value of the index register
Rm is added to or subtracted from the value of the base register Rn and written back to the base register Rn.
The conditions are defined in The condition field on page A3-5.
Syntax
[<Rn>], +/-<Rm>
where:
<Rn> Specifies the register containing the base address.
<Rm> Specifies the register containing the offset to add to or subtract from Rn.
Architecture version
Version 4 and above
Operation
address = Rn
if ConditionPassed(cond) then
if U == 1 then
Rn = Rn + Rm
else /* U == 0 */
Rn = Rn - Rm
A5-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-262-320.jpg)
![ARM Addressing Modes
The S bit For LDMs that load the PC, the S bit indicates that the CPSR is loaded from the SPSR. For
LDMs that do not load the PC and all STMs, the S bit indicates that when the processor is in
a privileged mode, the User mode banked registers are transferred instead of the registers of
the current mode.
The W bit Indicates that the base register is updated after the transfer. The base register is incremented
(U==1) or decremented (U==0) by four times the number of registers in the register list.
The L bit Distinguishes between Load (L==1) and Store (L==0) instructions.
Register list The register_list field of the instruction has one bit for each general-purpose register: bit[0]
for register zero through to bit[15] for register 15 (the PC). If no bits are set, the result is
UNPREDICTABLE.
The instruction syntax specifies the registers to load or store in <registers>, which is a
comma-separated list of registers, surrounded by { and }.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-49](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-265-320.jpg)
![ARM Addressing Modes
5.5 Addressing Mode 5 - Load and Store Coprocessor
There are four addressing modes which are used to calculate the address of a Load or Store Coprocessor
instruction. The general instruction syntax is:
<opcode>{<cond>}{L} <coproc>,<CRd>,<addressing_mode>
where <addressing_mode> is one of the following four options:
1. [<Rn>,#+/-<offset_8>*4]
See Load and Store Coprocessor - Immediate offset on page A5-58.
2. [<Rn>,#+/-<offset_8>*4]!
See Load and Store Coprocessor - Immediate pre-indexed on page A5-60.
3. [<Rn>],#+/-<offset_8>*4
See Load and Store Coprocessor - Immediate post-indexed on page A5-62.
4. [<Rn>],<option>
See Load and Store Coprocessor - Unindexed on page A5-64.
5.5.1 Encoding
The following diagram shows the encoding for this addressing mode:
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 P U N W L Rn CRd cp# offset_8
The P bit Has two meanings:
P == 1 Indicates the use of post-indexed addressing or unindexed addressing (the W bit
determines which). The base register value is used for the memory address.
P == 0 Indicates the use of offset addressing or pre-indexed addressing (the W bit
determines which). The memory address is generated by applying the offset to
the base register value.
The U bit Has two meanings:
U == 1 Indicates that the offset is added to the base.
U == 0 Indicates that he offset is subtracted from the base
The N bit The meaning of this bit is coprocessor-dependent. Its recommended use is to distinguish
between different-sized values to be transferred.
The W bit Has two meanings:
W == 1 Indicates that the memory address is written back to the base register.
W == 0 Indicates that the base register value is unchanged.
A5-56 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-272-320.jpg)
![ARM Addressing Modes
5.5.2 Load and Store Coprocessor - Immediate offset
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 1 U N 0 L Rn CRd cp_num offset_8
This addressing mode produces a sequence of consecutive addresses. The first address is calculated by
adding or subtracting four times the value of an immediate offset to or from the value of the base register
Rn. The subsequent addresses in the sequence are produced by incrementing the previous address by four
until the coprocessor signals the end of the instruction. This allows a coprocessor to access data whose size
is coprocessor-defined.
The coprocessor must not request a transfer of more than 16 words.
Syntax
[<Rn>, #+/-<offset_8>*4]
where:
<Rn> Specifies the register containing the base address.
<offset_8> Specifies the immediate offset that is multiplied by 4, then added to or subtracted
from the value of Rn to form the address.
Architecture version
Version 2 and above
Operation
if ConditionPassed(cond) then
if U == 1 then
address = Rn + offset_8 * 4
else /* U == 0 */
address = Rn - offset_8 * 4
start_address = address
while (NotFinished(coprocessor[cp_num]))
address = address + 4
end_address = address
A5-58 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-274-320.jpg)
![ARM Addressing Modes
5.5.3 Load and Store Coprocessor - Immediate pre-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 1 U N 1 L Rn CRd cp_num offset_8
This addressing mode produces a sequence of consecutive addresses. The first address is calculated by
adding or subtracting four times the value of an immediate offset to or from the value of the base register
Rn. If the condition specified in the instruction matches the condition code status, the first address is written
back to the base register Rn. The subsequent addresses in the sequence are produced by incrementing the
previous address by four until the coprocessor signals the end of the instruction. This allows a coprocessor
to access data whose size is coprocessor-defined.
The coprocessor must not request a transfer of more than 16 words.
Syntax
[<Rn>, #+/-<offset_8>*4]!
where:
<Rn> Specifies the register containing the base address.
<offset_8> Specifies the immediate offset that is multiplied by 4, then added to or subtracted
from the value of Rn to form the address.
! Sets the W bit, causing base register update.
Architecture version
Version 2 and above
Operation
if ConditionPassed(cond) then
if U == 1 then
Rn = Rn + offset_8 * 4
else /* U == 0 */
Rn = Rn - offset_8 * 4
start_address = Rn
address = start_address
while (NotFinished(coprocessor[cp_num]))
address = address + 4
end_address = address
A5-60 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-276-320.jpg)
![ARM Addressing Modes
5.5.4 Load and Store Coprocessor - Immediate post-indexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 0 U N 1 L Rn CRd cp_num offset_8
This addressing mode produces a sequence of consecutive addresses. The first address is the value of the
base register Rn. The subsequent addresses in the sequence are produced by incrementing the previous
address by four until the coprocessor signals the end of the instruction. This allows a coprocessor to access
data whose size is coprocessor-defined.
If the condition specified in the instruction matches the condition code status, the base register Rn is updated
by adding or subtracting four times the value of an immediate offset to or from the value of the base register
Rn.
The coprocessor must not request a transfer of more than 16 words.
Syntax
[<Rn>], #+/-<offset_8>*4
where:
<Rn> Specifies the register containing the base address.
<offset_8> Specifies the immediate offset that is multiplied by 4, then added to or subtracted
from the value of Rn to form the address.
Architecture version
Version 2 and above
Operation
if ConditionPassed(cond) then
start_address = Rn
if U == 1 then
Rn = Rn + offset_8 * 4
else /* U == 0 */
Rn = Rn - offset_8 * 4
address = start_address
while (NotFinished(coprocessor[cp_num]))
address = address + 4
end_address = address
A5-62 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-278-320.jpg)
![ARM Addressing Modes
5.5.5 Load and Store Coprocessor - Unindexed
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0
cond 1 1 0 0 U N 0 L Rn CRd cp_num option
This addressing mode produces a sequence of consecutive addresses. The first address is the value of the
base register Rn. The subsequent addresses in the sequence are produced by incrementing the previous
address by four until the coprocessor signals the end of the instruction. This allows a coprocessor to access
data whose size is coprocessor-defined.
The base register Rn is not updated. Bits[7:0] of the instruction are therefore not used by the ARM, either
for the address calculation or to calculate a new value for the base register, and so can be used to specify
additional instruction options to the coprocessor.
The coprocessor must not request a transfer of more than 16 words.
Syntax
[<Rn>], <option>
where:
<Rn> Specifies the register containing the base address.
<option> Specifies additional instruction options to the coprocessor. The <option> is specified in
the instruction syntax as an integer in the range 0-255, surrounded by { and }.
Architecture version
Version 2 and above
Operation
if ConditionPassed(cond) then
start_address = Rn
address = start_address
while (NotFinished(coprocessor[cp_num]))
address = address + 4
end_address = address
A5-64 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-280-320.jpg)
![ARM Addressing Modes
Notes
The N bit Is coprocessor-dependent.
The L bit Distinguishes between Load (L==1) and Store (L==0) instructions.
Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8.
The U bit If bit[23] (the Up/down bit) is not set, the result is either UNDEFINED or UNPREDICTABLE (see
Coprocessor instruction extension space on page A3-33).
Option bits Are unused by the ARM in this addressing mode, and therefore can be used to request
additional instruction options in a coprocessor-dependent fashion.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-65](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-281-320.jpg)
![The Thumb Instruction Set
6.1 About the Thumb instruction set
The Thumb instruction set is a re-encoded subset of the ARM instruction set. Thumb is designed to increase
the performance of ARM implementations that use a 16-bit or narrower memory data bus and to allow better
code density than ARM. T variants of the ARM architecture incorporate both a full 32-bit ARM instruction
set and the 16-bit Thumb instruction set. Every Thumb instruction is encoded in 16 bits.
Thumb does not alter the underlying programmer’s model of the ARM architecture. It merely presents
restricted access to it. All Thumb data-processing instructions operate on full 32-bit values, and full 32-bit
addresses are produced by both data-access instructions and instruction fetches.
When the processor is executing Thumb instructions, eight general-purpose integer registers are available,
R0 to R7, which are the same physical registers as R0 to R7 when executing ARM instructions. Some
Thumb instructions also access the Program Counter (ARM Register 15), the Link Register
(ARM Register 14) and the Stack Pointer (ARM Register 13). Further instructions allow limited access to
ARM registers 8 to 15, which are know as the high registers.
When R15 is read, bit[0] is zero and bits[31:1] contain the PC. When R15 is written, bit[0] is IGNORED and
bits[31:1] are written to the PC. Depending on how it is used, the value of the PC is either the address of the
instruction plus 4 or is UNPREDICTABLE.
Thumb does not provide direct access to the CPSR or any SPSR (as the MSR and MRS instructions do in
the ARM instruction set)). Thumb execution is flagged by the T bit (bit[5]) in the CPSR:
T == 0 32-bit instructions are fetched (and the PC is incremented by four) and are executed as ARM
instructions.
T == 1 16-bit instructions are fetched (and the PC is incremented by two) and are executed as
Thumb instructions.
Note
The Thumb instruction set is only compatible with the 32-bit ARM architectures. Thumb is not
recommended for use with 26-bit architectures or with 26-bit compatibility options on 32-bit architectures.
6.1.1 Entering Thumb state
Thumb execution is normally entered by executing an ARM BX instruction (Branch and Exchange). This
instruction branches to the address held in a general-purpose register, and if bit[0] of that register is 1,
Thumb execution begins at the branch target address. If bit[0] of the target register is 0, ARM execution
continues from the branch target address. On architecture versions 5 and above, BLX instructions and
LDR/LDM instructions that load the PC can be used similarly.
Thumb execution can also be initiated by setting the T bit in the SPSR and executing an ARM instruction
which restores the CPSR from the SPSR (a data-processing instruction with the S bit set and the PC as the
destination, or a Load Multiple with Restore CPSR instruction). This allows an operating system to
automatically restart a process independent of whether that process is executing Thumb code or ARM code.
The result is UNPREDICTABLE if the T bit is altered directly by writing the CPSR.
A6-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-284-320.jpg)
![The Thumb Instruction Set
6.2 Instruction set encoding
Figure 6-1 shows the Thumb instruction set encoding. An entry in square brackets, for example [1],
indicates a note on the following page.
1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0
0 0 0 opcode [1] immediate Rm Rd
1 1 0
1 1 1
opc Rm Rn Rd
opc immediate Rn Rd
opcode Rd / Rn
immediate
Rm / Rs Rd / Rn
opcode
opcode [1] Rd / Rn
H1 H2 Rm
Branch/exchange 0 1 0 0 0 1 1 1 L H2 Rm SBZ
instruction set [3]
PC-relative offset
0 1 0 0 0 1
1
1 Rm Rn Rd
offset
opcode
B L Rn Rd
offset
0 L Rn Rd
SP-relative offset
0 0 0
0 Rd
1 0 0 1 L Rd
1 0 1 0 SP Rd immediate
Miscellaneous:
See Figure 6-2 1 0 1 1 x x x x x x x x x x x x
1 1 0 0 L Rn register list
1 cond [2] offset
1 1 0 1 1 1 1 0 x x x x x x x x
1 1 0 1 1 1 1 1 immediate
0 0 offset
BLX suffix [4] 1 1 1 0 1 offset 0
1 1 1 0 1 x x x x x x x x x x 1
1 0 offset
1 1 1 1 1 offset
Figure 6-1 Thumb instruction set overview
Shift by immediate
Add/subtract register
Add/subtract immediate
Add/subtract/compare/move immediate
Data-processing register
Special data processing
Load from literal pool
Load/store register offset
Load/store word/byte immediate offset
Load/store halfword immediate offset
Load/store to/from stack
Add to SP or PC
0 0 0
0 0 0
0 0 1
0 1 0
0 1 0
0 1 0
0 1 1
1 0 0
Load/store multiple
Conditional branch
Undefined instruction
Software interrupt
Unconditional branch
Undefined instruction
BL/BLX prefix
BL suffix
1 1 0
1 1 1
1 1 1
A6-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-286-320.jpg)
![The Thumb Instruction Set
1. The opc field is not allowed to be 11 in this line. Other lines deal with the case that the opc field is
11.
2. The cond field is not allowed to be 1110 or 1111 in this line. Other lines deal with the cases where
the cond field is 1110 or 1111.
3. The form with L==1 is UNPREDICTABLE prior to ARM architecture version 5.
4. This is an undefined instruction prior to ARM architecture version 5.
6.2.1 Miscellaneous instructions
Figure 6-2 lists miscellaneous Thumb instructions. An entry in square brackets, for example [1], indicates
a note below the figure.
1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0
1 0 1 1 0 0 0 0 opc immediate
1 0 1 1 L 1 0 R register list
Software breakpoint [1] 1 0 1 1 1 1 1 0 immediate
Figure 6-2 Miscellaneous Thumb instructions
Adjust stack pointer
Push/pop register list
1. This is an undefined instruction prior to ARM architecture version 5.
Note
Any instruction with bits[15:12] = 1011, and which is not shown in Figure 6-2, is an undefined instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-287-320.jpg)
![The Thumb Instruction Set
6.5 Load and Store Register instructions
Thumb supports eight types of Load and Store Register instructions. Two basic addressing modes are
available. These allow the load and store of words, halfwords and bytes, and also the load of signed
halfwords and bytes:
• register plus register
• register plus 5-bit immediate (not available for signed halfword and signed byte loads).
If an immediate offset is used, it is scaled by 4 for word access and 2 for halfword accesses.
In addition, three special instructions allow:
• words to be loaded using the PC as a base with a 1KB (word-aligned) immediate offset
• words to be loaded and stored with the stack pointer (R13) as the base and a 1KB (word-aligned)
immediate offset.
6.5.1 Formats
Load and Store Register instructions have the following formats:
Format 1
<opcode1> <Rd>, [<Rn>, #<5_bit_offset>]
<opcode1> := LDR|LDRH|LDRB|STR|STRH|STRB
15 11 10 6 5 3 2 0
Format 2
opcode1 5_bit_offset Rn Rd
<opcode2> <Rd>, [<Rn>, <Rm>]
<opcode2> := LDR|LDRH|LDRSH|LDRB|LDRSB|STR|STRH|STRB
15 9 8 6 5 3 2 0
Format 3
opcode2 Rm Rn Rd
LDR <Rd>, [PC, #<8_bit_offset>]
15 14 13 12 11 10 8 7 0
0 1 0 0 1 Rd 8_bit_immediate
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-297-320.jpg)
![The Thumb Instruction Set
Format 4
<opcode3> <Rd>, [SP, #<8_bit_offset>]
<opcode3> := LDR | STR
15 14 13 12 11 10 8 7 0
1 0 0 1 L Rd 8_bit_immediate
For example:
LDR R4, [R2, #4] ; Load word into R4 from address R2 + 4
LDR R4, [R2, R1] ; Load word into R4 from address R2 + R1
STR R0, [R7, #0x7C] ; Store word from R0 to address R7 + 124
STRB R1, [R5, #31] ; Store byte from R1 to address R5 + 31
STRH R4, [R2, R3] ; Store halfword from R4 to R2 + R3
LDRH R3, [R6, R5] ; Load word into R3 from R6 + R5
LDRB R2, [R1, #5] ; Load byte into R2 from R1 + 5
LDR R6, [PC, #0x3FC] ; Load R6 from PC + 0x3FC
LDR R5, [SP, #64] ; Load R5 from SP + 64
STR R4, [SP, #0x260] ; Load R5 from SP + 0x260
A6-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-298-320.jpg)
![Thumb Instructions
7.1.2 ADC
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 0 1 0 1 Rm Rd
The ADC (Add with Carry) instruction can be used to synthesize multi-word addition. The condition code
flags are updated, based on the result.
Syntax
ADC <Rd>, <Rm>
where:
<Rd> Holds the first value for the addition, and is the destination register for the operation.
<Rm> Specifies the register that contains the second operand for the addition.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd + Rm + C Flag
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = CarryFrom(Rd + Rm + C Flag)
V Flag = OverflowFrom(Rd + Rm + C Flag)
Equivalent ARM syntax and encoding
ADCS <Rd>, <Rd>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 0 1 0 1 1 Rd Rd 0 0 0 0 0 0 0 0 Rm
A7-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-308-320.jpg)
![Thumb Instructions
7.1.3 ADD (1)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 0 0 1 1 1 0 immed_3 Rn Rd
This form of ADD adds a small constant value to the value of a register and stores the result in a second
register. The condition code flags are updated, based on the result.
Syntax
ADD <Rd>, <Rn>, #<immed_3>
where:
<Rd> Is the destination register for the completed operation.
<Rn> Specifies the register that contains the operand for the addition.
<immed_3> Specifies a 3-bit immediate value that is added to the value of <Rn>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rn + immed_3
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = CarryFrom(Rn + immed_3)
V Flag = OverflowFrom(Rn + immed_3)
Equivalent ARM syntax and encoding
ADDS <Rd>, <Rn>, #<immed_3>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 0
1 1 1 0 0 0 1 0 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 0 immed_3
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-309-320.jpg)
![Thumb Instructions
7.1.4 ADD (2)
15 14 13 12 11 10 8 7 0
0 0 1 1 0 Rd immed_8
This form of ADD adds a large immediate value to the value of a register and stores the result back in the
same register. The condition code flags are updated, based on the result.
Syntax
ADD <Rd>, #<immed_8>
where:
<Rd> Holds the first operand for the addition, and is the destination register for the
completed operation.
<immed_8> Specifies an 8-bit immediate value that is added to the value of <Rd>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd + immed_8
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = CarryFrom(Rd + immed_8)
V Flag = OverflowFrom(Rd + immed_8)
Equivalent ARM syntax and encoding
ADDS <Rd>, <Rd>, #<immed_8>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0
1 1 1 0 0 0 1 0 1 0 0 1 Rd Rd 0 0 0 0 immed_8
A7-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-310-320.jpg)
![Thumb Instructions
7.1.5 ADD (3)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 0 0 1 1 0 0 Rm Rn Rd
This form of ADD adds the value of one register to the value of a second register, and stores the result in a
third register. The condition code flags are updated, based on the result.
Syntax
ADD <Rd>, <Rn>, <Rm>
where:
<Rd> Is the destination register for the completed operation.
<Rn> Specifies the register containing the first value for the addition.
<Rm> Specifies the register containing the second value for the addition.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rn + Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = CarryFrom(Rn + Rm)
V Flag = OverflowFrom(Rn + Rm)
Equivalent ARM syntax and encoding
ADDS <Rd>, <Rn>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 0 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-311-320.jpg)
![Thumb Instructions
7.1.10 AND
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 0 0 0 0 Rm Rd
The AND (Logical AND) instruction performs a bitwise AND of the values in two registers. The condition
code flags are updated, based on the result.
Syntax
AND <Rd>, <Rm>
where:
<Rd> Specifies the register containing the first operand, and is also the destination register.
<Rm> Specifies the register containing the second operand.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd AND Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected
V Flag = unaffected
Equivalent ARM syntax and encoding
ANDS <Rd>, <Rd>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 0 0 0 0 1 Rd Rd 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-13](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-317-320.jpg)
![Thumb Instructions
7.1.11 ASR (1)
15 14 13 12 11 10 6 5 3 2 0
0 0 0 1 0 immed_5 Rm Rd
This form of the ASR (Arithmetic Shift Right) instruction is used to provide the signed value of a register
divided by a constant power of 2. The condition code flags are updated, based on the result.
Syntax
ASR <Rd>, <Rm>, #<immed_5>
where:
<Rd> Is the destination register for the completed operation.
<Rm> Specifies the register that contains the value to be shifted.
<immed_5> Specifies the shift amount, in the range 1 to 32. Shifts by 1 to 31 are encoded directly
in immed_5. A shift by 32 is encoded as immed_5 == 0.
Architecture version
All T variants
Exceptions
None
Operation
if immed_5 == 0
C Flag = Rm[31]
if Rm[31] == 0 then
Rd = 0
else /* Rm[31] == 1 */]
Rd = 0xFFFFFFFF
else /* immed_5 > 0 */
C Flag = Rm[immed_5 - 1]
Rd = Rm Arithmetic_Shift_Right immed_5
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
V Flag = unaffected
A7-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-318-320.jpg)
![Thumb Instructions
7.1.12 ASR (2)
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 0 1 0 0 Rs Rd
This form of ASR is used to provide the signed value of a register divided by a variable power of 2. The
condition code flags are updated, based on the result.
Syntax
ASR <Rd>, <Rs>
where:
<Rd> Contains the value to be shifted, and is also the destination register for the completed
operation.
<Rs> Specifies the register that contains the value of the shift.
Architecture version
All T variants
Exceptions
None
Operation
if Rs[7:0] == 0 then
C Flag = unaffected
Rd = unaffected
else if Rs[7:0] < 32 then
C Flag = Rd[Rs[7:0] - 1]
Rd = Rd Arithmetic_Shift_Right Rs[7:0]
else /* Rs[7:0] >= 32 */
C Flag = Rd[31]
if Rd[31] == 0 then
Rd = 0
else /* Rd[31] == 1 */
Rd = 0xFFFFFFFF
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
V Flag = unaffected
A7-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-320-320.jpg)
![Thumb Instructions
7.1.15 BIC
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 1 1 0 Rm Rd
The BIC (Bit Clear) instruction performs a bitwise AND of the value of one register and the bitwise inverse
of the value of another register. The condition code flags are updated, based on the result.
Syntax
BIC <Rd>, <Rm>
where:
<Rd> Is the register containing the value to be ANDed, and is also the destination register for the
completed operation.
<Rm> Specifies the register that contains the value whose complement is ANDed with the value
in <Rd>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd AND NOT Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected
V Flag = unaffected
A7-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-326-320.jpg)
![Thumb Instructions
7.1.16 BKPT
15 14 13 12 11 10 9 8 7 0
1 0 1 1 1 1 1 0 immed_8
The BKPT (Breakpoint) instruction causes a software breakpoint to occur. This breakpoint can be handled
by an exception handler installed on the prefetch abort vector. In implementations which also include debug
hardware, the hardware can optionally override this behavior and handle the breakpoint itself. When this
occurs, the prefetch abort vector is not entered.
Syntax
BKPT <immed_8>
where:
<immed_8> Is an 8-bit immediate value, which is placed in bits[7:0] of the instruction. This value is
ignored by the ARM hardware, but can be used by a debugger to store additional
information about the breakpoint.
Architecture version
T variants of version 5 and above
Exceptions
Prefetch Abort
Operation
if (not overridden by debug hardware)
R14_abt = address of BKPT instruction + 4
SPSR_abt = CPSR
CPSR[4:0] = 0b10111 /* Enter Abort mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF000C
else
PC = 0x0000000C
A7-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-328-320.jpg)
![Thumb Instructions
Usage
The exact usage of the BKPT instruction depends on the debug system being used. A debug system can use
the BKPT instruction in two ways:
• Debug hardware (if present) does not override the normal behavior of the BKPT instruction, and so
the prefetch abort vector is entered. If the system also allows real prefetch aborts to occur, the
prefetch abort handler determines (in a system-dependent manner) whether the vector entry occurred
as a result of a BKPT instruction or as a result of a real prefetch abort, and branches to debug code or
prefetch abort code accordingly. Otherwise, the prefetch abort handler just branches straight to debug
code.
When used in this manner, the BKPT instruction must be avoided within abort handlers, as it corrupts
R14_abt and SPSR_abt. For the same reason, it must also be avoided within FIQ handlers, as an FIQ
interrupt can occur within an abort handler.
• Debug hardware overrides the normal behavior of the BKPT instruction and handles the software
breakpoint itself. When finished, it typically either resumes execution at the instruction following the
BKPT, or replaces the BKPT in memory with another instruction and resumes execution at that
instruction.
When BKPT is used in this manner, R14_abt and SPSR_abt are not corrupted, and so the above
restrictions about its use in abort and FIQ handlers do not apply.
Notes
Hardware override Debug hardware in an implementation is specifically permitted to override the
normal behavior of the BKPT instruction. Because of this, software must not use this
instruction for purposes other than those permitted by the debug system being used
(if any). In particular, software cannot rely on the Prefetch Abort exception
occurring, unless either there is guaranteed to be no debug hardware in the system
or the debug system specifies that it will occur.
For more information, consult the documentation for the debug system being used.
Equivalent ARM syntax and encoding
BKPT <immed_8>
31 30 29 28 27 26 25 24 23 22 21 10 19 18 17 6 15 14 13 12 11 8 7 4 3 0
1 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0
immed_8
[7:4]
0 1 1 1
immed_8
[3:0]
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-25](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-329-320.jpg)
![Thumb Instructions
7.1.17 BL, BLX(1)
15 14 13 12 11 10 0
1 1 1 H offset_11
This form of the BL (Branch with Link) instruction provides an unconditional subroutine call to another
Thumb routine. The return from subroutine is typically performed by one of the following:
• MOV PC,LR
• BX LR
• a POP instruction that loads the PC.
This form of the BLX (Branch with Link and Exchange instruction set) instruction provides an unconditional
subroutine call to an ARM routine. The return from subroutine is typically performed by a BX LR
instruction, or an LDR or LDM instruction that loads the PC.
To allow for a reasonably large offset to the target subroutine, each of these two instructions is automatically
translated by the assembler into a sequence of two 16-bit Thumb instructions:
• The first Thumb instruction has H == 10 and supplies the high part of the branch offset. This
instruction sets up for the subroutine call and is shared between the BL and BLX forms.
• The second Thumb instruction has H == 11 (for BL) or H == 01 (for BLX). It supplies the low part
of the branch offset and causes the subroutine call to take place.
Syntax
BL <target_addr>
BLX <target_addr>
where:
<target_addr> Specifies the address to branch to. The branch target address is calculated by:
1. Shifting the offset_11 field of the first instruction left twelve bits.
2. Sign-extending the result to 32 bits.
3. Adding this to the contents of the PC (which contains the address of the first
instruction plus 4).
4. Adding twice the offset_11 field of the second instruction. For BLX, the
resulting address is forced to be word-aligned by clearing bit[1].
The instruction can therefore specify a branch of approximately ±4MB.
Architecture version
BL (H == 10 and H == 11 forms) is in all T variants.
BLX (H == 01 form) is in T variants of version 5 and above.
A7-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-330-320.jpg)
![Thumb Instructions
Exceptions
None
Operation
if H == 10 then
LR = PC + (SignExtend(offset_11) << 12)
else if H == 11 then
PC = LR + (offset_11 << 1)
LR = (address of next instruction) | 1
else if H == 01 then
PC = (LR + (offset_11 << 1)) AND 0xFFFFFFFC
LR = (address of next instruction) | 1
T Flag = 0
Usage
To generate the correct pair of instructions, the assembler (or other toolkit component) must first generate
the branch offset, as follows:
1. Form the base address for the branch. This is the address of the first of the two Thumb instructions
(the one with H == 10), plus 4. In other words, the base address is equal to the PC value read by that
instruction.
2. If the instruction is BLX, set bit[1] of the target address to be equal to bit[1] of the base address. This
is an exception to the normal rule that bits[1:0] of the address of an ARM instruction are 0b00. This
adjustment is required to ensure that the restrictions associated with the H == 01 form of the
instruction are obeyed.
3. Subtract the base address from the target address to form the offset.
The resulting offset is always even. If the offset lies outside the range:
-222 <= offset <= +222 - 2
the target address lies outside the addressing range of these instructions. This results in alternative code or
an error, as appropriate.
If the offset is in range, a sequence of two Thumb instructions must be generated, both using the above form:
• The first with H == 10 and offset_11 = offset[22:12].
• The second with H == 11 (for BL) or H== 01 (for BLX) and offset_11 = offset[11:1].
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-27](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-331-320.jpg)
![Thumb Instructions
Notes
Encoding If H == 00, the instruction is an unconditional branch instruction instead (see the
Thumb instruction B (2) on page A7-20).
Bit[0] for BLX If H == 01, then bit[0] of the instruction must be zero, or the instruction is
UNDEFINED. The offset calculation method described in Usage above ensures that
the offset calculated for a BLX instruction is a multiple of four, and that this
restriction is obeyed.
Memory bounds Branching backwards past location zero and forwards over the end of the 32-bit
address space is UNPREDICTABLE.
Instruction pairs These Thumb instructions must always occur in the pairs described above.
Specifically:
• If a Thumb instruction at address A is the H==10 form of this instruction, the
Thumb instruction at address A+2 must be either the H==01 or the H==11
form of this instruction.
• If a Thumb instruction at address A is either the H==01 or the H==11 form
of this instruction, the Thumb instruction at address A-2 must be the H==10
form of this instruction.
Also, except as noted below under Exceptions, the second instruction of the pair
must not be the target of any branch, whether as the result of a branch instruction or
of some other instruction that changes the PC.
Failure to adhere to any of these restrictions can result in UNPREDICTABLE behavior.
Exceptions It is IMPLEMENTATION DEFINED whether processor exceptions can occur between
the two instructions of a BL or BLX pair. If they can, the ARM instructions designed
for use for exception returns must be capable of returning correctly to the second
instruction of the pair. So, exception handlers need take no special precautions
about returning to the second instruction of a BL or BLX pair.
A7-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-332-320.jpg)
![Thumb Instructions
Equivalent ARM syntax and encoding
Close equivalents to these instruction pairs are as follows.
To call a Thumb subroutine:
BLX <target_addr>
31 30 29 28 27 26 25 24 23 22 21 20 0
1 1 1 1 1 0 1 L offset sign offset[22:2]
where L == offset[1].
To call an ARM routine:
BL <target_addr>
31 30 29 28 27 26 25 24 23 22 21 20 0
1 1 1 0 1 0 1 1 offset sign offset[22:2]
These differ slightly from the Thumb instruction pairs because of the different values of the PC in ARM and
Thumb code. This can be compensated for by adjusting the offset by 4.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-29](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-333-320.jpg)
![Thumb Instructions
7.1.18 BLX(2)
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 1 1 1 1 H2 Rm SBZ
This form of BLX is used to call an ARM or Thumb subroutine from the Thumb instruction set, at an address
specified in a register. This instruction branches and selects the instruction decoder to use to decode the
instructions at the branch destination.
The T flag is updated with bit[0] of the value of register Rm. To return from the subroutine to the caller, use
BX R14.
Syntax
BLX <Rm>
where:
<Rm> Is the register that contains the branch target address. It can be any of R0 to R14. The register
number is encoded in the instruction in H2 (most significant bit) and Rm (remaining three
bits). If R15 is specified for <Rm>, the results are UNPREDICTABLE.
Architecture version
T variants of version 5 and above
Exceptions
None
Operation
LR = (address of the instruction after this BLX) | 1
T Flag = Rm[0]
PC = Rm[31:1] << 1
Notes
Encoding Bit 7 is the H1 bit for some of the other instructions that access the high registers. If it is 0
for this instruction, rather than 1 as shown, the instruction is a BX instruction instead (see
BX on page A7-32).
ARM/Thumb state transfers
If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned
addresses are impossible in ARM state.
A7-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-334-320.jpg)
![Thumb Instructions
7.1.19 BX
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 1 1 1 0 H2 Rm SBZ
The BX (Branch and Exchange) instruction is used to branch between ARM code and Thumb code.
Syntax
BX <Rm>
where:
<Rm> Is the register that contains the branch target address. It can be any of R0 to R15. The register
number is encoded in the instruction in H2 (most significant bit) and Rm (remaining three
bits).
Architecture version
All T variants
Exceptions
None
Operation
T Flag = Rm[0]
PC = Rm[31:1] << 1
Usage
The normal subroutine return instruction in Thumb code is BX R14. The following subroutine call
instructions leave a suitable return value in R14:
• ARM BLX instructions (See BLX (1) on page A4-16 and BLX (2) on page A4-18)
• Thumb BL and BLX instructions (see BL, BLX(1) on page A7-26 and BLX(2) on page A7-30).
In T variants of ARM architecture version 4, a subroutine call to an ARM routine can be performed by a
code sequence of the form:
<Put address of routine to call in Ra>
MOV LR,PC ; Return to second following instruction
BX Ra
In T variants of ARM architecture 5 and above, a subroutine call to an ARM routine can be performed more
efficiently with a BLX instruction (see BL, BLX(1) on page A7-26 and BLX(2) on page A7-30).
A7-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-336-320.jpg)
![Thumb Instructions
Notes
Encoding Bit 7 is the H1 bit for some of the other instructions that access the high registers. If it is 1
for this instruction, rather than 0 as shown, the instruction is:
• a BLX instruction instead in ARM architecture version 5 and above (see BLX(2) on
page A7-30)
• UNPREDICTABLE prior to ARM architecture version 5.
ARM/Thumb state transfers
If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned
addresses are impossible in ARM state.
Use of R15 Register 15 can be specified for <Rm>. If this is done, R15 is read as normal for Thumb
code, that is, it is the address of the BX instruction itself plus 4. If the BX instruction is at a
word-aligned address, this results in a branch to the next word, executing in ARM state.
However, if the BX instruction is not at a word-aligned address, this means that the results
of the instruction are UNPREDICTABLE (because the value read for R15 has bits[1:0]==0b10).
Equivalent ARM syntax and encoding
A close equivalent is:
BX <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 0 1 H2 Rm
This ARM instruction is not quite equivalent to the Thumb instruction, because their specified behavior
differs when <Rm> is R15.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-33](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-337-320.jpg)
![Thumb Instructions
7.1.20 CMN
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 0 1 1 Rm Rn
The CMN (Compare Negative) instruction compares a register value with the negation of another register
value. The condition flags are updated, based on the result of adding the two register values, so that
subsequent instructions can be conditionally executed (using a conditional branch).
Syntax
CMN <Rn>, <Rm>
where:
<Rn> Is the register containing the first value for comparison.
<Rm> Is the register containing the second value for comparison.
Architecture version
All T variants
Exceptions
None
Operation
alu_out = Rn + Rm
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn + Rm)
V Flag = OverflowFrom(Rn + Rm)
Equivalent ARM syntax and encoding
CMN <Rn>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 0 1 1 1 Rn SBZ 0 0 0 0 0 0 0 0 Rm
A7-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-338-320.jpg)
![Thumb Instructions
7.1.21 CMP (1)
15 14 13 12 11 10 8 7 0
0 0 1 0 1 Rn immed_8
This form of the CMP (Compare) instruction compares a register value with a large immediate value. The
condition flags are updated, based on the result of subtracting the constant from the register value, so that
subsequent instructions can be conditionally executed (using a conditional branch).
Syntax
CMP <Rn>, #<immed_8>
where:
<Rn> Is the register containing the first value for comparison.
<immed_8> Is the 8-bit second value for comparison.
Architecture version
All T variants
Exceptions
None
Operation
alu_out = Rn - immed_8
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - immed_8)
V Flag = OverflowFrom(Rn - immed_8)
Equivalent ARM syntax and encoding
CMP <Rn>, #<immed_8>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0
1 1 1 0 0 0 1 1 0 1 0 1 Rn SBZ 0 0 0 0 immed_8
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-35](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-339-320.jpg)
![Thumb Instructions
7.1.22 CMP (2)
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 0 1 0 Rm Rn
This form of CMP compares two register values. The condition code flags are updated, based on the result
of subtracting the second register value from the first, so that subsequent instructions can be conditionally
executed (using a conditional branch).
Syntax
CMP <Rn>, <Rm>
where:
<Rn> Is the register containing the first value for comparison.
<Rm> Is the register containing the second value for comparison.
Architecture version
All T variants
Exceptions
None
Operation
alu_out = Rn - Rm
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - Rm)
V Flag = OverflowFrom(Rn - Rm)
Equivalent ARM syntax and encoding
CMP <Rn>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 0 1 0 1 Rn SBZ 0 0 0 0 0 0 0 0 Rm
A7-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-340-320.jpg)
![Thumb Instructions
7.1.23 CMP (3)
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 1 0 1 H1 H2 Rm Rn
This form of CMP compares the values of two registers, one or both of which are high registers. The
condition flags are updated, based on the result of subtracting the second register value from the first, so that
subsequent instructions can be conditionally executed (using a conditional branch).
Syntax
CMP <Rn>, <Rm>
where:
<Rn> Is the register containing the first value. It can be any of R0 to R15. Its number is encoded
in the instruction in H1 (most significant bit) and Rn (remaining three bits).
<Rm> Is the register containing the second value. It can be any of R0 to R15. Its number is encoded
in the instruction in H2 (most significant bit) and Rm (remaining three bits).
Architecture version
All T variants
Exceptions
None
Operation
alu_out = Rn - Rm
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - Rm)
V Flag = OverflowFrom(Rn - Rm)
Notes
Operand restriction If a low register is specified for both <Rn> and <Rm> (H1==0 and H2==0), the
result is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-37](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-341-320.jpg)
![Thumb Instructions
7.1.24 EOR
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 0 0 0 1 Rm Rd
The EOR (Exclusive OR) instruction performs a bitwise EOR of the values in two registers. The condition
code flags are updated, based on the result.
Syntax
EOR <Rd>, <Rm>
where:
<Rd> Specifies the register containing the first operand, and is also the destination register.
<Rm> Specifies the register containing the second operand.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd EOR Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected
V Flag = unaffected
Equivalent ARM syntax and encoding
EORS <Rd>, <Rd>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 0 0 0 1 1 Rd Rd 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-39](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-343-320.jpg)
![Thumb Instructions
7.1.25 LDMIA
15 14 13 12 11 10 8 7 0
1 1 0 0 1 Rn register_list
The LDMIA (Load Multiple Increment After) instruction loads a non-empty subset, or possibly all, of the
general-purpose registers from sequential memory locations.
Syntax
LDMIA <Rn>!, <registers>
where:
<Rn> Is the register containing the start address for the instruction.
! Causes base register writeback, and is not optional.
<registers> Is a list of registers to be loaded, separated by commas and surrounded by { and }.
The list is encoded in the register_list field of the instruction, by setting bit[i] to 1 if
register Ri is included in the list and to 0 otherwise, for each of i=0 to 7.
At least one register must be loaded. If bits[7:0] are all zero, the result is
UNPREDICTABLE.
The registers are loaded in sequence, the lowest-numbered register from the lowest
memory address (start_address), through to the highest-numbered register
from the highest memory address (end_address).
The start_address is the value of the base register <Rn>. Subsequent
addresses are formed by incrementing the previous address by four. One address is
produced for each register that is specified in <registers>.
The end_address value is four less than the sum of the value of the base register
and four times the number of registers specified in <registers>.
Finally, the base register <Rn> is incremented by four times the numbers of
registers in <registers>.
Architecture version
All T variants
Exceptions
Data Abort
A7-40 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-344-320.jpg)
![Thumb Instructions
Operation
start_address = Rn
end_address = Rn + (Number_Of_Set_Bits_In(register_list) * 4) - 4
address = start_address
for i = 0 to 7
if register_list[i] == 1
Ri = Memory[address,4]
address = address + 4
assert end_address == address - 4
Rn = Rn + (Number_Of_Set_Bits_In(register_list) * 4)
Usage
The LDMIA instruction is useful as a block load instruction. Combined with STMIA (Store Multiple), it
allows efficient block copy.
Notes
Operand restrictions
If the base register <Rn> is specified in <registers>, the final value of <Rn> is the
loaded value (not the written-back value).
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment Load Multiple instructions ignore the least significant two bits of address.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
Time order The time order of the accesses to individual words of memory generated by this instruction
is only defined in some circumstances. See Data accesses to memory-mapped I/O on
page A2-32 for details.
Equivalent ARM syntax and encoding
If <Rn> is not in the register list (W == 1):
LDMIA <Rn>!, <registers>
If <Rn> is in the register list (W == 0):
LDMIA <Rn>, <registers>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 14 13 12 11 10 9 8 7 0
1 1 1 0 1 0 0 0 1 0 W 1 Rn 0 0 0 0 0 0 0 0 register_list
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-41](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-345-320.jpg)
![Thumb Instructions
7.1.26 LDR (1)
15 14 13 12 11 10 6 5 3 2 0
0 1 1 0 1 immed_5 Rn Rd
This form of the LDR (Load Register) instruction allows 32-bit memory data to be loaded into a
general-purpose register. The addressing mode is useful for accessing structure (record) fields.
With an offset of zero, the address produced is the unaltered value of the base register <Rn>.
Syntax
LDR <Rd>, [<Rn>, #<immed_5> * 4]
where:
<Rd> Is the destination register for the word loaded from memory.
<Rn> Is the register containing the base address for the instruction.
<immed_5> Is a 5-bit value that is multiplied by 4 and added to the value of <Rn> to form the memory
address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + (immed_5 * 4)
if address[1:0] == 0b00
data = Memory[address,4]
else
data = UNPREDICTABLE
Rd = data
A7-42 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-346-320.jpg)
![Thumb Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not word-aligned and no data abort occurs, the value written to the
destination register is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception (a type of data abort).
Equivalent ARM syntax and encoding
LDR <Rd>, [<Rn>, #<immed_5> * 4]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 2 1 0
1 1 1 0 0 1 0 1 1 0 0 1 Rn Rd 0 0 0 0 0 immed_5 0 0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-43](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-347-320.jpg)
![Thumb Instructions
7.1.27 LDR (2)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 1 0 0 Rm Rn Rd
This form of LDR allows 32-bit memory data to be loaded into a general-purpose register. The addressing
mode is useful for pointer+large offset arithmetic and for accessing a single element of an array.
Syntax
LDR <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the destination register for the word loaded from memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register containing the second value used in forming the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
if address[1:0] == 0b00
data = Memory[address,4]
else
data = UNPREDICTABLE
Rd = data
A7-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-348-320.jpg)
![Thumb Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not word-aligned and no data abort occurs, the value written to the
destination register is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception (a type of data abort).
Equivalent ARM syntax and encoding
LDR <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 1 1 1 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-45](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-349-320.jpg)
![Thumb Instructions
7.1.28 LDR (3)
15 14 13 12 11 10 8 7 0
0 1 0 0 1 Rd immed_8
This form of LDR allows 32-bit memory data to be loaded into a general-purpose register. The addressing
mode is useful for accessing PC-relative data.
Syntax
LDR <Rd>, [PC, #<immed_8> * 4]
where:
<Rd> Is the destination register for the word loaded from memory.
PC Is the program counter. Its value is used to calculate the memory address. Bit 1 of the PC
value is forced to zero for the purpose of this calculation, so the address is always
word-aligned.
<immed_8> Is an 8-bit value that is multiplied by 4 and added to the value of the PC to form the memory
address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = (PC[31:2] << 2) + (immed_8 * 4)
Rd = Memory[address, 4]
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
A7-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-350-320.jpg)
![Thumb Instructions
Equivalent ARM syntax and encoding
A close equivalent is:
LDR <Rd>, [PC, #<immed_8> * 4]
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 2 1 0
1 1 1 0 0 1 0 1 1 0 0 1 1 1 1 1 Rd 0 0 immed_8 0 0
There are slight differences caused by the different definitions of the PC and the fact that the Thumb
instruction ignores bit[1] of the PC.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-47](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-351-320.jpg)
![Thumb Instructions
7.1.29 LDR (4)
15 14 13 12 11 10 8 7 0
1 0 0 1 1 Rd immed_8
This form of LDR allows 32-bit memory data to be loaded into a general-purpose register. The addressing
mode is useful for accessing stack data.
Syntax
LDR <Rd>, [SP, #<immed_8> * 4]
where:
<Rd> Is the destination register for the word loaded from memory.
SP Is the stack pointer. Its value is used to calculate the memory address.
<immed_8> Is an 8-bit value that is multiplied by 4 and added to the value of the SP to form the memory
address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = SP + (immed_8 * 4)
if address[1:0] == 0b00
data = Memory[address,4]
else
data = UNPREDICTABLE
Rd = data
A7-48 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-352-320.jpg)
![Thumb Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not word-aligned and no data abort occurs, the value written to the
destination register is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception (a type of data abort).
Equivalent ARM syntax and encoding
LDR <Rd>, [SP, #<immed_8> * 4]
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 2 1 0
1 1 1 0 0 1 0 1 1 0 0 1 1 1 0 1 Rd 0 0 immed_8 0 0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-49](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-353-320.jpg)
![Thumb Instructions
7.1.30 LDRB (1)
15 14 13 12 11 10 6 5 3 2 0
0 1 1 1 1 immed_5 Rn Rd
This form of the LDRB (Load Register Byte) instruction loads a byte from memory and zero-extends it to
form a 32-bit word which is written to a general-purpose register. The addressing mode is useful for
accessing structure (record) fields. With an offset of zero, the address produced is the unaltered value of the
base register <Rn>.
Syntax
LDRB <Rd>, [<Rn>, #<immed_5>]
where:
<Rd> Is the destination register for the byte loaded from memory.
<Rn> Is the register containing the base address for the instruction.
<immed_5> Is a 5-bit value that is added to the value of <Rn> to form the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + immed_5
Rd = Memory[address,1]
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Equivalent ARM syntax and encoding
LDRB <Rd>, [<Rn>, #<immed_5>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 0
1 1 1 0 0 1 0 1 1 1 0 1 Rn Rd 0 0 0 0 0 0 0 immed_5
A7-50 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-354-320.jpg)
![Thumb Instructions
7.1.31 LDRB (2)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 1 1 0 Rm Rn Rd
This form of LDRB loads a byte from memory and zero-extends it to form a 32-bit word, which is written
to a general-purpose register. The addressing mode is useful for pointer+large offset arithmetic and for
accessing a single element of an array.
Syntax
LDRB <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the destination register for the byte loaded from memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register containing the second value used in forming the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
Rd = Memory[address,1]
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Equivalent ARM syntax and encoding
LDRB <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 1 1 1 1 1 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-51](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-355-320.jpg)
![Thumb Instructions
7.1.32 LDRH (1)
15 14 13 12 11 10 6 5 3 2 0
1 0 0 0 1 immed_5 Rn Rd
This form of the LDRH (Load Register Halfword) instruction loads a halfword (16 bits) from memory and
zero-extends it to form a 32-bit word, which is written to a general-purpose register. The addressing mode
is useful for accessing structure (record) fields. With an offset of zero, the address produced is the unaltered
value of the base register <Rn>.
Syntax
LDRH <Rd>, [<Rn>, #<immed_5> * 2]
where:
<Rd> Is the destination register for the halfword loaded from memory.
<Rn> Is the register containing the base address for the instruction.
<immed_5> Is a 5-bit value that is multiplied by 2, then added to the value of <Rn> to form the memory
address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + (immed_5 * 2)
if address[0] == 0
data = Memory[address,2]
else
data = UNPREDICTABLE
Rd = data
A7-52 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-356-320.jpg)
![Thumb Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written
to the destination register is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes
an alignment exception (a type of data abort).
Equivalent ARM syntax and encoding
LDRH <Rd>, [<Rn>, #<immed_5> * 2]
31 30 29 28 27 26 25 24 23 22 21 10 19 16 15 12 11 10 9 8 7 6 5 4 3 1 0
1 1 1 0 0 0 0 1 1 1 0 1 Rn Rd 0 0
immed
[4:3]
1 0 1 1
immed
[2:0]
0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-53](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-357-320.jpg)
![Thumb Instructions
7.1.33 LDRH (2)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 1 0 1 Rm Rn Rd
This form of LDRH loads a halfword (16 bits) from memory and zero-extends it to form a 32-bit word, which
is written to a general-purpose register. The addressing mode is useful for pointer + large offset arithmetic
and for accessing a single element of an array.
Syntax
LDRH <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the destination register for the halfword loaded from memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register containing the second value used in forming the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
if address[0] == 0
data = Memory[address,2]
else
data = UNPREDICTABLE
Rd = data
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written
to the destination register is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes
an alignment exception (a type of data abort).
A7-54 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-358-320.jpg)
![Thumb Instructions
Equivalent ARM syntax and encoding
LDRH <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 1 0 0 1 Rn Rd SBZ 1 0 1 1 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-55](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-359-320.jpg)
![Thumb Instructions
7.1.34 LDRSB
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 0 1 1 Rm Rn Rd
The LDRSB (Load Register Signed Byte) instruction loads a byte from memory and sign-extends it to form
a 32-bit word, which is written to a general-purpose register.
Syntax
LDRSB <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the destination register for the byte loaded from memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register containing the second value used in forming the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
Rd = SignExtend(Memory[address,1])
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Equivalent ARM syntax and encoding
LDRSB <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 1 0 0 1 Rn Rd SBZ 1 1 0 1 Rm
A7-56 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-360-320.jpg)
![Thumb Instructions
7.1.35 LDRSH
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 1 1 1 Rm Rn Rd
The LDRSH (Load Register Signed Halfword) instruction loads a halfword from memory and sign-extends
it to form a 32-bit word, which is written to a general-purpose register.
Syntax
LDRSH <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the destination register for the halfword loaded from memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register containing the second value used in forming the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
if address[0] == 0
data = Memory[address,2]
else
data = UNPREDICTABLE
Rd = SignExtend(data)
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written
to the destination register is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes
an alignment exception (a type of data abort).
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-57](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-361-320.jpg)
![Thumb Instructions
Equivalent ARM syntax and encoding
LDRSH <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 1 0 0 1 Rn Rd SBZ 1 1 1 1 Rm
A7-58 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-362-320.jpg)
![Thumb Instructions
7.1.36 LSL (1)
15 14 13 12 11 10 6 5 3 2 0
0 0 0 0 0 immed_5 Rm Rd
This form of the LSL (Logical Shift Left) instruction is used to provide either the value of a register directly
(LSL #0), or the value of a register multiplied by a constant power of two. Zeros are inserted into the bit
positions vacated by the shift, and the condition code flags are updated, based on the result.
Syntax
LSL <Rd>, <Rm>, #<immed_5>
where:
<Rd> Is the register that stores the result of the operation.
<Rm> Is the register containing the value to be shifted.
<immed_5> Specifies the shift amount, in the range 0 to 31.
Architecture version
All T variants
Exceptions
None
Operation
if immed_5 == 0
C Flag = unaffected
Rd = Rm
else /* immed_5 > 0 */
C Flag = Rm[32 - immed_5]
Rd = Rm Logical_Shift_Left immed_5
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
V Flag = unaffected
Equivalent ARM syntax and encoding
MOVS <Rd>, <Rm>, LSL #<immed_5>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0
1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd immed_5 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-59](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-363-320.jpg)
![Thumb Instructions
7.1.37 LSL (2)
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 0 0 1 0 Rs Rd
This form of LSL is used to provide the value of a register multiplied by a variable power of two. Zeros are
inserted into the vacated bit positions. The condition code flags are updated, based on the result.
Syntax
LSL <Rd>, <Rs>
where:
<Rd> Contains the value to be shifted, and is the destination register for the result of the operation.
<Rs> Is the register containing the shift value. The value is held in the least significant byte.
Architecture version
All T variants
Exceptions
None
Operation
if Rs[7:0] == 0
C Flag = unaffected
Rd = unaffected
else if Rs[7:0] < 32 then
C Flag = Rd[32 - Rs[7:0]]
Rd = Rd Logical_Shift_Left Rs[7:0]
else if Rs[7:0] == 32 then
C Flag = Rd[0]
Rd = 0
else /* Rs[7:0] > 32 */
C Flag = 0
Rd = 0
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
V Flag = unaffected
A7-60 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-364-320.jpg)
![Thumb Instructions
7.1.38 LSR (1)
15 14 13 12 11 10 6 5 3 2 0
0 0 0 0 1 immed_5 Rm Rd
This form of the LSR (Logical Shift Right) instruction is used to provide the unsigned value of a register,
divided by a constant power of two. LSR performs a logical shift right of the value of register <Rm>, and
zeros are inserted into the vacated bit positions. The condition code flags are updated, based on the result.
Syntax
LSR <Rd>, <Rm>, #<immed_5>
where:
<Rd> Is the destination register for the operation.
<Rm> Is the register containing the value to be shifted.
<immed_5> Specifies the shift amount, in the range 1 to 32. Shifts by 1 to 31 are encoded directly
in immed_5. A shift by 32 is encoded as immed_5 == 0.
Architecture version
All T variants
Exceptions
None
Operation
if immed_5 == 0
C Flag = Rd[31]
Rd = 0
else /* immed_5 > 0 */
C Flag = Rd[immed_5 - 1]
Rd = Rm Logical_Shift_Right immed_5
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
V Flag = unaffected
A7-62 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-366-320.jpg)
![Thumb Instructions
7.1.39 LSR (2)
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 0 0 1 1 Rs Rd
This form of LSR is used to provide the unsigned value of a register divided by a variable power of two.
Zeros are inserted into the vacated bit positions. The condition code flags are updated, based on the result.
Syntax
LSR <Rd>, <Rs>
where:
<Rd> Contains the value to be shifted, and is the destination register for the result of the operation.
<Rs> Is the register containing the shift value. The value is held in the least significant byte.
Architecture version
All T variants
Exceptions
None
Operation
if Rs[7:0] == 0 then
C Flag = unaffected
Rd = unaffected
else if Rs[7:0] < 32 then
C Flag = Rd[Rs[7:0] - 1]
Rd = Rd Logical_Shift_Right Rs[7:0]
else if Rs[7:0] == 32 then
C Flag = Rd[31]
Rd = 0
else /* Rs[7:0] > 32 */
C Flag = 0
Rd = 0
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
V Flag = unaffected
A7-64 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-368-320.jpg)
![Thumb Instructions
7.1.40 MOV (1)
15 14 13 12 11 10 8 7 0
0 0 1 0 0 Rd immed_8
This form of the MOV (Move) instruction moves a large immediate value to a register. The condition code
flags are updated, based on the result.
Syntax
MOV <Rd>, #<immed_8>
where:
<Rd> Is the destination register for the operation.
<immed_8> Is an 8-bit immediate value, in the range 0 to 255, to move into <Rd>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = immed_8
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected
V Flag = unaffected
Equivalent ARM syntax and encoding
MOVS <Rd>, #<immed_8>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0
1 1 1 0 0 0 1 1 1 0 1 1 SBZ Rd 0 0 0 0 immed_8
A7-66 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-370-320.jpg)
![Thumb Instructions
7.1.41 MOV (2)
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 0 0 1 1 1 0 0 0 0 Rn Rd
This form of MOV is used to move a value from one low register to another, and the flags are set according
to that value.
Syntax
MOV <Rd>, <Rn>
where:
<Rd> Is the destination register for the operation.
<Rn> Is the register containing the value to be copied.
Architecture Version
All T variants
Exceptions
None
Operation
Rd = Rn
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = 0
V Flag = 0
Notes
Encoding This instruction is encoded as ADD Rd, Rn, #0.
See also ADD (1) on page A7-5.
Equivalent ARM syntax and encoding
ADDS <Rd>, <Rn>, #0
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0
1 1 1 0 0 0 1 0 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 0 0 0 0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-67](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-371-320.jpg)
![Thumb Instructions
7.1.43 MUL
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 1 0 1 Rm Rd
The MUL (Multiply) instruction multiplies signed or unsigned variables to produce a 32-bit result. The
condition code flags are updated, based on the result.
Syntax
MUL <Rd>, <Rm>
where:
<Rd> Contains the value to be multiplied with the value of <Rm>, and is also the destination
register for the operation.
<Rm> Is the register containing the value to be multiplied with the value of <Rd>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = (Rm * Rd)[31:0]
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected /* See "C flag" note */
V Flag = unaffected
A7-70 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-374-320.jpg)
![Thumb Instructions
7.1.44 MVN
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 1 1 1 Rm Rd
The MVN (Move NOT) instruction is used to complement a register value, often to form a bit mask. The
condition code flags are updated, based on the result.
Syntax
MVN <Rd>, <Rm>
where:
<Rd> Is the destination register for the operation.
<Rm> Is the register containing the value whose one’s complement is written to <Rd>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = NOT Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected
V Flag = unaffected
Equivalent ARM syntax and encoding
MVNS <Rd>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 1 1 1 1 SBZ Rd 0 0 0 0 0 0 0 0 Rm
A7-72 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-376-320.jpg)
![Thumb Instructions
7.1.45 NEG
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 0 0 1 Rm Rd
The NEG (Negate) instruction negates the value of one register and stores the result in a second register. The
condition code flags are updated (based on the result).
Syntax
NEG <Rd>, <Rm>
where:
<Rd> Is the destination register for the operation.
<Rm> Is the register containing the value that is subtracted from zero.
Architecture version
All T variants
Exceptions
None
Operation
Rd = 0 - Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(0 - Rm)
V Flag = OverflowFrom(0 - Rm)
Equivalent ARM syntax and encoding
RSBS <Rd>, <Rm>, #0
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0
1 1 1 0 0 0 1 0 0 1 1 1 Rm Rd 0 0 0 0 0 0 0 0 0 0 0 0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-73](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-377-320.jpg)
![Thumb Instructions
7.1.46 ORR
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 1 0 0 Rm Rd
The ORR (Logical OR) instruction performs a bitwise OR of the values in two registers. The condition code
flags are updated, based on the result.
Syntax
ORR <Rd>, <Rm>
where:
<Rd> Is the destination register for the operation.
<Rm> Is the register containing the value that is ORed with the value of <Rd>. The operation is a
bitwise inclusive OR.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd OR Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = unaffected
V Flag = unaffected
Operation
ORRS <Rd>, <Rd>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 1 0 0 1 Rd Rd 0 0 0 0 0 0 0 0 Rm
A7-74 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-378-320.jpg)
![Thumb Instructions
7.1.47 POP
15 14 13 12 11 10 9 8 7 0
1 0 1 1 1 1 0 R register_list
The POP (Pop Multiple Registers) instruction loads a subset (or possibly all) of the general-purpose
registers R0-R7 and the PC from the stack.
The general-purpose registers loaded can include the PC. If they do, the word loaded for the PC is treated
as an address and a branch occurs to that address. In ARM architecture version 5 and above, bit[0] of the
loaded value determines whether execution continues after this branch in ARM state or in Thumb state, as
though the following instruction had been executed:
BX (loaded_value)
In T variants of architecture version 4, bit[0] of the loaded value is ignored and execution continues in
Thumb state, as though the following instruction had been executed:
MOV PC,(loaded_value)
Syntax
POP <registers>
where:
<registers> Is the list of registers, separated by commas and surrounded by { and }. The list is
encoded in the register_list field of the instruction, by setting bit[i] to 1 if register
Ri is included in the list and to 0 otherwise, for each of i=0 to 7. The R bit (bit[8])
is set to 1 if the PC is in the list and to 0 otherwise.
At least one register must be loaded. If bits[8:0] are all zero, the result is
UNPREDICTABLE.
The registers are loaded in sequence, the lowest-numbered register from the lowest
memory address (start_address), through to the highest-numbered register
from the highest memory address (end_address). If the PC is specified in the
register list (opcode bit[8] is set), the instruction causes a branch to the address
(data) loaded into the PC.
The <start_address> is the value of the SP.
Subsequent addresses are formed by incrementing the previous address by four.
One address is produced for each register that is specified in <registers>.
The end_address value is four less than the sum of the value of the SP and four
times the number of registers specified in <registers>.
The SP register is incremented by four times the numbers of registers in
<registers>.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-75](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-379-320.jpg)
![Thumb Instructions
Architecture version
All T variants
Exceptions
Data Abort
Operation
start_address = SP
end_address = SP + 4*(R + Number_Of_Set_Bits_In(register_list))
address = start_address
for i = 0 to 7
if register_list[i] == 1 then
Ri = Memory[address,4]
address = address + 4
if R == 1 then
value = Memory[address,4]
PC = value AND 0xFFFFFFFE
if (architecture version 5 or above) then
T Bit = value[0]
address = address + 4
assert end_address = address
SP = end_address
Usage
The POP instruction is useful for stack operations. A POP instruction with the PC in the register list can be
used for an efficient procedure exit, as it restores saved registers, loads the PC with the return address, and
updates the stack pointer with a single instruction.
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
CPSR The CPSR is not updated when POP loads the PC.
Alignment POP instructions ignore the least significant two bits of address.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
A7-76 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-380-320.jpg)
![Thumb Instructions
ARM/Thumb state transfers
In ARM architecture 5 and above, if bits[1:0] of a value loaded for R15 are 0b10, the result
is UNPREDICTABLE, as branches to non word-aligned addresses are not possible in ARM
state.
Time order The time order of the accesses to individual words of memory generated by this instruction
is only defined in some circumstances. See Data accesses to memory-mapped I/O on
page A2-32 for details.
Equivalent ARM syntax and encoding
LDMIA SP!, <registers>
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 0
1 1 1 0 1 0 0 0 1 0 1 1 1 1 0 1 R 0 0 0 0 0 0 0 register_list
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-77](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-381-320.jpg)
![Thumb Instructions
7.1.48 PUSH
15 14 13 12 11 10 9 8 7 0
1 0 1 1 0 1 0 R register_list
The PUSH (Push Multiple Registers) instruction stores a subset (or possibly all) of the general-purpose
registers R0-R7 and the LR to the stack.
Syntax
PUSH <registers>
where:
<registers> Is the list of registers to be stored, separated by commas and surrounded by { and }.
The list is encoded in the register_list field of the instruction, by setting bit[i] to 1 if
register Ri is included in the list and to 0 otherwise, for each of i=0 to 7. The R bit
(bit[8]) is set to 1 if the LR is in the list and to 0 otherwise.
At least one register must be stored. If bits[8:0] are all zero, the result is
UNPREDICTABLE.
The registers are stored in sequence, the lowest-numbered register to the lowest
memory address (start_address), through to the highest-numbered register to
the highest memory address (end_address)
The start_address is the value of the SP minus 4 times the number of registers
to be stored.
Subsequent addresses are formed by incrementing the previous address by four.
One address is produced for each register that is specified in <registers>.
The end_address value is four less than the original value of SP.
The SP register is decremented by four times the numbers of registers in
<registers>.
Architecture version
All T variants
Exceptions
Data Abort
A7-78 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-382-320.jpg)
![Thumb Instructions
Operation
start_address = SP - 4*(R + Number_Of_Set_Bits_In(register_list))
end_address = SP - 4
address = start_address
for i = 0 to 7
if register_list[i] == 1
Memory[address,4] = Ri
address = address + 4
if R == 1
Memory[address,4] = LR
address = address + 4
assert end_address == address - 4
SP = SP - 4*(R + Number_Of_Set_Bits_In(register_list))
Usage
PUSH is useful for stack operations. A PUSH instruction with the LR in the register list can be used for an
efficient procedure entry, as it saves registers (including the return address) on the stack and updates the
stack pointer with a single instruction. A matching POP instruction can be used later to return from the
procedure.
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment PUSH instructions ignore the least significant two bits of address.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
Time order The time order of the accesses to individual words of memory generated by this instruction
is only defined in some circumstances. See Data accesses to memory-mapped I/O on
page A2-32 for details.
Equivalent ARM syntax and encoding
STMDB SP!, <registers>
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 0
1 1 1 0 1 0 0 1 0 0 1 0 1 1 0 1 0 R 0 0 0 0 0 0 register_list
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-79](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-383-320.jpg)
![Thumb Instructions
7.1.49 ROR
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 0 1 1 1 Rs Rd
The ROR (Rotate Right Register) instruction is used to provide the value of a register rotated by a variable
value (from another register). The bits that are rotated off the right end are inserted into the vacated bit
positions on the left. The condition code flags are updated, based on the result.
Syntax
ROR <Rd>, <Rs>
where:
<Rd> Contains the value to be rotated, and is also the destination register for the operation.
<Rs> Is the register containing the rotation applied to the value of <Rd>. The value of the rotation
is stored in the least significant byte.
Architecture version
All T variants
Exceptions
None
Operation
if Rs[7:0] == 0 then
C Flag = unaffected
Rd = unaffected
else if Rs[4:0] == 0 then
C Flag = Rd[31]
Rd = unaffected
else /* Rs[4:0] > 0 */
C Flag = Rd[Rs[4:0] - 1]
Rd = Rd Rotate_Right Rs[4:0]
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
V Flag = unaffected
A7-80 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-384-320.jpg)
![Thumb Instructions
7.1.50 SBC
15 14 13 12 11 10 9 9 8 6 5 3 2 0
0 1 0 0 0 0 0 1 1 0 Rm Rd
The SBC (Subtract with Carry) instruction can be used to synthesize multi-word subtraction. It subtracts the
value of register <Rm> and the value of NOT(Carry Flag) from the value of register <Rd>. The condition
code flags are updated, based on the result.
Syntax
SBC <Rd>, <Rm>
where:
<Rd> Contains the first operand for the subtraction, and is also the destination register for the
operation.
<Rm> Contains the value to be subtracted from <Rd>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd - Rm - NOT(C Flag)
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rd - Rm - NOT(C Flag))
V Flag = OverflowFrom(Rd - Rm - NOT(C Flag))
A7-82 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-386-320.jpg)
![Thumb Instructions
7.1.51 STMIA
15 14 13 12 11 10 8 7 0
1 1 0 0 0 Rn register_list
The STMIA (Store Multiple Increment After) instruction stores a non-empty subset, or possibly all, of the
general-purpose registers to sequential memory locations.
Syntax
STMIA <Rn>!, <registers>
where:
<Rn> Is the register containing the start address for the instruction.
! Causes base register writeback, and is not optional.
<registers> Is a list of registers to be stored, separated by commas and surrounded by { and }.
The list is encoded in the register_list field of the instruction, by setting bit[i] to 1 if
register Ri is included in the list and to 0 otherwise, for each of i=0 to 7.
At least one register must be stored. If bits[7:0] are all zero, the result is
UNPREDICTABLE.
The registers are stored in sequence, the lowest-numbered register to the lowest
memory address (start_address), through to the highest-numbered register to
the highest memory address (end_address).
The start_address is the value of the base register <Rn>. Subsequent
addresses are formed by incrementing the previous address by four. One address is
produced for each register that is specified in <registers>.
The end_address value is four less than the sum of the value of the base register
and four times the number of registers specified in <registers>.
Finally, the base register <Rn> is incremented by 4 times the numbers of registers
in <registers>.
Architecture version
All T variants
Exceptions
Data Abort
A7-84 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-388-320.jpg)
![Thumb Instructions
Operation
start_address = Rn
end_address = Rn + (Number_Of_Set_Bits_In(register_list) * 4) - 4
address = start_address
for i = 0 to 7
if register_list[i] == 1
Memory[address,4] = Ri
address = address + 4
assert end_address == address - 4
Rn = Rn + (Number_Of_Set_Bits_In(register_list) * 4)
Usage
STMIA is useful as a block store instruction. Combined with LDMIA (Load Multiple), it allows efficient
block copy.
Notes
Operand restrictions
If <Rn> is specified in <registers> and writeback is specified:
• If <Rn> is the lowest-numbered register specified in <registers>, the original
value of <Rn> is stored.
• Otherwise, the stored value of <Rn> is UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment Store Multiple instructions ignore the least significant two bits of address.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception.
Time order The time order of the accesses to individual words of memory generated by this instruction
is only defined in some circumstances. See Data accesses to memory-mapped I/O on
page A2-32 for details.
Equivalent ARM syntax and encoding
STMIA <Rn>!, <registers>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 14 13 12 11 10 9 8 7 0
1 1 1 0 1 0 0 0 1 0 1 0 Rn 0 0 0 0 0 0 0 0 register_list
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-85](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-389-320.jpg)
![Thumb Instructions
7.1.52 STR (1)
15 14 13 12 11 10 6 5 3 2 0
0 1 1 0 0 immed_5 Rn Rd
This form of the STR (Store Register) instruction allows 32-bit data from a general-purpose register to be
stored to memory. The addressing mode is useful for accessing structure (record) fields. With an offset of
zero, the address produced is the unaltered value of the base register <Rn>.
Syntax
STR <Rd>, [<Rn>, #<immed_5> * 4]
where:
<Rd> Is the register that contains the word to be stored to memory.
<Rn> Is the register containing the base address for the instruction.
<immed_5> Is a 5-bit value that is multiplied by 4 and added to the value of <Rn> to form the memory
address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + (immed_5 * 4)
if address[1:0] == 0b00
Memory[address,4] = Rd
else
Memory[address,4] = UNPREDICTABLE
A7-86 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-390-320.jpg)
![Thumb Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not word-aligned and no data abort occurs, the value written to
memory is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception (a type of data abort).
Equivalent ARM syntax and encoding
STR <Rd>, [<Rn>, #<immed_5> * 4]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 2 1 0
1 1 1 0 0 1 0 1 1 0 0 0 Rn Rd 0 0 0 0 0 immed_5 0 0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-87](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-391-320.jpg)
![Thumb Instructions
7.1.53 STR (2)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 0 0 0 Rm Rn Rd
This form of STR allows 32-bit data from a general-purpose register to be stored to memory. The addressing
mode is useful for pointer + large offset arithmetic, and for accessing a single element of an array.
Syntax
STR <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the register that contains the word to be stored to memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register containing the second value used in forming the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
if address[1:0] == 0b00
Memory[address,4] = Rd
else
Memory[address,4] = UNPREDICTABLE
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not word-aligned and no data abort occurs, the value written to
memory is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception (a type of data abort).
A7-88 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-392-320.jpg)
![Thumb Instructions
Equivalent ARM syntax and encoding
STR <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 1 1 1 1 0 0 0 Rn Rd 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-89](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-393-320.jpg)
![Thumb Instructions
7.1.54 STR (3)
15 14 13 12 11 10 8 7 0
1 0 0 1 0 Rd immed_8
This form of STR allows 32-bit data from a general-purpose register to be stored to memory. The addressing
mode is useful for accessing stack data. In this case, STR stores a word from register <Rd> to memory.
Syntax
STR <Rd>, [SP, #<immed_8> * 4]
where:
<Rd> Is the register that contains the word to be stored to memory.
SP Is the stack pointer. Its value is used to calculate the memory address.
<immed_8> Is an 8-bit value that is multiplied by 4 and added to the value of the SP to form the memory
address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = SP + (immed_8 * 4)
if address[1:0] == 0b00
Memory[address,4] = Rd
else
Memory[address,4] = UNPREDICTABLE
A7-90 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-394-320.jpg)
![Thumb Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not word-aligned and no data abort occurs, the value written to
memory is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00
causes an alignment exception (a type of data abort).
Equivalent ARM syntax and encoding
STR <Rd>, [SP, #<immed_8> * 4]
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 2 1 0
1 1 1 0 0 1 0 1 1 0 0 0 1 1 0 1 Rd 0 0 immed_8 0 0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-91](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-395-320.jpg)
![Thumb Instructions
7.1.55 STRB (1)
15 14 13 12 11 10 6 5 3 2 0
0 1 1 1 0 immed_5 Rn Rd
This form of the STRB (Store Register Byte) instruction allows 8-bit data from a general-purpose register
to be stored to memory. The addressing mode is useful for accessing structure (record) fields.
With an offset of zero, the address produced is the unaltered value of the base register <Rn>.
Syntax
STRB <Rd>, [<Rn>, #<immed_5>]
where:
<Rd> Is the register whose least significant byte is stored to memory.
<Rn> Is the register containing the base address for the instruction.
<immed_5> Is a 5-bit immediate value that is added to the value of <Rn> to form the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + immed_5
Memory[address,1] = Rd[7:0]
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Equivalent ARM syntax and encoding
STRB <Rd>, [<Rn>, #<immed_5>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 0
1 1 1 0 0 1 0 1 1 1 0 0 Rn Rd 0 0 0 0 0 0 0 immed_5
A7-92 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-396-320.jpg)
![Thumb Instructions
7.1.56 STRB (2)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 0 1 0 Rm Rn Rd
This form of STRB allows 8-bit data from a general-purpose register to be stored to memory. The
addressing mode is useful for pointer + large offset arithmetic, and for accessing a single element of an
array.
Syntax
STRB <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the register whose least significant byte is stored to memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register whose value is added to <Rn> to form the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
Memory[address,1] = Rd[7:0]
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Equivalent ARM syntax and encoding
STRB <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 1 1 1 1 1 0 0 Rn Rd 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-93](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-397-320.jpg)
![Thumb Instructions
7.1.57 STRH (1)
15 14 13 12 11 10 6 5 3 2 0
1 0 0 0 0 immed_5 Rn Rd
This form of the STRH (Store Register Halfword) instruction allows 16-bit data from a general-purpose
register to be stored to memory. The addressing mode is useful for accessing structure (record) fields. With
an offset of zero, the address produced is the unaltered value of the base register <Rn>.
Syntax
STRH <Rd>, [<Rn>, #<immed_5> * 2]
where:
<Rd> Is the register whose least significant halfword is stored to memory.
<Rn> Is the register containing the base address for the instruction.
<immed_5> Is a 5-bit immediate value that is multiplied by two and added to the value of <Rn> to form
the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + (immed_5 * 2)
if address[1:0] == 0
Memory[address,2] = Rd[15:0]
else
Memory[address,2] = UNPREDICTABLE
A7-94 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-398-320.jpg)
![Thumb Instructions
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written
to memory is UNPREDICTABLE.
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes
an alignment exception (a type of data abort).
Equivalent ARM syntax and encoding
STRH <Rd>, [<Rn>, #<immed_5> * 2]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 1 0
1 1 1 0 0 0 0 1 1 1 0 0 Rn Rd 0 0
immed
[4:3]
1 0 1 1
immed
[2:0]
0
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-95](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-399-320.jpg)
![Thumb Instructions
7.1.58 STRH (2)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 1 0 1 0 0 1 Rm Rn Rd
This form of STRH allows 16-bit data from a general-purpose register to be stored to memory. The
addressing mode is useful for pointer + large offset arithmetic and for accessing a single element of an array.
Syntax
STRH <Rd>, [<Rn>, <Rm>]
where:
<Rd> Is the register whose least significant halfword is stored to memory.
<Rn> Is the register containing the first value used in forming the memory address.
<Rm> Is the register whose value is added to <Rn> to form the memory address.
Architecture version
All T variants
Exceptions
Data Abort
Operation
address = Rn + Rm
if address[1:0] == 0
Memory[address,2] = Rd[15:0]
else
Memory[address,2] = UNPREDICTABLE
Notes
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written
to memory is UNPREDICTABLE
If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes
an alignment exception (a type of data abort).
A7-96 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-400-320.jpg)
![Thumb Instructions
Equivalent ARM syntax and encoding
STRH <Rd>, [<Rn>, <Rm>]
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 2 1 0
1 1 1 0 0 0 0 1 1 0 0 0 Rn Rd SBZ 1 0 1 1 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-97](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-401-320.jpg)
![Thumb Instructions
7.1.59 SUB (1)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 0 0 1 1 1 1 immed_3 Rn Rd
This form of the SUB (Subtract) instruction subtracts a small constant value from the value of a register and
stores the result in a second register. The condition code flags are updated, based on the result.
Syntax
SUB <Rd>, <Rn>, #<immed_3>
where:
<Rd> Is the destination register for the operation.
<Rn> Is the register containing the first operand for the subtraction.
<immed_3> Is a 3-bit immediate value (values 0 to 7) that is subtracted from <Rn>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rn - immed_3
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - immed_3)
V Flag = OverflowFrom(Rn - immed_3)
Equivalent ARM syntax and encoding
SUBS <Rd>, <Rn>, #<immed_3>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 0
1 1 1 0 0 0 1 0 0 1 0 1 Rn Rd 0 0 0 0 0 0 0 0 0 immed_3
A7-98 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-402-320.jpg)
![Thumb Instructions
7.1.60 SUB (2)
15 14 13 12 11 10 8 7 0
0 0 1 1 1 Rd immed_8
This form of SUB subtracts a large immediate value from the value of a register and stores the result back
in the same register. The condition code flags are updated, based on the result.
Syntax
SUB <Rd>, #<immed_8>
where:
<Rd> Is the register containing the first operand for the subtraction, and is also the
destination register for the operation.
<immed_8> Is an 8-bit immediate value (values 0 to 255) that is subtracted from <Rd>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rd - immed_8
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rd - immed_8)
V Flag = OverflowFrom(Rd - immed_8)
Equivalent ARM syntax and encoding
SUBS <Rd, <Rd>, #<immed_8>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0
1 1 1 0 0 0 1 0 0 1 0 1 Rd Rd 0 0 0 0 immed_8
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-99](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-403-320.jpg)
![Thumb Instructions
7.1.61 SUB (3)
15 14 13 12 11 10 9 8 6 5 3 2 0
0 0 0 1 1 0 1 Rm Rn Rd
This form of SUB subtracts the value of one register from the value of a second register and stores the result
in a third register. The condition code flags are updated, based on the result.
Syntax
SUB <Rd>, <Rn>, <Rm>
where:
<Rd> Is the destination register for the operation.
<Rn> Is the register containing the first operand for the subtraction.
<Rm> Is the register whose value is subtracted from <Rn>.
Architecture version
All T variants
Exceptions
None
Operation
Rd = Rn - Rm
N Flag = Rd[31]
Z Flag = if Rd == 0 then 1 else 0
C Flag = NOT BorrowFrom(Rn - Rm)
V Flag = OverflowFrom(Rn - Rm)
Equivalent ARM syntax and encoding
SUBS <Rd>, <Rn>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 0 0 1 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm
A7-100 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-404-320.jpg)
![Thumb Instructions
7.1.63 SWI
15 14 13 12 11 10 9 8 7 0
1 1 0 1 1 1 1 1 immed_8
The SWI (Software Interrupt) instruction is used as an operating system service call. A SWI exception is
generated, which is handled by an operating system to provide the requested service. See Exceptions on
page A2-13.
Syntax
SWI <immed_8>
where:
<immed_8> Is an 8-bit immediate value that is put into bits[7:0] of the instruction. This value is
ignored by the processor, but can be used by an operating system’s SWI exception
handler to determine which operating system service is being requested.
Architecture version
All T variants
Exceptions
Software Interrupt
Operation
R14_svc = address of next instruction after the SWI instruction
SPSR_svc = CPSR
CPSR[4:0] = 0b10011 /* Enter Supervisor mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
if high vectors configured then
PC = 0xFFFF0008
else
PC = 0x00000008
Equivalent ARM syntax and encoding
SWI <immed_8>
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 0
1 1 1 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 immed_8
A7-102 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-406-320.jpg)
![Thumb Instructions
7.1.64 TST
15 14 13 12 11 10 9 8 7 6 5 3 2 0
0 1 0 0 0 0 1 0 0 0 Rm Rn
The TST (Test) instruction is used to determine whether a particular subset of bits in a register includes at
least one set bit. A very common use for TST is to test whether a single bit is set or clear. The condition
code flags are updated, based on the result.
Syntax
TST <Rn>, <Rm>
where:
<Rn> Is the register containing the first operand for the instruction.
<Rm> Is the register whose value is logically ANDed with the value of <Rn>.
Architecture version
All T variants
Exceptions
None
Operation
alu_out = Rn AND Rm
N Flag = alu_out[31]
Z Flag = if alu_out == 0 then 1 else 0
C Flag = unaffected
V Flag = unaffected
Equivalent ARM syntax and encoding
TST <Rn>, <Rm>
31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
1 1 1 0 0 0 0 1 0 0 0 1 Rn SBZ 0 0 0 0 0 0 0 0 Rm
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-103](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-407-320.jpg)
![The 26-bit Architectures
8.2 Format of register 15
31 30 29 28 27 26 25 2 1 0
N Z C V I F Program Counter M1M0
Bits[25:2] are collectively known as the Program Counter (PC). Because the PC occupies only 24 bits of
register 15, only 224 instructions (226 bytes) can be addressed, giving a maximum addressable program size
of 64MB.
Bits[31:26] and bits[1:0] are collectively known as the Program Status Register (PSR).
The N, Z, C, V, I, and F bits have the same meaning in both 26-bit and 32-bit architectures. M[1:0] also have
the same meaning in both architectures.
Abort, Undefined, and System mode are not supported in 26-bit architectures. Aborts and Undefined
Instruction exceptions have exactly the same actions in both modes, except that in 26-bit architectures,
Supervisor mode is entered instead of Abort or Undefined mode.
The precise effect of an exception on a 26-bit architecture can be derived from the rules in Exceptions on
page A2-13, as follows:
• The banked version of R14 has bits[25:2] set to the specified address, and bits[31:26, 1, 0] set to
copies of the corresponding bits in R15.
• The I, F, M1, and M0 bits are modified in the same way as CPSR[7], CPSR[6], CPSR[1], and
CPSR[0] respectively, on a 32-bit architecture.
The I, F and M[1:0] bits cannot be written directly when the processor is in User mode. In User mode they
are only changed by an exception occurring.
8.2.1 Reading register 15
In 26-bit architectures, the value of R15 is read in five different ways:
• Most importantly, if R15 has an UNPREDICTABLE value in the 32-bit architecture, it also has an
UNPREDICTABLE value when used in the same way in the 26-bit architecture.
• If R15 is specified in bits[19:16] of an instruction (and its value is not UNPREDICTABLE), only the PC
(bits[25:2]) is used. All other bits read as zero.
• If R15 is specified in bits[3:0] of an instruction (and its value is not UNPREDICTABLE), all 32 bits are
used.
• If R15 is stored using STR or STM, the value of the PC (bits[25:2]) is IMPLEMENTATION DEFINED, but
all 32 bits of the register are stored.
• All 32 bits are stored in the Link register (R14) after a Branch with Link instruction or an exception
entry.
A8-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-414-320.jpg)
![The 26-bit Architectures
8.3 26-bit PSR update instructions
31 27 26 25 24 23 22 21 20 19 16 15 12 11 0
cond 0 0 I 1 0 opc 1 Rn 1 1 1 1 shifter_operand
In 26-bit architectures, the MSR and MRS instructions are not supported. Instead, variants of the CMN, CMP,
TEQ and TST instructions are used to write just the PSR part of R15 without affecting the PC part. These
26-bit-only variants are called CMNP, CMPP, TEQP and TSTP, and are distinguished by having instruction
bits[15:12] equal to 0b1111. These bits are equal to 0b0000 in the normal variants of these instructions.
These instructions write their ALU result directly to the PSR part of register 15 (only N, Z, C, and V are
affected in User mode).
When the processor is in User mode, only the condition codes are affected. All other modes allow all PSR
bits to be altered.
8.3.1 Syntax
CMN{<cond>}P Rn, <shifter_operand>
CMP{<cond>}P Rn, <shifter_operand>
TEQ{<cond>}P Rn, <shifter_operand>
TST{<cond>}P Rn, <shifter_operand>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rn> Specifies the register that contains the first operand for the instruction.
<shifter_operand>
Specifies the second operand for the instruction. Its options are described in Addressing
Mode 1 - Data-processing operands on page A5-2, including how each option causes the I
bit (bit 25) and the <shifter_operand> bits (bits 11:0) to be set in the instruction.
8.3.2 Exceptions
None
A8-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-416-320.jpg)
![The 26-bit Architectures
8.3.3 Operation
if ConditionPassed(<cond>) then
case <opc> of
0b00 /* TSTP */
<alu_out> = Rn AND <shifter_operand>
0b01 /* TEQP */
<alu_out> = Rn EOR <shifter_operand>
0b10 /* CMPP */
<alu_out> = Rn - <shifter_operand>
0b11 /* CMNP */
<alu_out> = Rn + <shifter_operand>
endcase
if R15[1:0] == 0b00 then /* M[1:0] == 0b00, User mode */
R15[31:28] = <alu_out>[31:28] /* update just NZCV */
else /* a privileged mode */
R15[31:26] = <alu_out>[31:26] /* update NZCVIF and ... */
R15[1:0] = <alu_out>[1:0] /* ... update M[1:0] */
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-417-320.jpg)
![The 26-bit Architectures
8.4 Address exceptions
In 26-bit architectures, all data addresses are checked to ensure that they are between 0 and 64MB. If a data
address is produced with a 1 in any of the top 6 bits, an address exception is generated. When an address
exception is generated, the following actions are performed:
R14_svc[25:2] = address of instruction + 8
R14_svc[31:26,1,0] = R15[31:26,1,0]
M[1:0] = 0b11 ; Supervisor mode
F = unchanged
I = 1 ; (normal) interrupts disabled
PC = 0x14
The address of the instruction which caused the address exception is the value in R14 minus 8.
8.4.1 Returning from an address exception
As this exception implies a programming error, it is not usual to return from address exceptions, but if a
return is required, use:
SUBS PC,R14,#8
This restores both the PC and PSR (from R14_svc) and returns to the instruction that generated the address
exception.
A8-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-418-320.jpg)
![The 26-bit Architectures
8.5 Backwards compatibility from 32-bit architectures
As described in Overview of the 26-bit architectures on page A8-2, some 32-bit architectures include
mandatory or optional backwards-compatibility support for 26-bit architectures. Architecture versions with
this support implement the four 26-bit modes described in Overview of the 26-bit architectures on
page A8-2. When the processor is in one of these modes, the PC behaves as explained in Format of register
15 on page A8-4 and the instructions described in 26-bit PSR update instructions on page A8-6 are
available. This allows older 26-bit programs to be executed in a suitable 26-bit mode.
In architecture versions without 26-bit backwards-compatibility support, CPSR bit[4] (M[4]) always reads
as 1, and all writes to it are ignored.
The complete list of processor modes is shown in Table 8-1.
Table 8-1 32-bit and 26-bit modes
M[4:0] Mode Accessible registers
0b00000 User_26 R0 to R14, PC, (CPSR)
0b00001 FIQ_26 R0 to R7, R8_fiq to R14_fiq, PC, (CPSR, SPSR_fiq)
0b00010 IRQ_26 R0 to R12, R13_irq, R14_irq, PC, (CPSR, SPSR_irq)
0b00011 SVC_26 R0 to R12, R13_svc, R14_svc, PC, (CPSR, SPSR_svc)
0b10000 User_32 R0 to R14, PC, CPSR
0b10001 FIQ_32 R0 to R7, R8_fiq to R14_fiq, PC, CPSR, SPSR_fiq
0b10010 IRQ_32 R0 to R12, R13_irq, R14_irq, PC, CPSR, SPSR_irq
0b10011 SVC_32 R0 to R12, R13_svc, R14_svc, PC, CPSR, SPSR_svc
0b10111 Abort_32 R0 to R12, R13_abt, R14_abt, PC, CPSR, SPSR_abt
0b11011 Undef_32 R0 to R12, R13_und, R14_und, PC, CPSR, SPSR_und
0b11111 System_32 R0 to R14, PC, CPSR (architecture version 4 and above)
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-419-320.jpg)
![The 26-bit Architectures
Note
The MRS and MSR instructions are supported in all cases, despite the fact that they do not exist in pure 26-bit
architectures. This is to assist conversion to a 32-bit architecture, as the CMNP, CMPP, TEQP, and TSTP
instructions cannot enter a 32-bit mode from a 26-bit mode.
If an MSR instruction which lies outside the 26-bit address space is used to switch into a 26-bit mode, the
results are UNPREDICTABLE.
The results of an MSR instruction which switches between a 32-bit mode and a 26-bit mode (in either
direction) are also UNPREDICTABLE if it is executed at any point where predictable subsequent execution can
reach either of the following:
• the end of the 26-bit address space
• a branch instruction whose target addresses in the 26-bit and 32-bit address spaces are different.
For this purpose, predictable subsequent execution is as defined in Other uses for IMBs on page A2-30.
8.5.1 32-bit and 26-bit configuration
Architecture versions with 26-bit backwards compatibility support optionally incorporate two hardware
signals that control 32-bit instruction accesses and 32-bit data accesses. The signals are mapped to two bits
in register 1 of the system control coprocessor. These signals are:
• PROG32
• DATA32.
Note
If an implementation does not support these signals but does provide 32-bit modes, it must behave as though
both signals were 1.
32-bit configuration
1. If PROG32 is active and DATA32 is active, the processor switches to a 32-bit mode when
processing exceptions (including Reset), using the _32 modes for handling all exceptions. This is
called a 32-bit configuration. Abort_32 mode is used for handling memory aborts, and Undef_32 for
handling Undefined Instruction exceptions. A 26-bit mode can be selected by putting a 26-bit mode
number into the M[4:0] bits of the CPSR (either using MSR or an exception return sequence). A
32-bit mode can also be entered from a 26-bit mode using the MSR instruction. Once in a 26-bit mode,
another 26-bit mode can be entered using one of the CMNP, CMPP, TEQP and TSTP instructions, or
the MSR instruction.
If an exception occurs when the processor is in a 26-bit mode, only the PC bits from R15[25:2] are
copied to the link register. The remaining bits in the link register are zeroed. The PSR bits from
R15[31:26] and R15[1:0] are copied into the SPSR, ready for a normal 32-bit return sequence.
2. If PROG32 is active and DATA32 is not active (32-bit programs with 26-bit data), the result is
UNPREDICTABLE.
A8-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-420-320.jpg)
![The 26-bit Architectures
26-bit configuration
1. If PROG32 is not active, the processor is locked into 26-bit modes (that is, cannot be placed into a
32-bit mode by any means) and handles exceptions in 26-bit modes. This is called a 26-bit
configuration. In this configuration, CMNP, CMPP, TEQP and TSTP instructions, or the MSR
instruction can be used to switch to 26-bit modes. Attempts to write CPSR bits[4:2] (M[4:2]) are
ignored, stopping any attempts to switch to a 32-bit mode, and SVC_26 mode is used to handle
memory aborts and Undefined Instruction exceptions. The PC is limited to 24 bits, limiting the
addressable program memory to 64MB.
2. If PROG32 is not active, DATA32 has the following actions:
• If DATA32 is not active, all data addresses are checked to ensure that they are between 0 and
64MB. If a data address is produced with a 1 in any of the top 6 bits, an address exception is
generated.
• If DATA32 is active, full 32-bit addresses can be produced and are not checked for address
exceptions. This allows 26-bit programs to access data in the full 32-bit address space.
8.5.2 Vector exceptions
When the processor is in a 32-bit configuration (PROG32 is active) and in a 26-bit mode (CPSR[4] == 0),
data access (but not instruction fetches) to the exception vectors (address 0x0 to 0x1F) causes a data abort.
This is known as a vector exception.
Vector exceptions are always produced if the exception vectors are written in a 32-bit configuration and a
26-bit mode. It is IMPLEMENTATION DEFINED whether reading the exception vectors in a 32-bit
configuration and a 26-bit mode also causes a vector exception.
Vector exceptions are provided to support 26-bit backwards compatibility. When a vector exception is
generated, it indicates that a 26-bit mode process is trying to install a (26-bit) vector handler. Because the
processor is in a 32-bit configuration, exceptions are handled in a 32-bit mode, so a veneer must be used to
change from the 32-bit exception mode to a 26-bit mode before calling the 26-bit exception handler.
This veneer can be installed on each vector and can switch to a 26-bit mode before calling any 26-bit
handlers.
The return from the 26-bit exception handler might also need to be veneered. Some SWI handlers return
status information in the processor flags, and this information needs to be transferred from the link register
to the SPSR with a return veneer for the SWI handler.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-11](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-421-320.jpg)
![ARM Code Sequences
9.3 Load and Store instructions
Load and Store instructions are the best way to load or store a single word. They are also the only
instructions that can load or store a byte or halfword.
9.3.1 Linked lists
The following code searches for an element in a linked list that has two elements (a single byte value and a
pointer to the next record) in each record. A null next pointer indicates this is the last element in the list:
; Linked list search
; On entry : R0 holds a pointer to the first record in the list
; : R1 holds the byte we are searching for
; : Call this code with a BL
; On exit : R0 holds the address of the first record matched
; : or a null pointer if no match was found
; : R2 is destroyed
llsearch
CMP R0, #0 ; null pointer?
LDRNEB R2, [R0] ; load the byte value from this record
CMPNE R1, R2 ; compare with the looked-for value
LDRNE R0, [R0, #4] ; if not found, follow the link to the
BNE llsearch ; next record and then keep looking
MOV PC, LR ; return with pointer in R0
9.3.2 Simple string compare
The following code performs a very simple string compare on two zero-terminated strings:
; String compare
; On entry : R0 points to the first string
; : R1 points to the second string
; : Call this code with a BL
; On exit : R0 is < 0 if the first string is less than the second
; : R0 is = 0 if the first string is equal to the second
; : R0 is > 0 if the first string is greater than the second
; : R1, R2 and R3 are destroyed
strcmp
LDRB R2, [R0], #1 ; Get a byte from the first string
LDRB R3, [R1], #1 ; Get a byte from the second string
CMP R2, #0 ; Have we reached the end of either
CMPNE R3, #0 ; string?
BEQ return ; Go to return code if so
CMP R2, R3 ; Are the strings the same so far?
BEQ strcmp ; Repeat for next character if so
return
SUB R0, R2, R3 ; Calculate result value and return
MOV PC, LR ; by copying R14 (LR) into the PC
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-429-320.jpg)
![ARM Code Sequences
The following code performs a more optimized string compare:
int strcmp(char *s1, char *s2)
{
unsigned int ch1, ch2;
do
{
ch1 = *s1++;
ch2 = *s2++;
} while (ch1 >= 1 && ch1 == ch2);
return ch1 - ch2;
}
This code uses an unsigned comparison with 1 to test for a null character, rather than the normal comparison
with 0.
The corresponding ARM code is:
strcmp
LDRB R2,[R0],#1
LDRB R3,[R1],#1
CMP R2,#1
CMPCS R2,R3
BEQ strcmp
SUB R0,R2,R3
MOV PC,LR
The change in the way that null characters are detected allows the condition tests to be combined:
• If R2 == 0, the CMP instruction sets Z = 0, C = 0. Neither the CMPCS instruction nor the BEQ
instruction is executed, and the loop terminates.
• If R2 != 0 and R3 == 0, the CMP instruction sets C = 1, then the CMPCS instruction is executed and
sets Z = 0. So, the BEQ instruction is not executed and the loop terminates.
• If R2 != 0 and R3 != 0, the CMP instruction sets C = 1, then the CMPCS instruction is executed and
sets Z according to whether R2 == R3. So, the BEQ instruction is executed if R2 == R3 and the loop
terminates if R2 != R3.
Much faster string comparison routines are possible by loading one word of each string at a time and
comparing all four bytes.
A9-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-430-320.jpg)
![ARM Code Sequences
9.3.3 Long branch
A Load instruction can be used to generate a branch to anywhere in the 4GB address space. By manually
setting the value of the link register (R14), a subroutine call can be made to anywhere in the address space.
; Long branch (and link)
ADD LR, PC, #4 ; set the return address to be 8 bytes
; after the next instruction
LDR PC, [PC, #-4] ; get the address from the next word
DCD function ; store the address of the function
; (DCD is an assembler directive)
return_here ; return to here
This code uses the location after the load to hold the address of the function to call. In practice, this location
can be anywhere as long as it is within 4KB of the load instruction. Notice also that this code is
position-independent except for the address of the function to call. Full position-independence can be
achieved by storing the offset of the branch target after the load, and using an ADD instruction to add it to
the PC.
9.3.4 Multi-way branches
The following code improves on the multi-way branch code shown above by using a table of addresses of
functions to call:
; Multi-way branch
; On entry: R0 holds the branch index
CMP R0, #maxindex ; checks the index is in the range
; by using an unsigned compare.
LDRLO PC, [PC, R0, LSL #2] ; convert the index to a word offset
; do a look up in the table put the loaded
; value into the PC and jump there
B IndexOutOfRange ; jump to the error handler
DCD Handler0 ; DCD is an assembler directive to
DCD Handler1 ; store a word (in this case an
DCD Handler2 ; address in memory).
DCD Handler3
....
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-431-320.jpg)
![ARM Code Sequences
9.5 Semaphore instructions
This code controls the entry and exit from a critical section of code. The semaphore instruction (SWP) does
not provide a compare and conditional write facility, so this must be done explicitly. The following code
achieves this by using a semaphore value to indicate that the lock is being inspected.
The code below causes the calling process to busy-wait until the lock is free. To ensure progress, three OS
calls need to be made (one before each loop branch) to sleep the process if the lock cannot be accessed.
; Critical section entry and exit
; The code uses a process ID to identify the lock owner
; An ID of zero indicates the lock is free
; An ID of -1 indicates the lock is being inspected
; On entry: R0 holds the address of the semaphore
; R1 holds the ID of the process requesting the lock
MVN R2, #0 ; load the ‘looking’ value (-1) in R2
spinin SWP R3, R2, [R0] ; look at the lock, and lock others out
CMN R3, #1 ; anyone else trying to look?
....
Insert conditional OS call to sleep process here
....
BEQ spinin ; yes, so wait our turn
CMP R3, #0 ; no-one looking, is the lock free?
STRNE R3, [R0] ; no, then restore the previous owner
....
Insert conditional OS call to sleep process here
....
BNE spinin ; and wait again
STR R1, [R0] ; otherwise grab the lock
.....
Insert critical code here
.....
spinout SWP R3, R2, [R0] ; look at the lock, and lock others out
CMN R3, #1 ; anyone else trying to look ?
....
Insert conditional OS call to sleep process here
....
BEQ spinout ; yes, so wait our turn
CMP R3, R1 ; check we own it
BNE CorruptSemaphore ; we should have been the owner!
MOV R2, #0 ; load the ‘free’ value
STR R2, [R0] ; and open the lock
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-11](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-433-320.jpg)
![ARM Code Sequences
9.6 Other code examples
The following sequences illustrate some other applications of ARM assembly language.
9.6.1 Software interrupt dispatch
This code segment dispatches software interrupts (SWIs) to individual handlers. For it to work, the
instruction at the software interrupt vector (memory location 0x00000008) must branch to the first
instruction of this code. The SWI instruction has a 24-bit field that can be used for specific SWI functions.
This code also handles the 16-bit Thumb SWI instruction, which has an 8-bit SWI number field rather than
a 24-bit field.
This example assumes that the code to handle each of the individual SWIs only modifies r0-r3, r12, lr and
the PC. If more registers are needed, the example should be modified to include the extra registers needed
in the register lists of the STMFD and LDMFD instructions. This makes the extra registers available to all of
the SWI handlers, but the code will typically take longer to execute because of the extra memory accesses.
Alternatively, if only a few of the individual SWI handlers require extra registers, use extra STMFD and
LDMFD instructions within those handlers. This ensures that SWIs which do not require the extra registers
are not slowed down.
SWIHandler
STMFD sp!, {r0-r3,r12,lr} ; Store the registers
MRS r0, spsr ; Move SPSR into general purpose
; register
TST r0, #0x20 ; Test the SPSR T bit to discover
; ARM/Thumb state when SWI occurred
LDRNEH r0, [lr, #-2] ; T bit set so load halfword (Thumb)
BICNE r0, r0, #0xff00 ; and clear top 8 bits of halfword
; (LDRH clears top 16 bits of word)
LDREQ r0, [lr, #-4] ; T bit clear so load word (ARM)
BICEQ r0, r0, #0xff000000 ; and clear top 8 bits of word
CMP r0, #MaxSWI ; Check the SWI number is in range
LDRLS pc, [pc, r0, LSL #2] ; If so, jump to the correct routine
B SWIOutOfRange
switable
DCD do_swi_0
DCD do_swi_1
:
:
do_swi_0
.....
Insert code to handle SWI 0 here
.....
LDMFD sp!, {r0-r3,r12,pc}^ ; Restore the registers and return.
do_swi_1
:
A9-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-434-320.jpg)
![ARM Code Sequences
9.6.2 Single-channel DMA transfer
The following code is an interrupt handler to perform interrupt driven input/output to memory transfers
(soft DMA). The code is written as an FIQ handler, and uses the banked FIQ registers to maintain state
between interrupts. Therefore this code is best situated at location 0x1C. The entire sequence to handle a
normal transfer is just four instructions. Code situated after the conditional return is used to signal that the
transfer is complete.
LDR r11, [r8, #IOData] ; load port data from the I/O device
STR r11, [r9], #4 ; store it to memory: update the pointer
CMP r9, r10 ; reached the end?
SUBLTS pc, lr, #4 ; no, so return
; Insert transfer complete code here
where:
R8 Points to the base address of the input/output device that data is read from.
IOData Is the offset from the base address to the 32-bit data register that is read. Reading this
register disables the interrupt.
R9 Points to the memory location where data is being transferred.
R10 Points to the last address to transfer to.
Of course, byte transfers can be made by replacing the load and store instructions with Load and Store byte
instructions, and changing the offset in the store instruction from 4 to 1. Transfers from memory to an
input/output device are made by swapping the addressing modes between the Load instruction and the Store
instruction.
9.6.3 Dual-channel DMA transfer
This code is similar to the example in Single-channel DMA transfer on page A9-13, except that it handles
two channels (which can be the input and output side of the same channel). Again, this code is written as an
FIQ handler, and uses the banked FIQ registers to maintain state between interrupts. Therefore this code is
best situated at location 0x1C.
The entire sequence to handle a normal transfer is just nine instructions. Code situated after the conditional
return is used to signal that the transfer is complete.
LDR r13, [r8, #IOStat] ; load status register to find ....
TST r13, #IOPort1Active ; .... which port caused the interrupt?
LDREQ r13, [r8, #IOPort1] ; load port 1 data
LDRNE r13, [r8, #IOPort2] ; load port 2 data
STREQ r13, [r9], #4 ; store to buffer 1
STRNE r13, [r10], #4 ; store to buffer 2
CMP r9, r11 ; reached the end?
CMPNE r10, r12 ; on either channel?
SUBNES pc, lr, #4 ; return
; Insert transfer complete code here
where:
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-13](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-435-320.jpg)
![ARM Code Sequences
9.6.4 Interrupt prioritization
This code dispatches up to 32 interrupt sources to their appropriate handler routines. This code is intended
to use the normal interrupt vector, so memory location 0x00000018 must contain an instruction that
branches to the first instruction of this code.
External hardware is used to prioritize the interrupt and present the number of the highest-priority active
interrupt in an input register. Interrupts are re-enabled after 10 instructions (including the branch to this
code).
; first save the critical state
;
SUB r14, r14, #4 ; adjust return address before saving it
STMFD r13!, {r12, r14} ; stack return address and working register
MRS r12, SPSR ; get the SPSR ...
STMFD r13!, {r12} ; ... and stack that too
;
; now get the priority level of the highest priority active interrupt
MOV r12, #IntBase ; get interrupt controller’s base address
LDR r12, [r12, #IntLevel] ; get the interrupt level (0 to 31)
;
; now read-modify-write the CPSR to enable interrupts
MRS r14, CPSR ; read the status register
BIC r14, r14, #0x80 ; clear the I bit (use 0x40 for the F bit)
MSR CPSR_c, r14 ; write it back to re-enable interrupts
; jump to the correct handler
LDR PC, [PC, r12, LSL #2] ; and jump to the correct handler. PC base
; address points to this instruction + 8
NOP ; pad so the PC indexes this table
;
; table of handler start addresses
;
DCD Priority0Handler
DCD Priority1Handler ........
Priority0Handler
STMFD r13!, {r0 - r11} ; save working registers
;
; insert handler code here
;
........
MRS r12, CPSR ; Read-modify-write the CPSR to disable
ORR r12, r12, #0x80 ; interrupts (use 0x40 instead for FIQs)
MSR CPSR_c, r12 ; Note: Do not use r14 instead of r12. It
; will be corrupted if an interrupt occurs
LDMFD r13!, {r0-r12} ; Recover the working registers and SPSR
MSR SPSR_cxsf, r12 ; Put the SPSR back
LDMFD r13!, {r12, PC}^ ; Restore last working register and return
Priority1Handler
........
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-437-320.jpg)
![ARM Code Sequences
If the normal interrupt processing code instead switches to another User mode process, it puts pointers to
the PCBs of the old and new processes in R0 and R1 respectively and branches to the following code:
; First store the old process’s User mode state to the PCB pointed to by R0.
MRS R12, SPSR ; Get CPSR of interrupted process
STR R12, [R0], #8 ; Store CPSR to PCB, point R0 at
; PCB location for R0 value
LDMFD R13!, {R2, R3} ; Reload R0/R1 of interrupted
; process from stack
STMIA R0!, {R2, R3} ; Store R0/R1 values to PCB, point
; R0 at PCB location for R2 value
LDMFD R13!, {R2, R3, R12, R14} ; Reload remaining stacked values
STR R14, [R0, #-12] ; Store R14_irq, the interrupted
; process’s restart address
STMIA R0, {R2-R14}^ ; Store user R2-R14 - see Note 1
; Then load the new process’s User mode state and return to it.
LDMIA R1!, {R12, R14} ; Put interrupted process’s CPSR
MSR SPSR_fsxc, R12 ; and restart address in SPSR_irq
; and R14_irq
LDMIA R1, {R0-R14}^ ; Load user R0-R14 - see Note 2
NOP ; Note: Cannot use banked register
; immediately after User mode LDM
MOVS PC, R14 ; Return to address in R14_irq,
; with SPSR_irq -> CPSR transfer
Note
1. This instruction is an example of the form of STM described in STM (2) on page A4-86. It stores the
registers R2, R3, ..., R12, R13_usr, R14_usr to the correct places in the PCB.
2. This instruction is an example of the form of LDM described in LDM (2) on page A4-32. It loads the
registers R0, R1, ..., R12, R13_usr, R14_usr from the correct places in the PCB.
A9-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-440-320.jpg)
![Enhanced DSP Extension
10.4 The Q flag
The enhanced DSP extension incorporates a mechanism to determine whether saturation or overflow has
occurred in the course of a calculation. Bit[27] of the CPSR is a sticky overflow flag, also known as the Q
flag. This flag is set to 1 if any of the following occurs:
• Saturation of the addition result in a QADD or QDADD instruction
• Saturation of the subtraction result in a QSUB or QDSUB instruction
• Saturation of the doubling intermediate result in a QDADD or QDSUB instruction
• Signed overflow during an SMLA<x><y> or SMLAW<y> instruction.
Note
The Q flag is not affected by overflow during any other arithmetic instruction, such as ADD, SUB, or MLA.
The Q flag is sticky in that once it has been set to 1, it is not affected by whether subsequent calculations
saturate and/or overflow. Its intended usage is:
1. Use an MSR CPSR_f,#0 instruction to clear the Q flag (this also clears the condition code flags).
2. Perform a sequence of calculations.
3. Use an MRS Rn,CPSR instruction to read the CPSR, then test the value of the Q flag. If it is still 0,
none of the above types of saturation or overflow occurred during step 2. Otherwise, at least one
instance of saturation or overflow occurred.
Each SPSR also has a Q flag into which the CPSR Q flag is copied when an exception occurs as part of the
general CPSR ® SPSR copy performed on exception entry. Similarly, the exception return instructions
which atomically return to the correct address and perform an SPSR ® CPSR transfer will copy the SPSR
Q flag back to the CPSR. Between them, these ensure that the value of the Q flag is not changed by an
interrupt or other exception. For more details of this, see Exceptions on page A2-13.
Except as described above, the only instructions that affect or are affected by the Q flags are MSR
instructions which write to the flags byte of the destination PSR, and MRS instructions.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-445-320.jpg)
![Enhanced DSP Extension
10.5 Enhanced DSP instructions
The enhanced DSP instructions can be divided into the following groups:
• Integer multiply and multiply-accumulate instructions on page A10-6
• Saturated addition and subtraction instructions on page A10-7
• Two-word load and store instructions on page A10-7
• Cache preload instruction on page A10-7
• Two-word coprocessor register transfer instructions on page A10-7.
10.5.1 Integer multiply and multiply-accumulate instructions
The enhanced DSP instructions augment the normal integer multiply and multiply-accumulate instructions
with versions designed to work on 16-bit signed data. These form one or both of the operands to the
multiplication by taking either the bottom half (bits[15:0]) or the top half (bits[31:16]) of a source register
and sign-extending it, while ignoring the other half of the source register.
These instructions allow multiplications to be performed on 16-bit data values that are held packed in
registers, without the need to unpack them. The data values can be loaded in this format by LDR or LDM
instructions, which generally results in more efficient use of memory bandwidth than would be obtained by
the use of LDRH or LDRSH instructions.
The instructions available are:
• 16 ´ 16 ® 32 bit signed multiply instructions SMUL<x><y>. These instructions are very similar to
the normal MUL instruction, apart from the fact that their operands are generated as described above.
• 16 ´ 16 + 32 ® 32 bit signed multiply-accumulate instructions SMLA<x><y>. These instructions
are very similar to the normal MLA instruction, apart from the fact that their operands are generated
as described above.
• 32 ´ 16 ® 32 bit signed multiply instructions SMULW<y>. These instructions produce the upper 32
bits of the 48-bit product.
• 32 ´ 16 + 32 ® 32 bit signed multiply-accumulate instructions SMLAW<y>. The multiplication in
these instructions produces the upper 32 bits of the 48-bit product, and these 32 bits are then added
to the accumulate value.
• 16 ´ 16 + 64 ® 64 bit signed multiply instructions SMLAL<x><y>. These instructions are very
similar to the normal SMLAL instruction, apart from the fact that their operands are generated as
described above.
In all cases, <y> is either the letter B or T, depending on whether the bottom or top half of the second
operand register should be used. Where the first operand is a sign-extended 16-bit value, <x> specifies
whether the bottom or top half of the first operand register should be used in a similar fashion.
A10-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-446-320.jpg)
![Enhanced DSP Extension
Exceptions
Data abort
Operation
if ConditionPassed(cond) then
if (Rd is even-numbered) then
if (address[2:0] == 0b000) and (Rd is not R14) then
Rd = Memory[address,4]
R(d+1) = Memory[address+4,4]
else
UNPREDICTABLE
else
UNDEFINED
Notes
Operand restrictions
If <addressing_mode> performs base register writeback and the base register <Rn> is
one of the two destination registers of the instruction, the results are UNPREDICTABLE.
If <addressing_mode> specifies an index register <Rm> and <Rm> is one of the two
destination registers of the instruction, the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non doubleword-aligned addresses
If the load address is not doubleword-aligned, the results are UNPREDICTABLE.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, then:
• an address with bits[1:0] != 0b00 causes an alignment exception
• an address with bits[2:0] == 0b000 does not cause an alignment exception
• it is IMPLEMENTATION DEFINED whether an address with bits[2:0] == 0b100 causes
an alignment exception.
Time order The time order of the accesses to the two memory words is not architecturally defined. In
particular, an implementation is allowed to perform the two 32-bit memory accesses in
either order, or to combine them into a single 64-bit memory access.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-449-320.jpg)
![Enhanced DSP Extension
10.6.2 MCRR
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 4 3 0
cond 1 1 0 0 0 1 0 0 Rn Rd cp_num opcode CRm
The MCRR instruction passes the values of ARM registers <Rd> and <Rn> to the coprocessor whose
number is cp_num. If no coprocessors indicate that they can execute the instruction, an undefined
instruction exception is generated.
Syntax
MCRR{<cond>} <coproc>, <opcode>, <Rd>, <Rn>, <CRm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to
be placed in the cp_num field of the instruction. The standard generic coprocessor names
are p0, p1, …, p15.
<opcode> Is a coprocessor-specific opcode.
<Rd> Is the first ARM register whose value is transferred to the coprocessor. If R15 is specified
for <Rd>, the result is UNPREDICTABLE.
<Rn> Is the second ARM register whose value is transferred to the coprocessor. If R15 is specified
for <Rn>, the result is UNPREDICTABLE.
<CRm> Is the destination coprocessor register.
Architecture version
E variants of version 5 and above, excluding ARMv5TExP
Exceptions
Undefined instruction
Operation
if ConditionPassed(cond) then
send Rd value to Coprocessor[cp_num]
send Rn value to Coprocessor[cp_num]
A10-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-450-320.jpg)
![Enhanced DSP Extension
Usage
MCRR is used to initiate coprocessor operations that depend on values in two ARM registers. An example
for a floating-point coprocessor is an instruction to transfer a double-precision floating-point number held
in two ARM registers to a floating-point register.
Notes
Coprocessor fields
Only instruction bits[31:8] are defined by the ARM architecture. The remaining fields are
recommendations, for compatibility with ARM Development Systems.
Unimplemented coprocessor instructions
Hardware coprocessor support is optional, regardless of the architecture version. An
implementation may choose to implement a subset of the coprocessor instructions, or no
coprocessor instructions at all. Any coprocessor instructions that are not implemented
instead cause an undefined instruction trap.
Order of transfers
If a coprocessor uses these instructions, it will define how each of the values of <Rd> and
<Rn> is used. There is no architectural requirement for the two register transfers to occur
in any particular time order. It is IMPLEMENTATION DEFINED whether Rd is transferred
before Rn, after Rn, or at the same time as Rn.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-11](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-451-320.jpg)
![Enhanced DSP Extension
10.6.3 MRRC
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 4 3 0
cond 1 1 0 0 0 1 0 1 Rn Rd cp_num opcode CRm
The MRRC instruction causes the coprocessor whose number is cp_num to transfer values to two ARM
registers <Rd> and <Rn>. If no coprocessors indicate that they can execute the instruction, an undefined
instruction exception is generated.
Syntax
MRRC{<cond>} <coproc>, <opcode>, <Rd>, <Rn>, <CRm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to
be placed in the cp_num field of the instruction. The standard generic coprocessor names
are p0, p1, …, p15.
<opcode> Is a coprocessor-specific opcode.
<Rd> Is the first destination ARM register. If R15 is specified for <Rd>, the result is
UNPREDICTABLE.
<Rn> Is the second destination ARM register. If R15 is specified for <Rn>, the result is
UNPREDICTABLE.
<CRm> Is the coprocessor register which supplies the data to be transferred.
Architecture version
E variants of version 5 and above, excluding ARMv5TExP
Exceptions
Undefined instruction
Operation
if ConditionPassed(cond) then
Rd = first value from Coprocessor[cp_num]
Rn = second value from Coprocessor[cp_num]
A10-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-452-320.jpg)
![Enhanced DSP Extension
Usage
MRRC is used to initiate coprocessor operations that write values to two ARM registers. An example for a
floating-point coprocessor is an instruction to transfer a double-precision floating-point number held in a
floating-point register to two ARM registers.
Notes
Operand restrictions
Specifying the same register for <Rd> and <Rn> has UNPREDICTABLE results.
Coprocessor fields
Only instruction bits[31:8] are defined by the ARM architecture. The remaining fields are
recommendations, for compatibility with ARM Development Systems.
Unimplemented coprocessor instructions
Hardware coprocessor support is optional, regardless of the architecture version. An
implementation may choose to implement a subset of the coprocessor instructions, or no
coprocessor instructions at all. Any coprocessor instructions that are not implemented
instead cause an undefined instruction trap.
Order of transfers
If a coprocessor uses these instructions, it will define which value is written to <Rd> and
which value to <Rn>. There is no architectural requirement for the two register transfers to
occur in any particular time order. It is IMPLEMENTATION DEFINED whether Rd is transferred
before Rn, after Rn, or at the same time as Rn.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-13](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-453-320.jpg)
![Enhanced DSP Extension
Notes
Condition Unlike most other ARM instructions, this instruction cannot be executed conditionally.
Writeback Clearing bit[24] (the P bit) or setting bit[21] (the W bit) has UNPREDICTABLE results.
Data aborts This instruction never generates a data abort, nor does it signal any sort of memory system
exception detected for the address generated by <addressing_mode> in any other way.
All such memory system exceptions must be ignored by the memory system. Typically, the
memory system does this by treating the PLD instruction as a NOP if any exceptional case
is encountered while handling it.
Alignment There are no alignment restrictions on the address generated by <addressing_mode>.
If an implementation contains a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), it will not generate an alignment exception for any PLD instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-455-320.jpg)
![Enhanced DSP Extension
10.6.9 SMLA<x><y>
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 0 0 0 Rd Rn Rs 1 y x 0 Rm
The SMLA<x><y> instructions (SMLABB, SMLABT, SMLATB and SMLATT) perform a signed
multiply-accumulate operation. The multiply acts on two signed 16-bit quantities, taken from either the
bottom or the top half of their respective source registers. The other halves of these source registers are
ignored. The 32-bit product is added to a 32-bit accumulate value and the result is written to the destination
register.
If overflow occurs during the addition of the accumulate value, the instruction sets the Q flag in the CPSR.
It is not possible for overflow to occur during the multiplication.
Syntax
SMLA<x><y>{<cond>} <Rd>, <Rm>, <Rs>, <Rn>
where:
<x> Specifies which half of the source register <Rm> is used as the first multiply operand. If <x>
is B, then x == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rm> is
used. If <x> is T, then x == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rm> is used.
<y> Specifies which half of the source register <Rs> is used as the second multiply operand. If
<y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs>
is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rs> is used.
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register of the instruction.
<Rm> Specifies the source register whose bottom or top half (selected by <x>) is the first multiply
operand.
<Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second
multiply operand.
<Rn> Specifies the register which contains the accumulate value.
Architecture version
E variants of version 5 and above
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-23](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-463-320.jpg)
![Enhanced DSP Extension
Exceptions
None
Operation
if ConditionPassed(cond) then
if (x == 0) then
operand1 = SignExtend(Rm[15:0])
else /* x == 1 */
operand1 = SignExtend(Rm[31:16])
if (y == 0) then
operand2 = SignExtend(Rs[15:0])
else /* y == 1 */
operand2 = SignExtend(Rs[31:16])
Rd = (operand1 * operand2) + Rn
if OverflowFrom((operand1 * operand2) + Rn) then
Q Flag = 1
Usage
In addition to its straightforward uses for integer multiply-accumulates, these instructions sometimes
provide a faster alternative to Q15 ´ Q15 + Q31 ® Q31 multiply-accumulates synthesized from
SMUL<x><y> and QDADD instructions. The main circumstances under which this is possible are:
• if it is known that saturation and/or overflow cannot occur during the calculation
• if saturation and/or overflow can occur during the calculation but the Q flag is going to be used to
detect this and take remedial action if it does occur.
For example, the following code produces the dot product of the four Q15 numbers in R0 and R1 by the four
Q15 numbers in R2 and R3:
SMULBB R4, R0, R2
QADD R4, R4, R4
SMULTT R5, R0, R2
QDADD R4, R4, R5
SMULBB R5, R1, R3
QDADD R4, R4, R5
SMULTT R5, R1, R3
QDADD R4, R4, R5
In the absence of saturation, the following code provides a faster alternative:
A10-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-464-320.jpg)
![Enhanced DSP Extension
10.6.10 SMLAL<x><y>
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 1 0 0 RdHi RdLo Rs 1 y x 0 Rm
The SMLAL<x><y> instructions (SMLALBB, SMLALBT, SMLALTB and SMLALTT) perform a signed
multiply-accumulate operation. The multiply acts on two signed 16-bit quantities, taken from either the
bottom or the top half of their respective source registers. The other halves of these source registers are
ignored. The 32-bit product is sign-extended and added to the 64-bit accumulate value held in <RdHi> and
<RdLo>, and the result is written back to <RdHi> and <RdLo>.
Overflow is possible during this instruction, but only as a result of the 64-bit addition. This overflow is not
detected if it occurs. Instead, the result wraps around modulo 264.
Syntax
SMLAL<x><y>{<cond>} <RdLo>, <RdHi>, <Rm>, <Rs>
where:
<x> Specifies which half of the source register <Rm> is used as the first multiply operand. If <x>
is B, then x == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rm> is
used. If <x> is T, then x == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rm> is used.
<y> Specifies which half of the source register <Rs> is used as the second multiply operand. If
<y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs>
is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rs> is used.
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<RdLo> Supplies the lower 32 bits of the 64-bit accumulate value to be added to the product, and is
the destination register for the lower 32 bits of the 64-bit result.
<RdHi> Supplies the upper 32 bits of the 64-bit accumulate value to be added to the product, and is
the destination register for the upper 32 bits of the 64-bit result.
<Rm> Specifies the source register whose bottom or top half (selected by <x>) is the first multiply
operand.
<Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second
multiply operand.
Architecture version
E variants of version 5 and above
A10-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-466-320.jpg)
![Enhanced DSP Extension
Exceptions
None
Operation
if ConditionPassed(cond) then
if (x == 0) then
operand1 = SignExtend(Rm[15:0])
else /* x == 1 */
operand1 = SignExtend(Rm[31:16])
if (y == 0) then
operand2 = SignExtend(Rs[15:0])
else /* y == 1 */
operand2 = SignExtend(Rs[31:16])
RdLo = RdLo + (operand1 * operand2)
RdHi = RdHi + (if (operand1*operand2) < 0 then 0xFFFFFFFF else 0)
+ CarryFrom(RdLo + (operand1 * operand2))
Usage
These instructions allow a long sequence of multiply-accumulates of signed 16-bit integers or Q15 numbers
to be performed, with sufficient guard bits to ensure that the result cannot overflow the 64-bit destination
in practice. It would take more than 233 consecutive multiply-accumulates to cause such overflow.
If the overall calculation does not overflow a signed 32-bit number, then <RdLo> will hold the result of the
calculation.
A simple test to determine whether such a calculation has overflowed <RdLo> is to execute the instruction:
CMP <RdHi>, <RdLo>, ASR #31
at the end of the calculation. If the Z flag is set, <RdLo> holds an accurate final result. If the Z flag is clear,
the final result has overflowed a signed 32-bit destination.
Notes
Use of R15 Specifying R15 for register <RdLo>, <RdHi>, <Rm>, or <Rs> has
UNPREDICTABLE results.
Operand restriction If <RdLo> and <RdHi> are the same register, the results are UNPREDICTABLE.
Condition flags The SMLAL<x><y> instructions do not affect the N, Z, C, V, or Q flags.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-27](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-467-320.jpg)
![Enhanced DSP Extension
10.6.11 SMLAW<y>
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 0 1 0 Rd Rn Rs 1 y 0 0 Rm
The SMLAW<y> instructions (SMLAWB and SMLAWT) perform a signed multiply-accumulate operation. The
multiply acts on a signed 32-bit quantity and a signed 16-bit quantity, with the latter being taken from either
the bottom or the top half of its source register. The other half of the second source register is ignored. The
top 32 bits of the 48-bit product are added to a 32-bit accumulate value and the result is written to the
destination register. The bottom 16 bits of the 48-bit product are ignored. If overflow occurs during the
addition of the accumulate value, the instruction sets the Q flag in the CPSR. No overflow can occur during
the multiplication, because of the use of the top 32 bits of the 48-bit product.
Syntax
SMLAW<y>{<cond>} <Rd>, <Rm>, <Rs>, <Rn>
where:
<y> Specifies which half of the source register <Rs> is used as the second multiply operand. If
<y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs>
is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rs> is used.
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register of the instruction.
<Rm> Specifies the source register which contains the 32-bit first multiply operand.
<Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second
multiply operand.
<Rn> Specifies the register which contains the accumulate value.
Architecture version
E variants of version 5 and above
Exceptions
None
A10-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-468-320.jpg)
![Enhanced DSP Extension
Operation
if ConditionPassed(cond) then
if (y == 0) then
operand2 = SignExtend(Rs[15:0])
else /* y == 1 */
operand2 = SignExtend(Rs[31:16])
Rd = (Rm * operand2)[47:16] + Rn /* Signed multiplication */
if OverflowFrom((Rm * operand2)[47:16] + Rn) then
Q Flag = 1
Usage
In addition to its straightforward uses for integer multiply-accumulates, these instructions sometimes
provide a faster alternative to Q31 ´ Q15 + Q31 ® Q31 multiply-accumulates synthesized from
SMULW<y> and QDADD instructions. The circumstances under which this is possible and the benefits it
provides are very similar to those for the SMLA<x><y> instructions. See Usage on page A10-24 for more
details.
Notes
Use of R15 Specifying R15 for register <Rd>, <Rm>, <Rs>, or <Rn> has UNPREDICTABLE
results.
Condition flags The SMLAW<y> instructions do not affect the N, Z, C, or V flags.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-29](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-469-320.jpg)
![Enhanced DSP Extension
10.6.12 SMUL<x><y>
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 1 1 0 Rd SBZ Rs 1 y x 0 Rm
The SMUL<x><y> instructions (SMULBB, SMULBT, SMULTB, and SMULTT) perform a signed multiply
operation. The multiply acts on two signed 16-bit quantities, taken from either the bottom or the top half of
their respective source registers. The other halves of these source registers are ignored. No overflow is
possible during this instruction.
Syntax
SMUL<x><y>{<cond>} <Rd>, <Rm>, <Rs>
where:
<x> Specifies which half of the source register <Rm> is used as the first multiply operand. If <x>
is B, then x == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rm> is
used. If <x> is T, then x == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rm> is used.
<y> Specifies which half of the source register <Rs> is used as the second multiply operand. If
<y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs>
is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rs> is used.
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register of the instruction.
<Rm> Specifies the source register whose bottom or top half (selected by <x>) is the first multiply
operand.
<Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second
multiply operand.
Architecture version
E variants of version 5 and above
Exceptions
None
A10-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-470-320.jpg)
![Enhanced DSP Extension
Operation
if ConditionPassed(cond) then
if (x == 0) then
operand1 = SignExtend(Rm[15:0])
else /* x == 1 */
operand1 = SignExtend(Rm[31:16])
if (y == 0) then
operand2 = SignExtend(Rs[15:0])
else /* y == 1 */
operand2 = SignExtend(Rs[31:16])
Rd = operand1 * operand2
Usage
In addition to its straightforward uses for integer multiplies, this instruction can be used in combination with
the QADD, QDADD, and QDSUB instructions to perform multiplies, multiply-accumulates, and
multiply-subtracts on Q15 numbers. See the Usage sections on page A10-17, page A10-19, and
page A10-21 for examples.
Notes
Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rs> has UNPREDICTABLE results.
Condition flags The SMUL<x><y> instructions do not affect the N, Z, C, V, or Q flags.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-31](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-471-320.jpg)
![Enhanced DSP Extension
10.6.13 SMULW<y>
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0
cond 0 0 0 1 0 0 1 0 Rd SBZ Rs 1 y 1 0 Rm
The SMULW<y> instructions (SMULWB and SMULWT) perform a signed multiply operation. The multiply
acts on a signed 32-bit quantity and a signed 16-bit quantity, with the latter being taken from either the
bottom or the top half of its source register. The other half of the second source register is ignored. The top
32 bits of the 48-bit product are written to the destination register. The bottom 16 bits of the 48-bit product
are ignored.
No overflow is possible during this instruction.
Syntax
SMULW<y>{<cond>} <Rd>, <Rm>, <Rs>
where:
<y> Specifies which half of the source register <Rs> is used as the second multiply operand. If
<y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs>
is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of
<Rs> is used.
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination register of the instruction.
<Rm> Specifies the source register which contains the 32-bit first operand.
<Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second
operand.
Architecture version
E variants of version 5 and above
Exceptions
None
A10-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-472-320.jpg)
![Enhanced DSP Extension
Operation
if ConditionPassed(cond) then
if (y == 0) then
operand2 = SignExtend(Rs[15:0])
else /* y == 1 */
operand2 = SignExtend(Rs[31:16])
Rd = (Rm * operand2)[47:16] /* Signed multiplication */
Usage
In addition to its straightforward uses for integer multiplies, this instruction can be used in combination with
the QADD, QDADD, and QDSUB instructions to perform multiplies, multiply-accumulates and
multiply-subtracts between Q31 and Q15 numbers. See the Usage sections on page A10-17, page A10-19,
and page A10-21 for examples.
Notes
Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rs> has UNPREDICTABLE results.
Condition flags The SMULW<y> instructions do not affect the N, Z, C, V, or Q flags.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-33](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-473-320.jpg)
![Enhanced DSP Extension
Operation
if ConditionPassed(cond) then
if (Rd is even-numbered) then
if (address[2:0] == 0b000) and (Rd is not R14) then
Memory[address,4] = Rd
Memory[address+4,4] = R(d+1)
else
UNPREDICTABLE
else
UNDEFINED
Notes
Operand restrictions
If <addressing_mode> performs base register writeback and the base register <Rn> is
one of the two source registers of the instruction, the results are UNPREDICTABLE.
Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted
instructions on page A2-17.
Non doubleword-aligned addresses
If the store address is not doubleword-aligned, the results are UNPREDICTABLE.
Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System
Control Coprocessor), and alignment checking is enabled, then:
• an address with bits[1:0] != 0b00 causes an alignment exception
• an address with bits[2:0] == 0b000 does not cause an alignment exception
• it is IMPLEMENTATION DEFINED whether an address with bits[2:0] == 0b100 causes
an alignment exception.
Time order The time order of the accesses to the two memory words is not architecturally defined. In
particular, an implementation is allowed to perform the two 32-bit memory accesses in
either order, or to combine them into a single 64-bit memory access.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-35](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-475-320.jpg)
![Introduction to Memory and System Architectures
1.2 System-level issues
This section lists a number of general and operating-system issues that the system designer needs to address
when using an ARM processor.
1.2.1 Memory systems, write buffers and caches
ARM processors and software are designed to be connected to a byte-addressed memory. Word and
halfword accesses to the memory ignore the alignment of the address and access the naturally-aligned value
that is addressed (so a memory access ignores address bits 0 and 1 for word access, and ignores bit 0 for
halfword accesses). The endianness of the ARM processor should normally match that of the memory
system, or be configured to match it before any non-word accesses occur (when the endianness is
configurable and CP15 is implemented, bit[7] of CP15 register 1 controls the endianness).
Memory that is used to hold programs and data should be marked as follows:
• Main (RAM) memory is normally set as cachable and bufferable.
• ROM memory is normally set as cachable, and should be marked as read only, so the bufferable
attribute is not used and should be 1.
Write buffers
Some ARM implementations incorporate a merging write buffer that subsumes multiple writes to the same
location into a single write to main memory. Furthermore, some write buffers re-order writes, so that writes
are issued to memory in a different order to the order in which they are issued by the processor. Therefore,
I/O locations should not normally be marked as bufferable, to ensure all writes are issued to the I/O device
in the correct order.
For writes to bufferable areas of memory, memory aborts can only be signaled to the processor as a result
of conditions that are detectable at the time the data is placed in the write buffer. Conditions that can only
be detected when the data is later written to main memory (such as a parity error from main memory) must
be handled by other methods (typically by raising an interrupt).
Caches
Frame buffers can be cachable, but frame buffers on writeback cache implementations must be copied back
to memory after the frame buffer has been updated. Frame buffers can be bufferable, but again the write
buffer must be written back to memory after the frame buffer has been updated.
ARM processors do not normally support cache coherence between the ARM and other system bus masters.
Bus snooping is not supported. If memory data is to be shared between multiple bus masters without taking
special software measures to ensure coherency, then the data must be mapped as:
• uncachable to ensure that all reads access main memory
• unbufferable to ensure that all write access main memory.
B1-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-482-320.jpg)
![The System Control Coprocessor
2.2 Registers
The System Control coprocessor can contain up to 16 primary registers, each of which is 32 bits long. For
some of these, additional bits in the register access instructions are used to identify a specific version of the
register and/or specific types of access to the register, so the number of physical 32-bit registers in CP15
can be more than 16. However, the 4-bit primary register number is used to identify registers in descriptions
of the System Control coprocessor, because it is the primary factor determining the function of the register.
CP15 registers can be read-only, write-only or read/write. The detailed descriptions of the registers specify:
• what types of access are allowed
• what functionality is invoked by each type of access
• whether a primary register identifies more than one physical register, and if so, how they are
distinguished
• any other details that are relevant to the use of the register.
2.2.1 Register access instructions
The only defined System Control coprocessor instructions are:
• MCR instructions to write an ARM register to a CP15 register
• MRC instructions to read the value of a CP15 register into an ARM register.
All CP15 CDP, LDC and STC instructions are UNDEFINED.
31 28 27 26 25 24 23 21 20 19 16 15 12 11 8 7 5 4 3 0
cond 1 1 1 0 SBZ L CRn Rd 1 1 1 1 opcode2 1 CRm
The MCR and MRC instructions to access the CP15 registers use the generic syntax for those instructions:
MCR{<cond>} p15, 0, <Rd>, <CRn>, <CRm>{, <opcode2>}
MRC{<cond>} p15, 0, <Rd>, <CRn>, <CRm>{, <opcode2>}
where:
<cond> This is the condition under which the instruction is executed. The conditions are
defined in The condition field on page A3-5. If <cond> is omitted, the AL (always)
condition is used.
Bits[23:21] These bits of the instruction, which are the <opcode1> field in generic MRC and
MCR instructions, are always 0b000 in valid CP15 instructions. If they are not
0b000, the instruction is UNPREDICTABLE.
<Rd> This is the ARM register involved in the transfer (the source register for MCR and
the destination register for MRC). This register must not be R15, even though MCR
instructions normally allow it to be R15. If R15 is specified for <Rd> in a CP15 MRC
or MCR instruction, the instruction is UNPREDICTABLE.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-3](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-487-320.jpg)
![The System Control Coprocessor
2.3 Register 0: ID codes
CP15 register 0 contains one or more identification codes for the ARM and system implementation. When
this register is read, the opcode2 field of the MRC instruction selects which identification code is wanted, as
shown in Table 2-2, and the CRm field must be specified as c0 (if it is not, the instruction is
UNPREDICTABLE). Writing to CP15 register 0 is UNPREDICTABLE.
Table 2-2 System Control coprocessor ID registers
opcode2 Register Details in
0b000 Main ID register Main ID register
0b001 Cache Type register Cache Type register on page B2-9
other Reserved (see main text) -
It is recommended that all the ID registers in Table 2-2 are implemented, but only the main ID register
(<opcode2> == 0) is mandatory. Whether or not other ID registers are implemented is IMPLEMENTATION
DEFINED.
If an <opcode2> value corresponding to an unimplemented or reserved ID register is encountered, the
System Control coprocessor returns the value of the main ID register.
ID registers other than the main ID register are defined so that when implemented, their value cannot be
equal to that of the main ID register. Software can therefore determine whether they exist by reading both
the main ID register and the desired register and comparing their values. If the two values are not equal, the
desired register exists.
2.3.1 Main ID register
When CP15 register 0 is read with <opcode2> == 0, an identification code is returned from which, among
other things, the ARM architecture version number can be determined, as well as whether or not the Thumb
instruction set has been implemented.
Note
Only some of the fields in CP15 register 0 are architecturally defined. The rest are IMPLEMENTATION
DEFINED and provide more detailed information about the exact processor variant. Consult individual
datasheets for the precise identification codes used for each processor.
For historical reasons, there are three distinct ways in which the CP15 register 0 ID code might need to be
interpreted. To determine which to use, look at bits[15:12] of the ID code:
• if they are 0x0, this indicates a pre-ARM7 processor
• if they are 0x7, this indicates that the processor is in the ARM7 family
• otherwise, a more recent processor family than ARM7 is involved.
B2-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-490-320.jpg)
![The System Control Coprocessor
Post-ARM7 processors
If bits[15:12] of the ID code are neither 0x0 nor 0x7, the ID code is interpreted as follows:
31 24 23 20 19 16 15 4 3 0
Implementor Variant Architecture Primary part number Revision
Bits[3:0] Contain the IMPLEMENTATION DEFINED revision number for the processor.
Bits[15:4] Contain an IMPLEMENTATION DEFINED representation of the primary part number for the
processor. The top four bits of this number are not allowed to be 0x0 or 0x7.
Bits[19:16] Contain an architecture code. The following architecture codes are defined (all other values
of the architecture code are reserved by ARM Ltd):
0x1 Architecture 4
0x2 Architecture 4T
0x3 Architecture 5
0x4 Architecture 5T
0x5 Architecture 5TE.
Bits[23:20] Contain an IMPLEMENTATION DEFINED variant number. This is typically used to distinguish
two variants of the same primary part, for example, two different cache size variants.
Bits[31:24] Contain an implementor code. The following codes are defined (all other values of the
architecture code are reserved by ARM Ltd):
0x41 = A (ARM Ltd)
0x44 = D (Digital Equipment Corporation).
0x69 = i (Intel Corporation).
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-491-320.jpg)
![The System Control Coprocessor
ARM7 family processors
If bits[15:12] of the ID code are 0x7, the ID code is interpreted as follows:
31 24 23 22 16 15 4 3 0
Implementor A Variant Primary part number Revision
Bits[3:0] Contain the IMPLEMENTATION DEFINED revision number for the processor.
Bits[15:4] Contain an IMPLEMENTATION DEFINED representation of the primary part number for the
processor. The top four bits of this number are 0x7.
Bits[22:16] Contain an IMPLEMENTATION DEFINED variant number.
Bit[23] Indicates which of the two possible architectures for an ARM7-based processor is involved:
0 Architecture 3
1 Architecture 4T.
Bits[31:24] Contain an implementor code. See Post-ARM7 processors for these codes.
Pre-ARM7 processors
Four processors prior to ARM7 use ID codes in which bits[15:12] are 0x0, and no further processors will
be allocated such ID codes. They are interpreted as a 28-bit processor ID and a 4-bit revision number:
31 4 3 0
Processor ID Revision
The processor ID values are as follows:
0x4156030 ARM3 (Architecture 2)
0x4156060 ARM600 (Architecture 3)
0x4156061 ARM610 (Architecture 3)
0x4156062 ARM620 (Architecture 3).
B2-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-492-320.jpg)
![The System Control Coprocessor
The Read data block method of cleaning write-back caches encoded by ctype == 0b0001 consists of loading
a sequential block of data with size equal to that of the cache, and which is known not to be in the cache
already. It is only suitable for use when the cache organization guarantees that this causes the entire cache
to be reloaded. (For example, direct-mapped caches normally have this property, as do caches using some
types of round-robin replacement.)
Note
This method of cache cleaning must only be used if the Cache Type register has ctype == 0b0001, or if
implementation documentation states that it is a valid method for the implementation.
Register 7: Cache functions on page B5-15 gives details of the register 7 operations used for cleaning other
write-back caches.
For an explanation of cache lockdown and of the formats referred to in Table 2-3, see Register 9: Cache
lockdown on page B5-18.
2.3.3 Cache size fields
The Dsize and Isize fields in the Cache Type register have the same format, as follows:
11 9 8 6 5 3 2 1 0
0 0 0 size assoc M len
Bits[11:9] are reserved for future expansion.
The size of the cache is determined by the size field and M bit, as shown in Table 2-4.
Table 2-4 Cache sizes
size field Size if M == 0 Size if M == 1
0b000 0.5KB 0.75KB
0b001 1KB 1.5KB
0b010 2KB 3KB
0b011 4KB 6KB
0b100 8KB 12KB
0b101 16KB 24KB
0b110 32KB 48KB
0b111 64KB 96KB
B2-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-494-320.jpg)
![The System Control Coprocessor
2.4 Register 1: Control register
This register contains:
• Enable/disable bits for the caches, MMUs, and other memory system blocks that are primarily
controlled by other CP15 registers. This allows these memory system blocks to be programmed
correctly before they are enabled.
• Various configuration bits for memory system blocks and for the ARM processor itself.
Note
Extra bits of both varieties might be added in the future. Because of this, this register should normally be
updated using read/modify/write techniques, to ensure that currently unallocated bits are not needlessly
modified. Failure to observe this rule might result in code which has unexpected side-effects on future
processors. One exception that may be useful in some circumstances is that 0 can be written to this register
to restore it to its reset state.
31 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
UNP/SBZP L4RRV I Z F R S B L D P W C A M
Reading from CP15 register 1 reads the control bits. The <CRm> and <opcode2> fields are ignored when
reading CP15 register 1, and should be zero.
Writing to CP15 register 1 sets the control bits. The <CRm> and <opcode2> fields are not used when
writing CP15 register 1, and should be zero.
When a control bit in CP15 register 1 is not applicable to a particular implementation, it reads as the value
that most closely reflects that implementation, and ignores writes. (Specific examples of this general rule
are documented in the individual bit descriptions below.) Apart from bits that read as 1 according to this
rule, all bits in CP15 register 1 are set to 0 on reset.
M (bit[0]) This is the enable/disable bit for the MMU or Protection Unit:
0 = MMU or Protection Unit disabled
1 = MMU or Protection Unit enabled.
On systems without an MMU or Protection Unit, this bit reads as 0 and ignores writes.
A (bit[1]) For memory systems which optionally allow the alignment of data memory accesses to be
checked, this bit enables and disables alignment fault checking:
0 = Alignment fault checking disabled
1 = Alignment fault checking enabled.
For other memory systems, this bit ignores writes, and reads as 1 or 0 according to whether
the memory system does or does not check the alignment of data memory accesses.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-13](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-497-320.jpg)
![The System Control Coprocessor
C (bit[2]) If a unified cache is used, this is the enable/disable bit for the unified cache. If separate
caches are used, this is the enable/disable bit for the data cache. In either case:
0 = Cache disabled
1 = Cache enabled.
If the cache is not implemented, this bit reads as 0 and ignores writes. If the cache cannot
be disabled, this bit reads as 1 and ignores writes.
W (bit[3]) This is the enable/disable bit for the write buffer:
0 = Write buffer disabled
1 = Write buffer enabled.
If the write buffer is not implemented, this bit reads as 0 and ignores writes. If the write
buffer cannot be disabled, this bit reads as 1 and ignores writes.
P (bit[4]) On ARM processors which support 26-bit backwards-compatibility configurations, this bit
controls the PROG32 configuration signal:
0 = Exception handlers entered in 26-bit modes
1 = Exception handlers entered in 32-bit modes.
For details, see 32-bit and 26-bit configuration on page A8-10.
On ARM processors which do not support 26-bit backwards-compatibility configurations,
this bit reads as 1 and ignores writes.
D (bit[5]) On ARM processors which support 26-bit backwards-compatibility configurations, this bit
controls the DATA32 configuration signal:
0 = 26-bit address exception checking enabled
1 = 26-bit address exception checking disabled.
For details, see 32-bit and 26-bit configuration on page A8-10.
On ARM processors which do not support 26-bit backwards-compatibility configurations,
this bit reads as 1 and ignores writes.
L (bit[6]) In some ARMv3 and earlier implementations, the abort model of the processor could be
configured:
0 = Early Abort Model selected (now obsolete)
1 = Late Abort Model selected (same as Base Updated Abort Model).
For details, see Effects of data-aborted instructions on page A2-17.
On later processors, this bit reads as 1 and ignores writes.
B (bit[7]) On ARM processors which support both little-endian and big-endian memory systems, this
bit is used to configure the ARM processor to the endianness of the memory system:
0 = Configured for little-endian memory system
1 = Configured for big-endian memory system.
For details, see Endianness on page A2-23.
B2-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-498-320.jpg)
![The System Control Coprocessor
On ARM processors that support only little-endian memory systems, this bit reads as 0 and
ignores writes. On ARM processors that support only big-endian memory systems, this bit
reads as 1 and ignores writes.
S (bit[8]) System protection bit (in an MMU-based memory system). The effect of this bit is described
in Access permissions on page B3-16.
R (bit[9]) ROM protection bit (in an MMU-based memory system). The effect of this bit is described
in Access permissions on page B3-16.
F (bit[10]) The meaning of this bit is IMPLEMENTATION DEFINED.
Z (bit[11]) On ARM processors which support branch prediction, this is the enable/disable bit for
branch prediction:
0 = Branch prediction disabled
1 = Branch prediction enabled.
If branch prediction cannot be disabled, this bit reads as 1 and ignores writes.
On ARM processors that do not support branch prediction, this bit reads as 0 and ignores
writes.
I (bit[12]) If separate caches are used, this is the enable/disable bit for the instruction cache:
0 = Instruction cache disabled
1 = Instruction cache enabled.
If a unified cache is used or the instruction cache is not implemented, this bit reads as 0 and
ignores writes. If the instruction cache cannot be disabled, this bit reads as 1 and ignores
writes.
V (bit[13]) On ARM processors which support the alternative high vectors described in High vectors
on page A2-21, this bit is used to select the location of the exception vector:
0 = Normal exception vectors selected (address range 0x00000000 - 0x0000001C)
1 = High exception vectors selected (address range 0xFFFF0000 - 0xFFFF001C).
On ARM processors that do not support high vectors, this bit reads as 0 and ignores writes.
RR (bit[14]) If the cache allows an alternative replacement strategy to be used which has more easily
predictable worst-case performance, this bit selects it:
0 = Normal replacement strategy (for example, random replacement)
1 = Predictable strategy (for example, round-robin replacement).
If the cache does not allow a choice of replacement strategy this bit should ignore writes,
and read as 0 or 1 according to whether the replacement strategy has a reasonably easily
predictable worst-case performance.
L4 (bit[15]) For some ARM processors that support architecture version 5 or above, this bit controls a
backwards-compatibility feature with previous versions of the architecture.
0 = normal behaviour for the architecture
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-499-320.jpg)
![The System Control Coprocessor
1 = load instructions that load the PC and use bit[0] of the loaded value to determine whether
to enter ARM or Thumb state (that is, to set the new value of the CPSR T bit) instead ignore
bit[0] of the T bit and stay in the current execution state.
The instructions affected by this are:
• LDM (1) on page A4-30
• LDR on page A4-37
• POP on page A7-75
For ARM processors that support architecture versions prior to version 5, this bit should be
treated as UNP/SBZP. For ARM processors that support architecture versions 5 or above,
but which do not support this backwards-compatibility feature, this bit reads as zero and
ignores writes.
Bits[31:16] When these bits are read, they return an UNPREDICTABLE value. When they are written, the
value written must either be zero or a value previously read from the same bits.
B2-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-500-320.jpg)
![Memory Management Unit
3.2.1 Enabling and disabling the MMU
The MMU can be enabled and disabled by writing bit[0] of register 1 of the System Control coprocessor.
On reset, this bit is cleared to 0, so the MMU is disabled after a reset.
When the MMU is disabled, memory accesses are treated as follows:
• It is IMPLEMENTATION DEFINED whether caches and write buffers are allowed to be enabled while the
MMU is disabled.
— For caches and write buffers which are not allowed to be enabled while the MMU is disabled,
the values of the C and B bits for a memory access are irrelevant.
— For caches and write buffers which are allowed to be enabled while the MMU is disabled, a
data access is treated as uncachable and unbufferable (C == 0, B == 0). An instruction fetch is
treated as uncachable (C == 0) in a system with a unified TLB, and as cachable (C == 1) in a
system with a separate instruction TLB.
• No memory access permission checks are performed, and no aborts are generated by the MMU.
• The physical address for every access is equal to its virtual address (this is known as a flat address
mapping).
Before the MMU is enabled, suitable translation tables must be set up in memory and all relevant CP15
registers must be programmed.
Note
Enabling or disabling the MMU effectively changes the virtual-to-physical address mapping (unless the
translation tables are set up to implement a flat address mapping). It is therefore likely that any caches, for
example, that are enabled at the time will need to be flushed (see Address mapping changes on page B5-10).
In addition, if the physical address of the code that enables or disables the MMU differs from its virtual
address, instruction prefetching can cause complications (see Prefetching and self-modifying code on
page A2-27). It is therefore strongly recommended that code which enables or disables the MMU has
identical virtual and physical addresses.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-507-320.jpg)
![Memory Management Unit
3.3 Translation process
The MMU supports memory accesses based on sections or pages:
Sections Are comprised of 1MB blocks of memory.
Three different page sizes are supported:
Tiny pages Consist of 1KB blocks of memory.
Small pages Consist of 4KB blocks of memory.
Large pages Consist of 64KB blocks of memory.
Sections and large pages are supported to allow mapping of a large region of memory while using only a
single entry in the TLB. Additional access control mechanisms are extended within small pages to 1KB
subpages, and within large pages to 16KB subpages. There are no similar subpages for tiny pages, so access
controls for tiny pages apply to the entire page.
The translation table held in main memory has two levels:
First-level table Holds both section translations and pointers to second-level tables.
Second-level tables Hold both large and small page translations. One type of second-level table can also
hold tiny page translations.
The MMU translates virtual addresses generated by the CPU into physical addresses to access external
memory, and also derives and checks the access permission. There are four routes by which the address
translation (and therefore permission check) takes place. The route taken depends on whether the address
in question has been marked as a section-mapped access or a page-mapped access. A page-mapped access
can be a large, small, or tiny page access.
However, the translation process always starts out in the same way, as described below, with a first-level
fetch. A section-mapped access only requires a first level fetch, but a page-mapped access also requires a
second-level fetch.
3.3.1 Translation table base
The translation process is initiated when the on-chip TLB does not contain an entry for the requested virtual
address. The Translation Table Base Register (CP15 register 2) holds the physical address of the base of the
first-level table. Only bits[31:14] of the Translation Table Base Register are significant, and bits[13:0]
should be zero. Therefore, the first-level page table must reside on a 16KB boundary.
3.3.2 First-level fetch
Bits[31:14] of the Translation Table Base register are concatenated with bits[31:20] of the virtual address
and two zero bits to produce a 32-bit physical address as illustrated in Figure 3-2. This address selects a
four-byte translation table entry which is a first-level descriptor for a section or a pointer to a second-level
page table.
B3-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-508-320.jpg)
![Memory Management Unit
31 14 13
Translation base SBZ
31 20 19
Table index
31 14 13 2 1
Translation base Table index
0
0
0 0
0
Figure 3-2 Accessing the translation table first-level descriptors
3.3.3 First-level descriptors
Each entry in the first-level table is a descriptor of how its associated 1MB virtual address range is mapped.
There are the following possibilities, selected by bits[1:0] of the descriptor as shown in Table 3-1:
• If bits[1:0] == 0b00, the associated virtual addresses are unmapped, and attempts to access them
generate a translation fault (see Aborts on page B3-18). Software can use bits[31:2] for its own
purposes in such a descriptor, as they are ignored by the hardware. Where appropriate, it is suggested
that bits[31:2] continue to hold valid access permissions for the descriptor.
• If bits[1:0] == 0b10, the entry is a section descriptor for its associated virtual addresses. See Section
descriptor and translating section references on page B3-8 for details of how it is interpreted.
• If bit[0] == 1, the entry gives the physical address of a coarse second-level table (if bit[1] == 0) or of
a fine second-level table (if bit[1] == 1). Each type of table specifies how the associated 1MB virtual
address range is mapped. Coarse tables are smaller, at 1KB per table (compared with 4KB per table
for fine tables). However, coarse tables can only map large pages and small pages, while fine tables
can map large, small and tiny pages. The two types of table are described in Coarse page table
descriptor on page B3-9 and Fine page table descriptor on page B3-10.
Table 3-1 First-level descriptor format
31 20 19 12 11 10 9 8 5 4 3 2 1 0
Fault IGN 0 0
Coarse
page table
Coarse page table base address
S
B
Z
Domain IMP 0 1
Section Section base address SBZ AP
S
B
Z
Domain
I
M
P
C B 1 0
Fine page
table
Fine page table base address SBZ Domain IMP 1 1
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-509-320.jpg)
![Memory Management Unit
3.3.4 Section descriptor and translating section references
If the first-level descriptor is a section descriptor, the fields have the following meanings:
Bits[1:0] Identify the type of descriptor (0b10 marks a section descriptor).
Bits[3:2] The cachable and bufferable bits (see Cachability and bufferability on page B5-8).
Bit[4] The meaning of this bit is IMPLEMENTATION DEFINED.
Bits[8:5] The domain field specifies one of the sixteen possible domains for the section controlled by
this descriptor.
Bit[9] This bit is not currently used, and should be zero.
Bits[11:10] Access permissions. These bits control the access to the section. See Table 3-3 on
page B3-16 for the interpretation of these bits.
Bits[19:12] These bits are not currently used, and should be zero.
Bits[31:20] The Section Base Address forms the top 12 bits of the physical address.
Figure 3-3 illustrates the complete section translation sequence.
Note
The access permissions in the first-level descriptor must be checked before the physical address is
generated. The sequence for checking access permissions is given in Access permissions on page B3-16.
Translation base SBZ
31 20 19
Table index
Virtual
address
31 14 13 2 1
0
0
0 0
0
Section index
31 20 19 12 11 10 9 8 5 4 3 2 1 0
Section base address SBZ AP
SBZ
Domain
I
MP
C B 1 0
31 20 19 0
Section base address Section index
Figure 3-3 Section translation
31 14 13
Translation base Table index
Translation
table base
Physical
address
Address of
first-level descriptor
First-level fetch
First-level descriptor
B3-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-510-320.jpg)
![Memory Management Unit
3.3.5 Coarse page table descriptor
If the first-level descriptor is a coarse page table descriptor, the fields have the following meanings:
Bits[1:0] Identify the type of descriptor (0b01 marks a coarse page table descriptor).
Bits[4:2] The meaning of these bits is IMPLEMENTATION DEFINED.
Bits[8:5] The domain field specifies one of the sixteen possible domains for all the pages controlled
by this descriptor.
Bit[9] This bit is not currently used, and should be zero.
Bits[31:10] The Page Table Base Address is a pointer to a coarse second-level page table, giving the
base address for a second-level fetch to be performed. Coarse second-level page tables must
be aligned on a 1KB boundary.
If a coarse page table descriptor is returned from the first-level fetch, a second-level fetch is initiated to
retrieve a second-level descriptor, as shown in Figure 3-4.
31 14 13 0
Translation base SBZ
Virtual
address
31 20 19 12 11 0
First-level
table index
Second-level
table index
31 14 13 2 1 0
First-level
table index
Translation base 0 0
First-level fetch
First-level descriptor
10
31 9 8 5 4 2 1 0
S
Page table base address Domain IMP 0 1
BZ
31 9 2 1 0
10
Page table base address Second-level 0 0
table index
Address of
second-level descriptor
Figure 3-4 Accessing coarse page table second-level descriptors
Translation
table base
Address of
first-level descriptor
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-511-320.jpg)
![Memory Management Unit
3.3.6 Fine page table descriptor
If the first-level descriptor is a fine page table descriptor, the fields have the following meanings:
Bits[1:0] Identify the type of descriptor (0b11 marks a fine page table descriptor).
Bits[4:2] The meaning of these bits is IMPLEMENTATION DEFINED.
Bits[8:5] The domain field specifies one of the sixteen possible domains for all the pages controlled
by this descriptor.
Bit[11:9] These bits are not currently used, and should be zero.
Bits[31:12] The Page Table Base Address is a pointer to a fine second-level page table, giving the base
address for a second-level fetch to be performed. Fine second-level page tables must be
aligned on a 4KB boundary.
If a fine page table descriptor is returned from the first-level fetch, a second-level fetch is initiated to retrieve
a second-level descriptor, as shown in Figure 3-5.
31 14 13 0
Translation base SBZ
Virtual
address
31 20 19 10 9 0
First-level
table index
31 14 13 2 1 0
First-level
table index
Translation base 0 0
First-level fetch
First-level descriptor
Second-level
table index
31 12 11 9
8 5 4 2 1 0
Page table base address SBZ
Domain IMP 1 1
31 12 11 2 1 0
Page table base address Second-level 0 0
table index
Address of
second-level descriptor
Figure 3-5 Accessing fine page table second-level descriptors
Translation
table base
Address of
first-level descriptor
B3-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-512-320.jpg)
![Memory Management Unit
3.3.7 Second-level descriptor
Each entry in a coarse second-level table corresponds to how a 4KB virtual address range is mapped, and
each entry in a fine second-level table corresponds to how a 1KB virtual address range is mapped. The
entries are page descriptors, and can describe a page which is larger than 4KB or 1KB respectively. When
this happens, the descriptor must be repeated enough times in the second-level table to ensure that the same
descriptor is always used for the page, regardless of which virtual address is being accessed within the page.
There are four possibilities for a second-level descriptor, selected by bits[1:0] of the descriptor as shown in
Table 3-2:
• If bits[1:0] == 0b00, the associated virtual addresses are unmapped, and attempts to access them
generate a translation fault (see Aborts on page B3-18). Software can use bits[31:2] for its own
purposes in such a descriptor, as they are ignored by the hardware. Where appropriate, it is suggested
that bits[31:2] continue to hold valid access permissions for the descriptor.
• If bits[1:0] == 0b01, the entry is a large page descriptor, describing 64KB of virtual addresses. See
Translating large page references on page B3-13 for details.
A large page descriptor must be repeated 64 times in a fine second-level table or 16 times in a coarse
second-level table to ensure that all of its virtual addresses are described.
• If bits[1:0] == 0b10, the entry is a small page descriptor, describing 4KB of virtual addresses. See
Translating small page references on page B3-14 for details.
A small page descriptor must be repeated four times in a fine second-level table to ensure that all of
its virtual addresses are described. In a coarse second-level table, only one instance of each small
page descriptor is needed.
• If bits[1:0] == 0b11, the entry is a tiny page descriptor, describing 1KB of virtual addresses. See
Translating tiny page references on page B3-15 for details.
Only one instance of each tiny page descriptor is needed in a fine second-level table. Tiny page
descriptors must not appear in coarse second-level tables. If they do, the results are UNPREDICTABLE.
Table 3-2 Second-level descriptor format
31 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0
Fault IGN 0 0
Large page Large table base address SBZ AP3 AP2 AP1 AP0 C B 0 1
Small page Small page base address AP3 AP2 AP1 AP0 C B 1 0
Tiny page Tiny page base address SBZ AP C B 1 1
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-11](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-513-320.jpg)
![Memory Management Unit
Large page descriptor fields
The fields in a large page descriptor have the following meanings:
Bits[1:0] Identify the type of descriptor.
Bits[3:2] Are the cachable and bufferable bits (see Cachability and bufferability on page B5-8).
Bits[11:4] Are the access permission bits. These bits control access to the page. See Table 3-3 on
page B3-16 for the interpretation of these bits. Large pages are split into four subpages:
AP0 Encodes the access permissions for the first subpages.
AP1 Encodes the access permissions for the second subpages.
AP2 Encodes the access permissions for the third subpages.
AP3 Encodes the access permissions for the fourth (last) subpages.
Bits[15:12] Are not currently used, and must be zero.
Bits[31:16] Are used to form the corresponding bits of the physical address.
Small page descriptor fields
The fields in a small page descriptor have the following meanings:
Bits[1:0] Identify the type of descriptor.
Bits[3:2] Are the cachable and bufferable bits (see Cachability and bufferability on page B5-8).
Bits[11:4] Are the access permission bits. These bits control access to the page. See Table 3-3 on
page B3-16 for the interpretation of these bits. Small pages are split into four subpages:
AP0 Encodes the access permissions for the first subpages.
AP1 Encodes the access permissions for the second subpages.
AP2 Encodes the access permissions for the third subpages.
AP3 Encodes the access permissions for the fourth (last) subpages.
Bits[31:12] Are used to form the corresponding bits of the physical address.
Tiny page descriptor fields
The fields in a tiny page descriptor have the following meanings:
Bits[1:0] Identify the type of descriptor.
Bits[3:2] Are the cachable and bufferable bits (see Cachability and bufferability on page B5-8).
Bits[5:4] Are the access permission bits. See Table 3-3 on page B3-16 for the interpretation of these
bits, which apply to the entire tiny page.
Bits[9:6] Are not currently used, and must be zero.
Bits[31:10] Are used to form the corresponding bits of the physical address.
B3-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-514-320.jpg)
![Memory Management Unit
3.6 Aborts
Mechanisms that can cause the ARM processor to halt execution because of memory access restrictions are:
MMU fault The MMU detects the restriction and signals the processor.
External abort The external memory system signals an illegal memory access.
Collectively, MMU faults and external aborts are just called aborts. Accesses that cause aborts are said to
be aborted.
If the memory request that is aborted is an instruction fetch, then a Prefetch Abort exception is raised if and
when the processor attempts to execute the instruction corresponding to the illegal access. If the aborted
access is a data access, a Data Abort exception is raised. See Exceptions on page A2-13 for more
information about Prefetch and Data Aborts.
3.6.1 MMU faults
The MMU generates four types of fault:
• alignment fault
• translation fault
• domain fault
• permission fault.
The memory system can abort three types of access:
• line fetches
• memory accesses (uncached or unbuffered accesses)
• translation table accesses.
Aborts that are detected by the MMU are stopped before any external memory access takes place. It is the
responsibility of the external system to stop external accesses that cause external aborts.
The System Control coprocessor contains two registers which are updated when a data access is aborted.
These registers are not updated for Prefetch Aborts, as the aborted instruction might not be executed due to
changes in program flow.
Fault Address Register (FAR) and Fault Status Register (FSR)
Aborts resulting from data accesses (Data Aborts) are immediately acted upon by the CPU. The Fault Status
Register (FSR) is updated with a 4-bit Fault Status (FS[3:0]) and the domain number of the access. In
addition, the virtual address which caused the Data Abort is written into the Fault Address Register (FAR).
If a data access simultaneously generates more than one type of Data Abort, they are prioritized in the order
given in Table 3-5 on page B3-19.
Aborts arising from instruction fetches are flagged as the instruction enters the instruction pipeline. Only
when (and if) the instruction is executed does it cause a Prefetch Abort exception. An abort resulting from
an instruction fetch is not acted upon if the instruction is not used (for example, if it is branched around).
Normally, the fault address associated with a Prefetch Abort exception is determined from the value saved
in R14_abt when the Prefetch Abort exception vector is entered. It is IMPLEMENTATION DEFINED whether
B3-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-520-320.jpg)
![Memory Management Unit
the FSR and FAR are updated for an abort arising from an instruction fetch, and if so, what useful
information they contain about the fault. However, an abort arising from an instruction fetch will never
update the FSR and the FAR between the time that an abort arising from a data access updates them and the
time of the corresponding entry into the Data Abort exception vector. In other words, a Data Abort handler
can rely upon its FAR and FSR values not being corrupted by an abort arising from an instruction fetch that
was not acted upon.
Priority Sources
Highest Terminal Exception 0b0010 Invalid IMPLEMENTATION
Vector Exception 0b0000 Invalid Valid
Alignment 0b00x1 Invalid Valid
External Abort on Translation First level
Translation Section
Domain Section
Permission Section
External Abort on Linefetch Section
Lowest External Abort on
Non-linefetch
Note
Table 3-5 Priority encoding of fault status
FS
[3:0]
Domain
[3:0]
FAR
DEFINED
Second level
0b1100
0b1110
Invalid
Valid
Valid
Valid
Page
0b0101
0b0111
Invalid
Valid
Valid
Valid
Page
0b1001
0b1011
Valid
Valid
Valid
Valid
Page
0b1101
0b1111
Valid
Valid
Valid
Valid
Page
0b0100
0b0110
Valid
Valid
Valid
Valid
Section
Page
0b1000
0b1010
Valid
Valid
Valid
Valid
Alignment faults can write either 0b0001 or 0b0011 into FS[3:0].
Invalid values in Domain[3:0] occur because the fault is raised before a valid domain field has been loaded.
Any abort masked by the priority encoding can be regenerated by fixing the primary abort and restarting the
instruction.
The FS[3:0] encoding for Vector Exception breaks from the pattern that FS[0] is zero for all external aborts.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-19](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-521-320.jpg)
![Memory Management Unit
Terminal exception
This indicates that an irrecoverable fault has occurred. The circumstances under which this can happen (if
at all) are IMPLEMENTATION DEFINED.
Vector exception
When the processor is in a 32-bit configuration (PROG32 is active) and in a 26-bit mode (CPSR[4] == 0),
data access (but not instruction fetches) to the normal exception vectors (address 0x0 to 0x1F) cause a Data
Abort, known as a vector exception. It is IMPLEMENTATION DEFINED if vector exceptions are generated when
the MMU is not enabled. For more information, see Vector exceptions on page A8-11.
Alignment fault
If alignment faults are enabled, an alignment fault is generated on any data word access whose address is
not word-aligned (virtual address bits [1:0] != 0b00), or any halfword access that is not halfword-aligned
(virtual address bit[0] != 0). Alignment faults are not generated on any instruction fetch, or on any byte
access.
Note
If the access generates an alignment fault, it is aborted without reference to further permission checks. It is
IMPLEMENTATION DEFINED if alignment exceptions are generated when the MMU is not enabled.
Translation fault
There are two types of translation fault:
Section This is generated if the first-level descriptor is marked as invalid. This happens if bits[1:0]
of the descriptor are both 0.
Page This is generated if the second-level descriptor is marked as invalid. This happens if
bits[1:0] of the descriptor are both 0.
Domain fault
There are two types of domain fault:
• Section
• Page.
In both cases, the first descriptor holds the 4-bit Domain field which selects one of the sixteen 2-bit domains
in the Domain Access Control Register. The two bits of the specified domain are then checked for access
permissions as detailed in Table 3-4 on page B3-17:
• In the case of a section, the domain is checked when the first-level descriptor is returned.
• In the case of a page, the domain is checked when the second-level descriptor is returned.
If the specified access is marked as No Access in the Domain Access Control Register, either a Section
Domain Fault or Page Domain Fault occurs.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-21](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-523-320.jpg)
![Memory Management Unit
3.7 CP15 registers
The MMU is controlled with the System Control coprocessor registers 2, 3, 4, 5, 6, 8 and 10, and some bits
of register 1.
3.7.1 MMU control bits in register 1
The following bits in the System Control coprocessor register 1 are used to control the MMU:
M (bit[0]) This is the enable/disable bit for the MMU:
0 = MMU disabled
1 = MMU enabled.
On systems without an MMU or Protection Unit, this bit must read as zero and ignore
writes.
A (bit[1]) This bit enables and disables alignment fault checking (see Alignment fault on page B3-21):
0 = Alignment fault checking disabled
1 = Alignment fault checking enabled.
S (bit[8]) This is the system protection bit. The effect of this bit is described in Access permissions on
page B3-16.
R (bit[9]) This is the ROM protection bit. The effect of this bit is described in Access permissions on
page B3-16.
3.7.2 Register 2: Translation table base
31 14 13 0
Translation Table Base UNP/SBZP
Reading from CP15 register 2 returns the physical address of the currently active first-level translation table
in bits[31:14] and an UNPREDICTABLE value in bits[13:0]. The CRm and opcode2 fields are ignored when
reading CP15 register 2 and should be zero.
Writing to CP15 register 2 updates the physical address of the currently active first-level translation table
from the value in bits[31:14] of the written value. Bits[13:0] must be written as zero or as a value previously
read from bits[13:0] of this register. The CRm and opcode2 fields are ignored when writing CP15 register 2
and should be zero.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-23](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-525-320.jpg)
![Memory Management Unit
3.7.3 Register 3: Domain access control
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Reading from CP15 register 3 returns the value of the Domain Access Control Register. The CRm and
opcode2 fields are ignored when reading CP15 register 3 and should be zero.
Writing to CP15 register 3 writes the value of the Domain Access Control Register. The CRm and opcode2
fields are ignored when writing CP15 register 3 and should be zero.
The Domain Access Control Register consists of sixteen 2-bit fields (see Domains on page B3-17), each
defining the access permissions for one of the sixteen Domains (D15–D0).
3.7.4 Register 4: Reserved
Reading and writing CP15 register 4 is UNPREDICTABLE.
3.7.5 Register 5: Fault status
31 9 8 7 4 3 0
UNP/SBZP 0 Domain Status
Reading CP15 register 5 returns the value of the Fault Status Register (FSR). The FSR contains the source
of the last data fault. Only the bottom nine bits are returned. The upper 23 bits are UNPREDICTABLE. The
FSR indicates the domain and type of access being attempted when an abort occurred.
Bit[8] Returns zero.
Bits[7:4] Specify which of the sixteen domains (D15–D0) was being accessed when a fault occurred.
Bits[3:0] Indicate the type of access being attempted. The encoding of these bits is shown in Table
3-5 on page B3-19.
The FSR is updated for data faults. It is IMPLEMENTATION DEFINED whether the FSR is updated for prefetch
faults (for details, see Fault Address Register (FAR) and Fault Status Register (FSR) on page B3-18). The
CRm and opcode2 fields are ignored when reading CP15 register 5 and should be zero.
Writing CP15 register 5 sets the Fault Status Register to the value of the data written. This is useful for a
debugger to restore the value of the FSR. The upper 24 bits written should be zero or a value previously read
from the FSR. The CRm and opcode2 fields are ignored when writing CP15 register 5 and should be zero.
B3-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-526-320.jpg)
![Protection Unit
4.1.3 Enabling the Protection Unit
The Protection Unit can be enabled by writing 1 to CP15 register 1 bit[0], and can be disabled by writing 0
to the same bit. On reset, this bit is cleared and all protection regions are invalidated.
Before the Protection Unit is enabled, at least one valid protection region must be specified and its C, B and
AP bits must be set. In the case of a Protection Unit with separate sets of protection regions for instruction
and data accesses, at least one valid protection region in each of the two sets must be programmed in this
manner.
Furthermore, the code that enables the Protection Unit must lie within a valid (instruction) protection region.
Enabling the Protection Unit without adhering to these rules can result in behavior which is at best
UNPREDICTABLE, and can even result in a hung state which can only be recovered from by resetting the
system.
While the Protection Unit is disabled, all memory accesses are treated as uncachable, unbufferable and
non-aborting.
B4-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-536-320.jpg)
![Protection Unit
4.3 CP15 registers
The Protection Unit is controlled with the System Control coprocessor registers 2, 3, 5, and 6, and one or
two bits of register 1.
4.3.1 Protection Unit control bits in register 1
M(bit[0]) in the System Control coprocessor register 1 is the enable/disable bit for the Protection Unit:
0 = Protection Unit disabled
1 = Protection Unit enabled.
On systems without a Protection Unit or MMU, this bit must read as zero and ignore writes.
A Protection Unit memory system can optionally implement alignment checking, as described in Alignment
fault on page B3-21. If it does, the A bit (bit[1]) of register 1 behaves as described there and in MMU control
bits in register 1 on page B3-23. Otherwise, the A bit should be zero.
4.3.2 Register 2: Cachability bits
31 8 7 6 5 4 3 2 1 0
UNP/SBZP C7C6C5C4C3C2C1C0
Reading from CP15 register 2 returns the cachable (C) bits for the eight protection regions in bits[7:0], with
bit[n] corresponding to region n, and an UNPREDICTABLE value in bits[31:8].
Writing to CP15 register 2 updates the cachable (C) bits of the eight protection regions, with the C bit of
region n being set to bit[n] of the value written. Bits[31:8] must be written as zero or as a value previously
read from bits[31:8] of this register.
In each case, the <CRm> field of the MRC or MCR instruction is ignored and must be c0. If the
implementation only has one set of eight protection regions, the <opcode2> field should be zero. If it has
separate sets of protection regions for instruction and data accesses, <opcode2> must be specified as 0 to
select the data protection regions and 1 to select the instruction protection regions.
For details of how the C bit affects cache and write buffer behavior, see Cachability and bufferability on
page B5-8.
4.3.3 Register 3: Bufferability bits
31 8 7 6 5 4 3 2 1 0
UNP/SBZP B7B6B5B4B3B2B1B0
Reading from CP15 register 3 returns the bufferable (B) bits for the eight protection regions in bits[7:0],
with bit[n] corresponding to region n, and an UNPREDICTABLE value in bits[31:8].
B4-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-538-320.jpg)
![Protection Unit
Writing to CP15 register 3 updates the bufferable (B) bits of the eight regions, with the B bit of region n
being set to bit[n] of the value written. Bits[31:8] must be written as zero or as a value previously read from
bits[31:8] of this register.
In each case, the <CRm> and <opcode2> fields of the MRC or MCR instruction are ignored and must be c0
and zero respectively.
For details of how the B bit affects cache and write buffer behavior, see Cachability and bufferability on
page B5-8.
4.3.4 Registers 4, 8, 10: Reserved
Accessing (reading or writing) any of these registers is UNPREDICTABLE.
4.3.5 Register 5: Access permission bits
31 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
UNP/SBZP AP7 AP6 AP5 AP4 AP3 AP2 AP1 AP0
Reading from CP15 register 3 returns the AP bits for the eight protection regions in bits[15:0], with
bits[2n+1:2n] corresponding to region n, and an UNPREDICTABLE value in bits[31:16].
Writing to CP15 register 3 updates the AP bits of the eight regions, with the AP bits of region n being set to
bits[2n+1:2n] of the value written. Bits[31:16] must be written as zero or as a value previously read from
bits[31:16] of this register.
In each case, the <CRm> field of the MRC or MCR instruction is ignored and must be c0. If the
implementation only has one set of eight protection regions, the <opcode2> field should be zero. If it has
separate sets of protection regions for instruction and data accesses, <opcode2> must be specified as 0 to
select the data protection regions and 1 to select the instruction protection regions.
The interpretation of each set of AP bits is as shown in Table 4-1 on page B4-7. If the requested type of
access is not permitted, an abort is signaled to the ARM processor.
Table 4-1 Protection Unit access permissions
AP
Privileged
permissions
User
permissions
0b00 No access No access
0b01 Read/write No access
0b10 Read/write Read only
0b11 Read/write Read/write
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B4-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-539-320.jpg)
![Protection Unit
4.3.6 Register 6: Protection area control
31 12 11 6 5 1 0
Base address UNP/SBZP Size E
Reading from CP15 register 6 returns the current base address, size and enabled/disabled status of a
protection region, in the format shown in the above diagram. The value read for bits[11:6] is
UNPREDICTABLE.
Writing to CP15 register 6 sets the base address, size and enabled/disabled status of a protection region, in
the format shown in the above diagram. The value written to bits[11:6] must either be zero or a value
previously read from bits[11:6] of CP15 register 6.
There is one version of register 6 for each protection region in the Protection Unit. The version used (and
therefore the protection region affected) is selected by the <CRm> and <opcode2> fields of the MCR or
MRC instruction used to access the register:
• <CRm> is used to select the number of the protection region, by specifying c0 to select protection
region 0, c1 to select protection region 1, and so on, through to c7 to select protection region 7.
• If the implementation only has one set of eight protection regions, the <opcode2> field should be
zero.
• If the implementation has separate sets of protection regions for instruction and data accesses,
<opcode2> must be specified as 0 to select a data protection region and 1 to select an instruction
protection region.
The meaning of the fields in the value read from or written to register 6 is as follows:
• The E bit enables or disables the associated protection region:
0 = protection region disabled
1 = protection region enabled.
A disabled protection region never matches any addresses, and therefore does not affect the memory
access sequence in any way. All protection regions are disabled on reset.
• The Size field selects the associated protection region's size, which can vary from 4KB to 4GB. The
encoding is shown in Table 4-2 on page B4-9.
• The Base address field specifies bits[31:12] of the address of the first byte in the associated protection
region.
The address of this first byte is required to be a multiple of the region size, so its bits[11:0] are always
zero and do not need to be programmed. Also, because of this requirement for the region to be aligned
to a multiple of its size, one or more further bits of the base address may also be constrained to be
zero. These additional constraints are shown in Table 4-2 on page B4-9. If they are broken, the
protection region is misaligned, and UNPREDICTABLE behavior might result.
B4-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-540-320.jpg)
![Protection Unit
Table 4-2 Region size encoding
Size field Area size Base area constraints
0b00000 to 0b01010 UNPREDICTABLE -
0b01011 4KB None
0b01100 8KB Bit[12] must be zero
0b01101 16KB Bits[13:12] must be zero
0b01110 32KB Bits[14:12] must be zero
0b01111 64KB Bits[15:12] must be zero
0b10000 128KB Bits[16:12] must be zero
0b10001 256KB Bits[17:12] must be zero
0b10010 512KB Bits[18:12] must be zero
0b10011 1MB Bits[19:12] must be zero
0b10100 2MB Bits[20:12] must be zero
0b10101 4MB Bits[21:12] must be zero
0b10110 8MB Bits[22:12] must be zero
0b10111 16MB Bits[23:12] must be zero
0b11000 32MB Bits[24:12] must be zero
0b11001 64MB Bits[25:12] must be zero
0b11010 128MB Bits[26:12] must be zero
0b11011 256MB Bits[27:12] must be zero
0b11100 512MB Bits[28:12] must be zero
0b11101 1GB Bits[29:12] must be zero
0b11110 2GB Bits[30:12] must be zero
0b11111 4GB Bits[31:12] must be zero
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B4-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-541-320.jpg)
![Caches and Write Buffers
5.2 Cache organization
The basic unit of storage in a cache is the cache line. A cache line is said to be valid when it contains cached
data or instructions, and invalid when it does not. All cache lines in a cache are invalidated on reset. A cache
line becomes valid when data or instructions are loaded into it from memory.
When a cache line is valid, it contains up-to-date values for a block of consecutive main memory locations.
The length of this block (and therefore the length of the cache line) is always a power of two, and is typically
16 bytes (4 words) or 32 bytes (8 words). If the cache line length is 2L bytes, the block of main memory
locations is always 2L-byte aligned. Such blocks of main memory locations are called memory cache lines
or (loosely) just cache lines.
Because of this alignment requirement, virtual address bits[31:L] are identical for all bytes in a cache line.
A cache hit occurs when bits[31:L] of the virtual address supplied by the ARM processor match the same
bits of the virtual address associated with a valid cache line.
To simplify and speed up the process of determining whether a cache hit occurs, a cache is usually divided
into a number of cache sets. The number of cache sets is always a power of two. If the cache line length is
2L bytes and there are 2S cache sets, bits[L+S-1:L] of the virtual address supplied by the ARM processor
are used to select a cache set. Only the cache lines in that set are allowed to hold the data or instructions at
the address.
The remaining bits of the virtual address (bits[31:L+S]) are known as its tag bits. A cache hit occurs if the
tag bits of the virtual address supplied by the ARM processor match the tag bits associated with a valid line
in the selected cache set.
Figure 5-1 illustrates how the virtual address is used to look up data or instructions in the cache.
31 L+S L+S-1 L L-1 0
tag set pos
S
Select one of 2
cache sets
Look for cache line with
tag in selected cache set
if not found if found
Cache hit
Return data at position pos
in cache line
Figure 5-1 Cache look-up
Virtual address
Cache miss
Get data from
main memory
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-3](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-545-320.jpg)
![Caches and Write Buffers
5.6 CP15 registers
Caches and write buffers are controlled with System Control coprocessor registers 7 and 9, and some bits
of register 1.
5.6.1 Cache and write buffer control bits in register 1
The following bits in register 1 of the System Control Coprocessor control caches and write buffers:
C (bit[2]) If a unified cache is used, this is the enable/disable bit for the unified cache. If separate
caches are used, this is the enable/disable bit for the data cache. In either case:
0 = Cache disabled
1 = Cache enabled.
If the cache is not implemented, this bit reads as 0 and ignores writes. If the cache cannot
be disabled, this bit reads as 1 and ignores writes.
W (bit[3]) This is the enable/disable bit for the write buffer:
0 = Write buffer disabled
1 = Write buffer enabled.
If the write buffer is not implemented, this bit reads as 0 and ignore writes. If the write buffer
cannot be disabled, this bit reads as 1 and ignores writes.
I (bit[12]) If separate caches are used, this is the enable/disable bit for the instruction cache:
0 = Cache disabled
1 = Cache enabled.
If a unified cache is used or the instruction cache is not implemented, this bit reads as 0 and
ignores writes. If the instruction cache cannot be disabled, this bit reads as 1 and ignores
writes.
RR (bit[14]) If the cache allows an alternative replacement strategy to be used which has a more easily
predictable worst-case performance, this bit selects it:
0 = Normal replacement strategy (for example, random replacement)
1 = Predictable strategy (for example, round-robin replacement).
B5-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-556-320.jpg)
![Caches and Write Buffers
5.6.2 Register 7: Cache functions
The System Control coprocessor register 7 is a write-only register which is used to control caches and write
buffers. It is also used to implement some similar functions on prefetch buffers and branch target caches, if
they exist, and to implement the wait for interrupt clock control function. Table 5-2 lists the functions
available by using an MCR p15,0,<Rd>,c7,<CRm>,<opcode2> instruction to write to register 7.
Writing to register 7 with a combination of <CRm> and <opcode2> which is not listed in Table 5-2 has
UNPREDICTABLE results.
Not all functions listed in Table 5-2 are necessarily relevant to or supported by an implementation.
Attempting to perform a function which is not supported by the implementation has UNPREDICTABLE results.
Reading register 7 with an MRC instruction has UNPREDICTABLE results.
In Table 5-2, the following terms apply:
Clean Applies to write-back data caches, and means that if the cache line contains stored
data that has not yet been written out to main memory, it is written to main memory
now.
Invalidate Means that the cache line (or all the lines in the cache) is marked as invalid, so that
no cache hits occur for that line until it is re-allocated to an address.
For write-back data caches, this does not include cleaning the cache line unless that
is also stated.
Prefetch Means the memory cache line at the specified virtual address (which must be cache
line aligned) is loaded into the cache.
Drain write buffer Stops the ARM from executing further until all data in the write buffer has been
stored to main memory. It can be used instead of unbufferable memory when the
timing of specific main memory stores needs to be controlled (for example, when a
store to an interrupt acknowledge location needs to complete before interrupts are
enabled).
Wait for interrupt Puts the ARM into a low power state and stops it executing further until an interrupt
occurs.
When an interrupt does occur, the MCR instruction completes and the IRQ or FIQ
handler is entered as normal. The return link in R14_irq or R14_fiq contains the
address of the MCR instruction plus 8, so that the normal instruction used for
interrupt return (SUBS PC,R14,#4) returns to the instruction following the MCR.
Prefetch buffer Operations are IMPLEMENTATION DEFINED.
Branch target cache Operations are IMPLEMENTATION DEFINED.
Data Is the value that is written to register 7, which is the value in the register <Rd>
specified in the MCR instruction.
If the data is stated to be a virtual address, it must be cache line aligned. This means
that if the cache line length is 2L bytes, bits[L-1:0] of the address must be zero. This
address is looked up in the cache. If a cache hit occurs, the specified operation
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-15](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-557-320.jpg)
![Fast Context Switch Extension
6.2 Modified virtual addresses
The 4GB virtual address space is divided into 128 process blocks, each of size 32MB. Each process block
can contain a program which has been compiled to use the address range 0x00000000 to 0x01FFFFFF.
For each of i=0 to 127, process block i runs from address (i ´ 0x02000000) to address (i ´ 0x02000000
+ 0x01FFFFFF).
The FCSE processes each virtual address for a memory access generated by the ARM processor to produce
a modified virtual address, which is sent to the rest of the memory system to be used in place of the normal
virtual address. For an MMU-based memory system, the process is illustrated in Figure 6-1:
Virtual
address
(VA)
Modified
virtual
address
(MVA)
Physical
address
(PA)
ARM FCSE MMU Main
memory
Cache
Figure 6-1 Address flow in MMU memory system with FCSE
When the ARM processor generates a memory access, the relationship between the Virtual Address (VA)
and Modified Virtual Address (MVA) is:
if (VA[31:25] == 0b0000000) then
MVA = VA | (PID << 25)
else
MVA = VA
where PID is a 7-bit number that identifies which process block the current process is loaded into. This is
also known as the process ID of the current process.
Note
Virtual addresses are sometimes passed to the memory system as data, as for example in some of the cache
control operations described in Register 7: Cache functions on page B5-15. For these operations, no address
modification occurs, and MVA = VA.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B6-3](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-567-320.jpg)
![Fast Context Switch Extension
6.4 CP15 registers
The FCSE only uses one coprocessor 15 register, namely register 13.
6.4.1 Register 13: Process ID
31 25 24 0
PID UNP/SBZP
Reading register 13 returns the PID in bits[31:25]. Bits[24:0] of the value read are UNPREDICTABLE.
Writing register 13 sets the PID to bits[31:25] of the value written. Bits[24:0] of the value written should be
zero or bits[24:0] of a value previously read from register 13. The results of writing any other value to
bits[24:0] are UNPREDICTABLE.
In MCR and MRC instructions used to write and read register 13, <CRm> should be c0 and <opcode2>
should be 0 (or omitted). If they have other values, the instruction is UNPREDICTABLE.
Note
When the PID is written, the overall virtual-to-physical address mapping changes. Because of this, care
must be taken to ensure that instructions which might have already been prefetched are not affected by the
address mapping change.
Formally, this means that an IMB sequence is required, as described in Instruction Memory Barriers (IMBs)
on page A2-28. However, the mapping from the modified virtual address to the physical address is
unchanged, so typically there is no need to flush caches, etc. This means that an implementation can usually
specify a restricted form of the IMB sequence to be used after using an MCR to write to register 13.
Typically, this means that all instructions reached by predictable subsequent execution from the MCR
instruction must be fetched with addresses whose top seven bits are not 0b0000000. For the definition of
predictable subsequent execution and further details, see Other uses for IMBs on page A2-30.
B6-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-570-320.jpg)
![VFP Programmer’s Model
These behave identically in most circumstances, including getting an equal result if +0 and -0
are compared as floating-point numbers. However, they yield different results in some
exceptional circumstances (for example, they affect the sign of the infinity produced as the
default result for a Division by Zero exception). They can also be distinguished from each
other by performing an integer comparison of the two words.
— If fraction != 0, the value is a denormalized number and is equal to:
-1S ´ 2-126 ´ (0.fraction)
In this case, the mantissa of the value has a zero before the binary point, rather than the one
used by a normalized number. It lies in the range 0 < mantissa < 1 and is a multiple of 2-23.
The value's unbiased exponent is -126.
The minimum positive denormalized number is 2-149, or approximately 1.401 ´ 10-45.
• If exponent == 0xFF, the value is either an infinity or a Not a Number (NaN), depending on the
fraction bits.
If fraction == 0, the value is an infinity. There are two infinities:
+¥ Has S==0 and represents all positive numbers which are too big to be represented
accurately as a normalized number.
-¥ Has S==1 and represents all negative numbers which are too big to be represented
accurately as a normalized number.
If fraction != 0, the value is a NaN, and can be either a quiet NaN or a signaling NaN (see NaNs on
page C2-5 for details of these types of NaN).
In the VFP architecture, the two types of NaN are distinguished on the basis of their most significant
fraction bit (bit[22]):
— If bit[22] == 0, the NaN is a signaling NaN. The sign bit can take any value, and the remaining
fraction bits can take any value except all zeros, so there are 2 ´ (222-1) = 8388606 possible
signaling NaNs.
— If bit[22] == 1, the NaN is a quiet NaN. The sign bit and remaining fraction bits can take any
value, so there are 2 ´ 222 = 8388608 possible quiet NaNs.
Two NaNs are treated as being different values in the VFP architecture if their sign bits and/or any
of their fraction bits differ. This implies that all 232 possible word values are treated as distinct from
each other by the VFP architecture.
Note
The fact that NaNs with different sign and/or fraction bits are distinct NaNs does not mean that
floating-point comparison instructions can be used to distinguish them. This is because the IEEE 754
standard specifies that a NaN compares as unordered with everything, including itself.
However, different NaNs can be distinguished by using integer comparisons. Also, the rules for handling
NaNs are designed not to arbitrarily change one NaN into another (see NaNs on page C2-5).
These rules about NaNs also ensure that single-precision registers can be used to hold integer values without
any risk of corrupting them (see Holding integers in single-precision registers on page C2-18).
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-3](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-585-320.jpg)
![VFP Programmer’s Model
2.1.2 Double-precision format
A double-precision value consists of two 32-bit words, with the following formats:
Most significant word:
31 30 20 19 0
S exponent fraction[51:32]
Least significant word:
31 0
fraction[31:0]
When held in memory, the two words must appear consecutively and must both be word-aligned. The order
of the two words depends on the endianness of the memory system:
• In a little-endian memory system, the least significant word appears at the lower memory address and
the most significant word at the higher memory address.
• In a big-endian memory system, the most significant word appears at the lower memory address and
the least significant word at the lower memory address.
A VFP implementation must use the same endianness as the ARM implementation it is attached to. If the
ARM implementation has configurable endianness, double-precision values must not be loaded or stored
before the ARM processor endianness has been set to match that of the memory system (see Endianness on
page A2-23 for more details).
Note
The word order defined here for the VFP architecture differs from that of the earlier FPA floating-point
architecture. In the FPA architecture, the most significant word always appeared at the lower memory
address, with the least significant word at the higher, regardless of the memory system endianness.
Double-precision values represent numbers, infinities and NaNs analogously to single-precision values:
• If 0 < exponent < 0x7FF, the value is a normalized number and is equal to:
-1S ´ 2exponent-1023 ´ (1.fraction)
The mantissa of the value is the number 1.fraction, consisting of a one, followed by a binary point,
followed by the 52 fraction bits. The mantissa therefore lies in the range 1 £ mantissa < 2 and is a
multiple of 2-52.
The unbiased exponent of the value is (exponent-1023).
The minimum positive normalized number is 2-1022, or approximately 2.225 ´ 10-308. The
maximum positive normalized number is (2-2-52) ´ 21023, or approximately 1.798 ´ 10308.
C2-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-586-320.jpg)
![VFP Programmer’s Model
• If exponent == 0, the value is either a zero or a denormalized number, depending on the fraction bits.
If fraction == 0, the value is a zero. There are two distinct zeros which behave analogously to the two
single-precision zeros:
+0 with S==0
-0 with S==1.
If fraction != 0, the value is a denormalized number and is equal to:
-1S ´ 2-1022 ´ (0.fraction)
In this case, the mantissa of the value has a zero before the binary point, rather than the one used by
a normalized number. It lies in the range 0 < mantissa < 1 and is a multiple of 2-52. The unbiased
exponent of the value is -1022.
The minimum positive denormalized number is 2-1074, or approximately 4.941 ´ 10-324.
• If exponent == 0x7FF, the value is either an infinity or a NaN, depending on the fraction bits.
— If fraction == 0, the value is an infinity. As for single precision, there are two infinities:
+¥ Plus infinity with S==0
+¥ Minus infinity with S==1.
— If fraction != 0, the value is a NaN. The rules about interpreting NaN values are analogous to
those for single precision:
— The type of a NaN is determined by the most significant fraction bit (bit[19] of the most
significant word). A 0 indicates a signaling NaN and a 1 indicates a quiet NaN.
— Two NaNs whose sign bits and/or fractions differ are different NaNs.
2.1.3 NaNs
NaNs are special floating-point values which can be used when neither a numeric value nor an infinity is
appropriate. There are two types of NaN, each of which can be used for a variety of purposes:
Quiet NaNs These propagate unchanged through most floating-point operations. They can be
generated by floating-point arithmetic operations in some rare circumstances when
there is no other sensible result. Any further calculations which depend on the result
of such an operation then also produce a quiet NaN result. (Quiet NaNs can only be
generated in this way if the associated Invalid Operation exception is untrapped. If
it is trapped, a trap handler is called instead.) Another typical use for quiet NaNs is
to represent missing or unavailable data values. The results of any calculations that
depend on the missing values are then also quiet NaNs.
Signaling NaNs These cause an Invalid Operation exception whenever any floating-point operation
receives a signaling NaN as an operand.
One possible use for signaling NaNs is in debugging, to track down some uses of
uninitialized variables. To do this, pre-load memory with copies of a signaling NaN,
then load and run the program with Invalid Operation traps enabled. Any
floating-point operation whose operand has been loaded from uninitialized memory
then calls the Invalid Operation trap handler.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-587-320.jpg)
![VFP Programmer’s Model
Special cases
There are two instructions whose operands and results have different floating-point formats. These have
special rules for handling NaNs, as follows:
• The FCVTDS instruction converts a single-precision value to double precision. If its operand is a
single-precision quiet NaN, the result is the double-precision quiet NaN with:
S = S bit of operand
fraction[51:29] = fraction[22:0] of operand
fraction[28:0] = 0
If its operand is a single-precision signaling NaN, an Invalid Operation exception is generated. If the
exception is untrapped, the result is the double-precision quiet NaN with:
S = S bit of operand
fraction[51] = 1
fraction[50:29] = fraction[21:0] of operand
fraction[28:0] = 0
• The FCVTSD instruction converts a double-precision value to single precision. If its operand is a
double-precision quiet NaN, the result is the single-precision quiet NaN with:
S = S bit of operand
fraction[22:0] = fraction[51:29] of operand
If its operand is a double-precision signaling NaN, an Invalid Operation exception is generated. If the
exception is untrapped, the result is the single-precision quiet NaN with:
S = S bit of operand
fraction[22] = 1
fraction[21:0] = fraction[50:29] of operand
C2-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-590-320.jpg)
![VFP Programmer’s Model
2.3 Floating-point exceptions
The IEEE 754 standard specifies five classes of floating-point exception:
Invalid Operation exceptions
Occur in various cases where neither a numeric value nor an infinity is a sensible
result of a floating-point operation, and also when an operand of a floating-point
operation is a signaling NaN. For more details of Invalid Operation exceptions, see
NaNs on page C2-5.
Division by Zero exceptions
Occur when a normalized or denormalized number is divided by a zero.
Overflow exceptions
Occur when result of a floating-point operation is too big in magnitude for it to be
represented in the destination format without an unusually large rounding error for
the rounding mode in use. More precisely, the ideal rounded result of a
floating-point operation is defined to be the result that its rounding mode would
produce if the destination format had no limits on the unbiased exponent range. If
the ideal rounded result has unbiased exponent too big for the destination format
(that is, >127 for single precision or >1023 for double precision), it differs from the
actual rounded result, and an Overflow exception occurs.
Underflow exceptions
Occur when a floating-point operation’s result is too small in magnitude for it to be
represented in the destination format without an unusually large rounding error for
the rounding mode in use. The exact conditions for an Underflow exception to occur
vary slightly depending on the value of the Underflow exception enable (UFE) bit
(which is bit[11] of the FPSCR), that is, on whether the Underflow exception is to
be trapped.
In each case, one condition is that the ideal rounded result (as defined above) has an
unbiased exponent too small for the destination format (that is, < -126 for single
precision or < -1022 for double precision). If UFE == 1, this is the only condition
and a trapped Underflow exception occurs when it is detected. If UFE == 0, an
untrapped Underflow exception only occurs if the ideal rounded result is also
different from the result produced by actual rounding (which might be a zero, a
denormalized number or the minimum possible normalized number).
Note
The IEEE 754 standard leaves two choices open in its definition of the underflow
exception. In the terminology of the standard, the above description means that the
VFP architecture requires these choices to be:
• the after rounding form of tininess
• the denormalization loss form of loss of accuracy.
C2-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-592-320.jpg)
![VFP Programmer’s Model
The only operation which is normally performed on data stored with FSTMX is to reload it using a matching
FLDMX. However, debug software might need to interpret and/or modify the contents of stack frames or
process control blocks, and so might need to know the FSTMX/FLDMX memory format. There are two
standard options for the format:
• In standard format 1, FSTMX stores the register contents in exactly the same way as an FSTMD of the
same registers would store them. The first double-precision register value is stored as two words, in
the correct order for the configured endianness of the processor. Then the next double-precision
register value is stored similarly, and so on, until the N registers have been stored in 2N memory
words. The (2N+1)th memory word is unused.
The matching FLDMX instruction reloads the data as double-precision values, precisely like an
FLDMD of the same registers. This format is only used for implementations in which storing and
reloading double-precision registers in this fashion correctly reload their contents even if they happen
to contain single-precision values.
• In standard format 2, the first word stored is a format word, and then each double-precision register
specified in the instruction is stored as two words, in ascending order of register number. This option
uses all 2N+1 available words of memory.
The format word has the following format:
31 16 15 0
SBZ format bits
For each double-precision register Di which is stored by the FSTMX instruction, bit[i] of the format
word indicates how it is stored:
— If bit[i] == 0, the register contained single-precision data, and the two words stored are the
contents of S(2 ´ i) followed by the contents of S(2 ´ i + 1), both in single-precision format.
— If bit[i] == 1, the register contained double-precision data, and the two words stored are the
contents of Di, stored as two words in the correct order for the configured endianness of the
processor.
For each double-precision register Di which is not stored by the FSTMX instruction, bit[i] of the
format word is UNPREDICTABLE.
Example
For example, Figure 2-2 on page C2-17 shows how the instruction:
FSTMIAX Rn,{D4-D6}
would store the registers using each standard option, on the assumption that D4 and D6 contain
double-precision values and S10 and S11 (which overlap D5) contain single-precision values.
C2-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-598-320.jpg)
![VFP Programmer’s Model
2.6.1 FPSID
The FPSID has the following format:
31 24 23 22 21 20 19 16 15 8 7 4 3 0
implementor SW format SNG architecture part number variant revision
Bits[31:24] Contain an implementor code. The following code is defined:
0x41 = A (ARM Ltd)
All other values of the implementor code are reserved by ARM Ltd.
Bit[23] Contains 0 if the implementation contains a hardware coprocessor, or 1 if it is a pure
software implementation.
Bits[22:21] Indicate which FSTMX/FLDMX format is used (see Storing and reloading values of unknown
precision on page C2-15):
0b00 Indicates standard format 1.
0b01 Indicates standard format 2.
0b10 Is reserved.
0b11 Indicates a non-standard format.
Bit[20] Contains 0 if the implementation supports both single precision and double precision (a D
variant of the architecture), or 1 if it only supports single precision (a non-D variant).
Bits[19:16] Contain the architecture version number, encoded as follows:
0b0000 indicates VFPv1.
All other values of this architecture version code are reserved by ARM Ltd.
Bits[15:8] Contain an IMPLEMENTATION DEFINED representation of the primary part number of the
VFP implementation.
Bits[7:4] Contain an IMPLEMENTATION DEFINED variant number. This is typically used to distinguish
variants of the same primary part. For example, two variants of the same VFP
implementation might have hardware coprocessor interfaces designed to work with
different ARM processors.
Bits[3:0] Contain the IMPLEMENTATION DEFINED revision number of the part.
The FPSID register is read-only, and can be accessed in both privileged and unprivileged modes. Attempts
to write the FPSID register are ignored.
C2-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-602-320.jpg)
![VFP Programmer’s Model
2.6.2 FPSCR
The FPSCR has the following format:
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
N Z C V DNM FZRMODESTRIDE
D
N
M
LEN DNM
I
X
E
U
F
E
O
F
E
D
Z
E
I
O
E
DNM
I
X
C
U
F
C
O
F
C
D
Z
C
I
O
C
All of these bits can be read and written, and can be accessed in both privileged and unprivileged modes.
Note
All bits described as DNM (Do Not Modify) in the diagram are reserved for future expansion. They are
initialized to zeros. Non-initialization code must use read/modify/write techniques when handling the
FPSCR, in order to ensure that these bits are not modified. Failure to observe this rule can result in code
which has unexpected side effects on future systems.
The FPSCR bits are described in the following subsections.
Condition flags
Bits[31:28] of the FPSCR contain the results of the most recent floating-point comparison:
N Is 1 if the comparison produced a less than result
Z Is 1 if the comparison produced an equal result
C Is 1 if the comparison produced an equal, greater than or unordered result
V Is 1 if the comparison produced an unordered result.
These condition flags do not directly affect conditional execution, either of ARM instructions or of VFP
instructions. A comparison instruction is normally followed by an FMSTAT instruction. This transfers the
FPSCR condition flags to the ARM CPSR flags, after which they can affect conditional execution.
For more details of how comparisons are performed, see Comparison instructions on page C3-6.
Flush-to-zero mode control
Bit[24] of the FPSCR is the FZ bit and controls flush-to-zero mode. See Flush-to-zero mode on page C2-13
for details of this processing mode.
FZ == 0 Flush-to-zero mode is disabled and the behavior of the floating-point system is fully
compliant with the IEEE 754 standard.
FZ == 1 Flush-to-zero mode is enabled.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-21](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-603-320.jpg)
![VFP Programmer’s Model
Rounding mode control
Bits[23:22] of the FPSCR select the current rounding mode. This rounding mode is used for almost all
floating-point instructions. The only floating-point instructions which do not use it are FTOSIZD,
FTOSIZS, FTOUIZD and FTOUIZS, which always use RZ mode.
The rounding modes are encoded as follows:
0b00 Indicates Round to Nearest (RN) mode
0b01 Indicates Round towards Plus Infinity (RP) mode
0b10 Indicates Round towards Minus Infinity (RM) mode
0b11 Indicates Round towards Zero (RZ) mode.
See Rounding on page C2-9 for details of the rounding modes.
Vector length/stride control
The LEN field (bits[18:16]) of the FPSCR controls the vector length for VFP instructions that operate on
short vectors, that is, how many registers are in a vector operand. Similarly, the STRIDE field (bits[21:20])
controls the vector stride, that is, how far apart the registers in a vector lie in the register bank. The allowed
combinations of LEN and STRIDE are shown in Table 2-2 on page C2-23.
All other combinations of LEN and STRIDE produce UNPREDICTABLE results.
The combination LEN == 0b000, STRIDE == 0b00 is sometimes called scalar mode. When it is in effect,
all arithmetic instructions specify simple scalar operations. Otherwise, most arithmetic instructions specify
a scalar operation if their destination lies in the range S0-S7 (for single precision) or D0-D3 (for double
precision). The full rules used to determine which operands are vectors and full details of how vector
operands are specified can be found in Chapter C5 VFP Addressing Modes and in the individual instruction
descriptions.
The rules for vector operands do not allow the same register to appear twice or more in a vector. The allowed
LEN/STRIDE combinations listed in Table 2-2 never cause this to happen for single-precision instructions,
so single-precision scalar and vector instructions can be used with all of these LEN/STRIDE combinations.
For double-precision vector instructions, some of the allowed LEN/STRIDE combinations would cause the
same register to appear twice in a vector. If a double-precision vector instruction is executed with such a
LEN/STRIDE combination in effect, the instruction is UNPREDICTABLE. The last column of Table 2-2
indicates which LEN/STRIDE combinations this applies to. Double-precision scalar instructions work
normally with all of the allowed LEN/STRIDE combinations.
C2-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-604-320.jpg)
![VFP Programmer’s Model
Table 2-2 Vector length/stride combinations
LEN STRIDE
Vector
length
Vector
stride
Double-precision vector instructions
0b000 0b00 1 - All instructions are scalar
0b001 0b00 2 1 Work normally
0b001 0b11 2 2 Work normally
0b010 0b00 3 1 Work normally
0b010 0b11 3 2 UNPREDICTABLE
0b011 0b00 4 1 Work normally
0b011 0b11 4 2 UNPREDICTABLE
0b100 0b00 5 1 UNPREDICTABLE
0b101 0b00 6 1 UNPREDICTABLE
0b110 0b00 7 1 UNPREDICTABLE
0b111 0b00 8 1 UNPREDICTABLE
Exception status and control
Bits[12:8] and bits[4:0] of the FPSCR are the trap enable bits and cumulative exception bits respectively for
the five types of exception. For details of what these do, see Floating-point exceptions on page C2-10.
Table 2-3 shows which bits are associated with each exception.
Table 2-3 Exception status and control bits
Exception type Trap enable bit Cumulative exception bit
Invalid Operation IOE (bit[8]) IOC (bit[0])
Division by Zero DZE (bit[9]) DZC (bit[1])
Overflow OFE (bit[10]) OFC (bit[2])
Underflow UFE (bit[11]) UFC (bit[3])
Inexact IXE (bit[12]) IXC (bit[4])
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-23](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-605-320.jpg)
![VFP Programmer’s Model
2.6.3 FPEXC
The FPEXC register has the following format:
31 30 29 0
EXEN IMPLEMENTATION DEFINED
This register can only be accessed in privileged modes.
The EX bit
The EX bit (bit[31]) is a status bit which specifies how much information needs to be saved to record the
state of the floating-point system. It can be read on all VFP implementations, and is mainly of interest to
process swap code.
EX == 0 In this case, the only significant state in the floating-point system is the contents of the
architecturally defined writable registers, that is, of the general-purpose registers, FPSCR
and FPEXC. If EX == 0 when a process is swapped out, only these registers need to be
saved, or reloaded when the process is swapped back in. Also, no unexpected ARM
exceptions (such as an undefined instruction exception to process a pending exception in the
hardware) must occur during the saving and reloading of the registers.
EX == 1 Here, there is additional IMPLEMENTATION DEFINED significant state in the floating-point
system which process swap code needs to handle. This typically occurs when VFP hardware
requires support code assistance to handle a potential exception, and one or more of the
additional hardware system registers contains details of the potential exception. (Some
implementations describe this by saying that the hardware is in an exceptional state.) The
actions required to swap a process out when EX == 1 and to swap such a process back in
are IMPLEMENTATION DEFINED.
The behavior of the EX bit when FPEXC is written is IMPLEMENTATION DEFINED, subject to the constraint
that writing a 0 to the EX bit must be a legitimate action. Otherwise, the process swap technique described
above for the case EX == 0 cannot work.
The EN bit
The EN bit (bit[30]) is a global enable bit, and can be both read and written.
EN == 1 In this case, the floating-point system is enabled and operates normally.
EN == 0 Here, the floating-point system is disabled. In this state, all VFP instructions are treated as
undefined instructions when executed in an unprivileged ARM processor mode, and all
except the following are treated as undefined instructions when executed in a privileged
ARM processor mode:
• an FMXR to the FPEXC or FPSID register
• an FMRX from the FPEXC or FPSID register.
C2-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-606-320.jpg)
![VFP Instruction Set Overview
Table 3-5 VFP load and store instructions
cp
num
offset
[0]
Instruction
L==0
Instruction
L==1
Addr
mode
Registers transferred
0 0 x x x UNDEFINED - - -
0 1 0 0b1010 x FSTMS FLDMS Unindexed (offset) single-precision registers
0 1 0 0b1011 0 FSTMD FLDMD Unindexed (offset)/2 double-precision registers
0 1 0 0b1011 1 FSTMX FLDMX Unindexed (offset-1)/2 double-precision registers
0 1 1 0b1010 x FSTMS FLDMS Increment (offset) single-precision registers
0 1 1 0b1011 0 FSTMD FLDMD Increment (offset)/2 double-precision registers
0 1 1 0b1011 1 FSTMX FLDMX Increment (offset-1)/2 double-precision registers
1 0 0 0b1010 x FSTS FLDS Negative
offset
One single-precision register
1 0 0 0b1011 x FSTD FLDD Negative
offset
One double-precision register
1 0 1 0b1010 x FSTMS FLDMS Decrement (offset) single-precision registers
1 0 1 0b1011 0 FSTMD FLDMD Decrement (offset)/2 double-precision registers
1 0 1 0b1011 1 FSTMX FLDMX Decrement (offset-1)/2 double-precision registers
1 1 0 0b1010 x FSTS FLDS Positive
offset
One single-precision register
1 1 0 0b1011 x FSTD FLDD Positive
offset
One double-precision register
1 1 1 x x UNDEFINED - - -
All load instructions perform a copy of the loaded value(s) from memory, and all store instructions perform
a copy of the stored value(s) to memory. No exceptions are ever raised and the value(s) transferred are not
changed, except possibly for a reversible conversion to the internal register format of an
implementation.The copy is treated as a non floating-point operation for the purposes of NaN handling and
flush-to-zero mode. In particular, the VFP architecture requires:
• a load or store of a signaling NaN not to raise an Invalid Operation exception, nor to change the
signaling NaN into a quiet NaN
• a load or store of a denormalized number in flush-to-zero mode not to change it into +0.
PUW
C3-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-622-320.jpg)
![VFP Instruction Set Overview
Table 3-6 shows the assignment of register transfer opcodes and other details of the instructions:
Table 3-6 VFP register transfer instructions
opcode cp_num L Instruction name Instruction functionality
000 0b1010 0 FMSR Sn = Rd
000 0b1010 1 FMRS Rd = Sn
000 0b1011 0 FMDLR Dn[31:0] = Rd
000 0b1011 1 FMRDL Rd = Dn[31:0]
001 0b1010 x - UNDEFINED
001 0b1011 0 FMDHR Dn[63:32] = Rd
001 0b1011 1 FMRDH Rd = Dn[63:32]
01x 0b101x x - UNDEFINED
10x 0b101x x - UNDEFINED
110 0b101x x - UNDEFINED
111 0b1010 0 FMXR SystemReg(Fn,N) = Rd
111 0b1010 1 FMRX Rd = SystemReg(Fn,N)
111 0b1011 x - UNDEFINED
Table 3-7 shows how system registers are encoded in FMXR and FMRX instructions:
Table 3-7 VFP system register encodings
Fn N System register
0b0000 0 FPSID
0b0001 0 FPSCR
0b1000 0 FPEXC
Encodings that are not shown in this table are:
• Reserved for future expansion if the top bit of Fn is 0. FMXR and FMRX instructions using these
encodings are UNDEFINED.
• Reserved for additional IMPLEMENTATION DEFINED system registers if the top bit of Fn is 1. FMXR
and FMRX instructions using these encodings are IMPLEMENTATION DEFINED.
C3-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-626-320.jpg)
![VFP Instructions
4.1 Alphabetical list of VFP instructions
Each VFP instruction is described in detail on the following pages.
4.1.1 FABSD
31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 0 1 1 0 0 0 0 Dd 1 0 1 1 1 1 0 0 Dm
The FABSD (Floating-point Absolute Value, Double-precision) instruction writes the absolute value of a
double-precision register to another double-precision register. It can also perform a vector version of this
operation.
Syntax
FABSD{<cond>} <Dd>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dm> Specifies the source register.
Architecture version
D variants only
Exceptions
None
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = abs(Dm[i])
C4-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-630-320.jpg)
![VFP Instructions
Notes
Absolute value function
The function abs(x) means a copy of x with its sign bit forced to zero, as defined in the
Appendix to the IEEE 754-1985 standard.
Flush-to-zero mode
The FZ bit of the FPSCR does not affect the operand or result of this instruction.
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FABSD performs
just one absolute value operation, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FABSD might perform
more than one absolute value operation. Addressing Mode 4 - Double-precision vectors
(monadic) on page C5-19 describes how FABSD encodes the registers it uses and how
vec_len, Dd[i], and Dm[i] are determined.
Signaling NaNs
To comply with the VFP architecture, FABSD must not generate an exception even if the
value in its source register is a signaling NaN. This is a more stringent requirement than the
one in the Appendix to the IEEE 754-1985 standard.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-3](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-631-320.jpg)
![VFP Instructions
4.1.2 FABSS
31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 D 1 1 0 0 0 0 Fd 1 0 1 0 1 1 M 0 Fm
The FABSS (Floating-point Absolute Value, Single-precision) instruction writes the absolute value of a
single-precision register to another single-precision register. It can also perform a vector version of this
operation.
Syntax
FABSS{<cond>} <Sd>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom
bit).
<Sm> Specifies the source register. Its number is encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
None
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = abs(Sm[i])
C4-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-632-320.jpg)
![VFP Instructions
Notes
Absolute value function
The function abs(x) means a copy of x with its sign bit forced to zero, as defined in the
Appendix to the IEEE 754-1985 standard.
Flush-to-zero mode
The FZ bit of the FPSCR does not affect the operand or result of this instruction.
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FABSS performs
just one absolute value operation, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FABSS might perform
more than one absolute value operation. Addressing Mode 3 - Single-precision vectors
(monadic) on page C5-14 describes how FABSS encodes the registers it uses and how
vec_len, Sd[i], and Sm[i] are determined.
Signaling NaNs
To comply with the VFP architecture, FABSS must not generate an exception even if the
value in its source register is a signaling NaN. This is a more stringent requirement than the
one in the Appendix to the IEEE 754-1985 standard.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-5](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-633-320.jpg)
![VFP Instructions
4.1.3 FADDD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 1 1 Dn Dd 1 0 1 1 0 0 0 0 Dm
The FADDD (Floating-point Addition, Double-precision) instruction adds together two double-precision
registers and writes the result to a third double-precision register. It can also perform a vector version of this
operation.
Syntax
FADDD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dn> Specifies the register that contains the first operand for the addition.
<Dm> Specifies the register that contains the second operand for the addition.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = Dn[i] + Dm[i]
C4-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-634-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FADDD performs
just one addition, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FADDD might perform
more than one addition. Addressing Mode 2 - Double-precision vectors (non-monadic) on
page C5-8 describes how FADDD encodes the registers it uses and how vec_len, Dd[i],
Dn[i], and Dm[i] are determined.
Rounding The operation is a fully-rounded addition. The rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-7](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-635-320.jpg)
![VFP Instructions
4.1.4 FADDS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 D 1 1 Fn Fd 1 0 1 0 N 0 M 0 Fm
The FADDS (Floating-point Addition, Single-precision) instruction adds together two single-precision
registers and writes the result to a third single-precision register. It can also perform a vector version of this
operation.
Syntax
FADDS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom
bit).
<Sn> Specifies the register that contains the first operand for the addition. Its number is encoded
as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the addition. Its number is
encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = Sn[i] + Sm[i]
C4-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-636-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FADDS performs
just one addition, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FADDS might perform
more than one addition. Addressing Mode 1 - Single-precision vectors (non-monadic) on
page C5-2 describes how FADDS encodes the registers it uses and how vec_len, Sd[i],
Sn[i], and Sm[i] are determined.
Rounding The operation is a fully-rounded addition. The rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-9](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-637-320.jpg)
![VFP Instructions
4.1.13 FCPYD
31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 0 1 1 0 0 0 0 Dd 1 0 1 1 0 1 0 0 Dm
The FCPYD (Floating-point Copy, Double-precision) instruction copies one double-precision register to
another double-precision register. It can also perform a vector version of this operation.
Syntax
FCPYD{<cond>} <Dd>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dm> Specifies the source register.
Architecture version
D variants only
Exceptions
None
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = Dm[i]
C4-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-654-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FCPYD performs
just one copy, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FCPYD might perform
more than one copy. Addressing Mode 4 - Double-precision vectors (monadic) on
page C5-19 describes how FCYPD encodes the registers it uses and how vec_len,
Dd[i], and Dm[i] are determined.
Flush-to-zero mode
The FZ bit of the FPSCR does not affect the operand or result of this instruction.
Signaling NaNs
To comply with the VFP architecture, FCPYD must not generate an exception even if the
value in its source register is a signaling NaN. This is a more stringent requirement than the
one in the IEEE 754-1985 standard.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-27](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-655-320.jpg)
![VFP Instructions
4.1.14 FCPYS
31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 D 1 1 0 0 0 0 Fd 1 0 1 0 0 1 M 0 Fm
The FCPYS (Floating-point Copy, Single-precision) instruction copies one single-precision register to
another single-precision register. It can also perform a vector version of this operation.
Syntax
FCPYS{<cond>} <Sd>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D
(bottom bit).
<Sm> Specifies the source register. The register number is encoded as Fm (top 4 bits) and M
(bottom bit).
Architecture version
All
Exceptions
None
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = Sm[i]
C4-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-656-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FCPYS performs
just one copy, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FCPYD might perform
more than one copy. Addressing Mode 3 - Single-precision vectors (monadic) on
page C5-14 describes how FCYPS encodes the registers it uses and how vec_len,
Sd[i], and Sm[i] are determined.
Flush-to-zero mode
The FZ bit of the FPSCR does not affect the operand or result of this instruction.
Signaling NaNs
To comply with the VFP architecture, FCPYS must not generate an exception even if the
value in its source register is a signaling NaN. This is a more stringent requirement than the
one in the IEEE 754-1985 standard.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-29](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-657-320.jpg)
![VFP Instructions
4.1.17 FDIVD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 0 0 0 Dn Dd 1 0 1 1 0 0 0 0 Dm
The FDIVD (Floating-point Divide, Double-precision) instruction divides one double-precision register by
another double-precision register and writes the result to a third double-precision register. It can also
perform a vector version of this operation.
Syntax
FDIVD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dn> Specifies the register that contains the first operand for the division.
<Dm> Specifies the register that contains the second operand for the division.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Division by Zero, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = Dn[i] / Dm[i]
Usage
Divisions take a large number of cycles on most implementations, and vector divisions take proportionately
longer. This can have a major effect on performance.
If a lot of divisions by the same number are wanted, the performance can usually be improved by using one
division to calculate the number’s reciprocal, followed by a lot of multiplications by that reciprocal. This
slightly reduces the accuracy of the calculations, since they incur two rounding errors rather than one, but
this is often an acceptable tradeoff.
Also see Interrupts on page C1-5 for a description of some implications for interrupt latency.
C4-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-660-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FDIVD performs
just one division, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FDIVD might perform
more than one division. Addressing Mode 2 - Double-precision vectors (non-monadic) on
page C5-8 describes how FDIVD encodes the registers it uses and how vec_len, Dd[i],
Dn[i], and Dm[i] are determined.
Rounding The operation is a fully-rounded division. The rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-33](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-661-320.jpg)
![VFP Instructions
4.1.18 FDIVS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 D 0 0 Fn Fd 1 0 1 0 N 0 M 0 Fm
The FDIVS (Floating-point Divide, Single-precision) instruction divides one single-precision register by
another single-precision register and writes the result to a third single-precision register. It can also perform
a vector version of this operation.
Syntax
FDIVS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D
(bottom bit).
<Sn> Specifies the register that contains the first operand for the division. The register number is
encoded as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the division. The register number
is encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Division by Zero, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = Sn[i] / Sm[i]
C4-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-662-320.jpg)
![VFP Instructions
Usage
Divisions take a large number of cycles on most implementations, and vector divisions take proportionately
longer. This can have a major effect on performance.
If a lot of divisions by the same number are wanted, the performance can usually be improved by using one
division to calculate the number’s reciprocal, followed by a lot of multiplications by that reciprocal. This
slightly reduces the accuracy of the calculations, since they incur two rounding errors rather than one, but
this is often an acceptable tradeoff.
Also see Interrupts on page C1-5 for a description of some implications for interrupt latency.
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FDIVS performs
just one division, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FDIVS might perform
more than one division. The way FDIVS encodes the registers it uses and how vec_len,
Sd[i], Sn[i], and Sm[i] are determined is described on Addressing Mode 1 -
Single-precision vectors (non-monadic) on page C5-2.
Rounding The operation is a fully-rounded division. The rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-35](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-663-320.jpg)
![VFP Instructions
4.1.19 FLDD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0
cond 1 1 0 1 U 0 0 1 Rn Dd 1 0 1 1 offset
The FLDD (Floating-point Load, Double-precision) instruction loads a double-precision register from
memory.
Syntax
FLDD{<cond>} <Dd>, [<Rn>{, #+/-(<offset>*4)}]
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Rn> Specifies the register holding the base address for the transfer.
<offset> Specifies an offset to be multiplied by 4, then added to the base address (if U == 1) or
subtracted from it (if U == 0) in order to form the actual address of the transfer. If this offset
is omitted, it defaults to +0.
Architecture version
D variants only
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
if (U == 1)
address = Rn + offset * 4
else
address = Rn - offset * 4
if (big-endian)
Dd = [Memory[address,4] << 32) OR Memory[address+4,4]
else
Dd = [Memory[address+4,4] << 32) OR Memory[address,4]
C4-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-664-320.jpg)
![VFP Instructions
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to (offset-2)/2
/* d is the number of register Dd; */
/* D(n) is the double-precision register numbered n */
if (big-endian)
D(d+i) = (Memory[address,] << 32) OR Memory[address+4,4]
else
D(d+i) = (Memory[address+4,4] << 32) OR Memory[address,4]
address = address + 8
assert end_address = address - 4
Notes
Encoding If P=1 and W=0, the instruction is an FLDD instruction instead. Otherwise, if offset is odd,
the instruction is an FLDMX instruction instead.
Vectors The FLDMD instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and
does not wrap around at bank boundaries in the way that vector operands to data-processing
instructions do. Registers are loaded in simple increasing order of register number.
Invalid register lists
If Dd and offset do not specify a valid register list, the instruction is UNPREDICTABLE.
This happens in two cases:
• if offset == 0, that is, if an attempt is made to transfer no registers
• if d + offset/2 > 16, that is, if an attempt is made to transfer another register
after D15.
Conversions In the programmer’s model, FLDMD does not perform any conversion on the value
transferred. Implementations are free to convert the value transferred to an internal format,
provided they can recover the correct double-precision value as necessary.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-39](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-667-320.jpg)
![VFP Instructions
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to offset-1
/* d is as defined for <registers> above; */
/* S(n) is the single-precision register numbered n */
S(d+i) = Memory[address,4]
address = address + 4
assert end_address = address - 4
Notes
Encoding If P=1 and W=0, the instruction is an FLDS instruction instead.
Vectors The FLDMS instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and
does not wrap around at bank boundaries in the way that vector operands to data-processing
instructions do. Registers are loaded in simple increasing order of register number.
Invalid register lists
If Fd, D and offset do not specify a valid register list, the instruction is UNPREDICTABLE.
This happens in two cases:
• if offset == 0, that is, if an attempt is made to transfer no registers
• if d + offset > 32, that is, if an attempt is made to transfer another register after
S31.
Conversions In the programmer’s model, FLDMS does not perform any conversion on the values
transferred. The memory words can hold either integers or single-precision floating-point
numbers. Most VFP arithmetic instructions treat the loaded values as single-precision
floating-point numbers. If they are integers, they need to be converted using the
integer-to-floating-point conversion instructions before such arithmetic instructions can
yield sensible results.Implementations are free to convert the values transferred to an
internal format, provided they can recover either the correct single-precision value or the
correct integer value for each one (depending on how the registers are subsequently used).
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-41](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-669-320.jpg)
![VFP Instructions
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
/* d is the number of register Dd; */
/* D(n) is the double-precision register numbered n */
Load registers D(d) to D(d+(offset-3)/2) from memory words
Memory[start_address,4] through to Memory[end_address-4,4]
Usage
FLDMX is used to reload VFP register values from memory when FSTMX was previously used to store them.
Typical cases in which it is used are:
• in procedure exit sequences when a callee-save procedure-calling standard is being used
• in process swap code.
Notes
Encoding If P=1 and W=0, the instruction is an FLDD instruction instead. Otherwise, if offset is
even, the instruction is an FLDMD instruction instead.
Vectors The FLDMX instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and
does not wrap around at bank boundaries in the way that vector operands to data-processing
instructions do. Registers are loaded in simple increasing order of register number.
Invalid register lists
If Dd and offset do not specify a valid register list, the instruction is UNPREDICTABLE.
This happens in two cases:
• if offset == 0, that is, if an attempt is made to transfer no registers
• if d + (offset-1)/2 > 16, that is, if an attempt is made to transfer another
register after D15.
Memory format
The memory format used is IMPLEMENTATION DEFINED. The only requirements on it are:
• FLDMX must reload the register values stored by a matching FSTMX instruction
correctly, regardless of whether they originally contained integers, single-precision
values or double-precision values. For an FLDMX instruction and an FSTMX
instruction to count as matching, they must specify identical register lists and must
generate the same start_address and end_address when executed.
• It must not use more than the allowed number of memory words, that is, 2N+1
memory words for storing N double-precision registers.
Non implementation-specific code must not do anything with FLDMX other than reload the
results of a matching FSTMX instruction.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-43](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-671-320.jpg)
![VFP Instructions
4.1.23 FLDS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0
cond 1 1 0 1 U D 0 1 Rn Fd 1 0 1 0 offset
The FLDS (Floating-point Load, Single-precision) instruction loads a single-precision register from
memory.
Syntax
FLDS{<cond>} <Sd>, [<Rn>{, #+/-(<offset>*4)}]
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom
bit).
<Rn> Specifies the register holding the base address for the transfer.
<offset> Specifies an offset to be multiplied by 4, then added to the base address (if U == 1) or
subtracted from it (if U == 0) in order to form the actual address of the transfer. If this offset
is omitted, it defaults to +0.
Architecture version
All
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
if (U == 1)
address = Rn + offset * 4
else
address = Rn - offset * 4
Sd = Memory[address,4]
C4-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-672-320.jpg)
![VFP Instructions
4.1.24 FMACD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 0 0 Dn Dd 1 0 1 1 0 0 0 0 Dm
The FMACD (Floating-point Multiply and Accumulate, Double-precision) instruction multiplies together
two double-precision registers, adds a third double-precision register to the product and writes the result to
the third register. It can also perform a vector version of this operation.
Syntax
FMACD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register, which is also used as the second operand for the addition.
<Dn> Specifies the register that contains the first operand for the multiplication.
<Dm> Specifies the register that contains the second operand for the multiplication.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = Dn[i] * Dm[i] + Dd[i]
C4-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-674-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMACD performs
just one multiply-add operation, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and
Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FMACD might perform
more than one multiply-add operation. Addressing Mode 2 - Double-precision vectors
(non-monadic) on page C5-8 describes how FMACD encodes the registers it uses and how
vec_len, Dd[i], Dn[i] and Dm[i] are determined.
Rounding The operation is a fully-rounded multiplication followed by a fully-rounded addition. The
rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-47](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-675-320.jpg)
![VFP Instructions
4.1.25 FMACS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 D 0 0 Fn Fd 1 0 1 0 N 0 M 0 Fm
The FMACS (Floating-point Multiply and Accumulate, Single-precision) instruction multiplies together two
single-precision registers, adds a third single-precision register to the product and writes the result to the
third register. It can also perform a vector version of this operation.
Syntax
FMACS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register, which is also used as the second operand for the addition.
The register number is encoded as Fd (top 4 bits) and D (bottom bit).
<Sn> Specifies the register that contains the first operand for the multiplication. The register
number is encoded as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the multiplication. The register
number is encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = Sn[i] * Sm[i] + Sd[i]
C4-48 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-676-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMACS performs
just one multiply-add operation, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and
Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FMACS might perform
more than one multiply-add operation. Addressing Mode 1 - Single-precision vectors
(non-monadic) on page C5-2 shows how FMACS encodes registers and determines
vec_len, Sd[i], Sn[i] and Sm[i].
Rounding The operation is a fully-rounded multiplication followed by a fully-rounded addition. The
rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-49](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-677-320.jpg)
![VFP Instructions
4.1.26 FMDHR
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 1 0 Dn Rd 1 0 1 1 0 SBZ 1 SBZ
The FMDHR (Floating-point Move to Double-precision High from Register) instruction transfers the
contents of the ARM register Rd to the upper half of the double-precision register Dn. It is used in
conjunction with FMDLR to transfer double-precision values between ARM registers and floating-point
registers.
Syntax
FMDHR{<cond>} <Dn>, <Rd>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dn> Specifies the destination register.
<Rd> Specifies the source ARM register.
Architecture version
D variants only
Exceptions
None
Operation
if ConditionPassed(cond) then
Dn[63:32] = Rd
C4-50 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-678-320.jpg)
![VFP Instructions
4.1.27 FMDLR
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 0 0 Dn Rd 1 0 1 1 0 SBZ 1 SBZ
The FMDLR (Floating-point Move to Double-precision Low from Register) instruction transfers the
contents of the ARM register Rd to the lower half of the double-precision register Dn. Used with FMDHR,
it transfers double-precision values between ARM registers and floating-point registers.
Syntax
FMDLR{<cond>} <Dn>, <Rd>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dn> Specifies the destination register.
<Rd> Specifies the source ARM register.
Architecture version
D variants only
Exceptions
None
Operation
if ConditionPassed(cond) then
Dn[31:0] = Rd
C4-52 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-680-320.jpg)
![VFP Instructions
4.1.28 FMRDH
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 1 1 Dn Rd 1 0 1 1 0 SBZ 1 SBZ
The FMRDH (Floating-point Move to Register from Double-precision High) instruction transfers the upper
half of the contents of the double-precision register Dn to the ARM register Rd. It is used in conjunction
with FMRDL to transfer double-precision values between ARM registers and floating-point registers.
Syntax
FMRDH{<cond>} <Rd>, <Dn>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination ARM register.
<Dn> Specifies the source register.
Architecture version
D variants only
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Dn[63:32]
Notes
Conversions If an implementation uses an internal format for double-precision values, it must convert
that format back to the external double-precision format. Otherwise, no conversion is
required.
C4-54 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-682-320.jpg)
![VFP Instructions
4.1.29 FMRDL
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 0 1 Dn Rd 1 0 1 1 0 SBZ 1 SBZ
The FMRDL (Floating-point Move to Register from Double-precision Low) instruction transfers the lower
half of the contents of the double-precision register Dn to the ARM register Rd. It is used in conjunction
with FMRDH to transfer double-precision values between ARM registers and floating-point registers.
Syntax
FMRDL{<cond>} <Rd>, <Dn>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Rd> Specifies the destination ARM register.
<Dn> Specifies the source register.
Architecture version
D variants only
Exceptions
None
Operation
if ConditionPassed(cond) then
Rd = Dn[31:0]
Notes
Conversions If an implementation uses an internal format for double-precision values, it must convert
that format back to the external double-precision format. Otherwise, no conversion is
required.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-55](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-683-320.jpg)
![VFP Instructions
Notes
Serialization FMRX is a serializing instruction. See System register transfer instructions on page C3-20
for details of what this means.
Exception processing
After serialization, if the VFP system contains a hardware coprocessor, that coprocessor
might have a pending exception to process. Whether the FMRX instruction triggers the
processing of such an instruction depends on which system register is being transferred, as
described in the following notes. If exception processing is triggered, this causes the FMRX
instruction to take the ARM’s undefined instruction trap.
Reading FPSID
An FMRX instruction with source FPSID can be executed in any ARM processor mode.
After serialization, it writes the value of the FPSID to Rd, and does not trigger exception
processing.
Reading FPSCR
An FMRX instruction with source FPSCR can be executed in any ARM processor mode.
After serialization, exception processing is triggered if necessary. Otherwise, the value of
the FPSCR is written to Rd.
Reading FPEXC
An FMRX instruction with source FPEXC can only be executed in privileged ARM
processor modes. An attempt to execute it in User mode causes the ARM’s undefined
instruction trap to be taken.
After serialization, it writes the value of FPEXC to Rd, and does not trigger exception
processing. Because all but bits[31:30] of FPEXC is IMPLEMENTATION DEFINED,
non implementation-specific code must only rely on bits[31:0] of the value written to Rd.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-59](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-687-320.jpg)
![VFP Instructions
4.1.32 FMSCD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 0 1 Dn Dd 1 0 1 1 0 0 0 0 Dm
The FMSCD (Floating-point Multiply and Subtract, Double-precision) instruction multiplies together two
double-precision registers, subtracts a third double-precision register from the product and writes the result
to the third register. It can also perform a vector version of this operation.
Syntax
FMSCD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register, which is also the second operand for the subtraction.
<Dn> Specifies the register that contains the first operand for the multiplication.
<Dm> Specifies the register that contains the second operand for the multiplication.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = Dn[i] * Dm[i] - Dd[i]
C4-60 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-688-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMSCD performs
just one multiply-subtract operation, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and
Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FMSCD might perform
more than one multiply-subtract operation. Addressing Mode 2 - Double-precision vectors
(non-monadic) on page C5-8 describes how FMSCD encodes the registers it uses and how
vec_len, Dd[i], Dn[i], and Dm[i] are determined.
Rounding The operation is a fully-rounded multiplication followed by a fully-rounded subtraction.
The rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-61](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-689-320.jpg)
![VFP Instructions
4.1.33 FMSCS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 D 0 1 Fn Fd 1 0 1 0 N 0 M 0 Fm
The FMSCS (Floating-point Multiply and Subtract, Single-precision) instruction multiplies together two
single-precision registers, subtracts a third single-precision register from the product and writes the result
to the third register. It can also perform a vector version of this operation.
Syntax
FMSCS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register, which is also used as the second operand for the
subtraction. The register number is encoded as Fd (top 4 bits) and D (bottom bit).
<Sn> Specifies the register that contains the first operand for the multiplication. The register
number is encoded as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the multiplication. The register
number is encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = Sn[i] * Sm[i] - Sd[i]
C4-62 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-690-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMSCS performs
just one multiply-subtract operation, where vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and
Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FMSCS might perform
more than one multiply-subtract operation. Addressing Mode 1 - Single-precision vectors
(non-monadic) on page C5-2 shows how FMSCS encodes registers and determines
vec_len, Sd[i], Sn[i], and Sm[i].
Rounding The operation is a fully-rounded multiplication followed by a fully-rounded subtraction.
The rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-63](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-691-320.jpg)
![VFP Instructions
4.1.36 FMULD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 1 0 Dn Dd 1 0 1 1 0 0 0 0 Dm
The FMULD (Floating-point Multiply, Double-precision) instruction multiplies together two
double-precision registers and writes the result to a third double-precision register. It can also perform a
vector version of this operation.
Syntax
FMULD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dn> Specifies the register that contains the first operand for the multiplication.
<Dm> Specifies the register that contains the second operand for the multiplication.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = Dn[i] * Dm[i]
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMULD performs
one multiplication, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FMULD might perform
more than one multiplication. Addressing Mode 2 - Double-precision vectors
(non-monadic) on page C5-8 describes how FMULD encodes the registers it uses and how
vec_len, Dd[i], Dn[i], and Dm[i] are determined.
Rounding This is a fully-rounded multiplication. The rounding mode is determined by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-67](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-695-320.jpg)
![VFP Instructions
4.1.37 FMULS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 D 1 0 Fn Fd 1 0 1 0 N 0 M 0 Fm
The FMULS (Floating-point Multiply, Single-precision) instruction multiplies together two single-precision
registers and writes the result to a third single-precision register. It can also perform a vector version of this
operation.
Syntax
FMULS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom
bit).
<Sn> Specifies the register that contains the first operand for the multiplication. Its number is
encoded as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the multiplication. Its number is
encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = Sn[i] * Sm[i]
C4-68 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-696-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMULS performs
just one multiplication, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FMULS might perform
more than one multiplication. Addressing Mode 1 - Single-precision vectors (non-monadic)
on page C5-2 shows how FMULS encodes the registers it uses and determines vec_len,
Sd[i], Sn[i], and Sm[i].
Rounding The operation is a fully-rounded multiplication. The rounding mode is determined by the
FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-69](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-697-320.jpg)
![VFP Instructions
Notes
Serialization FMXR is a serializing instruction. See System register transfer instructions on page C3-20
for details of what this means.
Exception processing
After serialization, if the VFP system contains a hardware coprocessor, that coprocessor
might have a pending exception to process. Whether the FMXR instruction triggers the
processing of such an instruction depends on which system register is being transferred, as
described in the following notes. If exception processing is triggered, this causes the FMXR
instruction to take the ARM’s undefined instruction trap.
Writing FPSID
An FMXR instruction with destination FPSID can be executed in any ARM processor mode.
It is a serializing no-op, because FPSID is a read-only register, and does not trigger
exception processing.
Writing FPSCR
An FMXR instruction with destination FPSCR can be executed in any ARM processor mode.
After serialization, exception processing is triggered if necessary. Otherwise, the value of
Rd is written to the FPSCR.
Writing FPEXC
An FMXR instruction with destination FPEXC can only be executed in privileged ARM
processor modes. An attempt to execute it in User mode causes the ARM’s undefined
instruction trap to be taken.
After serialization, it writes the value of Rd to FPEXC, and does not trigger exception
processing. Because all but bits[31:30] of FPEXC is IMPLEMENTATION DEFINED,
non implementation-specific code must only use such an instruction as part of a
read/modify bits[31:0]/write sequence.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-71](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-699-320.jpg)
![VFP Instructions
4.1.39 FNEGD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 0 1 1 0 0 0 1 Dd 1 0 1 1 0 1 0 0 Dm
The FNEGD (Floating-point Negate, Double-precision) instruction negates the value of a double-precision
register and writes the result to another double-precision register. It can also perform a vector version of this
operation.
Syntax
FNEGD{<cond>} <Dd>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dm> Specifies the source register.
Architecture version
D variants only
Exceptions
None
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = -(Dm[i])
C4-72 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-700-320.jpg)
![VFP Instructions
Notes
Negation The function -(x) means a copy of x with its sign bit reversed, as defined in the Appendix
to the IEEE 754-1985 standard.
Flush-to-zero mode
The FZ bit of the FPSCR does not affect the operand or result of this instruction.
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNEGD performs
just one negation operation, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FNEGD might perform
more than one negation operation. Addressing Mode 4 - Double-precision vectors
(monadic) on page C5-19 shows how FNEGD encodes its registers and determines the
values of vec_len, Dd[i], and Dm[i].
Signaling NaNs
To comply with the VFP architecture, FNEGD must not generate an exception even if the
value in its source register is a signaling NaN. This is a more stringent requirement than the
one in the Appendix to the IEEE 754-1985 standard.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-73](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-701-320.jpg)
![VFP Instructions
4.1.40 FNEGS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 D 1 1 0 0 0 1 Fd 1 0 1 0 0 1 M 0 Fm
The FNEGS (Floating-point Negate, Single-precision) instruction negates the value of a single-precision
register and writes the result to another single-precision register. It can also perform a vector version of this
operation.
Syntax
FNEGS{<cond>} <Sd>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom
bit).
<Sm> Specifies the source register. Its number is encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
None
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = -(Sm[i])
C4-74 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-702-320.jpg)
![VFP Instructions
Notes
Negation The function -(x) means a copy of x with its sign bit reversed, as defined in the Appendix
to the IEEE 754-1985 standard.
Flush-to-zero mode
The FZ bit of the FPSCR does not affect the operand or result of this instruction.
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNEGS performs
just one negation operation, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FNEGS might perform
more than one negation operation. Addressing Mode 3 - Single-precision vectors (monadic)
on page C5-14 shows how FNEDS encodes its registers and determines vec_len, Sd[i],
and Sm[i].
Signaling NaNs
To comply with the VFP architecture, FNEGS must not generate an exception even if the
value in its source register is a signaling NaN. This is a more stringent requirement than the
one in the Appendix to the IEEE 754-1985 standard.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-75](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-703-320.jpg)
![VFP Instructions
4.1.41 FNMACD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 0 0 Dn Dd 1 0 1 1 0 1 0 0 Dm
The FNMACD (Floating-point Negated Multiply and Accumulate, Double-precision) instruction multiplies
together two double-precision registers, adds a third double-precision register to the negation of the product
and writes the result to the third register. It can also perform a vector version of this operation.
Syntax
FNMACD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register, which is also used as the second operand for the addition.
<Dn> Specifies the register that contains the first operand for the multiplication.
<Dm> Specifies the register that contains the second operand for the multiplication.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = -(Dn[i] * Dm[i]) + Dd[i]
C4-76 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-704-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMACD
performs just one multiply-negate-add operation, and vec_len=1, Dd[0]=Dd,
Dn[0]=Dn, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FNMACD might perform
more than one multiply-negate-add operation. Addressing Mode 4 - Double-precision
vectors (monadic) on page C5-19 shows how FNMACD encodes its registers and determines
vec_len, Dd[i], Dn[i], and Dm[i].
Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the
FPSCR, followed by reversal of the sign bit and a fully-rounded addition, using the same
rounding mode.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-77](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-705-320.jpg)
![VFP Instructions
4.1.42 FNMACS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 D 0 0 Fn Fd 1 0 1 0 N 1 M 0 Fm
The FNMACS (Floating-point Negated Multiply and Accumulate, Single-precision) instruction multiplies
together two single-precision registers, adds a third single-precision register to the negation of the product
and writes the result to the third register. It can also perform a vector version of this.
Syntax
FNMACS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register, which is also used as the second operand for the addition.
Its number is encoded as Fd (top 4 bits) and D (bottom bit).
<Sn> Specifies the register that contains the first operand for the multiplication. Its number is
encoded as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the multiplication. Its number is
encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = -(Sn[i] * Sm[i]) + Sd[i]
C4-78 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-706-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMACS
performs just one multiply-negate-add operation, and vec_len=1, Sd[0]=Sd,
Sn[0]=Sn, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FNMACS might perform
more than one multiply-negate-add operation. Addressing Mode 1 - Single-precision vectors
(non-monadic) on page C5-2 describes how FNMACS encodes the registers it uses and how
vec_len, Sd[i], Sn[i], and Sm[i] are determined.
Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the
FPSCR, followed by reversal of the sign bit and a fully-rounded addition, using the same
rounding mode.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-79](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-707-320.jpg)
![VFP Instructions
4.1.43 FNMSCD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 0 1 Dn Dd 1 0 1 1 0 1 0 0 Dm
The FNMSCD (Floating-point Negated Multiply and Subtract, Double-precision) instruction multiplies
together two double-precision registers, subtracts a third double-precision register from the negation of the
product and writes the result to the third register. It can also perform a vector version of this operation.
Syntax
FNMSCD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register, and is also used as the second operand for the subtraction.
<Dn> Specifies the register that contains the first operand for the multiplication.
<Dm> Specifies the register that contains the second operand for the multiplication.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = -(Dn[i] * Dm[i]) - Dd[i]
C4-80 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-708-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMSCD
performs just one multiply-negate-subtract operation, and vec_len=1, Dd[0]=Dd,
Dn[0]=Dn, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FNMSCD might perform
more than one multiply-negate-subtract operation. Addressing Mode 2 - Double-precision
vectors (non-monadic) on page C5-8 describes how FNMSCD encodes the registers it uses
and how vec_len, Dd[i], Dn[i], and Dm[i] are determined.
Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the
FPSCR, followed by reversal of the sign bit and a fully-rounded subtraction, using the same
rounding mode.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-81](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-709-320.jpg)
![VFP Instructions
4.1.44 FNMSCS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 D 0 1 Fn Fd 1 0 1 0 N 1 M 0 Fm
The FNMSCS Floating-point Negated Multiply and Subtract, Single-precision() instruction multiplies
together two single-precision registers, subtracts a third single-precision register from the negation of the
product and writes the result to the third register. It can also perform a vector version of this operation.
Syntax
FNMSCS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register, which is also used as the second operand for the
subtraction. The register number is encoded as Fd (top 4 bits) and D (bottom bit).
<Sn> Specifies the register that contains the first operand for the multiplication. The register
number is encoded as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the multiplication. The register
number is encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = -(Sn[i] * Sm[i]) - Sd[i]
C4-82 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-710-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMSCS
performs just one multiply-negate-subtract operation, and vec_len=1, Sd[0]=Sd,
Sn[0]=Sn, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FNMSCS might perform
more than one multiply-negate-subtract operation. Addressing Mode 1 - Single-precision
vectors (non-monadic) on page C5-2 describes how FNMSCS encodes the registers it uses
and how vec_len, Sd[i], Sn[i], and Sm[i] are determined.
Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the
FPSCR, followed by reversal of the sign bit and a fully-rounded subtraction, using the same
rounding mode.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-83](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-711-320.jpg)
![VFP Instructions
4.1.45 FNMULD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 0 1 0 Dn Dd 1 0 1 1 0 1 0 0 Dm
The FNMULD (Floating-point Negated Multiply, Double-precision) instruction multiplies together two
double-precision registers, and writes the negation of the result to a third double-precision register. It can
also perform a vector version of this operation.
Syntax
FNMULD{<cond>} <Dd>, <Dn>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dn> Specifies the register that contains the first operand for the multiplication.
<Dm> Specifies the register that contains the second operand for the multiplication.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = -(Dn[i] * Dm[i])
C4-84 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-712-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMULD
performs just one negated multiplication, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and
Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FNMULD might perform
more than one negated multiplication. Addressing Mode 2 - Double-precision vectors
(non-monadic) on page C5-8 describes how FNMULD encodes the registers it uses and how
vec_len, Dd[i], Dn[i], and Dm[i] are determined.
Rounding The operation is a fully-rounded multiplication. The rounding mode is determined by the
FPSCR, followed by reversal of the sign bit of the result.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-85](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-713-320.jpg)
![VFP Instructions
4.1.46 FNMULS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 0 D 1 0 Fn Fd 1 0 1 0 N 1 M 0 Fm
The FNMULS (Floating-point Negated Multiply, Single-precision) instruction multiplies together two
single-precision registers, and writes the negation of the result to a third single-precision register. It can also
perform a vector version of this operation.
Syntax
FNMULS{<cond>} <Sd>, <Sn>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D
(bottom bit).
<Sn> Specifies the register that contains the first operand for the multiplication. The register
number is encoded as Fn (top 4 bits) and N (bottom bit).
<Sm> Specifies the register that contains the second operand for the multiplication. The register
number is encoded as Fm (top 4 bits) and M (bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = -(Sn[i] * Sm[i])
C4-86 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-714-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMULS
performs just one negated multiplication, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and
Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FNMULS might perform
more than one negated multiplication. Addressing Mode 1 - Single-precision vectors
(non-monadic) on page C5-2 shows how FNMULS encodes its registers and determines
vec_len, Sd[i], Sn[i], and Sm[i].
Rounding The operation is a fully-rounded multiplication. The rounding mode is determined by the
FPSCR, followed by reversal of the sign bit of the result.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-87](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-715-320.jpg)
![VFP Instructions
4.1.49 FSQRTD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 0 1 1 0 0 0 1 Dd 1 0 1 1 1 1 0 0 Dm
The FSQRTD (Floating-point Square Root, Double-precision) instruction calculates the square root of the
value in a double-precision register and writes the result to another double-precision register. It can also
perform a vector version of this operation.
Syntax
FSQRTD{<cond>} <Dd>, <Dm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the destination register.
<Dm> Specifies the source register.
Architecture version
D variants only
Exceptions
Floating-point exceptions: Invalid Operation, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Dd[i] = sqrt(Dm[i])
Usage
Square roots take a large number of cycles on most implementations, and vector square roots take
proportionately longer. This can have a major effect on performance, and so the use of large numbers of
square roots should be avoided where possible.
Also see Interrupts on page C1-5 for a description of some implications for interrupt latency.
C4-90 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-718-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FSQRTD
performs just one square root operation, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm.
When the LEN field indicates a vector mode (vector length > 1), FSQRTD might perform
more than one square root operation. Addressing Mode 4 - Double-precision vectors
(monadic) on page C5-19 describes how FSQRTD encodes the registers it uses and how
vec_len, Dd[i], and Dm[i] are determined.
Rounding The operation is a fully-rounded square root operation. The rounding mode is determined
by the FPSCR.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-91](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-719-320.jpg)
![VFP Instructions
4.1.50 FSQRTS
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 1 0 1 D 1 1 0 0 0 1 Fd 1 0 1 0 1 1 M 0 Fm
The FSQRTS (Floating-point Square Root, Single-precision) instruction calculates the square root of the
value in a single-precision register and writes the result to another single-precision register. It can also
perform a vector version of this operation.
Syntax
FSQRTS{<cond>} <Sd>, <Sm>
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D
(bottom bit).
<Sm> Specifies the source register. The register number is encoded as Fm (top 4 bits) and M
(bottom bit).
Architecture version
All
Exceptions
Floating-point exceptions: Invalid Operation, Inexact
Operation
if ConditionPassed(cond) then
for i = 0 to vec_len-1
Sd[i] = sqrt(Sm[i])
Usage
Square roots take a large number of cycles on most implementations, and vector square roots take
proportionately longer. This can have a major effect on performance, and so the use of large numbers of
square roots should be avoided where possible.
Also see Interrupts on page C1-5 for a description of some implications for interrupt latency.
C4-92 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-720-320.jpg)
![VFP Instructions
Notes
Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FSQRTS
performs just one square root operation, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm.
When the LEN field indicates a vector mode (vector length > 1), FSQRTS might perform
more than one square root operation. Addressing Mode 3 - Single-precision vectors
(monadic) on page C5-14 describes how FSQRTS encodes the registers it uses and how
vec_len, Sd[i], and Sm[i] are determined.
Rounding This is a fully-rounded square root operation. The FPSCR determines the rounding mode.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-93](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-721-320.jpg)
![VFP Instructions
4.1.51 FSTD
31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0
cond 1 1 0 1 U 0 0 0 Rn Dd 1 0 1 1 offset
The FSTD (Floating-point Store, Double-precision) instruction stores a double-precision register to
memory.
Syntax
FSTD{<cond>} <Dd>, [<Rn>{, #+/-(<offset>*4)}]
where:
<cond> Is the condition under which the instruction is executed. The conditions are defined in The
condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used.
<Dd> Specifies the source register.
<Rn> Specifies the register holding the base address for the transfer.
<offset> Specifies an offset to be multiplied by 4, then added to the base address (if U == 1) or
subtracted from it (if U == 0) to form the actual address of the transfer. If offset is
omitted, it defaults to +0.
Architecture version
D variants only
Exceptions
Data Abort
Operation
if ConditionPassed(cond) then
if (U == 1)
address = Rn + offset * 4
else
address = Rn - offset * 4
if (big-endian)
Memory[address,4] = Dd[63:32]
Memory[address+4,4] = Dd[31:0]
else
Memory[address,4] = Dd[31:0]
Memory[address+4,4] = Dd[63:32]
C4-94 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-722-320.jpg)
![VFP Instructions
Operation
if ConditionPassed(cond) then
address = start_address
for i = 0 to (offset-2)/2
/* d is the number of register Dd; */
/* D(n) is the double-precision register numbered n */
if (big-endian)
Memory[address,4] = D(d+i)[63:32]
Memory[address+4,4] = D(d+i)[31:0]
else
Memory[address,4] = D(d+i)[31:0]
Memory[address+4,4] = D(d+i)[63:32]
address = address + 8
assert end_address = address - 4
Notes
Encoding If P=1 and W=0, the instruction is instead an FSTD instruction. Otherwise, if offset is odd,
the instruction is instead an FSTMX instruction.
Vectors The FSTMD instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and
does not wrap around at bank boundaries in the way that vector operands to data-processing
instructions do. Registers are stored in simple increasing order of register number.
Invalid register lists
If Dd and offset do not specify a valid register list, the instruction is UNPREDICTABLE.
This happens in two cases:
• if offset == 0, that is, if an attempt is made to transfer no registers
• if d + offset/2 > 16, that is, if an attempt is made to transfer another register
after D15.
Conversions If an implementation uses an internal format for double-precision values, it must convert
that format back to the external double-precision format. Otherwise, no conversion is
required.
ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-97](https://image.slidesharecdn.com/armarchitecturereferencemanual2ed-141024004907-conversion-gate01/85/Arm-architecture-reference-manual-2-ed-725-320.jpg)
Arm architecture reference manual 2 ed
- 1. ARM Architecture Reference Manual ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved.
- 2. Copyright © 1996–2000 ARM Limited. All rights reserved. Release Information The following changes have been made to this document. Change History Date Issue Change February 1996 A First edition. July 1997 B Updated and index added. April 1998 C Updated. February 2000 D Updated for ARM architecture v5. June 2000 E Updated for ARM architecture v5TE and corrections to Part B. Proprietary Notice ARM, the ARM Powered logo, Thumb, and StrongARM are registered trademarks of ARM Limited. The ARM logo, AMBA, Angel, ARMulator, EmbeddedICE, ModelGen, Multi-ICE, PrimeCell, ARM7TDMI, ARM7TDMI-S, ARM9TDMI, ARM9E-S, ETM7, ETM9, TDMI, STRONG, are trademarks of ARM Limited. All other products or services mentioned herein may be trademarks of their respective owners. Neither the whole nor any part of the information contained in, or the product described in, this document may be adapted or reproduced in any material form except with the prior written permission of the copyright holder. The product described in this document is subject to continuous developments and improvements. All particulars of the product and its use contained in this document are given by ARM in good faith. However, all warranties implied or expressed, including but not limited to implied warranties of merchantability, or fitness for purpose, are excluded. This document is intended only to assist the reader in the use of the product. ARM Limited shall not be liable for any loss or damage arising from the use of any information in this document, or any error or omission in such information, or any incorrect use of the product. ii Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 3. Preface This preface describes the versions of the ARM architecture and the contents of this manual, then lists the conventions and terminology it uses. • About this manual on page iv • Architecture versions and variants on page v • Using this manual on page x • Conventions on page xii. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. iii
- 4. Preface About this manual The purpose of this manual is to describe the ARM instruction set architecture, including its high code density Thumb subset, and two of its standard coprocessor extensions: • The standard System Control coprocessor (coprocessor 15), which is used to control memory system components such as caches, write buffers, Memory Management Units, and Protection Units. • The Vector Floating-point (VFP) architecture, which uses coprocessors 10 and 11 to supply a high-performance floating-point instruction set. These instruction sets are described primarily from the viewpoint of the instruction being a 32-bit word or 16-bit halfword. The precise effects of each instruction are described, including any restrictions on its use. This information is of primary importance to authors of compilers, assemblers, and other programs that generate ARM machine code. Assembler syntax is given for most of the instructions described in this manual, allowing instructions to be specified in textual form. This is of considerable use to assembly code writers, and also when debugging either assembler or high-level language code at the single instruction level. However, this manual is not intended as tutorial material for ARM assembler language, nor does it describe ARM assembler language at anything other than a very basic level. To make effective use of ARM assembler language, consult the documentation supplied with the assembler being used. Different assemblers vary considerably with respect to many aspects of assembler language, such as which assembler directives are accepted and how they are coded. A considerable amount of generic information is also included about how ARM processors access memory and other system components. Although this usually needs to be supplemented by detailed implementation-specific information from the technical reference manual of the device being used, this information is of use to designers of ARM-based systems. iv Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 5. Preface Architecture versions and variants The ARM instruction set architecture has evolved significantly since it was first developed, and will continue to be developed in the future. In order to be precise about which instructions exist in any particular ARM implementation, five major versions of the instruction set have been defined to date. These are denoted by the version numbers 1 to 5. Many of the versions can be qualified with variant letters to specify collections of additional instructions that are included in that version. These collections vary from being very small (the M variant denotes the addition of just four extra instructions) to very large (the T variant denotes the addition of the entire Thumb instruction set). The five versions of the ARM instruction set architecture to date are as follows: Version 1 This version was implemented only by ARM1, and was never used in a commercial product. It contained: • the basic data-processing instructions (not including multiplies) • byte, word, and multi-word load/store instructions • branch instructions, including a branch-and-link instruction designed for subroutine calls • a software interrupt instruction, for use in making Operating System calls. Version 1 only had a 26-bit address space, and is now obsolete. Version 2 This version extended architecture version 1 by adding: • multiply and multiply-accumulate instructions • coprocessor support • two more banked registers in fast interrupt mode • atomic load-and-store instructions called SWP and SWPB (in a slightly later variant called version 2a). Versions 2 and 2a still only had a 26-bit address space, and are now obsolete. Version 3 This architecture version extended the addressing range to 32 bits. Program status information which had previously been stored in R15 was moved to a new Current Program Status Register (CPSR), and Saved Program Status Registers (SPSRs) were added to preserve the CPSR contents when an exception occurred. As a result, the following changes occurred to the instruction set: • two instructions (MRS and MSR) were added to allow the new CPSR and SPSRs to be accessed. • the functionality of instructions previously used to return from exceptions was modified to allow them to continue to be used for that purpose. Version 3 also added two new processor modes in order to make it possible to use Data Abort, Prefetch Abort and Undefined Instruction exceptions effectively in Operating System code. Backwards-compatibility support for the 26-bit architectures was obligatory in version 3, except in a variant called version 3G. The distinction between versions 3 and 3G is now obsolete. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. v
- 6. Preface Version 4 This version extended architecture version 3 by adding: • halfword load/store instructions • instructions to load and sign-extend bytes and halfwords • in T variants, an instruction to transfer to Thumb state • a new privileged processor mode that uses the User mode registers. Version 4 also made it clearer which instructions should cause the Undefined Instruction exception to be taken. Backwards-compatibility support for 26-bit architectures ceased to be obligatory in version 4. Version 5 This version extends architecture version 4 by adding instructions and slightly modifying the definitions of some existing instructions to: • improve the efficiency of ARM/Thumb interworking in T variants • allow the same code generation techniques to be used for non-T variants as for T variants. Version 5 also: • adds a count leading zeros instruction, which (among other things) allows more efficient integer divide and interrupt prioritization routines • adds a software breakpoint instruction • adds more instruction options for coprocessor designers • tightens the definition of how flags are set by multiply instructions. The Thumb instruction set (T variants) The Thumb instruction set is a re-encoded subset of the ARM instruction set. Thumb instructions are half the size of ARM instructions (16 bits compared with 32), with the result that greater code density can usually be achieved by using the Thumb instruction set instead of the ARM instruction set. The Thumb instruction set is described in detail in Chapter A6 The Thumb Instruction Set and Chapter A7 Thumb Instructions. Two limitations of the Thumb instruction set compared with the ARM instruction set are: • Thumb code usually uses more instructions for the same job, so ARM code is usually best for maximizing the performance of time-critical code. • The Thumb instruction set does not include some instructions that are needed for exception handling, so ARM code needs to be used for at least the top-level exception handlers. Because of the second of these, the Thumb instruction set is always used in conjunction with a suitable version of the ARM instruction set. Its presence is denoted by the variant letter T, and it is not valid prior to ARM architecture version 4. vi Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 7. Preface Thumb instruction set versions There are two versions of the Thumb instruction set: • Thumb version 1 is used in T variants of ARM architecture version 4 • Thumb version 2 is used in T variants of ARM architecture version 5. Compared with Thumb version 1, Thumb version 2: • adds instructions and slightly modifies the definition of some existing instructions to improve the efficiency of ARM/Thumb interworking • adds a software breakpoint instruction • tightens the definition of how the Thumb multiply instruction sets the flags. These improvements are closely related to the changes between ARM architecture versions 4 and 5. Note In general, the Thumb instruction set version number is not used in this manual. Instead, the version number of the associated version of the ARM instruction set is used, to allow easy use with the naming scheme described in Naming of ARM/Thumb architecture versions on page viii. Long multiply instructions (M variants) M variants of the ARM instruction set include four extra instructions which perform 32 ´ 32 ® 64 multiplications and 32 ´ 32 + 64 ® 64 multiply-accumulates. These instructions imply the existence of a multiplier that is significantly larger than minimum, and so are sometimes omitted in implementations for which a small die size is very important and multiply performance is not very important. Their presence is denoted by the use of the variant letter M. These instructions were first defined as a variant of architecture version 3, and are included in similar variants of later architecture versions. Because the combination of requirements that leads to them being excluded does not arise very often in practice, inclusion of these instructions is standard in architecture versions 4 and above. Enhanced DSP instructions (E variants) E variants of the ARM instruction set include a number of extra instructions which enhance the performance of an ARM processor on typical digital signal processing (DSP) algorithms. These instructions are described in detail in Chapter A10 Enhanced DSP Extension, and include: • Several new multiply and multiply-accumulate instructions that act on 16-bit data items • Addition and subtraction instructions that perform saturated signed arithmetic. This is a form of integer arithmetic that produces the maximum negative or positive value instead of wrapping around if the calculation overflows the normal integer range. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. vii
- 8. Preface • Load (LDRD), store (STRD) and coprocessor register transfer (MCRR and MRRC) instructions that act on 2 words of data. • A cache preload instruction PLD. These instructions were first defined as a variant of architecture version 5T. Their presence is denoted by the variant letter E, and they are not valid prior to architecture version 5. They are also not valid in non-T or non-M variants of the architecture. The ARMv5TExP architecture version Some early implementations of the enhanced DSP variant of the ARM architecture omitted the LDRD, STRD, MCRR, MRRC and PLD instructions. Apart from this omission, all the ARM implementations concerned implemented the ARMv5TE architecture. In order to be able to name this architecture variant, the letter P can be used to exclude these five instructions from architecture version ARMv5TE, according to the rules in Naming of ARM/Thumb architecture versions on page viii. The resulting architecture variant is therefore named ARMv5TExP. This is the only use of the P variant letter. Naming of ARM/Thumb architecture versions To name a precise version and variant of the ARM/Thumb architecture, the following strings are concatenated: 1. The string ARMv. 2. The version number of the ARM instruction set. 3. Variant letters of the included variants, except that the M variant is standard in architecture versions 4 and above, and therefore not normally listed. 4. If any variants described as standard in 3 above are not present, the letter x followed by the letters of the excluded variants. In addition, the letter P can be used after x to denote the exclusion of certain instructions from architecture version ARMv5TE, as described in The ARMv5TExP architecture version. The table Architecture versions on page ix lists the standard names of the current (not obsolete) ARM/Thumb architecture versions described in this manual. These names provide a shorthand way of describing the precise instruction set implemented by an ARM processor. However, this manual normally uses descriptive phrases such as “M variants of architecture version 3 and above” to avoid the use of lists of architecture names which are already long and will grow further in the future. Obsolete architecture names are ARMv1, ARMv2, ARMv2a, and ARMv3G. These are the versions 1, 2, 2a, and 3G described in Architecture versions and variants on page v. viii Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 9. Preface Architecture versions Name ARM instruction set version Thumb instruction set version Long multiply instructions? Enhanced DSP instructions ARMv3 3 None No No ARMv3M 3 None Yes No ARMv4xM 4 None No No ARMv4 4 None Yes No ARMv4TxM 4 1 No No ARMv4T 4 1 Yes No ARMv5xM 5 None No No ARMv5 5 None Yes No ARMv5TxM 5 2 No No ARMv5T 5 2 Yes No ARMv5TExP 5 2 Yes All but LDRD, MCRR, MRRC, PLD, and STRD ARMv5TE 5 2 Yes Yes ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. ix
- 10. Preface Using this manual The information in this manual is organized into three parts, as described below. Part A - CPU Architectures Part A describes the ARM and Thumb instruction sets, and contains the following chapters: Chapter A1 Gives a quick overview of the ARM instruction set. Chapter A2 Describes the types of value that ARM instructions operate on, the general-purpose registers that contain those values, and the Program Status Registers. This chapter also describes how ARM processors handle interrupts and other exceptions, and contains general information about the memory interface of an ARM processor. Chapter A3 Gives a description of the ARM instruction set, organized by type of instruction. Chapter A4 Contains detailed reference material on each ARM instruction, arranged alphabetically by instruction mnemonic. Chapter A5 Contains detailed reference material on the addressing modes used by ARM instructions. The term addressing mode is interpreted broadly in this manual, to mean a procedure shared by many different instructions, for generating values used by the instructions. For four of the addressing modes described in this chapter, the values generated are memory addresses (which is the traditional role of an addressing mode). The remaining addressing mode generates values to be used as operands by data-processing instructions. Chapter A6 Gives a description of the Thumb instruction set, organized by type of instruction. This chapter also contains information about how to switch between the ARM and Thumb instruction sets, and how exceptions that arise during Thumb state execution are handled. Chapter A7 Contains detailed reference material on each Thumb instruction, arranged alphabetically by instruction mnemonic. Chapter A8 Gives information on the 26-bit architectures (ARMv1, ARMv2, and ARMv2a), and about the backwards-compatibility support for these architectures that is built into some later ARM processors. All of these features are now obsolete, and information about them is only relevant to historical systems. Chapter A9 Contains some examples of using the ARM instruction set. Chapter A10 Gives a description of the extra instructions added in the enhanced DSP extension (see Enhanced DSP instructions (E variants) on page vii). x Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 11. Preface Part B - Memory and System Architectures Part B describes standard memory system features that are normally implemented by the System Control coprocessor (coprocessor 15) in an ARM-based system. It contains the following chapters: Chapter B1 Gives a brief overview of this part of the manual. Chapter B2 Gives a general description of the System Control coprocessor and its use. Chapter B3 Describes the standard ARM memory and system architecture based on the use of a Memory Management Unit (MMU). (Chapter B2 and Chapter B5 are also relevant to this architecture.) Chapter B4 Gives a description of the simpler standard ARM memory and system architecture based on the use of a Protection Unit. (Chapter B2 and Chapter B5 are also relevant to this architecture.) Chapter B5 Gives a description of the standard ways to control caches and write buffers in ARM memory systems. This chapter is relevant both to systems based on an MMU and to systems based on a Protection Unit. Chapter B6 Describes the Fast Context Switch Extension (FCSE), which allows switching between multiple small processes (£ 32MB in size) without incurring large performance costs due to cache flushing and similar overheads. Part C - Vector Floating-point Architecture Part C describes the Vector Floating-point (VFP) architecture. This is a coprocessor extension to the ARM architecture designed for high floating-point performance on typical graphics and DSP algorithms. Chapter C1 Gives a brief overview of the VFP architecture and information about its compliance with the IEEE 754-1985 floating-point arithmetic standard. Chapter C2 Describes the floating-point formats supported by the VFP instruction set, the floating-point general-purpose registers that hold those values, and the VFP system registers. Chapter C3 Describes the VFP coprocessor instruction set, organized by type of instruction. Chapter C4 Contains detailed reference material on the VFP coprocessor instruction set, organized alphabetically by instruction mnemonic. Chapter C5 Contains detailed reference material on the addressing modes used by VFP instructions. One of these is a traditional addressing mode, generating addresses for load/store instructions. The remainder specify how the floating-point general-purpose registers and instructions can be used to hold and perform calculations on vectors of floating-point values. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. xi
- 12. Preface Conventions This manual employs typographic and other conventions intended to improve its ease of use. General typographic conventions typewriter Is used for assembler syntax descriptions, pseudo-code descriptions of instructions, and source code examples. In the cases of assembler syntax descriptions and pseudo-code descriptions, see the additional conventions below. The typewriter font is also used in the main text for instruction mnemonics and for references to other items appearing in assembler syntax descriptions, pseudo-code descriptions of instructions and source code examples. italic Highlights important notes, introduces special terminology, and denotes internal cross-references and citations. bold Is used for emphasis in descriptive lists and elsewhere, where appropriate. SMALL CAPITALS Are used for a few terms which have specific technical meanings. Their meanings can be found in the Glossary. Pseudo-code descriptions of instructions A form of pseudo-code is used to provide precise descriptions of what instructions do. This pseudo-code is written in a typewriter font, and uses the following conventions for clarity and brevity: • Indentation is used to indicate structure. For example, the range of statements that a for statement loops over, goes from the for statement to the next statement at the same or lower indentation level as the for statement (both ends exclusive). • Comments are bracketed by /* and */, as in the C language. • English text is occasionally used outside comments to describe functionality that is hard to describe otherwise. • All keywords and special functions used in the pseudo-code are described in the Glossary. • Assignment and equality tests are distinguished by using = for an assignment and == for an equality test, as in the C language. • Instruction fields are referred to by the names shown in the encoding diagram for the instruction. When an instruction field denotes a register, a reference to it means the value in that register, rather than the register number, unless the context demands otherwise. For example, a Rn == 0 test is checking whether the value in the specified register is 0, but a Rd is R15 test is checking whether the specified register is register 15. • When an instruction uses an addressing mode, the pseudo-code for that addressing mode generates one or more values that are used in the pseudo-code for the instruction. For example, the AND instruction described in AND on page A48 uses ARM addressing mode 1 (see Addressing Mode 1 - Data-processing operands on page A52). The pseudo-code for the addressing mode generates two values shifter_operand and shifter_carry_out, which are used by the pseudo-code for the AND instruction. xii Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 13. Preface Assembler syntax descriptions This manual contains numerous syntax descriptions for assembler instructions and for components of assembler instructions. These are shown in a typewriter font, and are as follows: < > Any item bracketed by < and > is a short description of a type of value to be supplied by the user in that position. A longer description of the item is normally supplied by subsequent text. Such items often correspond to a similarly named field in an encoding diagram for an instruction. When the correspondence simply requires the binary encoding of an integer value or register number to be substituted into the instruction encoding, it is not described explicitly. For example, if the assembler syntax for an ARM instruction contains an item <Rn> and the instruction encoding diagram contains a 4-bit field named Rn, the number of the register specified in the assembler syntax is encoded in binary in the instruction field. If the correspondence between the assembler syntax item and the instruction encoding is more complex than simple binary encoding of an integer or register number, the item description indicates how it is encoded. { } Any item bracketed by { and } is optional. A description of the item and of how its presence or absence is encoded in the instruction is normally supplied by subsequent text. | This indicates an alternative character string. For example, LDM|STM is either LDM or STM. spaces Single spaces are used for clarity, to separate items. When a space is obligatory in the assembler syntax, two or more consecutive spaces are used. +/- This indicates an optional + or - sign. If neither is coded, + is assumed. * When used in a combination like <immed_8> * 4, this describes an immediate value which must be a specified multiple of a value taken from a numeric range. In this instance, the numeric range is 0 to 255 (the set of values that can be represented as an 8-bit immediate) and the specified multiple is 4, so the value described must be a multiple of 4 in the range 4*0 = 0 to 4*255 = 1020. All other characters must be encoded precisely as they appear in the assembler syntax. Apart from { and }, the special characters described above do not appear in the basic forms of assembler instructions documented in this manual. The { and } characters need to be encoded in a few places as part of a variable item. When this happens, the long description of the variable item indicates how they must be used. Note This manual only attempts to describe the most basic forms of assembler instruction syntax. In practice, assemblers normally recognize a much wider range of instruction syntaxes, as well as various directives to control the assembly process and additional features such as symbolic manipulation and macro expansion. All of these are beyond the scope of this manual. For descriptions of the extra facilities provided by the assemblers included in ARM Development Systems, see the ARM Software Development Toolkit Reference Guide (ARM DUI 0041) for SDT 2.50, or the ARM Developer Suite Tools Guide (ARM DUI 0067) for ADS 1.0. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. xiii
- 14. Preface xiv Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 15. Contents ARM Architecture Reference Manual Preface About this manual ................................................................................................... iv Architecture versions and variants ........................................................................... v Using this manual .................................................................................................... x Conventions ............................................................................................................xii Part A: CPU Architecture Chapter A1 Introduction to the ARM Architecture A1.1 About the ARM architecture ............................................................................... A1-2 A1.2 ARM instruction set ............................................................................................ A1-5 Chapter A2 Programmer’s Model A2.1 Data types .......................................................................................................... A2-2 A2.2 Processor modes ............................................................................................... A2-3 A2.3 Registers ............................................................................................................ A2-4 A2.4 General-purpose registers ................................................................................. A2-5 A2.5 Program status registers .................................................................................... A2-9 A2.6 Exceptions ....................................................................................................... A2-13 A2.7 Memory and memory-mapped I/O ................................................................... A2-22 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. xv
- 16. Chapter A3 The ARM Instruction Set A3.1 Instruction set encoding ..................................................................................... A3-2 A3.2 The condition field .............................................................................................. A3-5 A3.3 Branch instructions ............................................................................................. A3-7 A3.4 Data-processing instructions .............................................................................. A3-9 A3.5 Multiply instructions .......................................................................................... A3-12 A3.6 Miscellaneous arithmetic instructions ............................................................... A3-14 A3.7 Status register access instructions ................................................................... A3-15 A3.8 Load and store instructions .............................................................................. A3-17 A3.9 Load and Store Multiple instructions ................................................................ A3-21 A3.10 Semaphore instructions ................................................................................... A3-23 A3.11 Exception-generating instructions .................................................................... A3-24 A3.12 Coprocessor instructions .................................................................................. A3-25 A3.13 Extending the instruction set ............................................................................ A3-27 Chapter A4 ARM Instructions A4.1 Alphabetical list of ARM instructions .................................................................. A4-2 A4.2 ARM instructions and architecture versions ................................................... A4-113 Chapter A5 ARM Addressing Modes A5.1 Addressing Mode 1 - Data-processing operands ............................................... A5-2 A5.2 Addressing Mode 2 - Load and Store Word or Unsigned Byte ........................ A5-18 A5.3 Addressing Mode 3 - Miscellaneous Loads and Stores ................................... A5-34 A5.4 Addressing Mode 4 - Load and Store Multiple ................................................. A5-48 A5.5 Addressing Mode 5 - Load and Store Coprocessor ......................................... A5-56 Chapter A6 The Thumb Instruction Set A6.1 About the Thumb instruction set ........................................................................ A6-2 A6.2 Instruction set encoding ..................................................................................... A6-4 A6.3 Branch instructions ............................................................................................. A6-6 A6.4 Data-processing instructions .............................................................................. A6-8 A6.5 Load and Store Register instructions ............................................................... A6-15 A6.6 Load and Store Multiple instructions ................................................................ A6-18 A6.7 Exception-generating instructions .................................................................... A6-20 A6.8 Undefined instruction space ............................................................................. A6-21 Chapter A7 Thumb Instructions A7.1 Alphabetical list of Thumb instructions ............................................................... A7-2 A7.2 Thumb instructions and architecture versions ................................................ A7-104 xvi Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 17. Chapter A8 The 26-bit Architectures A8.1 Overview of the 26-bit architectures .................................................................. A8-2 A8.2 Format of register 15 .......................................................................................... A8-4 A8.3 26-bit PSR update instructions .......................................................................... A8-6 A8.4 Address exceptions ............................................................................................ A8-8 A8.5 Backwards compatibility from 32-bit architectures ............................................. A8-9 Chapter A9 ARM Code Sequences A9.1 Arithmetic instructions ........................................................................................ A9-2 A9.2 Branch instructions ............................................................................................ A9-5 A9.3 Load and Store instructions ............................................................................... A9-7 A9.4 Load and Store Multiple instructions ................................................................ A9-10 A9.5 Semaphore instructions ................................................................................... A9-11 A9.6 Other code examples ....................................................................................... A9-12 Chapter A10 Enhanced DSP Extension A10.1 About the enhanced DSP instructions ............................................................. A10-2 A10.2 Saturated integer arithmetic ............................................................................. A10-3 A10.3 Saturated Q15 and Q31 arithmetic .................................................................. A10-4 A10.4 The Q flag ........................................................................................................ A10-5 A10.5 Enhanced DSP instructions ............................................................................. A10-6 A10.6 Alphabetical list of enhanced DSP instructions ................................................ A10-8 Part B: Memory and System Architectures Chapter B1 Introduction to Memory and System Architectures B1.1 About the memory system ................................................................................. B1-2 B1.2 System-level issues ........................................................................................... B1-4 Chapter B2 The System Control Coprocessor B2.1 About the System Control coprocessor ............................................................. B2-2 B2.2 Registers ............................................................................................................ B2-3 B2.3 Register 0: ID codes .......................................................................................... B2-6 B2.4 Register 1: Control register .............................................................................. B2-13 B2.5 Registers 2-15 .................................................................................................. B2-17 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. xvii
- 18. Chapter B3 Memory Management Unit B3.1 About the MMU architecture .............................................................................. B3-2 B3.2 Memory access sequence ................................................................................. B3-4 B3.3 Translation process ............................................................................................ B3-6 B3.4 Access permissions ......................................................................................... B3-16 B3.5 Domains ........................................................................................................... B3-17 B3.6 Aborts ............................................................................................................... B3-18 B3.7 CP15 registers ................................................................................................. B3-23 Chapter B4 Protection Unit B4.1 About the Protection Unit ................................................................................... B4-2 B4.2 Overlapping regions ........................................................................................... B4-5 B4.3 CP15 registers ................................................................................................... B4-6 Chapter B5 Caches and Write Buffers B5.1 About caches and write buffers .......................................................................... B5-2 B5.2 Cache organization ............................................................................................ B5-3 B5.3 Types of cache ................................................................................................... B5-5 B5.4 Cachability and bufferability ............................................................................... B5-8 B5.5 Memory coherency ........................................................................................... B5-10 B5.6 CP15 registers ................................................................................................. B5-14 Chapter B6 Fast Context Switch Extension B6.1 About the FCSE ................................................................................................. B6-2 B6.2 Modified virtual addresses ................................................................................. B6-3 B6.3 Enabling the FCSE ............................................................................................. B6-5 B6.4 CP15 registers ................................................................................................... B6-6 Part C: Vector Floating-point Architecture Chapter C1 Introduction to the Vector Floating-point Architecture C1.1 About the Vector Floating-point architecture ......................................................C1-2 C1.2 Overview of the VFP architecture ......................................................................C1-3 C1.3 Compliance with the IEEE 754 standard ............................................................C1-7 C1.4 IEEE 754 implementation choices .....................................................................C1-8 xviii Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 19. Chapter C2 VFP Programmer’s Model C2.1 Floating-point formats ........................................................................................ C2-2 C2.2 Rounding ............................................................................................................ C2-9 C2.3 Floating-point exceptions ................................................................................. C2-10 C2.4 Flush-to-zero mode .......................................................................................... C2-13 C2.5 Floating-point general-purpose registers ......................................................... C2-14 C2.6 System registers .............................................................................................. C2-19 C2.7 Reset behavior and initialization ...................................................................... C2-26 Chapter C3 VFP Instruction Set Overview C3.1 Data-processing instructions .............................................................................. C3-2 C3.2 Load and Store instructions ............................................................................. C3-13 C3.3 Register transfer instructions ........................................................................... C3-17 Chapter C4 VFP Instructions C4.1 Alphabetical list of VFP instructions ................................................................... C4-2 Chapter C5 VFP Addressing Modes C5.1 Addressing Mode 1 - Single-precision vectors (non-monadic) .......................... C5-2 C5.2 Addressing Mode 2 - Double-precision vectors (non-monadic) ......................... C5-8 C5.3 Addressing Mode 3 - Single-precision vectors (monadic) ................................ C5-14 C5.4 Addressing Mode 4 - Double-precision vectors (monadic) .............................. C5-19 C5.5 Addressing Mode 5 - VFP load/store multiple .................................................. C5-24 Glossary Index ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. xix
- 20. xx Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 21. Part A CPU Architecture
- 23. Chapter A1 Introduction to the ARM Architecture This chapter introduces the ARM architecture and contains the following sections: • About the ARM architecture on page A1-2 • ARM instruction set on page A1-5. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A1-1
- 24. Introduction to the ARM Architecture 1.1 About the ARM architecture The ARM architecture has been designed to allow very small, yet high-performance implementations. The architectural simplicity of ARM processors leads to very small implementations, and small implementations allow devices with very low power consumption. The ARM is a Reduced Instruction Set Computer (RISC), as it incorporates these typical RISC architecture features: • a large uniform register file • a load/store architecture, where data-processing operations only operate on register contents, not directly on memory contents • simple addressing modes, with all load/store addresses being determined from register contents and instruction fields only • uniform and fixed-length instruction fields, to simplify instruction decode. In addition, the ARM architecture gives you: • control over both the Arithmetic Logic Unit (ALU) and shifter in every data-processing instruction to maximize the use of an ALU and a shifter • auto-increment and auto-decrement addressing modes to optimize program loops • Load and Store Multiple instructions to maximize data throughput • conditional execution of all instructions to maximize execution throughput. These enhancements to a basic RISC architecture allow ARM processors to achieve a good balance of high performance, low code size, low power consumption and low silicon area. 1.1.1 ARM registers ARM has 31 general-purpose 32-bit registers. At any one time, 16 of these registers are visible. The other registers are used to speed up exception processing. All the register specifiers in ARM instructions can address any of the 16 visible registers. The main bank of 16 registers is used by all unprivileged code. These are the User mode registers. User mode is different from all other modes as it is unprivileged, which means: • User mode is the only mode which cannot switch to another processor mode without generating an exception • memory systems and coprocessors might allow User mode less access to memory and coprocessor functionality than a privileged mode. A1-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 25. Introduction to the ARM Architecture Two of the 16 visible registers have special roles: Link register Register 14 is the Link Register (LR). This register holds the address of the next instruction after a Branch and Link (BL) instruction, which is the instruction used to make a subroutine call. At all other times, R14 can be used as a general-purpose register. Program counter Register 15 is the Program Counter (PC). It can be used in most instructions as a pointer to the instruction which is two instructions after the instruction being executed. All ARM instructions are four bytes long (one 32-bit word) and are always aligned on a word boundary. This means that the bottom two bits of the PC are always zero, and therefore the PC contains only 30 non-constant bits. The remaining 14 registers have no special hardware purpose. Their uses are defined purely by software. Software normally uses R13 as a Stack Pointer (SP). For more details on registers, please refer to Registers on page A2-4. 1.1.2 Exceptions ARM supports five types of exception, and a privileged processing mode for each type. The five types of exceptions are: • fast interrupt • normal interrupt • memory aborts, which can be used to implement memory protection or virtual memory • attempted execution of an undefined instruction • software interrupt (SWI) instructions which can be used to make a call to an operating system. When an exception occurs, some of the standard registers are replaced with registers specific to the exception mode. All exception modes have replacement banked registers for R13 and R14. The fast interrupt mode has more registers for fast interrupt processing. When an exception handler is entered, R14 holds the return address for exception processing. This is used to return after the exception is processed and to address the instruction that caused the exception. Register 13 is banked across exception modes to provide each exception handler with a private stack pointer. The fast interrupt mode also banks registers 8 to 12 so that interrupt processing can begin without the need to save or restore these registers. There is a sixth privileged processing mode, System mode, which uses the User mode registers. This is used to run tasks that require privileged access to memory and/or coprocessors, without limitations on which exceptions can occur during the task. For more details on exceptions, please refer to Exceptions on page A2-13. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A1-3
- 26. Introduction to the ARM Architecture The exception process When an exception occurs, the ARM processor halts execution after the current instruction and begins execution at one of a number of fixed addresses in memory, known as the exception vectors. There is a separate vector location for each exception. An operating system installs a handler on every exception at initialization. Privileged operating system tasks are normally run in System mode to allow exceptions to occur within the operating system without state loss. 1.1.3 Status registers All processor state other than the general-purpose register contents is held in status registers. The current operating processor status is in the Current Program Status Register (CPSR). The CPSR holds: • 4 condition code flags (Negative, Zero, Carry and oVerflow) • 2 interrupt disable bits, one for each type of interrupt • 5 bits which encode the current processor mode • 1 bit which encodes whether ARM or Thumb instructions are being executed. Each exception mode also has a Saved Program Status Register (SPSR) which holds the CPSR of the task immediately before the exception occurred. The CPSR and the SPSRs are accessed with special instructions. For more details on status registers, please refer to Program status registers on page A2-9. A1-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 27. Introduction to the ARM Architecture 1.2 ARM instruction set The ARM instruction set can be divided into six broad classes of instruction: • Branch instructions • Data-processing instructions on page A1-6 • Status register transfer instructions on page A1-7 • Load and store instructions on page A1-7 • Coprocessor instructions on page A1-8 • Exception-generating instructions on page A1-9. Most data-processing instructions and one type of coprocessor instruction can update the four condition code flags in the CPSR (Negative, Zero, Carry and oVerflow) according to their result. Almost all ARM instructions contain a 4-bit condition field. One value of this field specifies that the instruction is executed unconditionally. Fourteen other values specify conditional execution of the instruction. If the condition code flags indicate that the corresponding condition is true when the instruction starts executing, it executes normally. Otherwise, the instruction does nothing. The 14 available conditions allow: • tests for equality and non-equality • tests for <, <=, >, and >= inequalities, in both signed and unsigned arithmetic • each condition code flag to be tested individually. The sixteenth value of the condition field is used for a few instructions which do not allow conditional execution. 1.2.1 Branch instructions As well as allowing many data-processing or load instructions to change control flow by writing the PC, a standard Branch instruction is provided with a 24-bit signed offset, allowing forward and backward branches of up to 32MB. There is a Branch and Link (BL) option that also preserves the address of the instruction after the branch in R14, the LR. This provides a subroutine call which can be returned from by copying the LR into the PC. There are also branch instructions which can switch instruction set, so that execution continues at the branch target using the Thumb instruction set. These allow ARM code to call Thumb subroutines, and ARM subroutines to return to a Thumb caller. Similar instructions in the Thumb instruction set allow the corresponding Thumb ® ARM switches. An overview of the Thumb instruction set is provided in Chapter A6 The Thumb Instruction Set. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A1-5
- 28. Introduction to the ARM Architecture 1.2.2 Data-processing instructions The data-processing instructions perform calculations on the general-purpose registers. There are four types of data-processing instructions: • Arithmetic/logic instructions • Comparison instructions • Multiply instructions • Count Leading Zeros instruction on page A1-7. Arithmetic/logic instructions There are twelve arithmetic/logic instructions which share a common instruction format. These perform an arithmetic or logical operation on up to two source operands, and write the result to a destination register. They can also optionally update the condition code flags based on the result. Of the two source operands: • one is always a register • the other has two basic forms: — an immediate value — a register value, optionally shifted. If the operand is a shifted register, the shift amount can be either an immediate value or the value of another register. Four types of shift can be specified. Every arithmetic/logic instruction can therefore perform an arithmetic/logic and a shift operation. As a result, ARM does not have dedicated shift instructions. Because the Program Counter (PC) is a general-purpose register, arithmetic/logic instructions can write their results directly to the PC. This allows easy implementation of a variety of jump instructions. Comparison instructions There are four comparison instructions which use the same instruction format as the arithmetic/logic instructions. These perform an arithmetic or logical operation on two source operands, but do not write the result to a register. They always update the condition flags based on the result. The source operands of comparison instructions take the same forms as those of arithmetic/logic instructions, including the ability to incorporate a shift operation. Multiply instructions Multiply instructions come in two classes. Both types multiply two 32-bit register values and store their result: 32-bit result Normal. Stores the 32-bit result in a register. 64-bit result Long. Stores the 64-bit result in two separate registers. Both types of multiply instruction can optionally perform an accumulate operation. A1-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 29. Introduction to the ARM Architecture Count Leading Zeros instruction The Count Leading Zeros (CLZ) instruction determines the number of zero bits at the most significant end of a register value, up to the first 1 bit. This number is written to the destination register of the CLZ instruction. 1.2.3 Status register transfer instructions The status register transfer instructions transfer the contents of the CPSR or an SPSR to or from a general-purpose register. Writing to the CPSR can: • set the values of the condition code flags • set the values of the interrupt enable bits • set the processor mode. 1.2.4 Load and store instructions The following load and store instructions are available: • Load and Store Register • Load and Store Multiple registers on page A1-8 • Swap register and memory contents on page A1-8. Load and Store Register Load Register instructions can load a 32-bit word, a 16-bit halfword or an 8-bit byte from memory into a register. Byte and halfword loads can be automatically zero-extended or sign-extended as they are loaded. Store Register instructions can store a 32-bit word, a 16-bit halfword or an 8-bit byte from a register to memory. Load and Store Register instructions have three primary addressing modes, all of which use a base register and an offset specified by the instruction: • In offset addressing, the memory address is formed by adding or subtracting an offset to or from the base register value. • In pre-indexed addressing, the memory address is formed in the same way as for offset addressing. As a side-effect, the memory address is also written back to the base register. • In post-indexed addressing, the memory address is the base register value. As a side-effect, an offset is added to or subtracted from the base register value and the result is written back to the base register. In each case, the offset can be either an immediate or the value of an index register. Register-based offsets can also be scaled with shift operations. As the PC is a general-purpose register, a 32-bit value can be loaded directly into the PC to perform a jump to any address in the 4GB memory space. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A1-7
- 30. Introduction to the ARM Architecture Load and Store Multiple registers Load Multiple (LDM) and Store Multiple (STM) instructions perform a block transfer of any number of the general-purpose registers to or from memory. Four addressing modes are provided: • pre-increment • post-increment • pre-decrement • post-decrement. The base address is specified by a register value, which can be optionally updated after the transfer. As the subroutine return address and PC values are in general-purpose registers, very efficient subroutine entry and exit sequences can be constructed with LDM and STM: • A single STM instruction at subroutine entry can push register contents and the return address onto the stack, updating the stack pointer in the process. • A single LDM instruction at subroutine exit can restore register contents from the stack, load the PC with the return address, and update the stack pointer. LDM and STM instructions also allow very efficient code for block copies and similar data movement algorithms. Swap register and memory contents A swap (SWP) instruction performs the following sequence of operations: 1. It loads a value from a register-specified memory location. 2. It stores the contents of a register to the same memory location. 3. It writes the value loaded in step 1 to a register. By specifying the same register for steps 2 and 3, the contents of a memory location and a register are interchanged. The swap operation performs a special indivisible bus operation that allows atomic update of semaphores. Both 32-bit word and 8-bit byte semaphores are supported. 1.2.5 Coprocessor instructions There are three types of coprocessor instructions: Data-processing instructions These start a coprocessor-specific internal operation. Data transfer instructions These transfer coprocessor data to or from memory. The address of the transfer is calculated by the ARM processor. Register transfer instructions These allow a coprocessor value to be transferred to or from an ARM register. A1-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 31. Introduction to the ARM Architecture 1.2.6 Exception-generating instructions Two types of instruction are designed to cause specific exceptions to occur. Software interrupt instructions SWI instructions cause a software interrupt exception to occur. These are normally used to make calls to an operating system, to request an OS-defined service. The exception entry caused by a SWI instruction also changes to a privileged processor mode. This allows an unprivileged task to gain access to privileged functions, but only in ways permitted by the OS. Software breakpoint instructions BKPT instructions cause an abort exception to occur. If suitable debugger software is installed on the abort vector, an abort exception generated in this fashion is treated as a breakpoint. If debug hardware is present in the system, it can instead treat a BKPT instruction directly as a breakpoint, preventing the abort exception from occurring. In addition to the above, the following types of instruction cause an Undefined Instruction exception to occur: • coprocessor instructions which are not recognized by any hardware coprocessor • most instruction words that have not yet been allocated a meaning as an ARM instruction. In each case, this exception is normally used either to generate a suitable error or to initiate software emulation of the instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A1-9
- 32. Introduction to the ARM Architecture A1-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 33. Chapter A2 Programmer’s Model This chapter introduces the ARM programmer’s model. It contains the following sections: • Data types on page A2-2 • Processor modes on page A2-3 • Registers on page A2-4 • General-purpose registers on page A2-5 • Program status registers on page A2-9 • Exceptions on page A2-13 • Memory and memory-mapped I/O on page A2-22. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-1
- 34. Programmer’s Model 2.1 Data types ARM processors support the following data types: Byte 8 bits. Halfword 16 bits (halfwords must be aligned to two-byte boundaries). Word 32 bits (words must be aligned to four-byte boundaries). Note • All three types are supported in ARM architecture version 4 and above. Only bytes and words were supported prior to ARM architecture version 4. • When any of these types is described as unsigned, the N-bit data value represents a non-negative integer in the range 0 to +2N-1, using normal binary format. • When any of these types is described as signed, the N-bit data value represents an integer in the range -2N-1 to +2N-1-1, using two's complement format. • All data operations, for example ADD, are performed on word quantities. • Load and store operations can transfer bytes, halfwords and words to and from memory, automatically zero-extending or sign-extending bytes or halfwords as they are loaded. • ARM instructions are exactly one word (and are aligned on a four-byte boundary). Thumb instructions are exactly one halfword (and are aligned on a two-byte boundary). A2-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 35. Programmer’s Model 2.2 Processor modes The ARM architecture supports the seven processor modes shown in Table 2-1. Table 2-1 ARM version 4 processor modes Processor mode Description User usr Normal program execution mode FIQ fiq Supports a high-speed data transfer or channel process IRQ irq Used for general-purpose interrupt handling Supervisor svc A protected mode for the operating system Abort abt Implements virtual memory and/or memory protection Undefined und Supports software emulation of hardware coprocessors System sys Runs privileged operating system tasks (ARM architecture version 4 and above) Mode changes can be made under software control, or can be caused by external interrupts or exception processing. Most application programs execute in User mode. While the processor is in User mode, the program being executed is unable to access some protected system resources or to change mode, other than by causing an exception to occur (see Exceptions on page A2-13). This allows a suitably written operating system to control the use of system resources. The modes other than User mode are known as privileged modes. They have full access to system resources and can change mode freely. Five of them are known as exception modes: • FIQ • IRQ • Supervisor • Abort • Undefined. These are entered when specific exceptions occur. Each of them has some additional registers to avoid corrupting User mode state when the exception occurs (see Registers on page A2-4 for details). The remaining mode is System mode, and is only present in ARM architecture version 4 and above. It is not entered by any exception and has exactly the same registers available as User mode. However, it is a privileged mode and is therefore not subject to the User mode restrictions. It is intended for use by operating system tasks which need access to system resources, but wish to avoid using the additional registers associated with the exception modes. Avoiding such use ensures that the task state is not corrupted by the occurrence of any exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-3
- 36. Programmer’s Model 2.3 Registers The ARM processor has a total of 37 registers: • 31 general-purpose registers, including a program counter. These registers are 32 bits wide and are described in General-purpose registers on page A2-5. • 6 status registers. These registers are also 32 bits wide, but only 12 of the 32 bits are allocated or need to be implemented. These are described in Program status registers on page A2-9. Registers are arranged in partially overlapping banks, with a different register bank for each processor mode, as shown in Figure 2-1. At any time, 15 general-purpose registers (R0 to R14), one or two status registers and the program counter are visible. Each column of Figure 2-1 shows which general-purpose and status registers are visible in the indicated processor mode. Fast interrupt Privileged modes Exception modes Undefined R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13_und R14_und PC CPSR SPSR_und Interrupt R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13_irq R14_irq PC CPSR SPSR_irq R0 R1 R2 R3 R4 R5 R6 R7 R8_fiq R9_fiq R10_fiq R11_fiq R12_fiq R13_fiq R14_fiq PC CPSR SPSR_fiq Figure 2-1 Register organization System R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 PC CPSR Supervisor R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13_svc R14_svc PC CPSR SPSR_svc Modes Abort R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13_abt R14_abt PC CPSR SPSR_abt User R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 PC CPSR indicates that the normal register used by User or System mode has been replaced by an alternative register specific to the exception mode A2-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 37. Programmer’s Model 2.4 General-purpose registers The general-purpose registers R0-R15 can be split into three groups. These groups differ in the way they are banked and in their special-purpose uses: • The unbanked registers, R0-R7 • The banked registers, R8-R14 • Register 15, the PC, is described in The program counter, R15 on page A2-7. 2.4.1 The unbanked registers, R0-R7 Registers R0 to R7 are unbanked registers. This means that each of them refers to the same 32-bit physical register in all processor modes. They are completely general-purpose registers, with no special uses implied by the architecture, and can be used wherever an instruction allows a general-purpose register to be specified. 2.4.2 The banked registers, R8-R14 Registers R8 to R14 are banked registers. The physical register referred to by each of them depends on the current processor mode. Where a particular physical register is intended, without depending on the current processor mode, a more specific name (as described below) is used. Almost all instructions allow the banked registers to be used wherever a general-purpose register is allowed. Note A few exceptions to this rule are noted in the individual instruction descriptions. Where a restriction exists on the use of banked registers, it always applies to all of R8 to R14. For example, R8 to R12 are subject to such restrictions even in systems in which FIQ mode is never used and so only one physical version of the register is ever in use. Registers R8 to R12 have two banked physical registers each. One is used in all processor modes other than FIQ mode, and the other is used in FIQ mode. Where it is necessary to be specific about which version is being referred to, the first group of physical registers are referred to as R8_usr to R12_usr and the second group as R8_fiq to R12_fiq. Registers R8 to R12 do not have any dedicated special purposes in the architecture. However, for interrupts that are simple enough to be processed using registers R8 to R14 only, the existence of separate FIQ mode versions of these registers allows very fast interrupt processing. Examples of this usage can be found in Single-channel DMA transfer on page A9-13 and Dual-channel DMA transfer on page A9-13. Registers R13 and R14 have six banked physical registers each. One is used in User and System modes, while each of the remaining five is used in one of the five exception modes. Where it is necessary to be specific about which version is being referred to, you use names of the form: R13_<mode> R14_<mode> where <mode> is the appropriate one of usr, svc (for Supervisor mode), abt, und, irq and fiq. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-5
- 38. Programmer’s Model Register R13 is normally used as a stack pointer and is also known as the SP. In the ARM instruction set, this is by convention only, as there are no defined instructions or other functionality which use R13 in a special-case manner. However, there are such instructions in the Thumb instruction set, as described in Chapter A6 The Thumb Instruction Set. Each exception mode has its own banked version of R13, which should normally be initialized to point to a stack dedicated to that exception mode. On entry, the exception handler typically stores to this stack the values of other registers to be used. By reloading these values into the registers when it returns, the exception handler can ensure that it does not corrupt the state of the program that was being executed when the exception occurred. Register R14 (also known as the Link Register or LR) has two special functions in the architecture: • In each mode, the mode's own version of R14 is used to hold subroutine return addresses. When a subroutine call is performed by a BL or BLX instruction, R14 is set to the subroutine return address. The subroutine return is performed by copying R14 back to the program counter. This is typically done in one of the two following ways: — Execute either of these instructions: MOV PC,LR BX LR — On subroutine entry, store R14 to the stack with an instruction of the form: STMFD SP!,{<registers>,LR} and use a matching instruction to return: LDMFD SP!,{<registers>,PC} • When an exception occurs, the appropriate exception mode's version of R14 is set to the exception return address (offset by a small constant for some exceptions). The exception return is performed in a similar way to a subroutine return, but using slightly different instructions to ensure full restoration of the state of the program that was being executed when the exception occurred. See Exceptions on page A2-13 for more details. Register R14 can be treated as a general-purpose register at all other times. Note When nested exceptions are possible, the two special-purpose uses might conflict. For example, if an IRQ interrupt occurs when a program is being executed in User mode, none of the User mode registers are necessarily corrupted. But if an interrupt handler running in IRQ mode re-enables IRQ interrupts and a nested IRQ interrupt occurs, any value the outer interrupt handler is holding in R14_irq at the time is overwritten by the return address of the nested interrupt. System programmers need to be careful about such interactions. The usual way to deal with them is to ensure that the appropriate version of R14 does not hold anything significant at times that nested exceptions can occur. When this is hard to do in a straightforward way, it is usually best to change to another processor mode during entry to the exception handler, before re-enabling interrupts or otherwise allowing nested exceptions to occur. (In ARM architecture version 4 and above, System mode is usually the best mode to use for this purpose.) A2-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 39. Programmer’s Model 2.4.3 The program counter, R15 Register R15 holds the Program Counter (PC). It can often be used in place of one of the general-purpose registers R0 to R14, and is therefore considered one of the general-purpose registers. However, there are also many instruction-specific restrictions or special cases about its use. These are noted in the individual instruction descriptions. Usually, the instruction is UNPREDICTABLE if R15 is used in a manner that breaks these restrictions. The Program Counter is always used for a special purpose, as described in: • Reading the program counter • Writing the program counter on page A2-8. Reading the program counter When an instruction reads R15 without breaking any of the restrictions on its use, the value read is the address of the instruction plus 8 bytes. As ARM instructions are always word-aligned, bits[1:0] of the resulting value are always zero. (In T variants of the architecture, this behavior changes during Thumb state execution - see Chapter A6 The Thumb Instruction Set for details.) This way of reading the PC is primarily used for quick, position-independent addressing of nearby instructions and data, including position-independent branching within a program. An exception to the above rule occurs when an STR or STM instruction stores R15. Such instructions can store either the address of the instruction plus 8 bytes, like other instructions that read R15, or the instruction's own address plus 12 bytes. Whether the offset of 8 or the offset of 12 is used is IMPLEMENTATION DEFINED. An implementation must use the same offset for all STR and STM instructions that store R15. It cannot use 8 for some of them and 12 for others. Because of this exception, it is usually best to avoid the use of STR and STM instructions that store R15. If this is difficult, use a suitable instruction sequence in the program to ascertain which offset the implementation uses. For example, if R0 points to an available word of memory, then the following instructions put the offset of the implementation in R0: SUB R1, PC, #4 ; R1 = address of following STR instruction STR PC, [R0] ; Store address of STR instruction + offset, LDR R0, [R0] ; then reload it SUB R0, R0, R1 ; Calculate the offset as the difference Note The rules about how R15 is read apply only to reads by instructions. In particular, they do not necessarily describe the values placed on a hardware address bus during instruction fetches. Like all other details of hardware interfaces, such values are IMPLEMENTATION DEFINED. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-7
- 40. Programmer’s Model Writing the program counter When an instruction writes R15 without breaking any of the restrictions on its use, the normal result is that the value written to R15 is treated as an instruction address and a branch occurs to that address. Since ARM instructions are required to be word-aligned, values written to R15 are normally expected to have bits[1:0] == 0b00. The precise rules for this depend on the architecture version: • In ARM architecture versions 3 and below, bits[1:0] of a value written to R15 are ignored, so that the actual destination address of the instruction is (value written to R15) AND 0xFFFFFFFC. • In ARM architecture versions 4 and above, bits[1:0] of a value written to R15 in ARM state must be 0b00. If they are not, the results are UNPREDICTABLE. Similarly, in T variants of ARM architecture versions 4 and above, Thumb instructions are required to be halfword-aligned. Bit[0] of a value written to R15 in Thumb state is ignored, so that the actual destination address of the instruction is (value written to R15) AND 0xFFFFFFFE. Several instructions have their own rules for interpreting values written to R15. For example, BX and other instructions designed to transfer between ARM and Thumb states use bit[0] of the value to select whether to execute the code at the destination address in ARM state or Thumb state. Special rules of this type are described on the individual instruction pages, and override the general rules in this section. A2-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 41. Programmer’s Model 2.5 Program status registers The current program status register (CPSR) is accessible in all processor modes. It contains condition code flags, interrupt disable bits, the current processor mode, and other status and control information. Each exception mode also has a saved program status register (SPSR), that is used to preserve the value of the CPSR when the associated exception occurs. Note User mode and System mode do not have an SPSR, because they are not exception modes. All instructions which read or write the SPSR are UNPREDICTABLE when executed in User mode or System mode. The format of the CPSR and the SPSRs is shown below. 31 30 29 28 27 26 8 7 6 5 4 3 2 1 0 N Z C V Q DNM(RAZ) I F T 2.5.1 The condition code flags M 4 M 3 M 2 M 1 M 0 The N, Z, C, and V (Negative, Zero, Carry and oVerflow) bits are collectively known as the condition code flags, often referred to as flags. The condition code flags in the CPSR can be tested by most instructions to determine whether the instruction is to be executed. The condition code flags are usually modified by: • Execution of a comparison instruction (CMN, CMP, TEQ or TST). • Execution of some other arithmetic, logical or move instruction, where the destination register of the instruction is not R15. Most of these instructions have both a flag-preserving and a flag-setting variant, with the latter being selected by adding an S qualifier to the instruction mnemonic. Some of these instructions only have a flag-preserving version. This is noted in the individual instruction descriptions. In either case, the new condition code flags (after the instruction has been executed) usually mean: N Is set to bit 31 of the result of the instruction. If this result is regarded as a two's complement signed integer, then N = 1 if the result is negative and N = 0 if it is positive or zero. Z Is set to 1 if the result of the instruction is zero (which often indicates an equal result from a comparison), and to 0 otherwise. C Is set in one of four ways: • For an addition, including the comparison instruction CMN, C is set to 1 if the addition produced a carry (that is, an unsigned overflow), and to 0 otherwise. • For a subtraction, including the comparison instruction CMP, C is set to 0 if the subtraction produced a borrow (that is, an unsigned underflow), and to 1 otherwise. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-9
- 42. Programmer’s Model • For non-addition/subtractions that incorporate a shift operation, C is set to the last bit shifted out of the value by the shifter. • For other non-addition/subtractions, C is normally left unchanged (but see the individual instruction descriptions for any special cases). V Is set in one of two ways: • For an addition or subtraction, V is set to 1 if signed overflow occurred, regarding the operands and result as two's complement signed integers. • For non-addition/subtractions, V is normally left unchanged (but see the individual instruction descriptions for any special cases). The flags can be modified in these additional ways: • Execution of an MSR instruction, as part of its function of writing a new value to the CPSR or SPSR. • Execution of MRC instructions with destination register R15. The purpose of such instructions is to transfer coprocessor-generated condition code flag values to the ARM processor. • Execution of some variants of the LDM instruction. These variants copy the SPSR to the CPSR, and their main intended use is for returning from exceptions. • Execution of flag-setting variants of arithmetic and logical instructions whose destination register is R15. These also copy the SPSR to the CPSR, and are mainly intended for returning from exceptions. The Q flag In E variants of ARM architecture 5 and above, bit[27] of the CPSR is known as the Q flag and is used to indicate whether overflow and/or saturation has occurred in some of the enhanced DSP instructions. Similarly, bit[27] of each SPSR is a Q flag, and is used to preserve and restore the CPSR Q flag if an exception occurs. For more details of the Q flag, see Chapter A10 Enhanced DSP Extension. In architecture versions prior to version 5, and in non-E variants of architecture version 5 and above, bit[27] of the CPSR and SPSRs should be treated as described in Other bits on page A2-12. 2.5.2 The control bits The bottom eight bits of a Program Status Register (PSR), incorporating I, F, T and M[4:0], are known collectively as the control bits. The control bits change when an exception arises and can be altered by software only when the processor is in a privileged mode. Interrupt disable bits I and F are the interrupt disable bits: I bit Disables IRQ interrupts when it is set. F bit Disables FIQ interrupts when it is set. A2-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 43. Programmer’s Model The T bit The T bit should be zero (SBZ) on ARM architecture version 3 and below, and on non-T variants of ARM architecture version 4. No instructions exist in these architectures that can switch between ARM and Thumb states. On T variants of ARM architecture 4 and above, the T bit has the following meanings: T = 0 Indicates ARM execution. T = 1 Indicates Thumb execution. Instructions that switch between ARM and Thumb states can be used freely on implementations of these architectures. On non-T variants of ARM architecture version 5 and above, the T bit has the following meanings: T = 0 Indicates ARM execution. T = 1 Forces the next instruction executed to cause an undefined instruction exception (see Undefined Instruction exception on page A2-15). Instructions that switch between ARM and Thumb states can be used on implementations of these architectures, but only function correctly as long as the program remains in ARM state. If the program attempts to switch to Thumb state, the first instruction executed after the attempted switch causes an undefined instruction exception. Entry into that exception then switches back to ARM state. The exception handler can detect that this was the cause of the exception from the fact that the T bit of SPSR_und is set. Mode bits M0, M1, M2, M3, and M4 (M[4:0]) are the mode bits, and these determine the mode in which the processor operates. Their interpretation is shown in Table 2-2. Table 2-2 The mode bits M[4:0] Mode Accessible registers 0b10000 User PC, R14 to R0, CPSR 0b10001 FIQ PC, R14_fiq to R8_fiq, R7 to R0, CPSR, SPSR_fiq 0b10010 IRQ PC, R14_irq, R13_irq, R12 to R0, CPSR, SPSR_irq 0b10011 Supervisor PC, R14_svc, R13_svc, R12 to R0, CPSR, SPSR_svc 0b10111 Abort PC, R14_abt, R13_abt, R12 to R0, CPSR, SPSR_abt 0b11011 Undefined PC, R14_und, R13_und, R12 to R0, CPSR, SPSR_und 0b11111 System PC, R14 to R0, CPSR (ARM architecture v4 and above) ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-11
- 44. Programmer’s Model Not all combinations of the mode bits define a valid processor mode. Only those combinations explicitly described can be used. If any other value is programmed into the mode bits M[4:0], the result is UNPREDICTABLE. See also Table 8-1 on page A8-9 for details of the mode bits in the 26-bit architectures. 2.5.3 Other bits Other bits in the program status registers are reserved for future expansion. In general, programmers must take care to write code in such a way that these bits are never modified. Failure to do this might result in code which has unexpected side-effects on future versions of the architecture. See the usage notes for the MSR instruction on page A4-62 for more details. A2-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 45. Programmer’s Model 2.6 Exceptions Exceptions are generated by internal and external sources to cause the processor to handle an event, such as an externally generated interrupt or an attempt to execute an undefined instruction. The processor state just before handling the exception must be preserved so that the original program can be resumed when the exception routine has completed. More than one exception can arise at the same time. ARM supports seven types of exception. Table 2-3 lists the types of exception and the processor mode that is used to process that exception. When an exception occurs, execution is forced from a fixed memory address corresponding to the type of exception. These fixed addresses are called the exception vectors. Note The normal vector at address 0x00000014 and the high vector at address 0xFFFF0014 are not normally used and are reserved for future expansion. The reserved vector at address 0x00000014 was used for an Address Exception vector in earlier versions of the ARM architecture which had a 26-bit address space. See Chapter A8 The 26-bit Architectures for more information. Table 2-3 Exception processing modes Exception type Mode Normal address High vector address Reset Supervisor 0x00000000 0xFFFF0000 Undefined instructions Undefined 0x00000004 0xFFFF0004 Software interrupt (SWI) Supervisor 0x00000008 0xFFFF0008 Prefetch Abort (instruction fetch memory abort) Abort 0x0000000C 0xFFFF000C Data Abort (data access memory abort) Abort 0x00000010 0xFFFF0010 IRQ (interrupt) IRQ 0x00000018 0xFFFF0018 FIQ (fast interrupt) FIQ 0x0000001C 0xFFFF001C When an exception occurs, the banked versions of R14 and the SPSR for the exception mode are used to save state as follows: R14_<exception_mode> = return link SPSR_<exception_mode> = CPSR CPSR[4:0] = exception mode number CPSR[5] = 0 /* Execute in ARM state */ if <exception_mode> == Reset or FIQ then CPSR[6] = 1 /* Disable fast interrupts */ /* else CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ PC = exception vector address ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-13
- 46. Programmer’s Model To return after handling the exception, the SPSR is moved into the CPSR, and R14 is moved to the PC. This can be done atomically in two ways: • using a data-processing instruction with the S bit set, and the PC as the destination • using the Load Multiple with Restore CPSR instruction, as described in LDM (3) on page A4-34. The following sections show what happens automatically when the exception occurs, and also show the recommended data-processing instruction to use to return from each exception. This instruction is always a MOVS or SUBS instruction with the PC as its destination. Note When the recommended data-processing instruction is a SUBS and a Load Multiple with Restore CPSR instruction is used to return from the exception handler, the subtraction must still be performed. This is usually done at the start of the exception handler, before the return link is stored to memory. For example, an interrupt handler that wishes to store its return link on the stack might use instructions of the following form at its entry point: SUB R14, R14, #4 STMFD SP!, {<other_registers>, R14} and return using the instruction: LDMFD SP!, {<other_registers>, PC}^ 2.6.1 Reset When the Reset input is asserted on the processor, the ARM processor immediately stops execution of the current instruction. When Reset is de-asserted, the following actions are performed: R14_svc = UNPREDICTABLE value SPSR_svc = UNPREDICTABLE value CPSR[4:0] = 0b10011 /* Enter Supervisor mode */ CPSR[5] = 0 /* Execute in ARM state */ CPSR[6] = 1 /* Disable fast interrupts */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF0000 else PC = 0x00000000 After Reset, the ARM processor begins execution at address 0x00000000 or 0xFFFF0000 in Supervisor mode with interrupts disabled. See About the MMU architecture on page B3-2 for more information on the effects of Reset. Note There is no architecturally defined way of returning from a Reset. A2-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 47. Programmer’s Model 2.6.2 Undefined Instruction exception If the ARM processor executes a coprocessor instruction, it waits for any external coprocessor to acknowledge that it can execute the instruction. If no coprocessor responds, an Undefined Instruction exception occurs. If an attempt is made to execute an instruction that is UNDEFINED, an Undefined Instruction exception occurs (see Extending the instruction set on page A3-27). The Undefined Instruction exception can be used for software emulation of a coprocessor in a system that does not have the physical coprocessor (hardware), or for general-purpose instruction set extension by software emulation. When an Undefined Instruction exception occurs, the following actions are performed: R14_und = address of next instruction after the undefined instruction SPSR_und = CPSR CPSR[4:0] = 0b11011 /* Enter Undefined mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF0004 else PC = 0x00000004 To return after emulating the undefined instruction use: MOVS PC,R14 This restores the PC (from R14_und) and CPSR (from SPSR_und) and returns to the instruction following the undefined instruction. In some coprocessor designs, an internal exceptional condition caused by one coprocessor instruction is signaled imprecisely by refusing to respond to a later coprocessor instruction. In these circumstances, the Undefined Instruction handler takes whatever action is necessary to clear the exceptional condition, then returns to the second coprocessor instruction. To do this use: SUBS PC,R14,#4 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-15
- 48. Programmer’s Model 2.6.3 Software Interrupt exception The Software Interrupt instruction (SWI) enters Supervisor mode to request a particular supervisor (operating system) function. When a SWI is executed, the following actions are performed: R14_svc = address of next instruction after the SWI instruction SPSR_svc = CPSR CPSR[4:0] = 0b10011 /* Enter Supervisor mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF0008 else PC = 0x00000008 To return after performing the SWI operation, use the following instruction to restore the PC (from R14_svc) and CPSR (from SPSR_svc) and return to the instruction following the SWI: MOVS PC,R14 2.6.4 Prefetch Abort (instruction fetch memory abort) A memory abort is signaled by the memory system. Activating an abort in response to an instruction fetch marks the fetched instruction as invalid. A Prefetch Abort exception is generated if the processor tries to execute the invalid instruction. If the instruction is not executed (for example, as a result of a branch being taken while it is in the pipeline), no Prefetch Abort occurs. In ARM architecture version 5 and above, a Prefetch Abort exception can also be generated as the result of executing a BKPT instruction. For details, see BKPT on page A4-14 (ARM instruction) and BKPT on page A7-24 (Thumb instruction). When an attempt is made to execute an aborted instruction, the following actions are performed: R14_abt = address of the aborted instruction + 4 SPSR_abt = CPSR CPSR[4:0] = 0b10111 /* Enter Abort mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF000C else PC = 0x0000000C To return after fixing the reason for the abort, use: SUBS PC,R14,#4 This restores both the PC (from R14_abt) and CPSR (from SPSR_abt), and returns to the aborted instruction. A2-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 49. Programmer’s Model 2.6.5 Data Abort (data access memory abort) A memory abort is signaled by the memory system. Activating an abort in response to a data access (load or store) marks the data as invalid. A Data Abort exception occurs before any following instructions or exceptions have altered the state of the CPU. The following actions are performed: R14_abt = address of the aborted instruction + 8 SPSR_abt = CPSR CPSR[4:0] = 0b10111 /* Enter Abort mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF0010 else PC = 0x00000010 To return after fixing the reason for the abort use: SUBS PC,R14,#8 This restores both the PC (from R14_abt) and CPSR (from SPSR_abt), and returns to re-execute the aborted instruction. If the aborted instruction does not need to be re-executed use: SUBS PC,R14,#4 Effects of data-aborted instructions Instructions that access data memory can modify memory by storing one or more values. If a Data Abort occurs in such an instruction, the value of each memory location that the instruction stores to is: • unchanged if the memory system does not permit write access to the memory location • UNPREDICTABLE otherwise. Instructions that access data memory can modify registers in three ways: • By loading values into one or more of the general-purpose registers, which can include the PC. • By specifying base register writeback, in which the base register used in the address calculation has a modified value written to it. All instructions that allow this to be specified have UNPREDICTABLE results if base register writeback is specified and the base register is the PC, so only general-purpose registers other than the PC can legitimately be modified in this way. • By loading values into coprocessor registers. If a Data Abort occurs, the values left in these registers are determined by the following rules: 1. The PC value on entry to the data abort handler is 0x00000010 or 0xFFFF0010, and the R14_abt value is determined from the address of the aborted instruction. Neither is affected in any way by the results of any PC load specified by the instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-17
- 50. Programmer’s Model 2. If base register writeback is not specified, the base register value is unchanged. This applies even if the instruction loaded its own base register and the memory access to load the base register occurred earlier than the aborting access. For example, suppose the instruction is: LDMIA R0,{R0,R1,R2} and the implementation loads the new R0 value, then the new R1 value and finally the new R2 value. If a Data Abort occurs on any of the accesses, the value in the base register R0 of the instruction is unchanged. This applies even if it was the load of R1 or R2 that aborted, rather than the load of R0. 3. If base register writeback is specified, the value left in the base register is determined by the abort model of the implementation, as described in Abort models. 4. If the instruction only loads one general-purpose register, the value in that register is unchanged. 5. If the instruction loads more than one general-purpose register, UNPREDICTABLE values are left in destination registers which are neither the PC nor the base register of the instruction. 6. If the instruction loads coprocessor registers, UNPREDICTABLE values are left in the destination coprocessor registers, unless otherwise specified in the instruction set description of the specific coprocessor. Abort models The abort model used by an ARM implementation is IMPLEMENTATION DEFINED, and is one of the following: Base Restored Abort Model If a Data Abort occurs in an instruction which specifies base register writeback, the value in the base register is unchanged. Base Updated Abort Model If a Data Abort occurs in an instruction which specifies base register writeback, the base register writeback still occurs. In either case, the abort model applies uniformly across all instructions. An implementation does not use the Base Restored Abort Model for some instructions and the Base Updated Abort Model for others. Note In some ARMv3 and earlier implementations, a third abort model (the Early Abort Model) was used. In this model, base register writeback occurred for LDC, LDM, STC and STM instructions, and the base register was unchanged for all other instructions. The Early Abort Model is not valid in ARM architecture versions 3M, 4 and above. Some of these implementations optionally allowed a Late Abort Model to be selected. This is identical to the Base Updated Abort Model. A2-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 51. Programmer’s Model 2.6.6 Interrupt request (IRQ) exception The IRQ exception is generated externally by asserting the IRQ input on the processor. It has a lower priority than FIQ (see Table 2-4 on page A2-20), and is masked out when an FIQ sequence is entered. Interrupts are disabled when the I bit in the CPSR is set. If the I bit is clear, ARM checks for an IRQ at instruction boundaries. Note The I bit can only be changed from a privileged mode. When an IRQ is detected, the following actions are performed: R14_irq = address of next instruction to be executed + 4 SPSR_irq = CPSR CPSR[4:0] = 0b10010 /* Enter IRQ mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF0018 else PC = 0x00000018 To return after servicing the interrupt, use: SUBS PC,R14,#4 This restores both the PC (from R14_irq) and CPSR (from SPSR_irq), and resumes execution of the interrupted code. 2.6.7 Fast interrupt request (FIQ) exception The FIQ exception is generated externally by asserting the FIQ input on the processor. FIQ is designed to support a data transfer or channel process, and has sufficient private registers to remove the need for register saving in such applications, therefore minimizing the overhead of context switching. Fast interrupts are disabled when the F bit in the CPSR is set. If the F bit is clear, ARM checks for an FIQ at instruction boundaries. Note The F bit can only be changed from a privileged mode. When an FIQ is detected, the following actions are performed: ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-19
- 52. Programmer’s Model R14_fiq = address of next instruction to be executed + 4 SPSR_fiq = CPSR CPSR[4:0] = 0b10001 /* Enter FIQ mode */ CPSR[5] = 0 /* Execute in ARM state */ CPSR[6] = 1 /* Disable fast interrupts */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF001C else PC = 0x0000001C To return after servicing the interrupt, use: SUBS PC, R14,#4 This restores both the PC (from R14_fiq) and CPSR (from SPSR_fiq), and resumes execution of the interrupted code. The FIQ vector is deliberately the last vector to allow the FIQ exception-handler software to be placed directly at address 0x0000001C or 0xFFFF001C, without requiring a branch instruction from the vector. 2.6.8 Exception priorities Table 2-4 shows the exception priorities: Table 2-4 Exception priorities Priority Exception Highest 1 Reset 2 Data Abort 3 FIQ 4 IRQ 5 Prefetch Abort Lowest 6 Undefined instruction SWI Undefined instruction and software interrupt cannot occur at the same time, as they each correspond to particular (non-overlapping) decodings of the current instruction, and both must be lower priority than prefetch abort, as a prefetch abort indicates that no valid instruction was fetched. The priority of a Data Abort exception is higher than FIQ, which ensures that the data abort handler is entered before the FIQ handler is entered (so that the Data Abort is resolved after the FIQ handler has completed). A2-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 53. Programmer’s Model 2.6.9 High vectors Some ARM implementations allow the exception vector locations to be moved from their normal address range 0x00000000-0x0000001C at the bottom of the 32-bit address space, to an alternative address range 0xFFFF0000-0xFFFF001C near the top of the address space. These alternative locations are known as the high vectors. It is IMPLEMENTATION DEFINED whether the high vectors are supported. When they are, a hardware configuration input selects whether the normal vectors or the high vectors are to be used. The ARM instruction set does not contain any instructions which can directly change whether normal or high vectors are configured. However, if the standard System Control coprocessor is attached to an ARM processor which supports the high vectors, bit[13] of coprocessor 15 register 1 can be used to switch between using the normal vectors and the high vectors (see Register 1: Control register on page B2-13). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-21
- 54. Programmer’s Model 2.7 Memory and memory-mapped I/O This section discusses memory and memory-mapped I/O, mainly with regard to the assumptions ARM processor implementations make about how their memory systems behave and how programs should access memory. As a result, it describes the memory system from the outside, specifying how it should behave without going into much detail of how this behavior can or should be implemented. More details of how some standard memory systems behave can be found in Part B: Memory and System Architectures. The ARM architecture allows a wide variety of memory system designs, using the range of memory and I/O devices which are available. This makes it difficult to specify absolute rules about how a memory system should behave. Many of the rules below can be broken if the hardware and software are designed appropriately. However, breaking these rules is discouraged, for the following reasons: • It might make implementing the memory system more difficult. • It might cause difficulties in porting the system (hardware and/or software) to future ARM processors. • Standard software (such as compilers and other software toolkit components) might not work with the rule-breaking system. 2.7.1 Address space The ARM architecture uses a single, flat address space of 232 8-bit bytes. Byte addresses are treated as unsigned numbers, running from 0 to 232 - 1. This address space is regarded as consisting of 230 32-bit words, each of whose addresses is word-aligned, which means that the address is divisible by 4. The word whose word-aligned address is A consists of the four bytes with addresses A, A+1, A+2 and A+3. In ARM architecture version 4 and above, the address space is also regarded as consisting of 231 16-bit halfwords, each of whose addresses is halfword-aligned (divisible by 2). The halfword whose halfword-aligned address is A consists of the two bytes with addresses A and A+1. Note Prior to ARM architecture version 3, the address space was only 226 bytes, with addresses running from 0 to 226 - 1. This address space was split into 224 words. Some implementations of subsequent non-T variants of the ARM architecture include backwards-compatibility features to allow execution of code written for this address space. These features are described in Chapter A8 The 26-bit Architectures. Their use for any purpose other than executing old code is strongly discouraged. These backwards-compatibility features are not compatible with T variants of the architecture, due to conflicting uses of bits[1:0] of R15. A2-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 55. Programmer’s Model Address calculations are normally performed using ordinary integer instructions. This means that they normally wrap around if they overflow or underflow the address space. This means that the result of the calculation is reduced modulo 232. However, to minimize the chances of incompatibility if the address space is extended in the future, programs should not be written so that they rely on this behavior. Address calculations should be written so that their results would still lie in the range 0 to 232 - 1 if they were calculated without wrap-around. Most branch instructions calculate their targets by adding an instruction-specified offset to the value of the PC and writing the result back to the PC. If the overall effect of this calculation of: (address_of_current_instruction) + 8 + offset is to overflow or underflow the address space, the instruction is technically UNPREDICTABLE because it relies on address wrap-around. The result of this is that forward branches past address 0xFFFFFFFF and backward branches past address 0x00000000 should not be used. Also, normal sequential execution of instructions effectively calculates: (address_of_current_instruction) + 4 after each instruction to determine which instruction to execute next. If this calculation overflows the top of the address space, the result is again technically UNPREDICTABLE. In other words, programs should not rely on sequential execution of the instruction at address 0x00000000 after the instruction at address 0xFFFFFFFC. Note The above only applies to instructions that are executed, including those which fail their condition code check. Most ARM implementations prefetch instructions ahead of the currently-executing instruction. If this prefetching overflows the top of the address space, it does not cause the implementation’s behavior to become UNPREDICTABLE until and unless the prefetched instructions are actually executed. LDC, LDM, STC, and STM instructions access a sequence of words at increasing memory addresses, effectively incrementing a memory address by 4 for each load or store. If this calculation overflows the top of the address space, the result is again technically UNPREDICTABLE. In other words, programs should not use these instructions in such a way that they access the word at address 0x00000000 sequentially after the word at address 0xFFFFFFFC. 2.7.2 Endianness The rules in Address space on page A2-22 require that for a word-aligned address A: • The word at address A consists of the bytes at addresses A, A+1, A+2 and A+3. • The halfword at address A consists of the bytes at addresses A and A+1. • The halfword at address A+2 consists of the bytes at addresses A+2 and A+3. • The word at address A therefore consists of the halfwords at addresses A and A+2. However, this does not totally specify the mappings between words, halfwords and bytes. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-23
- 56. Programmer’s Model A memory system uses one of the two following mapping schemes. This choice is known as the endianness of the memory system. • In a little-endian memory system: — a byte or halfword at a word-aligned address is the least significant byte or halfword within the word at that address — a byte at a halfword-aligned address is the least significant byte within the halfword at that address. • In a big-endian memory system: — a byte or halfword at a word-aligned address is the most significant byte or halfword within the word at that address — a byte at a halfword-aligned address is the most significant byte within the halfword at that address. For a word-aligned address A, Table 2-5 and Table 2-6 show how the word at address A, the halfwords at addresses A and A+2, and the bytes at addresses A, A+1, A+2 and A+3 map on to each other for each endianness. Table 2-5 Big-endian memory system 31 24 23 16 15 8 7 0 Word at address A Halfword at address A Halfword at address A+2 Byte at address A Byte at address A+1 Byte at address A+2 Byte at address A+3 Table 2-6 Little-endian memory system 31 20 19 12 11 10 9 8 5 4 3 2 1 0 Word at address A Halfword at address A+2 Halfword at address A Byte at address A+3 Byte at address A+2 Byte at address A+1 Byte at address A It is IMPLEMENTATION DEFINED whether an ARM implementation supports little-endian memory systems, big-endian memory systems, or both. A2-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 57. Programmer’s Model The ARM instruction set does not contain any instructions that directly select the endianness. Instead, an ARM implementation which supports both endiannesses has a hardware input to configure it to match the endianness of the attached memory system. If a standard System Control coprocessor is attached to such an ARM implementation, this configuration input can be changed by writing to bit[7] of register 1 of the System Control coprocessor (see Register 1: Control register on page B2-13). If an ARM implementation is configured for a memory system of one endianness but is actually attached to a memory system of the opposite endianness, only word-sized instruction fetches, data loads and data stores can be relied upon. Other memory accesses have UNPREDICTABLE results. When the standard System Control coprocessor is attached to an ARM processor that supports both endiannesses, bit[7] of the coprocessor’s register 1 is cleared on reset. This means that the ARM processor is configured for a little-endian memory system immediately after reset. If it is attached to a big-endian memory system, one of the first things the reset handler must do is switch the configured endianness to big-endian, using an instruction sequence like: MRC p15, 0, r0, c1, c0 ; r0 := CP15 register 1 ORR r0, r0, #0x80 ; Set bit[7] in r0 MCR p15, 0, r0, c1, c0 ; CP15 register 1 := r0 This must be done before there is any possibility of a byte or halfword data access occurring, or of a Thumb instruction being executed. Note The rules on endianness imply that word loads and stores are not affected by the configured endianness. Because of this, it is not possible to reverse the order of the bytes in a word by storing it, changing the configured endianness, and reloading the stored word. Instead, use one of the code sequences in Swapping endianness on page A9-4. More generally, there is no point in changing the configured endianness of an ARM processor to be different from that of the memory system it is attached to, because no additional architecturally defined operations become available as a result of doing so. So normally, the only time the configured endianness is changed is at reset to make it match the memory system endianness. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-25
- 58. Programmer’s Model 2.7.3 Unaligned memory accesses The ARM architecture normally expects all memory accesses to be suitably aligned. In particular, the address used for a word access should normally be word-aligned, and the address used for a halfword access should normally be halfword-aligned. Memory accesses which are not aligned in this way are called unaligned memory accesses. Unaligned instruction fetches If an address which is not word-aligned is written to R15 during ARM state execution, the result is normally either UNPREDICTABLE or that bits[1:0] of the address are ignored. If an address which is not halfword-aligned is written to R15 during Thumb state execution, bit[0] of the address is normally ignored. See Writing the program counter on page A2-8 and individual instruction descriptions for more details. As a result, a value which is read from R15 during execution of valid code always has bits[1:0] zero for ARM state execution and bit[0] zero for Thumb state execution. When it is specified that these bits are ignored, ARM implementations are not required to ensure that they are cleared from the address sent to memory for the instruction fetch. They can instead send the value written to R15 unchanged to memory, and require the memory system to ignore bits[1:0] of the address for an ARM instruction fetch and bit[0] for a Thumb instruction fetch. Unaligned data accesses The architecturally defined behavior of a load/store instruction which generates an unaligned access is one of the following: • It is UNPREDICTABLE. • It ignores the low-order address bits that make the access unaligned. This means it effectively uses the formula (address AND 0xFFFFFFFE) for a halfword access, and uses the formula (address AND 0xFFFFFFFC)for a word access. • It ignores the low-order address bits that make the access unaligned for the memory access itself, but then uses those low-order bits to control a rotation of the loaded data. (This behavior applies only to the LDR and SWP instructions.) Which of these three options applies to a load/store instruction depends on which instruction it is, and is documented on the individual instruction pages. ARM implementations are not required to ensure that the low-order address bits that make an access unaligned are cleared from the address they send to memory. They can instead send the address as calculated by the load/store instruction unchanged to memory, and require the memory system to ignore address[0] for a halfword access and address[1:0] for a word access. Note When an instruction ignores the low-order address bits that make an access unaligned, the pseudo-code in the instruction description does not mask them out explicitly. Instead, the Memory[<address>,<size>] function used in the pseudo-code masks them out implicitly. This function is defined in the Glossary. A2-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 59. Programmer’s Model 2.7.4 Prefetching and self-modifying code Many ARM implementations fetch instructions from memory before execution of the instructions that precede them has completed. This behavior is known as prefetching the instruction. Prefetching an instruction does not commit the ARM implementation to actually executing the instruction. Two typical cases in which the instruction is not subsequently executed are: • When an exception occurs, execution of the current instruction is completed, all further prefetched instructions are discarded, and execution of the instructions at the exception vector is started. • When a branch is taken, any instructions that have already been prefetched from sequential locations beyond the branch are discarded. ARM implementations are free to choose how far ahead of the current point of execution they prefetch instructions, or even to have a dynamically varying number of prefetched instructions. The original ARM implementation prefetched two instructions ahead of the instruction currently being executed, but implementations are free to choose to prefetch more or less than this. Note When an instruction reads the PC, it gets the address of the instruction that is two after itself: • for ARM instructions, it gets its own address plus 8 • for Thumb instructions, it gets its own address plus 4. Historically, there is a link between this two-instruction offset for PC reads and the two-instruction prefetch of the original ARM implementation. However, this link is not architectural. An implementation which prefetches a different number of instructions still ensures that an instruction which reads the PC gets the address of the instruction two after itself. As well as being free to choose how many instructions to prefetch, an ARM implementation can choose which possible future execution path to prefetch along. For example, after a branch instruction, it can choose to prefetch either the instruction following the branch or the instruction at the branch target. This is known as branch prediction. A potential problem with all forms of instruction prefetching is that the instruction in memory might be changed after it was prefetched but before it is executed. If this happens, the modification to the instruction in memory does not normally prevent the already prefetched copy of the instruction from executing to completion. For example, in the following code sequence, the STR instruction replaces the SUB instruction that follows it by a copy of the ADD instruction: LDR r0, AddInstr STR r0, NextInstr NextInstr SUB r1, r1, #1 : : AddInstr ADD r1, r1, #1 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-27
- 60. Programmer’s Model However, the instruction executed after the STR instruction is normally the SUB instruction on the first occasion that this code is executed, because the SUB instruction was prefetched before the instruction in memory was changed. The ADD instruction is not executed until the second time that the code sequence is executed. Furthermore, even this behavior cannot be guaranteed, because: • On the first occasion that the code sequence is executed, it is possible that an interrupt will occur immediately after the STR. If it does, the SUB instruction that had been prefetched is discarded. When the interrupt handler returns, the instruction at NextInstr is prefetched again, and is the ADD instruction this time. So while the SUB instruction is normally executed on the first occasion that the code is executed, it is possible that the ADD instruction is executed instead. • Either the ARM processor or the memory system is allowed to keep copies of instructions fetched from memory and use those copies instead of repeating the instruction fetch if the instruction is executed again. If this occurs, a copy of the SUB instruction can be executed on the second or subsequent occasion that the code sequence is executed. The main reason that this might occur is that the memory system contains separate instruction and data caches (see Instruction cache coherency on page B5-11). However, other possibilities also exist. For example, some forms of branch prediction hardware keep copies of the instructions at branch targets. The overall result is that code which writes one or more instructions to memory and then executes them (known as self-modifying code) cannot be executed reliably on ARM processors without special precautions. Programming techniques that involve the use of self-modifying code are to be avoided as far as possible. Instruction Memory Barriers (IMBs) In many systems, however, it is impossible to avoid the use of self-modifying code entirely. For example, any system which allows a program to be loaded into memory and then executed is using self-modifying code. Each implementation therefore defines a sequence of operations that can be used in the middle of a self-modifying code sequence to make it execute reliably. This sequence is called an Instruction Memory Barrier (IMB), and often depends both on the ARM processor implementation and on the memory system implementation. The IMB sequence must be executed after the new instructions have been stored to memory and before they are executed, for example, after a program has been loaded and before its entry point is branched to. Any self-modifying code sequence which does not use an IMB in this way has UNPREDICTABLE behavior. Because the exact sequence of operations to be performed by an IMB depends on the ARM and memory system implementations, it is recommended that software is designed so that the IMB sequence is provided as a call to an easily replaceable system dependencies module, rather than being included in-line where it is needed. This eases porting to other ARM processors and memory systems. A2-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 61. Programmer’s Model Also, in many implementations, the IMB sequence includes operations that are only usable from privileged processor modes, such as the cache cleaning and invalidation operations supplied by the standard System Control coprocessor (see Chapter B5 Caches and Write Buffers). To allow User mode programs to use the IMB sequence, it is recommended that it is supplied as an operating system call, invoked by a SWI instruction. In systems that use the 24-bit immediate in a SWI instruction to specify the required operating system service, it is recommended that the IMB sequence is requested by the instruction: SWI 0xF00000 This call takes no parameters and does not return a result, and should use the same calling conventions as a call to a C function with prototype: void IMB(void); apart from the fact that a SWI instruction is used for the call, rather than a BL instruction. Some implementations can use knowledge of the range of addresses to which new instructions have been stored to reduce the execution time cost of an IMB. It is therefore also recommended that a second operating system call is supplied which does an IMB with respect to a specified address range only. On systems that use the 24-bit immediate in a SWI instruction to specify the required operating system service, this should be requested by the instruction: SWI 0xF00001 and should use similar calling conventions to those used by a call to a C function with prototype: void IMB_Range(unsigned long start_addr, unsigned long end_addr); where the address range runs from start_addr (inclusive) to end_addr (exclusive). Note • When the standard ARM Procedure Calling Standard is used, this means that start_addr is passed in R0 and end_addr in R1. • On some ARM implementations, the execution time cost of an IMB can be very large (many thousands of clock cycles), even when a small address range is specified. For small scale uses of self-modifying code, this is likely to lead to a major loss of performance. It is therefore recommended that self-modifying code is only used where it is unavoidable and/or it produces sufficiently large execution time benefits to offset the cost of the IMB. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-29
- 62. Programmer’s Model Other uses for IMBs Some memory systems allow virtual-to-physical address mapping, in which the physical memory location corresponding to an address generated by the ARM processor can be changed. If this address mapping is changed after an instruction has been prefetched but before it is executed, and the address of the instruction is affected by the change of address mapping, then the wrong instruction is executed. This is very similar to the situation that arises if a store occurs to an instruction address after it has been prefetched but before it is executed. In both cases, the instruction held at the memory address is being changed, either because a value is being stored to it or because a different physical memory location becomes associated with the address. The same solution is therefore used when the virtual-to-physical address mapping is changed. The IMB sequence must be executed after a change of virtual-to-physical address mapping and before any attempt to execute an instruction from a memory area whose address mapping has been changed. Another similar case occurs if memory access permissions are changed between prefetching and executing an instruction. If access was not permitted when the instruction was prefetched but is permitted when it is executed, an unexpected Prefetch Abort exception might occur. In the opposite case that access was permitted when the instruction was prefetched and is no longer permitted when it is executed, there might be a security hole in the system. Memory access permissions can typically be changed either by explicitly writing new access permission settings to the memory system, or because the memory system supports different access permissions for User mode and privileged modes and one of the following occurs: • An exception occurs in User mode, causing the processor to switch to a privileged mode. • Privileged code changes mode to User mode. All ARM implementations ensure that the following events do not cause any instructions to be executed after having been prefetched with the wrong access permissions: • An exception occurring in User mode. • Execution of one of the instructions designed for exception return causing a change from a privileged mode to User mode. These instructions are the ones which have a side-effect of copying the SPSR of the current mode to the CPSR, namely: — The data processing instructions ADCS, ADDS, ANDS, BICS, EORS, MOVS, MVNS, ORRS, RSBS, RSCS, SBCS and SUBS when their destination register is R15. (However, only MOVS and SUBS are commonly used for exception return.) — The form of the LDM instruction described in LDM (3) on page A4-34. The same is not guaranteed in the remaining cases where memory access permissions might change between prefetching and executing an instruction. These are: • Explicitly writing new access permission settings to the memory system. • Changing from a privileged mode to User mode by means of an MSR instruction. In these cases, an IMB sequence needs to be executed shortly after the change of access permissions, and none of the instructions executed after the change of access permissions and before the Instruction Memory Barrier should be affected by the change of access permissions. A2-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 63. Programmer’s Model However, the cost of a full IMB can often be avoided in these cases. In particular, the instruction word associated with any particular address has not changed, so it is usually possible to avoid cache flushes. An implementation can therefore define restricted versions of the IMB sequence to be used in these cases. In the case of an MSR instruction changing from a privileged mode to User mode, a restricted version of the IMB sequence that works on all ARM processors to date is simply to execute any instruction that writes to the PC, other than the branch instructions described in the following sections: • B, BL on page A4-10 • BLX (1) on page A4-16 • B (1) on page A7-18 • B (2) on page A7-20 • BL, BLX(1) on page A7-26. In other words, the mode change should not affect the access permissions of any instructions that can be reached from the MSR instruction by any combination of: • Normal sequential execution of instructions. • For each branch from the above list that can be reached in this way, execution of the instruction at its target. (The branch instructions in the list are precisely those that have a fixed, statically determined target.) This set of instructions is occasionally referred to elsewhere in this manual as the set of instructions that can be reached by predictable subsequent execution from the MSR instruction. 2.7.5 Memory-mapped I/O The standard way to perform I/O functions on ARM systems is by the use of memory-mapped I/O. This uses special memory addresses which supply I/O functions when they are loaded from or stored to. Typically, loading from a memory-mapped I/O address is used for input, and storing to a memory-mapped I/O address is used for output. Both loads and stores can also be used to perform control functions, either instead of or in addition to their normal input or output function. The behavior of a memory-mapped I/O location usually differs from that expected of a normal memory location. For example, two successive loads from a normal memory location return the same value each time unless there has been an intervening store to that location. For a memory-mapped I/O location, the value returned by the second load can be different from the value returned by the first load. Typically, this is because the first load has a side-effect (such as removing the loaded value from a buffer) or because of a side-effect of an intervening load or store to another memory-mapped I/O location. These differences in behavior mainly affect the use of caches and write buffers in the memory system. This is discussed in Chapter B5 Caches and Write Buffers. In short, memory-mapped I/O locations are normally marked as uncachable and unbufferable, to avoid changes to the number, type, order, or timing of the accesses made to them. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-31
- 64. Programmer’s Model Instruction fetches from memory-mapped I/O As described in Prefetching and self-modifying code on page A2-27, ARM implementations can vary considerably with regard to when they fetch instructions from memory. As a result, it is strongly recommended that memory-mapped I/O locations are only used for data loads and stores, not for instruction fetches. Any system design which relies on executing instructions fetched from a memory-mapped I/O location is likely to be hard to port to future ARM implementations. Data accesses to memory-mapped I/O An instruction sequence accesses data memory at various points during its execution, generating a sequence of load and store accesses. Provided these loads and stores access normal memory locations, they only interact with each other if they access the same memory location. As a result, loads and stores to distinct normal memory locations can be performed in a different order to that implied by the instruction sequence, without changing the final result of the sequence. This freedom to change the order of memory accesses can be exploited by a memory system to improve performance (for example, by the use of caches and write buffers). Furthermore, data accesses to the same normal memory location have other properties that can be exploited to improve performance. These include: • Successive loads from the same location without an intervening store generate identical results. • A load from a location returns the last value stored to that location. • Multiple accesses of one data size can sometimes be merged into a single, larger size access. For example, separate stores to the two halfwords contained within a word can be merged to produce a single word store. However, if the memory words, halfwords or bytes accessed by the code sequence are memory-mapped I/O locations, one access can generate a side-effect which changes the results of a subsequent access to a different location. If this happens, the time order of individual accesses makes a difference to the final results of the code sequence. Also, a load access to a memory-mapped I/O location can have a side-effect that changes the result of a subsequent access to the same location. Accesses to memory-mapped I/O locations must therefore not be optimized away, and their time order must not be changed. It is also important that for memory-mapped I/O, the data size of each memory access is maintained. For example, a code sequence that specifies 4 byte reads from 4 sequential byte addresses must not be merged into a single word read when accessing memory-mapped I/O. Such a system might cause the final results of the code sequence to be different from that intended. Similarly a system which splits word accesses up into many byte accesses might cause memory-mapped I/O devices not to operate as expected. Each ARM implementation provides a mechanism to ensure that no changes are made to the number of accesses in a sequence of data memory accesses, or to their data sizes, or time order. This mechanism consists of IMPLEMENTATION DEFINED requirements on the memory accesses whose number, data sizes, and time order are to be preserved. If these requirements are not adhered to for accesses to memory-mapped I/O locations, unexpected behavior might occur. A2-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 65. Programmer’s Model Typical requirements include: • Constraints on memory attributes of the memory-mapped I/O locations. For example, in the standard memory system architectures described in Part B: Memory and System Architectures, the memory locations must be uncachable and unbufferable. • Constraints on the sizes or alignments of the accesses to the memory-mapped I/O locations. For example, if an ARM implementation has a 16-bit external data bus, it might prohibit the use of 32-bit accesses to memory-mapped I/O locations, since they cannot be performed in a single bus cycle. • A requirement for additional external hardware. For example, an alternative possibility for an ARM implementation with a 16-bit external bus is to allow 32-bit accesses to memory-mapped I/O locations, but require external hardware to re-assemble the two 16-bit bus accesses into a single 32-bit access to the I/O device. If a sequence of data memory accesses includes some accesses which meet the requirements for memory-mapped I/O accesses and some which do not, then: • The number and data sizes of the accesses that meet the requirements are preserved. In particular, they are not merged with each other or with the accesses that do not meet the requirements in any way. The accesses which do not meet the requirements can be merged with each other. • The time order of the accesses which meet the requirements are preserved relative to each other. Their time order relative to accesses which do not meet the requirements is not guaranteed. Time ordering of LDM and STM instructions The LDM instruction performs a sequence of loads from successive words in memory, and the STM instruction performs a similar sequence of stores. The rules described above for accessing memory-mapped I/O apply to the sequence of word accesses within one of these instructions in the same way as they do to a series of separate memory access instructions. The time order of the sequence of memory accesses performed by an LDM or STM instruction is only architecturally defined under limited circumstances. The rules for this are: • If the register list in the instruction includes the PC, the time order of the sequence of memory accesses is not defined. (This means that such LDM and STM instructions are not suitable for accessing memory-mapped I/O.) • If the register list in the instruction does not include the PC, the time order of the sequence of memory accesses is in order of memory address, starting with the lowest address and ending with the highest address. (This order is identical to ascending register number order within the list of registers to be loaded or stored.) • If all of the memory accesses generated by an LDM or STM meet the IMPLEMENTATION DEFINED requirements to be treated as memory-mapped I/O locations, then their number, data sizes and time order are preserved. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A2-33
- 66. Programmer’s Model • If some of the memory accesses generated by an LDM or STM meet the IMPLEMENTATION DEFINED requirements to be treated as memory-mapped I/O locations, but others do not, then their number, data sizes and time order are not guaranteed to be preserved. In particular, the ARM processor and memory system do not even necessarily preserve the relative time order of the accesses that do meet the requirements. This is an exception to the normal rules that govern what happens when some accesses meet the requirements and others do not. For example, with the standard memory systems described in Part B: Memory and System Architectures, the time order of the memory accesses is not guaranteed to be preserved if the LDM or STM crosses the boundary between a cachable area of memory and an uncachable, unbufferable area. Such LDM and STM instructions are therefore not suitable for memory-mapped I/O. A2-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 67. Chapter A3 The ARM Instruction Set This chapter describes the ARM instruction set and contains the following sections: • Instruction set encoding on page A3-2 • The condition field on page A3-5 • Branch instructions on page A3-7 • Data-processing instructions on page A3-9 • Multiply instructions on page A3-12 • Miscellaneous arithmetic instructions on page A3-14 • Status register access instructions on page A3-15 • Load and store instructions on page A3-17 • Load and Store Multiple instructions on page A3-21 • Semaphore instructions on page A3-23 • Exception-generating instructions on page A3-24 • Coprocessor instructions on page A3-25 • Extending the instruction set on page A3-27. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-1
- 68. The ARM Instruction Set 3.1 Instruction set encoding Figure 3-1 shows the ARM instruction set encoding. All other bit patterns are UNPREDICTABLE or UNDEFINED. See Extending the instruction set on page A3-27 for a description of the cases where instructions are UNDEFINED. An entry in square brackets, for example [1], indicates that more information is given after the figure. 3 1 3 0 2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0 1 9 1 8 1 7 1 6 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0 Data processing immediate shift cond [1] 0 0 0 opcode S Rn Rd shift amount shift 0 Rm Miscellaneous instructions: x x x x x x x x x x x x x x x x x x x See Figure 3-3 cond [1] 0 0 0 1 0 x x 0 0 Data processing register shift [2] cond [1] 0 0 0 opcode S Rn Rd Rs 0 shift 1 Rm Miscellaneous instructions: x x x x x x x x x x x x 0 x x x x x x cond [1] 0 0 0 1 0 x x 0 1 Multiplies, extra load/stores: x x x x x x x x x x x x 1 x x x x x x cond [1] 0 0 0 x x x x x 1 See Figure 3-3 See Figure 3-2 Data processing immediate [2] cond [1] 0 0 1 opcode S Rn Rd rotate immediate Undefined instruction [3] cond [1] 0 0 1 1 0 x 0 0 x x x x x x x x x x x x x x x x x x x x Move immediate to status register cond [1] R Mask SBO rotate immediate 0 0 1 1 0 1 0 Load/store immediate offset cond [1] 0 1 0 P U B W L Rn Rd immediate Load/store register offset cond [1] 0 1 1 P U B W L Rn Rd shift amount shift 0 Rm Undefined instruction cond [1] 0 1 1 x x x x x x x x x x x x x x x x x x x x 1 x x x x Undefined instruction [4,7] 0 x x x x x x x x x x x x x x x x x x x x x x x x x x x 1 1 1 1 Load/store multiple cond [1] 1 0 0 P U S W L Rn register list Undefined instruction [4] 1 0 0 x x x x x x x x x x x x x x x x x x x x x x x x x 1 1 1 1 1 0 1 L Branch and branch with link cond [1] 24-bit offset Branch and branch with link and change to Thumb [4] 1 1 1 1 1 0 1 H 24-bit offset 1 1 0 P cond [5] U N W L Rn CRd cp_num 8-bit offset Coprocessor data processing cond [5] opcode1 CRn CRd cp_num opcode2 0 CRm 1 1 1 0 Coprocessor register transfers cond [5] opcode1 L CRn Rd cp_num opcode2 1 CRm 1 1 1 0 1 1 1 1 Software interrupt cond [1] swi number Undefined instruction [4] 1 1 1 1 x x x x x x x x x x x x x x x x x x x x x x x x Figure 3-1 ARM instruction set summary Coprocessor load/store and double register transfers [6] 1 1 1 1 A3-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 69. The ARM Instruction Set 1. The cond field is not allowed to be 1111 in this line. Other lines deal with the cases where bits[31:28] of the instruction are 1111. 2. If the opcode field is of the form 10xx and the S field is 0, one of the following lines applies instead. 3. UNPREDICTABLE prior to ARM architecture version 4. 4. UNPREDICTABLE prior to ARM architecture version 5. 5. If the cond field is 1111, this instruction is UNPREDICTABLE prior to ARM architecture version 5. 6. The coprocessor double register transfer instructions are described in Chapter A10 Enhanced DSP Extension. 7. In E variants of architecture version 5 and above, the cache preload instruction PLD uses a small number of these instruction encodings. 3.1.1 Multiplies and extra load/store instructions Figure 3-2 shows extra multiply and load/store instructions. An entry in square brackets, for example [1], indicates that more information is given below the figure. 3 1 3 0 2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0 1 9 1 8 1 7 1 6 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0 cond 0 0 0 0 0 0 A S Rd Rn Rs 1 0 0 1 Rm cond 0 0 0 0 1 U A S RdHi RdLo Rs 1 0 0 1 Rm cond 0 0 0 1 0 B 0 0 Rn Rd SBZ 1 0 0 1 Rm cond 0 0 0 P U 0 W L Rn Rd SBZ 1 0 1 1 Rm cond Rd Rn 0 0 0 1 U P L W HiOffset 1 0 1 1 LoOffset cond 0 0 0 P U 0 W 0 Rn Rd SBZ 1 1 S 1 Rm Load/store two words register offset [2] Load signed halfword/byte register offset [1] cond 0 0 0 P U 0 W 1 Rn Rd SBZ 1 1 H 1 Rm Load/store two words immediate offset [2] cond 0 0 0 P U 1 W 0 Rn Rd HiOffset 1 1 S 1 LoOffset immediate offset [1] cond 0 0 0 P U 1 W 1 Rn Rd HiOffset 1 1 H 1 LoOffset Figure 3-2 Multiplies and extra load/store instructions Multiply (accumulate) Multiply (accumulate) long Swap/swap byte Load/store halfword register offset [1] Load/store halfword immediate offset [1] Load signed halfword/byte 1. UNPREDICTABLE prior to ARM architecture version 4. 2. These instructions are described in Chapter A10 Enhanced DSP Extension. Note Any instruction with bits[27:25] = 000, bit[7] = 1, bit[4] = 1, and cond not equal to 1111, and which is not specified in Figure 3-2 or its notes, is an undefined instruction (or UNPREDICTABLE prior to ARM architecture version 4). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-3
- 70. The ARM Instruction Set 3.1.2 Miscellaneous instructions Figure 3-3 shows the remaining ARM instruction encodings. An entry in square brackets, for example [1], indicates that more information is given below the figure. 3 1 3 0 2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0 1 9 1 8 1 7 1 6 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0 Move status register to register cond 0 0 0 1 0 R 0 0 SBO Rd SBZ 0 0 0 0 SBZ Move register to status register cond 0 0 0 1 0 R 1 0 mask SBO SBZ 0 0 0 0 Rm Branch/exchange instruction set [1] cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 0 1 Rm Count leading zeros [2] cond 0 0 0 1 0 1 1 0 SBO Rd SBO 0 0 0 1 Rm Branch and link/exchange instruction set [2] cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 1 1 Rm Enhanced DSP add/subtracts [4] cond 0 0 0 1 0 op 0 Rn Rd SBZ 0 1 0 1 Rm Software breakpoint [2,3] cond 0 0 0 1 0 0 1 0 immed 0 1 1 1 immed Enhanced DSP multiplies[4] cond 0 0 0 1 0 op 0 Rd Rn Rs 1 y x 0 Rm Figure 3-3 Miscellaneous instructions 1. Defined in ARM architecture version 5 and above, and in T variants of ARM architecture version 4. 2. This is an undefined instruction is ARM architecture version 4, and is UNPREDICTABLE prior to ARM architecture version 4. 3. If the cond field of this instruction is not 1110, it is UNPREDICTABLE. 4. The enhanced DSP instructions are described in Chapter A10 Enhanced DSP Extension. Note Any instruction with bits[27:23] = 00010, bit[20] = 0, bit[7] and bit[4] not both 1, and cond is not equal to 1111, and which is not specified in Figure 3-3 or its notes, is an undefined instruction (or UNPREDICTABLE prior to architecture version 4). A3-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 71. The ARM Instruction Set 3.2 The condition field Almost all ARM instructions can be conditionally executed, which means that they only have their normal effect on the programmer’s model state, memory and coprocessors if the N, Z, C and V flags in the CPSR satisfy a condition specified in the instruction. If the flags do not satisfy this condition, the instruction acts as a NOP: that is, execution advances to the next instruction as normal, including any relevant checks for interrupts and prefetch aborts, but has no other effect. Prior to ARM architecture version 5, all ARM instructions could be conditionally executed. A few instructions have been introduced subsequently which can only be executed unconditionally. Every instruction contains a 4-bit condition code field in bits 31 to 28: 31 28 27 0 cond This field contains one of the 16 values described in Table 3-1 on page A3-6. Most instruction mnemonics can be extended with the letters defined in the mnemonic extension field. If the always (AL) condition is specified, the instruction is executed irrespective of the value of the condition code flags. The absence of a condition code on an instruction mnemonic implies the AL condition code. 3.2.1 Condition code 0b1111 As indicated in Table 3-1 on page A3-6, if the condition field is 0b1111, the behavior depends on the architecture version: • Prior to ARM architecture version 3, a condition field of 0b1111 meant that the instruction was never executed. The mnemonic extension for this condition was NV. Note Use of this condition is now obsolete and unsupported. • In ARM architecture version 3 and version 4, any instruction with a condition field of 0b1111 is UNPREDICTABLE. • In ARM architecture version 5 and above, a condition field of 0b1111 is used to encode various additional instructions which can only be executed unconditionally. All instruction encoding diagrams which show bits[31:28] as cond only match instructions in which these bits are not equal to 0b1111, unless otherwise stated in the individual instruction description. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-5
- 72. The ARM Instruction Set Table 3-1 Condition codes Opcode [31:28] Mnemonic extension Meaning Condition flag state 0000 EQ Equal Z set 0001 NE Not equal Z clear 0010 CS/HS Carry set/unsigned higher or same C set 0011 CC/LO Carry clear/unsigned lower C clear 0100 MI Minus/negative N set 0101 PL Plus/positive or zero N clear 0110 VS Overflow V set 0111 VC No overflow V clear 1000 HI Unsigned higher C set and Z clear 1001 LS Unsigned lower or same C clear or Z set 1010 GE Signed greater than or equal N set and V set, or N clear and V clear (N == V) 1011 LT Signed less than N set and V clear, or N clear and V set (N != V) 1100 GT Signed greater than Z clear, and either N set and V set, or N clear and V clear (Z == 0,N == V) 1101 LE Signed less than or equal Z set, or N set and V clear, or N clear and V set (Z == 1 or N != V) 1110 AL Always (unconditional) - 1111 (NV) See Condition code 0b1111 on page A3-5 - A3-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 73. The ARM Instruction Set 3.3 Branch instructions All ARM processors support a branch instruction that allows a conditional branch forwards or backwards up to 32MB. As the PC is one of the general-purpose registers (R15), a branch or jump can also be generated by writing a value to R15. A subroutine call can be performed by a variant of the standard branch instruction. As well as allowing a branch forward or backward up to 32MB, the Branch with Link (BL) instruction preserves the address of the instruction after the branch (the return address) in the LR (R14). In T variants of ARM architecture version 4, and in ARM architecture version 5 and above, the Branch and Exchange (BX) instruction copies the contents of a general-purpose register Rm to the PC (like a MOV PC,Rm instruction), with the additional functionality that if bit[0] of the transferred value is 1, the processor shifts to Thumb state. Together with the corresponding Thumb instructions, this allows interworking branches between ARM and Thumb code. Interworking subroutine calls can be generated by combining BX with an instruction to write a suitable return address to the LR, such as an immediately preceding MOV LR,PC instruction. In ARM architecture version 5 and above, there are also two types of Branch with Link and Exchange (BLX) instruction: • One type takes a register operand Rm, like a BX instruction. This instruction behaves like a BX instruction, and additionally writes the address of the next instruction into the LR. This provides a more efficient interworking subroutine call than a sequence of MOV LR,PC followed by BX Rm. • The other type behaves like a BL instruction, branching backwards or forwards by up to 32MB and writing a return link to the LR, but shifts to Thumb state rather than staying in ARM state as BL does. This provides a more efficient alternative to loading the subroutine address into Rm followed by a BLX Rm instruction when it is known that a Thumb subroutine is being called and that the subroutine lies within the 32MB range. A load instruction provides a way to branch anywhere in the 4GB address space (known as a long branch). A 32-bit value is loaded directly from memory into the PC, causing a branch. A long branch can be preceded by MOV LR,PC or another instruction that writes the LR to generate a long subroutine call. In ARM architecture version 5 and above, bit[0] of the value loaded by a long branch controls whether the subroutine is executed in ARM state or Thumb state, just like bit[0] of the value moved to the PC by a BX instruction. Prior to ARM architecture version 5, bits[1:0] of the value loaded into the PC are ignored, and a load into the PC can only be used to call a subroutine in ARM state. In non-T variants of ARM architecture version 5, the instructions described above can cause an entry into Thumb state despite the fact that the Thumb instruction set is not present. This causes the instruction at the branch target to enter the undefined instruction trap. See The control bits on page A2-10 for more details. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-7
- 74. The ARM Instruction Set 3.3.1 Examples B label ; branch unconditionally to label BCC label ; branch to label if carry flag is clear BEQ label ; branch to label if zero flag is set MOV PC, #0 ; R15 = 0, branch to location zero BL func ; subroutine call to function func . . MOV PC, LR ; R15=R14, return to instruction after the BL MOV LR, PC ; store the address of the instruction ; after the next one into R14 ready to return LDR PC, =func ; load a 32-bit value into the program counter 3.3.2 List of branch instructions B, BL Branch, and Branch with Link. See B, BL on page A4-10. BLX Branch with Link and Exchange. See BLX (1) on page A4-16 and BLX (2) on page A4-18. BX Branch and Exchange Instruction Set. See BX on page A4-19. A3-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 75. The ARM Instruction Set 3.4 Data-processing instructions ARM has 16 data-processing instructions, shown in Table 3-2. Table 3-2 Data-processing instructions Opcode Mnemonic Operation Action 0000 AND Logical AND Rd := Rn AND shifter_operand 0001 EOR Logical Exclusive OR Rd := Rn EOR shifter_operand 0010 SUB Subtract Rd := Rn - shifter_operand 0011 RSB Reverse Subtract Rd := shifter_operand - Rn 0100 ADD Add Rd := Rn + shifter_operand 0101 ADC Add with Carry Rd := Rn + shifter_operand + Carry Flag 0110 SBC Subtract with Carry Rd := Rn - shifter_operand - NOT(Carry Flag) 0111 RSC Reverse Subtract with Carry Rd := shifter_operand - Rn - NOT(Carry Flag) 1000 TST Test Update flags after Rn AND shifter_operand 1001 TEQ Test Equivalence Update flags after Rn EOR shifter_operand 1010 CMP Compare Update flags after Rn - shifter_operand 1011 CMN Compare Negated Update flags after Rn + shifter_operand 1100 ORR Logical (inclusive) OR Rd := Rn OR shifter_operand 1101 MOV Move Rd := shifter_operand (no first operand) 1110 BIC Bit Clear Rd := Rn AND NOT(shifter_operand) 1111 MVN Move Not Rd := NOT shifter_operand (no first operand) Most data-processing instructions take two source operands, though Move and Move Not take only one. The compare and test instructions only update the condition flags. Other data-processing instructions store a result to a register and optionally update the condition flags as well. Of the two source operands, one is always a register. The other is called a shifter operand and is either an immediate value or a register. If the second operand is a register value, it can have a shift applied to it. CMP, CMN, TST and TEQ always update the condition code flags. The assembler automatically sets the S bit in the instruction for them, and the corresponding instruction with the S bit clear is not a data-processing instruction, but instead lies in one of the instruction extension spaces (see Extending the instruction set on page A3-27). The remaining instructions update the flags if an S is appended to the instruction mnemonic (which sets the S bit in the instruction). See The condition code flags on page A2-9 for more details. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-9
- 76. The ARM Instruction Set 3.4.1 Instruction encoding <opcode1>{<cond>}{S} <Rd>, <shifter_operand> <opcode1> := MOV | MVN <opcode2>{<cond>} <Rn>, <shifter_operand> <opcode2> := CMP | CMN | TST | TEQ <opcode3>{<cond>}{S} <Rd>, <Rn>, <shifter_operand> <opcode3> := ADD | SUB | RSB | ADC | SBC | RSC | AND | BIC | EOR | ORR 31 28 27 26 25 24 21 20 19 16 15 12 11 0 cond 0 0 I opcode S Rn Rd shifter_operand I bit Distinguishes between the immediate and register forms of <shifter_operand>. S bit Signifies that the instruction updates the condition codes. Rn Specifies the first source operand register. Rd Specifies the destination register. shifter_operand Specifies the second source operand. See Addressing Mode 1 - Data-processing operands on page A5-2 for details of the shifter operands. A3-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 77. The ARM Instruction Set 3.4.2 List of data-processing instructions ADC Add with Carry. See ADC on page A4-4. ADD Add. See ADD on page A4-6. AND Logical AND. See AND on page A4-8. BIC Logical Bit Clear. See BIC on page A4-12. CMN Compare Negative. See CMN on page A4-23. CMP Compare. See CMP on page A4-25. EOR Logical EOR. See EOR on page A4-26. MOV Move. See MOV on page A4-56. MVN Move Negative. See MVN on page A4-68. ORR Logical OR. See ORR on page A4-70. RSB Reverse Subtract. See RSB on page A4-72. RSC Reverse Subtract with Carry. See RSC on page A4-74. SBC Subtract with Carry. See SBC on page A4-76. SUB Subtract. See SUB on page A4-98. TEQ Test Equivalence. See TEQ on page A4-106. TST Test. See TST on page A4-107. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-11
- 78. The ARM Instruction Set 3.5 Multiply instructions ARM has two classes of Multiply instruction: • normal, 32-bit result • long, 64-bit result. All Multiply instructions take two register operands as the input to the multiplier. The ARM processor does not directly support a multiply-by-constant instruction due to the efficiency of shift and add, or shift and reverse subtract instructions. 3.5.1 Normal multiply There are two Multiply instructions that produce 32-bit results: MUL Multiplies the values of two registers together, truncates the result to 32 bits, and stores the result in a third register. MLA Multiplies the values of two registers together, adds the value of a third register, truncates the result to 32 bits, and stores the result in a fourth register. This can be used to perform multiply-accumulate operations. Both Multiply instructions can optionally set the N (Negative) and Z (Zero) condition code flags. No distinction is made between signed and unsigned variants. Only the least significant 32 bits of the result are stored in the destination register, and the sign of the operands does not affect this value. 3.5.2 Long multiply There are four Multiply instructions that produce 64-bit results (long multiply). Two of the variants multiply the values of two registers together and store the 64-bit result in third and fourth registers. There are signed (SMULL) and unsigned (UMULL) variants. The signed variants produce a different result in the most significant 32 bits if either or both of the source operands is negative. The remaining two variants multiply the values of two registers together, add the 64-bit value from the third and fourth registers and store the 64-bit result back into those registers (third and fourth). There are signed (SMLAL) and unsigned (UMLAL) variants. These instructions perform a long multiply and accumulate. All four long multiply instructions can optionally set the N (Negative) and Z (Zero) condition code flags. 3.5.3 Examples MUL R4, R2, R1 ; Set R4 to value of R2 multiplied by R1 MULS R4, R2, R1 ; R4 = R2 x R1, set N and Z flags MLA R7, R8, R9, R3 ; R7 = R8 x R9 + R3 SMULL R4, R8, R2, R3 ; R4 = bits 0 to 31 of R2 x R3 ; R8 = bits 32 to 63 of R2 x R3 UMULL R6, R8, R0, R1 ; R8, R6 = R0 x R1 UMLAL R5, R8, R0, R1 ; R8, R5 = R0 x R1 + R8, R5 A3-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 79. The ARM Instruction Set 3.5.4 List of multiply instructions MLA Multiply Accumulate. See MLA on page A4-54. MUL Multiply. See MUL on page A4-66. SMLAL Signed Multiply Accumulate Long. See SMLAL on page A4-78. SMULL Signed Multiply Long. See SMULL on page A4-80. UMLAL Unsigned Multiply Accumulate Long. See UMLAL on page A4-109. UMULL Unsigned Multiply Long. See UMULL on page A4-111. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-13
- 80. The ARM Instruction Set 3.6 Miscellaneous arithmetic instructions In addition to the normal data-processing and multiply instructions, versions 5 and above of the ARM architecture include a Count Leading Zeros (CLZ) instruction. This instruction returns the number of 0 bits at the most significant end of its operand before the first 1 bit is encountered (or 32 if its operand is zero). Two typical applications for this are: • To determine how many bits the operand should be shifted left in order to normalize it, so that its most significant bit is 1. (This can be used in integer division routines.) • To locate the highest priority bit in a bit mask. 3.6.1 Instruction encoding CLZ{<cond>} <Rd>, <Rm> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 1 1 0 SBO Rd SBO 0 0 0 1 Rm Rd Specifies the destination register. Rm Specifies the operand register. 3.6.2 List of miscellaneous arithmetic instructions CLZ Count Leading Zeros. See CLZ on page A4-22. A3-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 81. The ARM Instruction Set 3.7 Status register access instructions There are two instructions for moving the contents of a program status register to or from a general-purpose register. Both the CPSR and SPSR can be accessed. Each status register is split into four 8-bit fields that can be individually written: Bits[31:24] The flags field. Bits[23:16] The status field. Bits[15:8] The extension field. Bits[7:0] The control field. To date, the ARM architecture does not use the status and extension fields, and three bits are unused in the flags field. The four condition code flags occupy bits[31:28]. In E variants of architecture versions 5 and above, the Q flag occupies bit[27]. See The Q flag on page A10-5 for more information on the Q flag. The control field contains two interrupt disable bits, five processor mode bits, and the Thumb bit on ARM architecture version 5 and above and on T variants of ARM architecture version 4 (see The T bit on page A2-11). The unused bits of the status registers might be used in future ARM architectures, and must not be modified by software. Therefore, a read-modify-write strategy must be used to update the value of a status register to ensure future compatibility. The status registers are readable to allow the read part of the read-modify-write operation, and to allow all processor state to be preserved (for instance, during process context switches). The status registers are writable to allow the write part of the read-modify-write operation, and allow all processor state to be restored. 3.7.1 CPSR value Altering the value of the CPSR has three uses: • sets the value of the condition code flags (and of the Q flag when it exists) to a known value • enables or disable interrupts • changes processor mode (for instance, to initialize stack pointers). Note The T bit must not be changed directly by writing to the CPSR, but only via the BX instruction, and in the implicit SPSR to CPSR moves in instructions designed for exception return. Attempts to enter or leave Thumb state by directly altering the T bit can have UNPREDICTABLE consequences. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-15
- 82. The ARM Instruction Set 3.7.2 Examples These examples assume that the ARM processor is already in a privileged mode. If the ARM processor starts in User mode, only the flag update has any effect. MRS R0, CPSR ; Read the CPSR BIC R0, R0, #0xF0000000 ; Clear the N, Z, C and V bits MSR CPSR_f, R0 ; Update the flag bits in the CPSR ; N, Z, C and V flags now all clear MRS R0, CPSR ; Read the CPSR ORR R0, R0, #0x80 ; Set the interrupt disable bit MSR CPSR_c, R0 ; Update the control bits in the CPSR ; interrupts (IRQ) now disabled MRS R0, CPSR ; Read the CPSR BIC R0, R0, #0x1F ; Clear the mode bits ORR R0, R0, #0x11 ; Set the mode bits to FIQ mode MSR CPSR_c, R0 ; Update the control bits in the CPSR ; now in FIQ mode 3.7.3 List of status register access instructions MRS Move PSR to General-purpose Register. See MRS on page A4-60. MSR Move General-purpose Register to PSR. See MSR on page A4-62. A3-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 83. The ARM Instruction Set 3.8 Load and store instructions The ARM architecture supports two broad types of instruction which load or store the value of a single register from or to memory: • The first type can load or store a 32-bit word or an 8-bit unsigned byte. • The second type can load or store a 16-bit unsigned halfword, and can load and sign extend a 16-bit halfword or an 8-bit byte. This type of instruction is only available in ARM architecture version 4 and above. 3.8.1 Addressing modes In both types of instruction, the addressing mode is formed from two parts: • the base register • the offset. The base register can be any one of the general-purpose registers (including the PC, which allows PC-relative addressing for position-independent code). The offset takes one of three formats: Immediate The offset is an unsigned number that can be added to or subtracted from the base register. Immediate offset addressing is useful for accessing data elements that are a fixed distance from the start of the data object, such as structure fields, stack offsets and input/output registers. For the word and unsigned byte instructions, the immediate offset is a 12-bit number. For the halfword and signed byte instructions, it is an 8-bit number. Register The offset is a general-purpose register (not the PC), that can be added to or subtracted from the base register. Register offsets are useful for accessing arrays or blocks of data. Scaled register The offset is a general-purpose register (not the PC) shifted by an immediate value, then added to or subtracted from the base register. The same shift operations used for data-processing instructions can be used (Logical Shift Left, Logical Shift Right, Arithmetic Shift Right and Rotate Right), but Logical Shift Left is the most useful as it allows an array indexed to be scaled by the size of each array element. Scaled register offsets are only available for the word and unsigned byte instructions. As well as the three types of offset, the offset and base register are used in three different ways to form the memory address. The addressing modes are described as follows: Offset The base register and offset are added or subtracted to form the memory address. Pre-indexed The base register and offset are added or subtracted to form the memory address. The base register is then updated with this new address, to allow automatic indexing through an array or memory block. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-17
- 84. The ARM Instruction Set Post-indexed The value of the base register alone is used as the memory address. The base register and offset are added or subtracted and this value is stored back in the base register, to allow automatic indexing through an array or memory block. 3.8.2 Load and Store word or unsigned byte instructions Load instructions load a single value from memory and write it to a general-purpose register. Store instructions read a value from a general-purpose register and store it to memory. Load and Store instructions have a single instruction format: LDR|STR{<cond>}{B}{T} Rd, <addressing_mode> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I P U B W L Rn Rd addressing_mode_specific I, P, U, W Are bits that distinguish between different types of <addressing_mode>. L bit Distinguishes between a Load (L==1) and a Store instruction (L==0). B bit Distinguishes between an unsigned byte (B==1) and a word (B==0) access. Rn Specifies the base register used by <addressing_mode>. Rd Specifies the register whose contents are to be loaded or stored. 3.8.3 Load and Store Halfword and Load Signed Byte Load instructions load a single value from memory and write it to a general-purpose register. Store instructions read a value from a general-purpose register and store it to memory. Load and Store Halfword and Load Signed Byte instructions have a single instruction format: LDR|STR{<cond>}H|SH|SB Rd, <addressing_mode> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U I W L Rn Rd addr_mode 1 S H 1 addr_mode addr_mode Are addressing-mode-specific bits. I, P, U, W Are bits that specify the type of addressing mode (see Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34). L bit Distinguishes between a Load (L==1) and a Store instruction (L==0). S bit Distinguishes between a signed (S==1) and an unsigned (S==0) halfword access. If the L bit is zero and S bit is one, the instruction is UNPREDICTABLE. A3-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 85. The ARM Instruction Set H bit Distinguishes between a halfword (H==1) and a signed byte (H==0) access. If the S bit and H bit are both zero, this instruction encodes a SWP or Multiply instruction. Rn Specifies the base register used by the addressing mode. Rd Specifies the register whose contents are to be loaded or stored. 3.8.4 Examples LDR R1, [R0] ; Load R1 from the address in R0 LDR R8, [R3, #4] ; Load R8 from the address in R3 + 4 LDR R12, [R13, #-4] ; Load R12 from R13 - 4 STR R2, [R1, #0x100] ; Store R2 to the address in R1 + 0x100 LDRB R5, [R9] ; Load byte into R5 from R9 ; (zero top 3 bytes) LDRB R3, [R8, #3] ; Load byte to R3 from R8 + 3 ; (zero top 3 bytes) STRB R4, [R10, #0x200] ; Store byte from R4 to R10 + 0x200 LDR R11, [R1, R2] ; Load R11 from the address in R1 + R2 STRB R10, [R7, -R4] ; Store byte from R10 to addr in R7 - R4 LDR R11, [R3, R5, LSL #2] ; Load R11 from R3 + (R5 x 4) LDR R1, [R0, #4]! ; Load R1 from R0 + 4, then R0 = R0 + 4 STRB R7, [R6, #-1]! ; Store byte from R7 to R6 - 1, ; then R6 = R6 - 1 LDR R3, [R9], #4 ; Load R3 from R9, then R9 = R9 + 4 STR R2, [R5], #8 ; Store R2 to R5, then R5 = R5 + 8 LDR R0, [PC, #40] ; Load R0 from PC + 0x40 (= address of ; the LDR instruction + 8 + 0x40) LDR R0, [R1], R2 ; Load R0 from R1, then R1 = R1 + R2 LDRH R1, [R0] ; Load halfword to R1 from R0 ; (zero top 2 bytes) LDRH R8, [R3, #2] ; Load halfword into R8 from R3 + 2 LDRH R12, [R13, #-6] ; Load halfword into R12 from R13 - 6 STRH R2, [R1, #0x80] ; Store halfword from R2 to R1 + 0x80 LDRSH R5, [R9] ; Load signed halfword to R5 from R9 LDRSB R3, [R8, #3] ; Load signed byte to R3 from R8 + 3 LDRSB R4, [R10, #0xC1] ; Load signed byte to R4 from R10 + 0xC1 LDRH R11, [R1, R2] ; Load halfword into R11 from address ; in R1 + R2 STRH R10, [R7, -R4] ; Store halfword from R10 to R7 - R4 LDRSH R1, [R0, #2]! ; Load signed halfword R1 from R0 + 2, ; then R0 = R0 + 2 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-19
- 86. The ARM Instruction Set LDRSB R7, [R6, #-1]! ; Load signed byte to R7 from R6 - 1, ; then R6 = R6 - 1 LDRH R3, [R9], #2 ; Load halfword to R3 from R9, ; then R9 = R9 + 2 STRH R2, [R5], #8 ; Store halfword from R2 to R5, ; then R5 = R5 + 8 3.8.5 List of load and store instructions LDR Load Word. See LDR on page A4-37. LDRB Load Byte. See LDRB on page A4-40. LDRBT Load Byte with User Mode Privilege. See LDRBT on page A4-42. LDRH Load Unsigned Halfword. See LDRH on page A4-44. LDRSB Load Signed Byte. See LDRSB on page A4-46. LDRSH Load Signed Halfword. See LDRSH on page A4-48. LDRT Load Word with User Mode Privilege. See LDRT on page A4-50. STR Store Word. See STR on page A4-88. STRB Store Byte. See STRB on page A4-90. STRBT Store Byte with User Mode Privilege. See STRBT on page A4-92. STRH Store Halfword. See STRH on page A4-94. STRT Store Word with User Mode Privilege. See STRT on page A4-96. A3-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 87. The ARM Instruction Set 3.9 Load and Store Multiple instructions Load Multiple instructions load a subset, or possibly all, of the general-purpose registers from memory. Store Multiple instructions store a subset, or possibly all, of the general-purpose registers to memory. Load and Store Multiple instructions have a single instruction format: LDM{<cond>}<addressing_mode> Rn{!}, <registers>{^} STM{<cond>}<addressing_mode> Rn{!}, <registers>{^} where: <addressing_mode> = IA | IB | DA | DB | FD | FA | ED | EA 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 P U S W L Rn register list register list The list of <registers> has one bit for each general-purpose register. Bit 0 is for R0, and bit 15 is for R15 (the PC). The register syntax list is an opening bracket, followed by a comma-separated list of registers, followed by a closing bracket. A sequence of consecutive registers can be specified by separating the first and last registers in the range with a minus sign. P, U, and W bits These distinguish between the different types of addressing mode (see Addressing Mode 4 - Load and Store Multiple on page A5-48). S bit For LDMs that load the PC, the S bit indicates that the CPSR is loaded from the SPSR after all the registers have been loaded. For all STMs, and LDMs that do not load the PC, it indicates that when the processor is in a privileged mode, the User mode banked registers are transferred and not the registers of the current mode. L bit This distinguishes between a Load (L==1) and a Store (L==0) instruction. Rn This specifies the base register used by the addressing mode. 3.9.1 Examples STMFD R13!, {R0 - R12, LR} LDMFD R13!, {R0 - R12, PC} LDMIA R0, {R5 - R8} STMDA R1!, {R2, R5, R7 - R9, R11} ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-21
- 88. The ARM Instruction Set 3.9.2 List of Load and Store Multiple instructions LDM Load Multiple. See LDM (1) on page A4-30. LDM User Registers Load Multiple. See LDM (2) on page A4-32. LDM Load Multiple with Restore CPSR. See LDM (3) on page A4-34. STM Store Multiple. See STM(1) on page A4-84. STM User Registers Store Multiple. See STM (2) on page A4-86. A3-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 89. The ARM Instruction Set 3.10 Semaphore instructions The ARM instruction set has two semaphore instructions: • Swap (S) WP • Swap Byte (SWPB). These instructions are provided for process synchronization. Both instructions generate an atomic load and store operation, allowing a memory semaphore to be loaded and altered without interruption. SWP and SWPB have a single addressing mode, whose address is the contents of a register. Separate registers are used to specify the value to store and the destination of the load. If the same register is specified for both of these, SWP exchanges the value in the register and the value in memory. The semaphore instructions do not provide a compare and conditional write facility. If wanted, this must be done explicitly. 3.10.1 Examples SWP R12, R10, [R9] ; load R12 from address R9 and ; store R10 to address R9 SWPB R3, R4, [R8] ; load byte to R3 from address R8 and ; store byte from R4 to address R8 SWP R1, R1, [R2] ; Exchange value in R1 and address in R2 3.10.2 List of semaphore instructions SWP Swap. See SWP on page A4-102. SWPB Swap Byte. See SWPB on page A4-104. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-23
- 90. The ARM Instruction Set 3.11 Exception-generating instructions The ARM instruction set provides two types of instruction whose main purpose is to cause a processor exception to occur: • The Software Interrupt (SWI) instruction is used to cause a SWI exception to occur (see Software Interrupt exception on page A2-16). This is the main mechanism in the ARM instruction set by which User mode code can make calls to privileged Operating System code. • The Breakpoint (BKPT) instruction is used for software breakpoints in ARM architecture versions 5 and above. Its default behavior is to cause a Prefetch Abort exception to occur (see Prefetch Abort (instruction fetch memory abort) on page A2-16). A debug monitor program which has previously been installed on the Prefetch Abort vector can handle this exception. If debug hardware is present in the system, it is allowed to override this default behavior. Details of whether and how this happens are IMPLEMENTATION DEFINED. 3.11.1 Instruction encodings SWI{<cond>} <immed_24> 31 28 27 26 25 24 23 0 cond 1 1 1 1 immed_24 BKPT <immediate> 31 28 27 26 25 24 23 22 21 20 19 8 7 4 3 0 1 1 1 0 0 0 0 1 0 0 1 0 immed 0 1 1 1 immed In both SWI and BKPT, the immediate fields of the instruction are ignored by the ARM processor. The SWI or Prefetch Abort handler can optionally be written to load the instruction that caused the exception and extract these fields. This allows them to be used to communicate extra information about the Operating System call or breakpoint to the handler. 3.11.2 List of exception-generating instructions BKPT Breakpoint. See BKPT on page A4-14. SWI Software Interrupt. See SWI on page A4-100. A3-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 91. The ARM Instruction Set 3.12 Coprocessor instructions The ARM instruction set provides three types of instruction for communicating with coprocessors. These allow: • the ARM processor to initiate a coprocessor data processing operation • ARM registers to be transferred to and from coprocessor registers • the ARM processor to generate addresses for the coprocessor Load and Store instructions. The instruction set distinguishes up to 16 coprocessors with a 4-bit field in each coprocessor instruction, so each coprocessor is assigned a particular number. Note One coprocessor can use more than one of the 16 numbers if a large coprocessor instruction set is required. Coprocessors execute the same instruction stream as ARM, ignoring ARM instructions and coprocessor instructions for other coprocessors. Coprocessor instructions that cannot be executed by coprocessor hardware cause an undefined instruction trap, allowing software emulation of coprocessor hardware. A coprocessor can partially execute an instruction and then cause an exception. This is useful for handling run-time-generated exceptions, like divide-by-zero or overflow. However, the partial execution is internal to the coprocessor and is not visible to the ARM processor. As far as the ARM processor is concerned, the instruction is held at the start of its execution and completes without exception if allowed to begin execution. Any decision on whether to execute the instruction or cause an exception is taken within the coprocessor before the ARM processor is allowed to start executing the instruction. Not all fields in coprocessor instructions are used by the ARM processor. Coprocessor register specifiers and opcodes are defined by individual coprocessors. Therefore, only generic instruction mnemonics are provided for coprocessor instructions. Assembler macros can be used to transform custom coprocessor mnemonics into these generic mnemonics, or to regenerate the opcodes manually. 3.12.1 Examples CDP p5, 2, c12, c10, c3, 4 ; Coproc 5 data operation ; opcode 1 = 2, opcode 2 = 4 ; destination register is 12 ; source registers are 10 and 3 MRC p15, 5, R4, c0, c2, 3 ; Coproc 15 transfer to ARM register ; opcode 1 = 5, opcode 2 = 3 ; ARM destination register = R4 ; coproc source registers are 0 and 2 MCR p14, 1, R7, c7, c12, 6 ; ARM register transfer to Coproc 14 ; opcode 1 = 1, opcode 2 = 6 ; ARM source register = R7 ; coproc dest registers are 7 and 12 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-25
- 92. The ARM Instruction Set LDC p6, CR1, [R4] ; Load from memory to coprocessor 6 ; ARM register 4 contains the address ; Load to CP reg 1 LDC p6, CR4, [R2, #4] ; Load from memory to coprocessor 6 ; ARM register R2 + 4 is the address ; Load to CP reg 4 STC p8, CR8, [R2, #4]! ; Store from coprocessor 8 to memory ; ARM register R2 + 4 is the address ; after the transfer R2 = R2 + 4 ; Store from CP reg 8 STC p8, CR9, [R2], #-16 ; Store from coprocessor 8 to memory ; ARM register R2 holds the address ; after the transfer R2 = R2 - 16 ; Store from CP reg 9 3.12.2 List of coprocessor instructions CDP Coprocessor Data Operations. See CDP on page A4-20. LDC Load Coprocessor Register. See LDC on page A4-28. MCR Move to Coprocessor from ARM Register. See MCR on page A4-52. MRC Move to ARM Register from Coprocessor. See MRC on page A4-58. STC Store Coprocessor Register. See STC on page A4-82. Note Coprocessor instructions are not implemented in ARM architecture version 1. A3-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 93. The ARM Instruction Set 3.13 Extending the instruction set Successive versions of the ARM architecture have extended the instruction set in a number of areas. This section describes the six areas where extensions have occurred, and where further extensions might occur in the future: • Undefined instruction space on page A3-28 • Arithmetic instruction extension space on page A3-29 • Control instruction extension space on page A3-30 • Load/store instruction extension space on page A3-32 • Coprocessor instruction extension space on page A3-33 • Unconditional instruction extension space on page A3-34. Instructions in these areas which have not yet been allocated a meaning are either UNDEFINED or UNPREDICTABLE. To determine which, use the following rules: 1. The decode bits of an instruction are defined to be bits[27:20] and bits[7:4]. In ARM architecture version 5 and above, the result of ANDing bits[31:28] together is also a decode bit. This bit determines whether the condition field is 0b1111, which is used in ARM architecture version 5 and above to encode various instructions which can only be executed unconditionally. See Condition code 0b1111 on page A3-5 and Unconditional instruction extension space on page A3-34 for more information. 2. If the decode bits of an instruction are equal to those of a defined instruction, but the whole instruction is not a defined instruction, then the instruction is UNPREDICTABLE. For example, suppose an instruction has: • bits[31:28] not equal to 0b1111 • bits[27:20] equal to 0b00010000 • bits[7:4] equal to 0b0000 but where: • bit[11] of the instruction is 1. Here, the instruction is in the control instruction extension space and has the same decode bits as an MRS instruction, but is not a valid MRS instruction because bit[11] of an MRS instruction should be zero. Using the above rule, this instruction is UNPREDICTABLE. 3. In ARM architecture version 4 and above, if the decode bits of an instruction are not equal to those of any defined instruction, then the instruction is UNDEFINED. 4. In ARM architecture version 3 and below, if the decode bits of an instruction are not equal to those of any defined instruction, then the instruction is: • UNDEFINED if it is in the undefined instruction space • UNPREDICTABLE if it is in any of the other five areas. Each of rules 2 to 4 above applies separately to each ARM architecture version. As a result, the status of an instruction might differ between architecture versions. Usually, this happens because an instruction which was UNPREDICTABLE or UNDEFINED in an earlier architecture version becomes a defined instruction in a later version. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-27
- 94. The ARM Instruction Set 3.13.1 Undefined instruction space Instructions with the following opcodes are undefined instruction space: opcode[27:25] = 0b011 opcode[4] = 1 31 28 27 26 25 24 5 4 3 0 cond 0 1 1 x x x x x x x x x x x x x x x x x x x x 1 x x x x The meaning of instructions in the undefined instruction space is UNDEFINED on all versions of the ARM architecture. In general, undefined instructions might be used to extend the ARM instruction set in the future. However, it is intended that instructions with the following encoding will not be used for this: 31 28 27 26 25 24 23 22 21 20 19 8 7 6 5 4 3 2 1 0 cond 0 1 1 1 1 1 1 1 x x x x x x x x x x x x 1 1 1 1 x x x x If a programmer wants to use an undefined instruction for software purposes, with minimal risk that future hardware will treat it as a defined instruction, one of the instructions with this encoding must be used. A3-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 95. The ARM Instruction Set 3.13.2 Arithmetic instruction extension space Instructions with the following opcodes are the arithmetic instruction extension space: opcode[27:24] == 0b0000 opcode[7:4] == 0b1001 opcode[31:28] != 0b1111 /* Only required for version 5 and above */ The field names given are guidelines suggested to simplify implementation. 31 28 27 26 25 24 23 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 op1 Rn Rd Rs 1 0 0 1 Rm Table 3-3 summarizes the instructions that have already been allocated in this area. Table 3-3 Arithmetic instruction space Instructions op1 Architecture versions MUL, MULS 000S Version 2 and above MLA, MLAS 001S Version 2 and above UMULL, UMULLS 100S All M variants UMLAL, UMLALS 101S All M variants SMULL, SMULLS 110S All M variants SMLAL, SMLALS 111S All M variants ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-29
- 96. The ARM Instruction Set 3.13.3 Control instruction extension space Instructions with the following opcodes are the control instruction space. opcode[27:26] == 0b00 opcode[24:23] == 0b10 opcode[20] == 0 opcode[31:28] != 0b1111 /* Only required for version 5 and above */ and not: opcode[25] == 0 opcode[7] == 1 opcode[4] == 1 The field names given are guidelines suggested to simplify implementation. 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 op1 0 Rn Rd Rs op2 0 Rm cond 0 0 0 1 0 op1 0 Rn Rd Rs 0 op2 1 Rm cond 0 0 1 1 0 op1 0 Rn Rd rotate_imm immed_8 Table 3-4 summarizes the instructions that have already been allocated in this area. Table 3-4 Control extension space instructions Instruction Bit[25] Bits[7:4] op1 Architecture versions MRS 0 0000 x0 Version 3 and above MSR (register form) 0 0000 x1 Version 3 and above BX 0 0001 01 Version 5 and above, plus T variants of version 4 CLZ 0 0001 11 Version 5 and above BLX (register form) 0 0011 01 Version 5 and above QADD 0 0101 00 E variants of version 5 and above QSUB 0 0101 01 E variants of version 5 and above QDADD 0 0101 10 E variants of version 5 and above QDSUB 0 0101 11 E variants of version 5 and above A3-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 97. The ARM Instruction Set Table 3-4 Control extension space instructions (continued) Instruction Bit[25] Bits[7:4] op1 Architecture versions BKPT 0 0111 01 Version 5 and above SMLA<x><y> 0 1yx0 00 E variants of version 5 and above SMLAW<y> 0 1y00 01 E variants of version 5 and above SMULW<y> 0 1y10 01 E variants of version 5 and above SMLAL<x><y> 0 1yx0 10 E variants of version 5 and above SMUL<x><y> 0 1yx0 11 E variants of version 5 and above MSR (immediate form) 1 xxxx x1 Version 3 and above ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-31
- 98. The ARM Instruction Set 3.13.4 Load/store instruction extension space Instructions with the following opcodes are the load/store instruction extension space: opcode[27:25] == 0b000 opcode[7] == 1 opcode[4] == 1 opcode[31:28] != 0b1111 /* Only required for version 5 and above */ and not: opcode[24] == 0 opcode[6:5] == 0 The field names given are guidelines suggested to simplify implementation. 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U B W L Rn Rd Rs 1 op1 1 Rm Table 3-5 summarizes the instructions that have already been allocated in this area. Table 3-5 Load/store instructions Instruction Bits[24:20] op1 Architecture versions SWP/SWPB 1 0 B 0 0 0 0 Version 3 and above, plus ARMv2a STRH P U I W 0 0 1 Version 4 and above LDRD P U I W 0 1 0 E variants of version 5 and above, except v5TExP STRD P U I W 0 1 1 E variants of version 5 and above, except v5TExP LDRH P U I W 1 0 1 Version 4 and above LDRSB P U I W 1 1 0 Version 4 and above LDRSH P U I W 1 1 1 Version 4 and above A3-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 99. The ARM Instruction Set 3.13.5 Coprocessor instruction extension space Instructions with the following opcodes are the coprocessor instruction extension space: opcode[27:23] == 0b11000 opcode[21] == 0 The field names given are guidelines suggested to simplify implementation. 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 0 0 x 0 x Rn CRd cp_num offset In ARM architecture version 3 and below, all instructions in the coprocessor instruction extension space are UNPREDICTABLE. In all variants of architecture version 4, and in non-E variants of architecture 5, all instructions in the coprocessor instruction extension space are UNDEFINED. It is IMPLEMENTATION DEFINED how an ARM processor achieves this. The options are: • The ARM processor might take the undefined instruction trap directly. • The ARM processor might require attached coprocessors not to respond to such instructions. This causes the undefined instruction trap to be taken (see Undefined Instruction exception on page A2-15). In E variants of architecture version 5, instructions in the coprocessor instruction extension space are treated as follows: • Instructions with bit[22] == 0 are UNDEFINED and are handled in precisely the same way as described above for non-E variants. • Instructions with bit[22] ==1 are the MCRR and MRRC instructions described in Chapter A10 Enhanced DSP Extension. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A3-33
- 100. The ARM Instruction Set 3.13.6 Unconditional instruction extension space In ARM architecture version 5 and above, instructions with the following opcode are the unconditional instruction space: opcode[31:28] == 0b1111 31 30 29 28 27 20 19 8 7 4 3 0 1 1 1 1 opcode1 x x x x x x x x x x x x opcode2 x x x x Table 3-6 summarizes the instructions that have already been allocated in this area. Table 3-6 Unconditional instruction extension space Instruction opcode1 opcode2 Architecture versions PLD 0 1 I 1 U 1 0 1 x x x x E variants of version 5 and above, except v5TExP BLX (address form) 1 0 1 x x x x x x x x x Version 5 and above STC2 1 1 0 x x x x 0 x x x x Version 5 and above LDC2 1 1 0 x x x x 1 x x x x Version 5 and above CDP2 1 1 1 0 x x x x x x x 0 Version 5 and above MCR2 1 1 1 0 x x x 0 x x x 1 Version 5 and above MRC2 1 1 1 0 x x x 1 x x x 1 Version 5 and above A3-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 101. Chapter A4 ARM Instructions This chapter describes the syntax and usage of every ARM instruction, in the sections: • Alphabetical list of ARM instructions on page A4-2 • ARM instructions and architecture versions on page A4-113. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-1
- 102. ARM Instructions 4.1 Alphabetical list of ARM instructions Every ARM instruction is listed on the following pages. Each instruction description shows: • the instruction encoding • the instruction syntax • the version of the ARM architecture where the instruction is valid • any exceptions that apply • an example in pseudo-code of how the instruction operates • notes on usage and special cases. 4.1.1 General notes These notes explain the types of information and abbreviations used on the instruction pages. Syntax abbreviations The following abbreviations are used in the instruction pages: immed_n This is an immediate value, where n is the number of bits. For example, an 8-bit immediate value is represented by: immed_8 offset_n This is an offset value, where n is the number of bits. For example, an 8-bit offset value is represented by: offset_8 The same construction is used for signed offsets. For example, an 8-bit signed offset is represented by: signed_offset_8 Encoding diagram and assembler syntax For the conventions used, see Assembler syntax descriptions on page Preface-xiii. Architecture versions This gives details of architecture versions where the instruction is valid. For details, see Architecture versions and variants on page Preface-v. Exceptions This gives details of which exceptions can occur during the execution of the instruction. Prefetch Abort is not listed in general, both because it can occur for any instruction and because if an abort occurred during instruction fetch, the instruction bit pattern is not known. (Prefetch Abort is however listed for BKPT, since it can generate a Prefetch Abort exception without these considerations applying.) A4-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 103. ARM Instructions Operation This gives a pseudo-code description of what the instruction does. For details of conventions used in this pseudo-code, see Pseudo-code descriptions of instructions on page Preface-xii. Information on usage Usage sections are included where appropriate to supply suggestions and other information about how to use the instruction effectively. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-3
- 104. ARM Instructions 4.1.2 ADC 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 1 0 1 S Rn Rd shifter_operand The ADC (Add with Carry) instruction adds the value of <shifter_operand> and the Carry flag to the value of <Rn> and stores the result in <Rd>. The condition code flags are optionally updated, based on the result. Syntax ADC{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the addition, and the C and V flags are set according to whether the addition generated a carry (unsigned overflow) and a signed overflow, respectively. The rest of the CPSR is unchanged. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the addition. <shifter_operand> Specifies the second operand for the addition. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not ADC. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 105. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn + shifter_operand + C Flag if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = CarryFrom(Rn + shifter_operand + C Flag) V Flag = OverflowFrom(Rn + shifter_operand + C Flag) Usage ADC is used to synthesize multi-word addition. If register pairs R0, R1 and R2, R3 hold 64-bit values (where R0 and R2 hold the least significant words) the following instructions leave the 64-bit sum in R4, R5: ADDS R4,R0,R2 ADC R5,R1,R3 If the second instruction is changed from: ADC R5,R1,R3 to: ADCS R5,R1,R3 the resulting values of the flags indicate: N The 64-bit addition produced a negative result. C An unsigned overflow occurred. V A signed overflow occurred. Z The most significant 32 bits are all zero. The following instruction produces a single-bit Rotate Left with Extend operation (33-bit rotate through the Carry flag) on R0: ADCS R0,R0,R0 See Data-processing operands - Rotate right with extend on page A5-17 for information on how to perform a similar rotation to the right. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-5
- 106. ARM Instructions 4.1.3 ADD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 1 0 0 S Rn Rd shifter operand The ADD instruction adds the value of <shifter_operand> to the value of register <Rn>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax ADD{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the addition, and the C and V flags are set according to whether the addition generated a carry (unsigned overflow) and a signed overflow, respectively. The rest of the CPSR is unchanged. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the addition. <shifter_operand> Specifies the second operand for the addition. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not ADD. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 107. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn + shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = CarryFrom(Rn + shifter_operand) V Flag = OverflowFrom(Rn + shifter_operand) Usage The ADD instruction is used to add two values together to produce a third. To increment a register value in Rx use: ADD Rx, Rx, #1 Constant multiplication of Rx by 2n+1 into Rd can be performed with: ADD Rd, Rx, Rx, LSL #n To form a PC-relative address use: ADD Rs, PC, #offset where the offset must be the difference between the required address and the address held in the PC, where the PC is the address of the ADD instruction itself plus 8 bytes. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-7
- 108. ARM Instructions 4.1.4 AND 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 0 0 0 S Rn Rd shifter_operand The AND instruction performs a bitwise AND of the value of register <Rn> with the value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax AND{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the operation, and the C flag is set to the carry output bit generated by the shifter (see Addressing Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the CPSR are unaffected. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the operation. <shifter_operand> Specifies the second operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not AND. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 109. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn AND shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected Usage AND is most useful for extracting a field from a register, by ANDing the register with a mask value that has 1s in the field to be extracted, and 0s elsewhere. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-9
- 110. ARM Instructions 4.1.5 B, BL 31 28 27 26 25 24 23 0 cond 1 0 1 L signed_immed_24 The B (Branch) and BL (Branch and Link) instructions cause a branch to a target address, and provide both conditional and unconditional changes to program flow. Syntax B{L}{<cond>} <target_address> where: L Causes the L bit (bit 24) in the instruction to be set to 1. The resulting instruction stores a return address in the link register (R14). If L is omitted, the L bit is 0 and the instruction simply branches without storing a return address. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <target_address> Specifies the address to branch to. The branch target address is calculated by: 1. Sign-extending the 24-bit signed (two’s complement) immediate to 32 bits. 2. Shifting the result left two bits. 3. Adding this to the contents of the PC, which contains the address of the branch instruction plus 8. The instruction can therefore specify a branch of approximately ±32MB. Architecture version All Exceptions None Operation if ConditionPassed(cond) then if L == 1 then LR = address of the instruction after the branch instruction PC = PC + (SignExtend(signed_immed_24) << 2) A4-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 111. ARM Instructions Usage The BL instruction is used to perform a subroutine call. The return from subroutine is achieved by copying the LR to the PC. Typically, this is done by one of the following methods: • Executing a BX R14 instruction, on architecture versions that support that instruction. • Executing a MOV PC,R14 instruction. • Storing a group of registers and R14 to the stack on subroutine entry, using an instruction of the form: STMFD R13!,{<registers>,R14} and then restoring the register values and returning with an instruction of the form: LDMFD R13!,{<registers>,PC} To calculate the correct value of signed_immed_24, the assembler (or other toolkit component) needs to: 1. Form the base address for this branch instruction. This is the address of the instruction, plus 8. In other words, this base address is equal to the PC value used by the instruction. 2. Subtract the base address from the target address to form a byte offset. This offset is always a multiple of four, because all ARM instructions are word-aligned. 3. If the byte offset is outside the range -33554432 to +33554428, use an alternative code-generation strategy or produce an error as appropriate. 4. Otherwise, set the signed_immed_24 field of the instruction to bits{25:2] of the byte offset. Notes Memory bounds Branching backwards past location zero and forwards over the end of the 32-bit address space is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-11
- 112. ARM Instructions 4.1.6 BIC 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 1 1 0 S Rn Rd shifter_operand The BIC (Bit Clear) instruction performs a bitwise AND of the value of register <Rn> with the complement of the value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax BIC{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the operation, and the C flag is set to the carry output bit generated by the shifter (see Addressing Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the CPSR are unaffected. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the operation. <shifter_operand> Specifies the second operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not BIC. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 113. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn AND NOT shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected Usage BIC can be used to clear selected bits in a register. For each bit, BIC with 1 clears the bit, and BIC with 0 leaves it unchanged. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-13
- 114. ARM Instructions 4.1.7 BKPT 31 28 27 26 25 24 23 22 21 20 19 8 7 4 3 0 1 1 1 0 0 0 0 1 0 0 1 0 immed 0 1 1 1 immed The BKPT (Breakpoint) instruction causes a software breakpoint to occur. This breakpoint can be handled by an exception handler installed on the prefetch abort vector. In implementations which also include debug hardware, the hardware can optionally override this behavior and handle the breakpoint itself. When this occurs, the prefetch abort vector is not entered. Syntax BKPT <immediate> where: <immediate> Is a 16-bit immediate value, the top 12 bits of which are placed in bits[19:8] of the instruction, and the bottom 4 bits of which are placed in bits[3:0] of the instruction. This value is ignored by the ARM hardware, but can be used by a debugger to store additional information about the breakpoint. Architecture version Version 5 and above Exceptions Prefetch Abort Operation if (not overridden by debug hardware) R14_abt = address of BKPT instruction + 4 SPSR_abt = CPSR CPSR[4:0] = 0b10111 /* Enter Abort mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF000C else PC = 0x0000000C A4-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 115. ARM Instructions Usage The exact usage of the BKPT instruction depends on the debug system being used. A debug system can use the BKPT instruction in two ways: • Debug hardware, if present, does not override the normal behavior of the BKPT instruction, and so the prefetch abort vector is entered. If the system also allows real prefetch aborts to occur, the prefetch abort handler determines, in a system-dependent manner, whether the vector entry occurred as a result of a BKPT instruction or as a result of a real prefetch abort, and branches to debug code or prefetch abort code accordingly. Otherwise, the prefetch abort handler just branches straight to debug code. When used in this manner, the BKPT instruction must be avoided within abort handlers, as it corrupts R14_abt and SPSR_abt. For the same reason, it must also be avoided within FIQ handlers, since an FIQ interrupt can occur within an abort handler. • Debug hardware does override the normal behavior of the BKPT instruction and handles the software breakpoint itself. When finished, it typically either resumes execution at the instruction following the BKPT, or replaces the BKPT in memory with another instruction and resumes execution at that instruction. When BKPT is used in this manner, R14_abt and SPSR_abt are not corrupted, and so the above restrictions about its use in abort and FIQ handlers do not apply. Notes Condition field The BKPT instruction must be unconditional. If bits[31:28] of the instruction encode a valid condition other than the AL (always) condition, the instruction is UNPREDICTABLE. Hardware override Debug hardware in an implementation is specifically permitted to override the normal behavior of the BKPT instruction. Because of this, software must not use this instruction for purposes other than those documented by the debug system being used (if any). In particular, software cannot rely on the Prefetch Abort exception occurring, unless either there is guaranteed to be no debug hardware in the system or the debug system specifies that it will occur. For more information, consult the documentation for the debug system being used. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-15
- 116. ARM Instructions 4.1.8 BLX (1) 31 30 29 28 27 26 25 24 23 0 1 1 1 1 1 0 1 H signed_immed_24 This form of the BLX (Branch with Link and Exchange) instruction is used to call a Thumb subroutine from the ARM instruction set at an address specified in the instruction. This instruction is unconditional (always causing a change in program flow) and preserves the address of the instruction following the branch in the link register (R14). Execution of Thumb instructions begins at the target address. Syntax BLX <target_addr> where: <target_addr> Specifies the address of the Thumb instruction to branch to. The branch target address is calculated by: 1. Sign-extending the 24-bit signed (two’s complement) immediate to 32 bits 2. Shifting the result left two bits 3. Setting bit[1] of the result of step 2 to the H bit 4. Adding the result of step 3 to the contents of the PC, which contains the address of the branch instruction plus 8. The instruction can therefore specify a branch of approximately ±32MB. Architecture version Version 5 and above Exceptions None Operation LR = address of the instruction after the BLX instruction T Flag = 1 PC = PC + (SignExtend(signed_immed_24) << 2) + (H << 1) A4-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 117. ARM Instructions Usage To return from a Thumb subroutine called via BLX to the ARM caller, use the Thumb instruction: BX R14 as described in BX on page A7-32, or use this instruction on subroutine entry: PUSH {<registers>,R14} and this instruction to return: POP {<registers>,PC} To calculate the correct value of signed_immed_24, the assembler (or other toolkit component) needs to: 1. Form the base address for this branch instruction. This is the address of the instruction, plus 8. In other words, this base address is equal to the PC value used by the instruction. 2. Subtract the base address from the target address to form a byte offset. This offset is always even, because all ARM instructions are word-aligned and all Thumb instructions are halfword-aligned. 3. If the byte offset is outside the range -33554432 to +33554430, use an alternative code-generation strategy or produce an error as appropriate. 4. Otherwise, set the signed_immed_24 field of the instruction to bits{25:2] of the byte offset, and the H bit of the instruction to bit[1] of the byte offset. Notes Condition Unlike most other ARM instructions, this instruction cannot be executed conditionally. Bit[24] This bit is used as bit[1] of the target address. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-17
- 118. ARM Instructions 4.1.9 BLX (2) 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 1 1 Rm This form of BLX is used to call an ARM or Thumb subroutine from the ARM instruction set, at an address specified in a register. The branch target address is the value of register Rm, with its bit[0] forced to zero. The instruction set to be used at the branch target is chosen by setting the CPSR T bit to bit[0] of Rm. Register 14 is set to a return address. To return from the subroutine, use a BX R14 instruction, or store R14 on the stack and re-load the stored value into the PC. Syntax BLX{<cond>} <Rm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rm> Is the register containing the address of the target instruction. Bit[0] of Rm is 0 to select a target ARM instruction, or 1 to select a target Thumb instruction. If R15 is specified for <Rm>, the results are UNPREDICTABLE. Architecture version Version 5 and above Exceptions None Operation if ConditionPassed(cond) then LR = address of instruction after the BLX instruction T Flag = Rm[0] PC = Rm AND 0xFFFFFFFE Notes ARM/Thumb state transfers If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned addresses are impossible in ARM state. A4-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 119. ARM Instructions 4.1.10 BX 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 0 1 Rm The BX (Branch and Exchange) instruction branches to an address held in a register Rm, with an optional switch to Thumb execution. The branch target address is the value of register Rm, with its bit[0] forced to zero. The instruction set to be used at the branch target is chosen by setting the CPSR T bit to bit[0] of Rm. Syntax BX{<cond>} <Rm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rm> Holds the value of the branch target address. Bit[0] of Rm is 0 to select a target ARM instruction, or 1 to select a target Thumb instruction. Architecture version Version 5 and above, plus T variants of version 4 Exceptions None Operation if ConditionPassed(cond) then T Flag = Rm[0] PC = Rm AND 0xFFFFFFFE Notes ARM/Thumb state transfers If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned addresses are impossible in ARM state. Use of R15 Register 15 can be specified for <Rm>, but doing so is discouraged. In a BX R15 instruction, R15 is read as normal for ARM code, that is, it is the address of the BX instruction itself plus 8. The result is to branch to the second following word, executing in ARM state. This is precisely the same effect that would have been obtained if a B instruction with an offset field of 0 had been executed, or an ADD PC,PC,#0 or MOV PC,PC instruction. In new code, use these instructions in preference to the more complex BX PC instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-19
- 120. ARM Instructions 4.1.11 CDP 31 28 27 26 25 24 23 20 19 16 15 12 11 8 7 5 4 3 0 cond 1 1 1 0 opcode_1 CRn CRd cp_num opcode_2 0 CRm The CDP (Coprocessor Data Processing) instruction tells the coprocessor whose number is cp_num to perform an operation that is independent of ARM registers and memory. If no coprocessors indicate that they can execute the instruction, an Undefined Instruction exception is generated. Syntax CDP{<cond>} <coproc>, <opcode_1>, <CRd>, <CRn>, <CRm>, <opcode_2> CDP2 <coproc>, <opcode_1>, <CRd>, <CRn>, <CRm>, <opcode_2> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. CDP2 Causes the condition field of the instruction to be set to 0b1111. This provides additional opcode space for coprocessor designers. The resulting instructions can only be executed unconditionally. <coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to be placed in the cp_num field of the instruction. The standard generic coprocessor names are p0, p1, ..., p15. <opcode_1> Specifies (in a coprocessor-specific manner) which coprocessor operation is to be performed. <CRd> Specifies the destination coprocessor register for the instruction. <CRn> Specifies the coprocessor register that contains the first operand for the instruction. <CRm> Specifies the coprocessor register that contains the second operand for the instruction. <opcode_2> Specifies (in a coprocessor-specific manner) which coprocessor operation is to be performed. Architecture version CDP is in Version 2 and above. CDP2 is in Version 5 and above. Exceptions Undefined Instruction A4-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 121. ARM Instructions Operation if ConditionPassed(cond) then Coprocessor[cp_num]-dependent operation Usage CDP is used to initiate coprocessor instructions that do not operate on values in ARM registers or in main memory. An example is a floating-point multiply instruction for a floating-point coprocessor. Notes Coprocessor fields Only instruction bits[31:24], bits[11:8], and bit[4] are architecturally defined. The remaining fields are recommendations, for compatibility with ARM Development Systems. Unimplemented coprocessor instructions Hardware coprocessor support is optional, regardless of the architecture version. An implementation can choose to implement a subset of the coprocessor instructions, or no coprocessor instructions at all. Any coprocessor instructions that are not implemented instead cause an undefined instruction trap. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-21
- 122. ARM Instructions 4.1.12 CLZ 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 1 1 0 SBO Rd SBO 0 0 0 1 Rm The CLZ (Count Leading Zeros) instruction returns the number of binary zero bits before the first binary one bit in a register value. The source register is scanned from the most significant bit (bit[31]) towards the least significant bit (bit[0]). The result value is 32 if no bits are set in the source register, and zero if bit[31] is set. This instruction does not update the condition code flags. Syntax CLZ{<cond>} <Rd>, <Rm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the operation. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <Rm> Specifies the source register for this operation. If R15 is specified for <Rm>, the result is UNPREDICTABLE. Architecture version Version 5 and above Exceptions None Operation if Rm == 0 Rd = 32 else Rd = 31 - (bit position of most significant ’1’ in Rm) Usage To normalize the value of register Rm, use CLZ followed by a left shift of Rm by the resulting Rd value. This shifts Rm so that its most significant 1 bit is in bit[31]. Using MOVS rather than MOV sets the Z flag in the special case that Rm is zero and so does not have a most significant 1 bit: CLZ Rd, Rm MOVS Rm, Rm, LSL Rd A4-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 123. ARM Instructions 4.1.13 CMN 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 0 1 1 1 Rn SBZ shifter_operand The CMN (Compare Negative) instruction compares a register value with the negative of another arithmetic value. The condition flags are updated, based on the result of adding the second arithmetic value to the register value, so that subsequent instructions can be conditionally executed. Syntax CMN{<cond>} <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the register that contains the first operand for the operation. <shifter_operand> Specifies the second operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not CMN. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All Exceptions None Operation if ConditionPassed(cond) then alu_out = Rn + shifter_operand N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = CarryFrom(Rn + shifter_operand) V Flag = OverflowFrom(Rn + shifter_operand) ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-23
- 124. ARM Instructions Usage CMN performs a comparison by adding the value of <shifter_operand> to the value of register <Rn>, and updates the condition code flags (based on the result). This is almost equivalent to subtracting the negative of the second operand from the first operand, and setting the flags on the result. The difference is that the flag values generated can differ when the second operand is 0 or 0x80000000. For example, this instruction always leaves the C flag = 1: CMP Rn, #0 while this instruction always leaves the C flag = 0: CMN Rn, #0 A4-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 125. ARM Instructions 4.1.14 CMP 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 0 1 0 1 Rn SBZ shifter_operand The CMP (Compare) instruction compares a register value with another arithmetic value. The condition flags are updated, based on the result of subtracting the second arithmetic value from the register value, so that subsequent instructions can be conditionally executed. Syntax CMP{<cond>} <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the register that contains the first operand for the operation. <shifter_operand> Specifies the second operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not CMP. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All Exceptions None Operation if ConditionPassed(cond) then alu_out = Rn - shifter_operand N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - shifter_operand) V Flag = OverflowFrom(Rn - shifter_operand) ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-25
- 126. ARM Instructions 4.1.15 EOR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 0 0 1 S Rn Rd shifter_operand The EOR (Exclusive OR) instruction performs a bitwise Exclusive-OR of the value of register <Rn> with the value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax EOR{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the operation, and the C flag is set to the carry output bit generated by the shifter (see Addressing Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the CPSR are unaffected. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the operation. <shifter_operand> Specifies the second operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not EOR. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 127. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn EOR shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected Usage EOR can be used to invert selected bits in a register. For each bit, EOR with 1 inverts that bit, and EOR with 0 leaves it unchanged. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-27
- 128. ARM Instructions 4.1.16 LDC 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 P U N W 1 Rn CRd cp_num 8_bit_word_offset The LDC (Load Coprocessor) instruction loads memory data from the sequence of consecutive memory addresses calculated by <addressing_mode> to the coprocessor whose number is cp_num. If no coprocessors indicate that they can execute the instruction, an Undefined Instruction exception is generated. Syntax LDC{<cond>}{L} <coproc>, <CRd>, <addressing_mode> LDC2{L} <coproc>, <CRd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. LDC2 Causes the condition field of the instruction to be set to 0b1111. This provides additional opcode space for coprocessor designers. The resulting instructions can only be executed unconditionally. L Sets the N bit (bit[22]) in the instruction to 1 and specifies a long load (for example, double-precision instead of single-precision data transfer). If L is omitted, the N bit is 0 and the instruction specifies a short load. <coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to be placed in the cp_num field of the instruction. The standard generic coprocessor names are p0, p1, ..., p15. <CRd> Specifies the coprocessor destination register of the instruction. <addressing_mode> Is described in Addressing Mode 5 - Load and Store Coprocessor on page A5-56. It determines the P, U, Rn, W and 8_bit_word_offset bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version LDC is in Version 2 and above. LDC2 is in Version 5 and above. A4-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 129. ARM Instructions Exceptions Undefined Instruction, Data Abort Operation if ConditionPassed(cond) then address = start_address load Memory[address,4] for Coprocessor[cp_num] while (NotFinished(Coprocessor[cp_num])) address = address + 4 load Memory[address,4] for Coprocessor[cp_num] assert address == end_address Usage LDC is useful for loading coprocessor data from memory. Notes Coprocessor fields Only instruction bits[31:23], bits[21:16], and bits[11:0] are ARM architecture-defined. The remaining fields (bit[22] and bits[15:12]) are recommendations, for compatibility with ARM Development Systems. In the case of the Unindexed addressing mode (P==0, U==1, W==0), instruction bits[7:0] are also not defined by the ARM architecture, and can be used to specify additional coprocessor options. Data abort For details of the effects of the instruction if a Data Abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses Load coprocessor register instructions ignore the least significant two bits of address. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Unimplemented coprocessor instructions Hardware coprocessor support is optional, regardless of the architecture version. An implementation can choose to implement a subset of the coprocessor instructions, or no coprocessor instructions at all. Any coprocessor instructions that are not implemented instead cause an undefined instruction trap. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-29
- 130. ARM Instructions 4.1.17 LDM (1) 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 P U 0 W 1 Rn register_list This form of the LDM (Load Multiple) instruction is useful for block loads, stack operations and procedure exit sequences. It loads a non-empty subset, or possibly all, of the general-purpose registers from sequential memory locations. The general-purpose registers loaded can include the PC. If they do, the word loaded for the PC is treated as an address and a branch occurs to that address. In ARM architecture version 5 and above, bit[0] of the loaded value determines whether execution continues after this branch in ARM state or in Thumb state, as though a BX (loaded_value) instruction had been executed. In earlier versions of the architecture, bits[1:0] of the loaded value are ignored and execution continues in ARM state, as though the instruction MOV PC,(loaded_value) had been executed. Syntax LDM{<cond>}<addressing_mode> <Rn>{!}, <registers> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <addressing_mode> Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines the P, U, and W bits of the instruction. <Rn> Specifies the base register used by <addressing_mode>. Using R15 as the base register <Rn> gives an UNPREDICTABLE result. ! Sets the W bit, causing the instruction to write a modified value back to its base register Rn as specified in Addressing Mode 4 - Load and Store Multiple on page A5-48. If ! is omitted, the W bit is 0 and the instruction does not change its base register in this way. (However, if the base register is included in <registers>, it changes when a value is loaded into it.) <registers> Is a list of registers, separated by commas and surrounded by { and }. It specifies the set of registers to be loaded by the LDM instruction. The registers are loaded in sequence, the lowest-numbered register from the lowest memory address (start_address), through to the highest-numbered register from the highest memory address (end_address). If the PC is specified in the register list (opcode bit[15] is set), the instruction causes a branch to the address (data) loaded into the PC. For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE. A4-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 131. ARM Instructions Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then address = start_address for i = 0 to 14 if register_list[i] == 1 then Ri = Memory[address,4] address = address + 4 if register_list[15] == 1 then value = Memory[address,4] if (architecture version 5 or above) then pc = value AND 0xFFFFFFFE T Bit = value[0] else pc = value AND 0xFFFFFFFC address = address + 4 assert end_address = address - 4 Notes Operand restrictions If the base register <Rn> is specified in <registers>, and base register writeback is specified, the final value of <Rn> is UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses Load Multiple instructions ignore the least significant two bits of address (the words are not rotated as for Load Word). Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. ARM/Thumb state transfers (ARM architecture version 5 and above) If bits[1:0] of a value loaded for R15 are 0b10, the result is UNPREDICTABLE, as branches to non word-aligned addresses are impossible in ARM state. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-31
- 132. ARM Instructions 4.1.18 LDM (2) 31 28 27 26 25 24 23 22 21 20 19 16 15 14 0 cond 1 0 0 P U 1 0 1 Rn 0 register_list This form of LDM loads User mode registers when the processor is in a privileged mode (useful when performing process swaps, and in instruction emulators). The instruction loads a non-empty subset of the User mode general-purpose registers from sequential memory locations. Syntax LDM{<cond>}<addressing_mode> <Rn>, <registers_without_pc>^ where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <addressing_mode> Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines the P and U bits of the instruction. Only the forms of this addressing mode with W == 0 are available for this form of the LDM instruction. <Rn> Specifies the base register used by <addressing_mode>. Using R15 as <Rn> gives an UNPREDICTABLE result. <registers_without_pc> Is a list of registers, separated by commas and surrounded by { and }. This list must not include the PC, and specifies the set of registers to be loaded by the LDM instruction. The registers are loaded in sequence, the lowest-numbered register from the lowest memory address (start_address), through to the highest-numbered register from the highest memory address (end_address). For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE. ^ For an LDM instruction that does not load the PC, this indicates that User mode registers are to be loaded. Architecture version All Exceptions Data Abort A4-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 133. ARM Instructions Operation if ConditionPassed(cond) then address = start_address for i = 0 to 14 if register_list[i] == 1 Ri_usr = Memory[address,4] address = address + 4 assert end_address == address - 4 Notes Banked registers This form of LDM must not be followed by an instruction which accesses banked registers (a following NOP is a good way to ensure this). Writeback Setting bit[21] (the W bit) has UNPREDICTABLE results. User and System mode This form of LDM is UNPREDICTABLE in User mode or System mode. Base register mode The base register is read from the current processor mode registers, not the User mode registers. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses LDM instructions ignore the least significant two bits of address (words are not rotated as for Load Word). Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-33
- 134. ARM Instructions 4.1.19 LDM (3) 31 28 27 26 25 24 23 22 21 20 19 16 15 14 0 cond 1 0 0 P U 1 W 1 Rn 1 register_list This form of is useful for returning from an exception. It loads a subset (or possibly all) of the general-purpose registers and the PC from sequential memory locations. Also, the SPSR of the current mode is copied to the CPSR. The value loaded for the PC is treated as an address and a branch occurs to that address. In ARM architecture version 5 and above, and in T variants of version 4, the value copied from the SPSR T bit to the CPSR T bit determines whether execution continues after the branch in ARM state or in Thumb state. In earlier architecture versions, it continues after the branch in ARM state (the only possibility in those architecture versions). Syntax LDM{<cond>}<addressing_mode> <Rn>{!}, <registers_and_pc>^ where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <addressing_mode> Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines the P, U, and W bits of the instruction. <Rn> Specifies the base register used by <addressing_mode>. Using R15 as <Rn> gives an UNPREDICTABLE result. ! Sets the W bit, and the instruction writes a modified value back to its base register Rn (see Addressing Mode 4 - Load and Store Multiple on page A5-48). If ! is omitted, the W bit is 0 and the instruction does not change its base register in this way. (However, if the base register is included in <registers>, it changes when a value is loaded into it.) <registers_and_pc> Is a list of registers, separated by commas and surrounded by { and }. This list must include the PC, and specifies the set of registers to be loaded by the LDM instruction. The registers are loaded in sequence, the lowest-numbered register from the lowest memory address (start_address), through to the highest-numbered register from the highest memory address (end_address). For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list and 0 otherwise. ^ For an LDM instruction that loads the PC, this indicates that the SPSR of the current mode is copied to the CPSR. A4-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 135. ARM Instructions Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then address = start_address for i = 0 to 14 if register_list[i] == 1 then Ri = Memory[address,4] address = address + 4 CPSR = SPSR value = Memory[address,4] if (architecture version 4T, 5 or above) and (T Bit == 1) then pc = value AND 0xFFFFFFFE else pc = value AND 0xFFFFFFFC address = address + 4 assert end_address = address - 4 Notes User and System mode This instruction is UNPREDICTABLE in User or System mode. Operand restrictions If the base register <Rn> is specified in <registers_and_pc>, and base register writeback is specified, the final value of <Rn> is UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses Load Multiple instructions ignore the least significant two bits of address (the words are not rotated as for Load Word). Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-35
- 136. ARM Instructions ARM/Thumb state transfers (ARM architecture versions 4T, 5 and above) If the SPSR T bit is 0 and bit[1] of the value loaded into the PC is 1, the results are UNPREDICTABLE because it is not possible to branch to an ARM instruction at a non word-aligned address. Note that no special precautions against this are needed on normal exception returns, because exception entries always either set the T bit of the SPSR to 1 or bit[1] of the return link value in R14 to 0. Time order The time order of the accesses to individual words of memory generated by this instruction is not defined. See Data accesses to memory-mapped I/O on page A2-32 for details. A4-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 137. ARM Instructions 4.1.20 LDR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I P U 0 W 1 Rn Rd addr_mode The LDR (Load Register) instruction loads a word from the memory address calculated by <addressing_mode> and writes it to register <Rd>. If the address is not word-aligned, the loaded value is rotated right by 8 times the value of bits[1:0] of the address. For a little-endian memory system, this rotation causes the addressed byte to occupy the least significant byte of the register. For a big-endian memory system, it causes the addressed byte to occupy bits[31:24] or bits[15:8] of the register, depending on whether bit[0] of the address is 0 or 1 respectively. If the PC is specified as register <Rd>, the instruction loads a data word which it treats as an address, then branches to that address. In ARM architecture version 5 and above, bit[0] of the loaded value determines whether execution continues after this branch in ARM state or in Thumb state, as though a BX (loaded_value) instruction had been executed. In earlier versions of the architecture, bits[1:0] of the loaded value are ignored and execution continues in ARM state, as though a MOV PC,(loaded_value) instruction had been executed. Syntax LDR{<cond>} <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the loaded value. <addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, P, U, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-37
- 138. ARM Instructions Operation if ConditionPassed(cond) then if address[1:0] == 0b00 then value = Memory[address,4] else if address[1:0] == 0b01 then value = Memory[address,4] Rotate_Right 8 else if address[1:0] == 0b10 then value = Memory[address,4] Rotate_Right 16 else /* address[1:0] == 0b11 */ value = Memory[address,4] Rotate_Right 24 if (Rd is R15) then if (architecture version 5 or above) then PC = value AND 0xFFFFFFFE T Bit = value[0] else PC = value AND 0xFFFFFFFC else Rd = value Usage Using the PC as the base register allows PC-relative addressing, which facilitates position-independent code. Combined with a suitable addressing mode, LDR allows 32-bit memory data to be loaded into a general-purpose register where its value can be manipulated. If the destination register is the PC, this instruction loads a 32-bit address from memory and branches to that address. To synthesize a Branch with Link, precede the LDR instruction with MOV LR, PC. A4-38 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 139. ARM Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Use of R15 If register 15 is specified for <Rd>, address[1:0] must be 0b00. If not, the result is UNPREDICTABLE. ARM/Thumb state transfers (ARM architecture version 5 and above) If bits[1:0] of the loaded value are 0b10, the result is UNPREDICTABLE, as branches to non word-aligned addresses are not possible in ARM state. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-39
- 140. ARM Instructions 4.1.21 LDRB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I P U 1 W 1 Rn Rd addr_mode The LDRB (Load Register Byte) instruction loads a byte from the memory address calculated by <addressing_mode>, zero-extends the byte to a 32-bit word, and writes the word to register <Rd>. Syntax LDR{<cond>}B <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the loaded value. If register 15 is specified for <Rd>, the result is UNPREDICTABLE. <addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, P, U, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then Rd = Memory[address,1] Usage Combined with a suitable addressing mode, LDRB allows 8-bit memory data to be loaded into a general-purpose register where it can be manipulated. Using the PC as the base register allows PC-relative addressing, to facilitate position-independent code. A4-40 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 141. ARM Instructions Notes Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-41
- 142. ARM Instructions 4.1.22 LDRBT 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I 0 U 1 1 1 Rn Rd addr_mode The LDRBT (Load Register Byte with Translation) instruction loads a byte from the memory address calculated by <post_indexed_addressing_mode>, zero-extends the byte to a 32-bit word, and writes the word to register <Rd>. If the instruction is executed when the processor is in a privileged mode, the memory system is signaled to treat the access as if the processor were in User mode. Syntax LDR{<cond>}BT <Rd>, <post_indexed_addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <post_indexed_addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W == 0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W == 1 instead, but the addressing mode is the same in all other respects. The syntax of all forms of <post_indexed_addressing_mode> includes a base register <Rn>. All forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then Rd = Memory[address,1] A4-42 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 143. ARM Instructions Usage LDRBT can be used by a (privileged) exception handler that is emulating a memory access instruction that would normally execute in User mode. The access is restricted as if it had User mode privilege. Notes User mode If this instruction is executed in User mode, an ordinary User mode access is performed. Operand restrictions If the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-43
- 144. ARM Instructions 4.1.23 LDRH 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U I W 1 Rn Rd addr_mode 1 0 1 1 addr_mode The LDRH (Load Register Halfword) instruction loads a halfword from the memory address calculated by <addressing_mode>, zero-extends the halfword to a 32-bit word, and writes the word to register <Rd>. If the address is not halfword-aligned, the result is UNPREDICTABLE. Syntax LDR{<cond>}H <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <addressing_mode> Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It determines the P, U, I, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version Version 4 and above Exceptions Data Abort Operation if ConditionPassed(cond) then if address[0] == 0 data = Memory[address,2] else /* address[0] == 1 */ data = UNPREDICTABLE Rd = data A4-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 145. ARM Instructions Usage Used with a suitable addressing mode, LDRH allows 16-bit memory data to be loaded into a general-purpose register where its value can be manipulated. Using the PC as the base register allows PC-relative addressing to facilitate position-independent code. Notes Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non halfword-aligned addresses If the load address is not halfword-aligned, the loaded value is UNPREDICTABLE. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-45
- 146. ARM Instructions 4.1.24 LDRSB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U I W 1 Rn Rd addr_mode 1 1 0 1 addr_mode The LDRSB (Load Register Signed Byte) instruction loads a byte from the memory address calculated by <addressing_mode>, sign-extends the byte to a 32-bit word, and writes the word to register <Rd>. Syntax LDR{<cond>}SB <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <addressing_mode> Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It determines the P, U, I, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version Version 4 and above Exceptions Data Abort Operation if ConditionPassed(cond) then data = Memory[address,1] Rd = SignExtend(data) Usage Used with a suitable addressing mode, LDRSB allows 8-bit signed memory data to be loaded into a general-purpose register where it can be manipulated. Using the PC as the base register allows PC-relative addressing, which facilitates position-independent code. A4-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 147. ARM Instructions Notes Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-47
- 148. ARM Instructions 4.1.25 LDRSH 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U I W 1 Rn Rd addr_mode 1 1 1 1 addr_mode The LDRSH (Load Register Signed Halfword) instruction loads a halfword from the memory address calculated by <addressing_mode>, sign-extends the halfword to a 32-bit word, and writes the word to register <Rd>. If the address is not halfword-aligned, the result is UNPREDICTABLE. Syntax LDR{<cond>}SH <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <addressing_mode> Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It determines the P, U, I, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version Version 4 and above Exceptions Data Abort Operation if ConditionPassed(cond) then if address[0] == 0 data = Memory[address,2] else /* address[0] == 1 */ data = UNPREDICTABLE Rd = SignExtend(data) A4-48 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 149. ARM Instructions Usage Used with a suitable addressing mode, LDRSH allows 16-bit signed memory data to be loaded into a general-purpose register where its value can be manipulated. Using the PC as the base register allows PC-relative addressing, which facilitates position-independent code. Notes Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non halfword-aligned addresses If the load address is not halfword-aligned, the loaded value is UNPREDICTABLE. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-49
- 150. ARM Instructions 4.1.26 LDRT 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I 0 U 0 1 1 Rn Rd addr_mode The LDRT (Load Register with Translation) instruction loads a word from the memory address calculated by <addressing_mode> and writes it to register <Rd>. If the address is not word-aligned, the loaded data is rotated as for the LDR instruction (see LDR on page A4-37). If the instruction is executed when the processor is in a privileged mode, the memory system is signaled to treat the access as if the processor were in User mode. Syntax LDR{<cond>}T <Rd>, <post_indexed_addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the loaded value. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <post_indexed_addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W == 0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W == 1 instead, but the addressing mode is the same in all other respects. The syntax of all forms of <post_indexed_addressing_mode> includes a base register <Rn>. All forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort A4-50 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 151. ARM Instructions Operation if ConditionPassed(cond) then if address[1:0] == 0b00 Rd = Memory[address,4] else if address[1:0] == 0b01 Rd = Memory[address,4] Rotate_Right 8 else if address[1:0] == 0b10 Rd = Memory[address,4] Rotate_Right 16 else /* address[1:0] == 0b11 */ Rd = Memory[address,4] Rotate_Right 24 Usage LDRT can be used by a (privileged) exception handler that is emulating a memory access instruction that would normally execute in User mode. The access is restricted as if it had User mode privilege. Notes User mode If this instruction is executed in User mode, an ordinary User mode access is performed. Operand restrictions If the same register is specified for <Rd> and <Rn> the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If an implementation includes a System Control coprocessor (See Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-51
- 152. ARM Instructions 4.1.27 MCR 31 28 27 26 25 24 23 21 20 19 16 15 12 11 8 7 5 4 3 0 cond 1 1 1 0 opcode_1 0 CRn Rd cp_num opcode_2 1 CRm The MCR (Move to Coprocessor from ARM Register) instruction passes the value of register <Rd> to the coprocessor whose number is cp_num. If no coprocessors indicate that they can execute the instruction, an Undefined Instruction exception is generated. Syntax MCR{<cond>} <coproc>, <opcode_1>, <Rd>, <CRn>, <CRm>{, <opcode_2>} MCR2 <coproc>, <opcode_1>, <Rd>, <CRn>, <CRm>{, <opcode_2>} where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. MCR2 Causes the condition field of the instruction to be set to 0b1111. This provides additional opcode space for coprocessor designers. The resulting instructions can only be executed unconditionally. <coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to be placed in the cp_num field of the instruction. The standard generic coprocessor names are p0, p1, ..., p15. <opcode_1> Is a coprocessor-specific opcode. <Rd> Is the ARM register whose value is transferred to the coprocessor. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <CRn> Is the destination coprocessor register. <CRm> Is an additional destination or source coprocessor register. <opcode_2> Is a coprocessor-specific opcode. If it is omitted, <opcode_2> is assumed to be 0. Architecture version MCR is in version 2 and above. MCR2 is in version 5 and above. Exceptions Undefined Instruction A4-52 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 153. ARM Instructions Operation if ConditionPassed(cond) then send Rd value to Coprocessor[cp_num] Usage MCR is used to initiate coprocessor instructions that operate on values in ARM registers. An example is a fixed-point to floating-point conversion instruction for a floating-point coprocessor. Notes Coprocessor fields Only instruction bits[31:24], bit[20], bits[15:8], and bit[4] are defined by the ARM architecture. The remaining fields are recommendations, for compatibility with ARM Development Systems. Unimplemented coprocessor instructions Hardware coprocessor support is optional, regardless of the architecture version. An implementation can choose to implement a subset of the coprocessor instructions, or no coprocessor instructions at all. Any coprocessor instructions that are not implemented instead cause an undefined instruction trap. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-53
- 154. ARM Instructions 4.1.28 MLA 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 0 0 1 S Rd Rn Rs 1 0 0 1 Rm The MLA (Multiply Accumulate) multiplies signed or unsigned operands to produce a 32-bit result, which is then added to a third operand, and written to the destination register. The condition code flags are optionally updated, based on the result. Syntax MLA{<cond>}{S} <Rd>, <Rm>, <Rs>, <Rn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR by setting the N and Z flags according to the result of the multiply-accumulate. If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by the instruction. <Rd> Specifies the destination register of the instruction. <Rm> Holds the value to be multiplied with the value of <Rs>. <Rs> Holds the value to be multiplied with the value of <Rm>. <Rn> Contains the value that is added to the product of <Rs> and <Rm>. Architecture version Version 2 and above Exceptions None Operation if ConditionPassed(cond) then Rd = (Rm * Rs + Rn)[31:0] if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected /* See "C flag" note */ V Flag = unaffected A4-54 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 155. ARM Instructions Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, <Rs>, or <Rn> has UNPREDICTABLE results. Operand restriction Specifying the same register for <Rd> and <Rm> has UNPREDICTABLE results. Early termination If the multiplier implementation supports early termination, it must be implemented on the value of the <Rs> operand. The type of early termination used (signed or unsigned) is IMPLEMENTATION DEFINED. Signed and unsigned Because the MLA instruction produces only the lower 32 bits of the 64-bit product, MLA gives the same answer for multiplication of both signed and unsigned numbers. C flag The MLAS instruction is defined to leave the C flag unchanged in ARM architecture version 5 and above. In earlier versions of the architecture, the value of the C flag was UNPREDICTABLE after a MLAS instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-55
- 156. ARM Instructions 4.1.29 MOV 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 1 0 1 S SBZ Rd shifter_operand The MOV (Move) instruction moves the value of <shifter_operand> to the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax MOV{<cond>}{S} <Rd>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the value moved (post-shift if a shift is specified), and the C flag is set to the carry output bit generated by the shifter (see Addressing Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the CPSR are unaffected. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <shifter_operand> Specifies the operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not MOV. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All Exceptions None A4-56 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 157. ARM Instructions Operation if ConditionPassed(cond) then Rd = shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected Usage MOV is used to: • Move a value from one register to another. • Put a constant value into a register. • Perform a shift without any other arithmetic or logical operation. A left shift by n can be used to multiply by 2n. • When the PC is the destination of the instruction, a branch occurs. The instruction: MOV PC, LR can therefore be used to return from a subroutine (see instructions B, BL on page A4-10). In T variants of architecture 4 and in architecture 5 and above, the instruction BX LR must be used in place of MOV PC, LR, as the BX instruction automatically switches back to Thumb state if appropriate. • When the PC is the destination of the instruction and the S bit is set, a branch occurs and the SPSR of the current mode is copied to the CPSR. This means that a MOVS PC, LR instruction can be used to return from some types of exception (see Exceptions on page A2-13). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-57
- 158. ARM Instructions 4.1.30 MRC 31 28 27 26 25 24 23 21 20 19 16 15 12 11 8 7 5 4 3 0 cond 1 1 1 0 opcode_1 1 CRn Rd cp_num opcode_2 1 CRm The MRC (Move to ARM Register from Coprocessor) instruction causes the coprocessor whose number is cp_num to transfer a value to an ARM register or to the condition flags. If no coprocessors indicate that they can execute the instruction an Undefined Instruction exception is generated. Syntax MRC{<cond>} <coproc>, <opcode_1>, <Rd>, <CRn>, <CRm>{, <opcode_2>} MRC2 <coproc>, <opcode_1>, <Rd>, <CRn>, <CRm>{, <opcode_2>} where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. MRC2 Causes the condition field of the instruction to be set to 0b1111. This provides additional opcode space for coprocessor designers. The resulting instructions can only be executed unconditionally. <coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to be placed in the cp_num field of the instruction. The standard generic coprocessor names are p0, p1, ..., p15. <opcode_1> Is a coprocessor-specific opcode. <Rd> Specifies the destination ARM register for the instruction. If R15 is specified for <Rd>, the condition code flags are updated instead of a general-purpose register. <CRn> Specifies the coprocessor register that contains the first operand for the instruction. <CRm> Is an additional coprocessor source or destination register. <opcode_2> Is a coprocessor-specific opcode. If it is omitted, <opcode_2> is assumed to be 0. Architecture version MRC is in version 2 and above. MRC2 is in version 5 and above. Exceptions Undefined Instruction A4-58 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 159. ARM Instructions Operation if ConditionPassed(cond) then data = value from Coprocessor[cp_num] if Rd is R15 then N flag = data[31] Z flag = data[30] C flag = data[29] V flag = data[28] else /* Rd is not R15 */ Rd = data Usage MRC has two uses: 1. If <Rd> specifies R15, the condition code flags bits are updated from the top four bits of the value from the coprocessor specified by <coproc> (to allow conditional branching on the status of a coprocessor) and the other 28 bits are ignored. An example of this use would be to transfer the result of a comparison performed by a floating-point coprocessor to the ARM’s condition flags. 2. Otherwise the instruction writes into register <Rd> a value from the coprocessor specified by <coproc>. An example of this use is a floating-point to integer conversion instruction in a floating-point coprocessor. Notes Coprocessor fields Only instruction bits[31:24], bit[20], bits[15:8] and bit[4] are defined by the ARM architecture. The remaining fields are recommendations, for compatibility with ARM Development Systems. Unimplemented coprocessor instructions Hardware coprocessor support is optional, regardless of the architecture version. An implementation can choose to implement a subset of the coprocessor instructions, or no coprocessor instructions at all. Any coprocessor instructions that are not implemented instead cause an undefined instruction trap. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-59
- 160. ARM Instructions 4.1.31 MRS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 0 1 0 R 0 0 SBO Rd SBZ The MRS (Move PSR to General-purpose Register) instruction moves the value of the CPSR or the SPSR of the current mode into a general-purpose register. In the general-purpose register, the value can be examined or manipulated with normal data-processing instructions. Syntax MRS{<cond>} <Rd>, CPSR MRS{<cond>} <Rd>, SPSR where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. If R15 is specified for <Rd>, the result is UNPREDICTABLE. Architecture version Version 3 and above Exceptions None Operation if ConditionPassed(cond) then if R == 1 then Rd = SPSR else Rd = CPSR A4-60 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 161. ARM Instructions Usage The MRS instruction is commonly used for three purposes: • As part of a read/modify/write sequence for updating a PSR. For more details, see MSR on page A4-62. • When an exception occurs and there is a possibility of a nested exception of the same type occurring, the SPSR of the exception mode is in danger of being corrupted. To deal with this, the SPSR value must be saved before the nested exception can occur, and later restored in preparation for the exception return. The saving is normally done by using an MRS instruction followed by a store instruction. Restoring the SPSR uses the reverse sequence of a load instruction followed by an MSR instruction. For an example of this usage, see Interrupt prioritization on page A9-15. • In process swap code, the programmer's model state of the process being swapped out needs to be saved, including relevant PSR contents, and similar state of the process being swapped in needs to be restored. Again, this involves the use of MRS/store and load/MSR instruction sequences. For an example of this usage, see Context switch on page A9-16. Notes Opcode [11:0] Execution of MRS instructions with opcode[11:0] != 0x000 is UNPREDICTABLE. Opcode [19:16] Execution of MRS instructions with opcode[19:16] != 0b1111 is UNPREDICTABLE. User mode SPSR Accessing the SPSR when in User mode or System mode is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-61
- 162. ARM Instructions 4.1.32 MSR Immediate operand: 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 0 0 1 1 0 R 1 0 field_mask SBO rotate_imm 8_bit_immediate Register operand: 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 R 1 0 field_mask SBO SBZ 0 0 0 0 Rm The MSR (Move to Status Register from ARM Register) instruction transfers the value of a general-purpose register or immediate constant to the CPSR or the SPSR of the current mode. Syntax MSR{<cond>} CPSR_<fields>, #<immediate> MSR{<cond>} CPSR_<fields>, <Rm> MSR{<cond>} SPSR_<fields>, #<immediate> MSR{<cond>} SPSR_<fields>, <Rm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <fields> Is a sequence of one or more of the following: c sets the control field mask bit (bit 16) x sets the extension field mask bit (bit 17) s sets the status field mask bit (bit 18) f sets the flags field mask bit (bit 19). <immediate> Is the immediate value to be transferred to the CPSR or SPSR. Allowed immediate values are 8-bit immediates (in the range 0x00 to 0xFF) and values that can be obtained by rotating them right by an even amount in the range 0–30. These immediate values are the same as those allowed in the immediate form as shown in Data-processing operands - Immediate on page A5-6. <Rm> Is the general-purpose register to be transferred to the CPSR or SPSR. A4-62 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 163. ARM Instructions Architecture version Version 3 and above Exceptions None Operation if ConditionPassed(cond) then if opcode[25] == 1 operand = 8_bit_immediate Rotate_Right (rotate_imm * 2) else /* opcode[25] == 0 */ operand = Rm if R == 0 then if field_mask[0] == 1 and InAPrivilegedMode() then CPSR[7:0] = operand[7:0] if field_mask[1] == 1 and InAPrivilegedMode() then CPSR[15:8] = operand[15:8] if field_mask[2] == 1 and InAPrivilegedMode() then CPSR[23:16] = operand[23:16] if field_mask[3] == 1 then CPSR[31:24] = operand[31:24] else /* R == 1 */ if field_mask[0] == 1 and CurrentModeHasSPSR() then SPSR[7:0] = operand[7:0] if field_mask[1] == 1 and CurrentModeHasSPSR() then SPSR[15:8] = operand[15:8] if field_mask[2] == 1 and CurrentModeHasSPSR() then SPSR[23:16] = operand[23:16] if field_mask[3] == 1 and CurrentModeHasSPSR() then SPSR[31:24] = operand[31:24] Usage This instruction is used to update the value of the condition code flags, interrupt enables, or the processor mode. The value of a PSR should normally be updated by moving the PSR to a general-purpose register (using the MRS instruction), modifying the relevant bits of the general-purpose register, and restoring the updated general-purpose register value back into the PSR (using the MSR instruction). For example, a good way to switch the ARM to Supervisor mode from another privileged mode is: MRS R0,CPSR ; Read CPSR BIC R0,R0,#0x1F ; Modify by removing current mode ORR R0,R0,#0x13 ; and substituting Supervisor mode MSR CPSR_c,R0 ; Write the result back to CPSR ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-63
- 164. ARM Instructions For maximum efficiency, MSR instructions should only write to those fields that they can potentially change. For example, the last instruction in the above code can only change the CPSR control field, as all bits in the other fields are unchanged since they were read from the CPSR by the first instruction. So it writes to CPSR_c, not CPSR_fsxc or some other combination of fields. However, if the only reason that an MSR instruction cannot change a field is that no bits are currently allocated to the field, then the field must be written, to ensure future compatibility. For example, when the process swap code in Context switch on page A9-16 restores the new process’s CPSR value, it writes to SPSR_fsxc (which is later copied to the CPSR by the LDM instruction which restarts the process). There are no bits allocated to the state or extension fields at present, so writing to SPSR_fc would work just as well in current architecture versions (5 and below). However, writing to SPSR_fsxc will continue to work correctly in future versions of the architecture that do have bits allocated in the state or extension fields, while writing to SPSR_fc will not. Note Due to deficiencies in the handling of the state and extension fields in versions 2.50 and below of the ARM Software Development Toolkit, only specify the flags and control fields when using these toolkit versions. In cases where the above guidelines mean that the state and extension fields should have been written for future compatibility, it is recommended that you keep a record of the need to change the set of fields specified when it becomes possible to do so. The immediate form of this instruction can be used to set any of the fields of a PSR, but you must take care to adhere to the read-modify-write technique described above. The immediate form of the instruction is equivalent to reading the PSR concerned, replacing all the bits in the fields concerned by the corresponding bits of the immediate constant and writing the result back to the PSR. The immediate form must therefore only be used when the intention is to modify all the bits in the specified fields and, in particular, must not be used if the specified fields include any as-yet-unallocated bits. Failure to observe this rule might result in code which has unanticipated side-effects on future versions of the ARM architecture. As an exception to the above rule, it is legitimate to use the immediate form of the instruction to modify the flags byte, despite the fact that bits[26:24] of the PSRs have no allocated function at present. For example, this instruction can be used to set all four flags (and clear the Q flag if the processor implements the Enhanced DSP extension): MSR CPSR_f,#0xF0000000 Any functionality allocated to bits[26:24] in a future version of the ARM architecture will be designed so that such code does not have unexpected side effects. A4-64 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 165. ARM Instructions Notes The R bit Bit[22] of the instruction is 0 if the CPSR is to be written and 1 if the SPSR is to be written. User mode CPSR Any writes to CPSR[23:0] in User mode are ignored (so that User mode programs cannot change to a privileged mode). User mode SPSR Accessing the SPSR when in User mode is UNPREDICTABLE. System mode SPSR Accessing the SPSR when in System mode is UNPREDICTABLE. Obsolete field specification The CPSR, CPSR_flg, CPSR_ctl, CPSR_all, SPSR, SPSR_flg, SPSR_ctl and SPSR_all forms of PSR field specification have been superseded by the csxf format shown on page A4-62. CPSR, SPSR, CPSR_all and SPSR_all produce a field mask of 0b1001. CPSR_flg and SPSR_flg produce a field mask of 0b1000. CPSR_ctl and SPSR_ctl produce a field mask of 0b0001. The T bit The MSR instruction must not be used to alter the T bit in the CPSR. If such an attempt is made, the results are UNPREDICTABLE. Addressing modes The immediate and register forms are specified in precisely the same way as the immediate and unshifted register forms of Addressing Mode 1 (see Addressing Mode 1 - Data-processing operands on page A5-2). All other forms of Addressing Mode 1 yield UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-65
- 166. ARM Instructions 4.1.33 MUL 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 0 0 0 S Rd SBZ Rs 1 0 0 1 Rm The MUL (Multiply) instruction is used to multiply signed or unsigned variables to produce a 32-bit result. The condition code flags are optionally updated, based on the result. Syntax MUL{<cond>}{S} <Rd>, <Rm>, <Rs> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR by setting the N and Z flags according to the result of the multiplication. If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by the instruction. <Rd> Specifies the destination register for the instruction. <Rm> Specifies the register that contains the first value to be multiplied. <Rs> Holds the value to be multiplied with the value of <Rm>. Architecture version Version 2 and above Exceptions None Operation if ConditionPassed(cond) then Rd = (Rm * Rs)[31:0] if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected /* See "C flag" note */ V Flag = unaffected A4-66 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 167. ARM Instructions Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rs> has UNPREDICTABLE results. Operand restriction Specifying the same register for <Rd> and <Rm> has UNPREDICTABLE results. Early termination If the multiplier implementation supports early termination, it must be implemented on the value of the <Rs> operand. The type of early termination used (signed or unsigned) is IMPLEMENTATION DEFINED. Signed and unsigned Because the MUL instruction produces only the lower 32 bits of the 64-bit product, MUL gives the same answer for multiplication of both signed and unsigned numbers. C flag The MULS instruction is defined to leave the C flag unchanged in ARM architecture version 5 and above. In earlier versions of the architecture, the value of the C flag was UNPREDICTABLE after a MULS instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-67
- 168. ARM Instructions 4.1.34 MVN 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 1 1 1 S SBZ Rd shifter_operand The MVN (Move Negative) instruction moves the logical one’s complement of the value of <shifter_operand> to the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax MVN{<cond>}{S} <Rd>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the operation, and the C flag is set to the carry output bit generated by the shifter (see Addressing Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the CPSR are unaffected. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <shifter_operand> Specifies the operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not MVN. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All Exceptions None A4-68 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 169. ARM Instructions Operation if ConditionPassed(cond) then Rd = NOT shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected Usage MVN is used to: • write a negative value into a register • form a bit mask • take the one’s complement of a value. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-69
- 170. ARM Instructions 4.1.35 ORR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 1 0 0 S Rn Rd shifter_operand The ORR (Logical OR) instruction performs a bitwise (inclusive) OR of the value of register <Rn> with the value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax ORR{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the operation, and the C flag is set to the carry output bit generated by the shifter (see Addressing Mode 1 - Data-processing operands on page A5-2). The V flag and the rest of the CPSR are unaffected. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the operation. <shifter_operand> Specifies the second operand for the operation. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not ORR. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-70 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 171. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn OR shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected Usage ORR can be used to set selected bits in a register. For each bit, OR with 1 sets the bit, and OR with 0 leaves it unchanged. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-71
- 172. ARM Instructions 4.1.36 RSB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 0 1 1 S Rn Rd shifter_operand The RSB (Reverse Subtract) instruction subtracts the value of register <Rn> from the value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax RSB{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction, and the C and V flags are set according to whether the subtraction generated a borrow (unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is unchanged. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the second operand for the subtraction. <shifter_operand> Specifies the first operand for the subtraction. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not RSB. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-72 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 173. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = shifter_operand - Rn if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(shifter_operand - Rn) V Flag = OverflowFrom(shifter_operand - Rn) Usage The following instruction stores the negation (two’s complement) of Rx in Rd: RSB Rd, Rx, #0 Constant multiplication (of Rx) by 2n–1 (into Rd) can be performed with: RSB Rd, Rx, Rx, LSL #n Notes C flag If S is specified, the C flag is set to: 1 if no borrow occurs 0 if a borrow does occur. In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow condition is usually compensated for by subsequent instructions. For example: • The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing a normal subtraction if C == 1 and subtracting one more than usual if C == 0. • The HS (unsigned higher or same) and LO (unsigned lower) conditions are equivalent to CS (carry set) and CC (carry clear) respectively. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-73
- 174. ARM Instructions 4.1.37 RSC 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 1 1 1 S Rn Rd shifter_operand The RSC (Reverse Subtract with Carry) instruction subtracts the value of register <Rn> and the value of NOT(Carry flag) from the value of <shifter_operand>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax RSC{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction, and the C and V flags are set according to whether the subtraction generated a borrow (unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is unchanged. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the second operand for the subtraction. <shifter_operand> Specifies the first operand for the subtraction. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not RSC. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-74 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 175. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = shifter_operand - Rn - NOT(C Flag) if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(shifter_operand - Rn - NOT(C Flag)) V Flag = OverflowFrom(shifter_operand - Rn - NOT(C Flag)) Usage To negate the 64-bit value in R0,R1, use the following sequence (R0 holds the least significant word) which stores the result in R2,R3: RSBS R2,R0,#0 RSC R3,R1,#0 Notes C flag If S is specified, the C flag is set to: 1 if no borrow occurs 0 if a borrow does occur. In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow condition is usually compensated for by subsequent instructions. For example: • The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing a normal subtraction if C == 1 and subtracting one more than usual if C == 0. • The HS (unsigned higher or same) and LO (unsigned lower) conditions are equivalent to CS (carry set) and CC (carry clear) respectively. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-75
- 176. ARM Instructions 4.1.38 SBC 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 1 1 0 S Rn Rd shifter_operand The SBC (Subtract with Carry) instruction is used to synthesize multi-word subtraction. SBC subtracts the value of <shifter_operand> and the value of NOT(Carry flag) from the value of register <Rn>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax SBC{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction, and the C and V flags are set according to whether the subtraction generated a borrow (unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is unchanged. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the subtraction. <shifter_operand> Specifies the second operand for the subtraction. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not SBC. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-76 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 177. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn - shifter_operand - NOT(C Flag) if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - shifter_operand - NOT(C Flag)) V Flag = OverflowFrom(Rn - shifter_operand - NOT(C Flag)) Usage If register pairs R0,R1 and R2,R3 hold 64-bit values (R0 and R2 hold the least significant words), the following instructions leave the 64-bit difference in R4,R5: SUBS R4,R0,R2 SBC R5,R1,R3 Notes C flag If S is specified, the C flag is set to: 1 if no borrow occurs 0 if a borrow does occur. In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow condition is usually compensated for by subsequent instructions. For example: • The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing a normal subtraction if C == 1 and subtracting one more than usual if C == 0. • The HS (unsigned higher or same) and LO (unsigned lower) conditions are equivalent to CS (carry set) and CC (carry clear) respectively. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-77
- 178. ARM Instructions 4.1.39 SMLAL 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 1 1 1 S RdHi RdLo Rs 1 0 0 1 Rm The SMLAL (Signed Multiply Accumulate Long) instruction multiplies the signed value of register <Rm> with the signed value of register <Rs> to produce a 64-bit product. This product is added to the 64-bit value held in <RdHi> and <RdLo>, and the sum is written back to <RdHi> and <RdLo>. The condition code flags are optionally updated, based on the result. Syntax SMLAL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR by setting the N and Z flags according to the result of the multiply-accumulate. If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by the instruction. <RdLo> Supplies the lower 32 bits of the value to be added to the product of <Rm> and <Rs>, and is the destination register for the lower 32 bits of the result. <RdHi> Supplies the upper 32 bits of the value to be added to the product of <Rm> and <Rs>, and is the destination register for the upper 32 bits of the result. <Rm> Holds the signed value to be multiplied with the value of <Rs>. <Rs> Holds the signed value to be multiplied with the value of <Rm>. Architecture version All M variants Exceptions None A4-78 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 179. ARM Instructions Operation if ConditionPassed(cond) then RdLo = (Rm * Rs)[31:0] + RdLo /* Signed multiplication */ RdHi = (Rm * Rs)[63:32] + RdHi + CarryFrom((Rm * Rs)[31:0] + RdLo) if S == 1 then N Flag = RdHi[31] Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0 C Flag = unaffected /* See "C and V flags" note */ V Flag = unaffected /* See "C and V flags" note */ Usage SMLAL multiplies signed variables to produce a 64-bit result, which is added to the 64-bit value in the two destination general-purpose registers. The result is written back to the two destination general-purpose registers. Notes Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has UNPREDICTABLE results. Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are UNPREDICTABLE. Early termination If the multiplier implementation supports early termination, it must be implemented on the value of the <Rs> operand. The type of early termination used (signed or unsigned) is IMPLEMENTATION DEFINED. C and V flags The SMLALS instruction is defined to leave the C and V flags unchanged in ARM architecture version 5 and above. In earlier versions of the architecture, the values of the C and V flags were UNPREDICTABLE after an SMLALS instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-79
- 180. ARM Instructions 4.1.40 SMULL 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 1 1 0 S RdHi RdLo Rs 1 0 0 1 Rm The SMULL (Signed Multiply Long) instruction multiplies the signed value of register <Rm> with the signed value of register <Rs> to produce a 64-bit result. The upper 32 bits of the result are stored in <RdHi>. The lower 32 bits are stored in <RdLo>. The condition code flags are optionally updated, based on the 64-bit result. Syntax SMULL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR by setting the N and Z flags according to the result of the multiplication. If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by the instruction. <RdLo> Stores the lower 32 bits of the result. <RdHi> Stores the upper 32 bits of the result. <Rm> Holds the signed value to be multiplied with the value of <Rs>. <Rs> Holds the signed value to be multiplied with the value of <Rm>. Architecture version All M variants Exceptions None A4-80 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 181. ARM Instructions Operation if ConditionPassed(cond) then RdHi = (Rm * Rs)[63:32] /* Signed multiplication */ RdLo = (Rm * Rs)[31:0] if S == 1 then N Flag = RdHi[31] Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0 C Flag = unaffected /* See "C and V flags" note */ V Flag = unaffected /* See "C and V flags" note */ Usage SMULL multiplies signed variables to produce a 64-bit result in two general-purpose registers. Notes Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has UNPREDICTABLE results. Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are UNPREDICTABLE. Early termination If the multiplier implementation supports early termination, it must be implemented on the value of the <Rs> operand. The type of early termination used (signed or unsigned) is IMPLEMENTATION DEFINED. C and V flags The SMULLS instruction is defined to leave the C and V flags unchanged in ARM architecture version 5 and above. In earlier versions of the architecture, the values of the C and V flags were UNPREDICTABLE after an SMULLS instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-81
- 182. ARM Instructions 4.1.41 STC 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 P U N W 0 Rn CRd cp_num 8_bit_word_offset The STC (Store Coprocessor) instruction stores data from the coprocessor whose name is cp_num to the sequence of consecutive memory addresses calculated by <addressing_mode>. If no coprocessors indicate that they can execute the instruction, an Undefined Instruction exception is generated. Syntax STC{<cond>}{L} <coproc>, <CRd>, <addressing_mode> STC2{L} <coproc>, <CRd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. STC2 Causes the condition field of the instruction to be set to 0b1111. This provides additional opcode space for coprocessor designers. The resulting instructions can only be executed unconditionally. L Sets the N bit (bit[22]) in the instruction to 1 and specifies a long store (for example, double-precision instead of single-precision data transfer). If L is omitted, the N bit is 0 and the instruction specifies a short store. <coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to be placed in the cp_num field of the instruction. The standard generic coprocessor names are p0, p1, ..., p15. <CRd> Specifies the coprocessor source register of the instruction. <addressing_mode> Is described in Addressing Mode 5 - Load and Store Coprocessor on page A5-56. It determines the P, U, Rn, W and 8_bit_word_offset bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version STC is in version 2 and above. STC2 is in version 5 and above. A4-82 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 183. ARM Instructions Exceptions Undefined Instruction, Data Abort Operation if ConditionPassed(cond) then address = start_address Memory[address,4] = value from Coprocessor[cp_num] while (NotFinished(coprocessor[cp_num])) address = address + 4 Memory[address,4] = value from Coprocessor[cp_num] assert address == end_address Usage STC is useful for storing coprocessor data to memory. The L (long) option controls the N bit and could be used to distinguish between a single- and double-precision transfer for a floating-point store instruction. Notes Coprocessor fields Only instruction bits[31:23], bits[21:16} and bits[11:0] are defined by the ARM architecture. The remaining fields (bit[22] and bits[15:12]) are recommendations, for compatibility with ARM Development Systems. In the case of the Unindexed addressing mode (P==0, U==1, W==0), instruction bits[7:0] are also not ARM architecture-defined, and can be used to specify additional coprocessor options. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses Store coprocessor register instructions ignore the least significant two bits of address. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Unimplemented coprocessor instructions Hardware coprocessor support is optional, regardless of the architecture version. An implementation can choose to implement a subset of the coprocessor instructions, or no coprocessor instructions at all. Any coprocessor instructions that are not implemented instead cause an undefined instruction trap. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-83
- 184. ARM Instructions 4.1.42 STM(1) 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 P U 0 W 0 Rn register_list This form of the STM (Store Multiple) instruction stores a non-empty subset (or possibly all) of the general-purpose registers to sequential memory locations. Syntax STM{<cond>}<addressing_mode> <Rn>{!}, <registers> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <addressing_mode> Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines the P, U, and W bits of the instruction. <Rn> Specifies the base register used by <addressing_mode>. If R15 is specified as <Rn>, the result is UNPREDICTABLE. ! Sets the W bit, causing the instruction to write a modified value back to its base register Rn as specified in Addressing Mode 4 - Load and Store Multiple on page A5-48. If ! is omitted, the W bit is 0 and the instruction does not change its base register in this way. <registers> Is a list of registers, separated by commas and surrounded by { and }. It specifies the set of registers to be stored by the STM instruction. The registers are stored in sequence, the lowest-numbered register to the lowest memory address (start_address), through to the highest-numbered register to the highest memory address (end_address). For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE. If R15 is specified in <registers>, the value stored is IMPLEMENTATION DEFINED. For more details, see Reading the program counter on page A2-7. Architecture version All Exceptions Data Abort A4-84 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 185. ARM Instructions Operation if ConditionPassed(cond) then address = start_address for i = 0 to 15 if register_list[i] == 1 Memory[address,4] = Ri address = address + 4 assert end_address == address - 4 Usage STM is useful as a block store instruction (combined with LDM it allows efficient block copy) and for stack operations. A single STM used in the sequence of a procedure can push the return address and general-purpose register values on to the stack, updating the stack pointer in the process. Notes Operand restrictions If <Rn> is specified as <registers> and base register writeback is specified: • If <Rn> is the lowest-numbered register specified in <register_list>, the original value of <Rn> is stored. • Otherwise, the stored value of <Rn> is UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses STM instructions ignore the least significant two bits of address. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-85
- 186. ARM Instructions 4.1.43 STM (2) 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 P U 1 0 0 Rn register_list This form of STM stores a subset (or possibly all) of the User mode general-purpose registers to sequential memory locations. Syntax STM{<cond>}<addressing_mode> <Rn>, <registers>^ where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <addressing_mode> Is described in Addressing Mode 4 - Load and Store Multiple on page A5-48. It determines the P and U bits of the instruction. Only the forms of this addressing mode with W == 0 are available for this form of the STM instruction. <Rn> Specifies the base register used by <addressing_mode>. If R15 is specified as the base register <Rn>, the result is UNPREDICTABLE. <registers> Is a list of registers, separated by commas and surrounded by { and }. It specifies the set of registers to be stored by the STM instruction. The registers are stored in sequence, the lowest-numbered register to the lowest memory address (start_address), through to the highest-numbered register to the highest memory address (end_address). For each of i=0 to 15, bit[i] in the register_list field of the instruction is 1 if Ri is in the list and 0 otherwise. If bits[15:0] are all zero, the result is UNPREDICTABLE. If R15 is specified in <registers> the value stored is IMPLEMENTATION DEFINED. For more details, see Reading the program counter on page A2-7. ^ For an STM instruction, indicates that User mode registers are to be stored. Architecture version All Exceptions Data Abort A4-86 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 187. ARM Instructions Operation if ConditionPassed(cond) then address = start_address for i = 0 to 15 if register_list[i] == 1 Memory[address,4] = Ri_usr address = address + 4 assert end_address == address - 4 Usage STM is used to store the User mode registers when the processor is in a privileged mode (useful when performing process swaps, and in instruction emulators). Notes Banked registers This instruction must not be followed by an instruction which accesses banked registers (a following NOP is a good way to ensure this). Writeback Setting bit 21 (the W bit) has UNPREDICTABLE results. User and System mode This instruction is UNPREDICTABLE in User or System mode. Base register mode For the purpose of address calculation, the base register is read from the current processor mode registers, not the User mode registers. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses STM instructions ignore the least significant two bits of address. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-87
- 188. ARM Instructions 4.1.44 STR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I P U 0 W 0 Rn Rd addr_mode The STR (Store Register) instruction stores a word from register <Rd> to the memory address calculated by <addressing_mode>. Syntax STR{<cond>} <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the value stored is IMPLEMENTATION DEFINED. For more details, see Reading the program counter on page A2-7. <addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, P, U, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then Memory[address,4] = Rd Usage Combined with a suitable addressing mode, STR stores 32-bit data from a general-purpose register into memory. Using the PC as the base register allows PC-relative addressing, which facilitates position-independent code. A4-88 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 189. ARM Instructions Notes Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non word-aligned addresses STR instructions ignore the least significant two bits of address. So if these bits are not 0b00, the effects of STR are not precisely opposite to those of LDR. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-89
- 190. ARM Instructions 4.1.45 STRB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I P U 1 W 0 Rn Rd addr_mode The STRB (Store Register Byte) instruction stores a byte from the least significant byte of register <Rd> to the memory address calculated by <addressing_mode>. Syntax STR{<cond>}B <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, P, U, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then Memory[address,1] = Rd[7:0] Usage Combined with a suitable addressing mode, STRB writes the least significant byte of a general-purpose register to memory. Using the PC as the base register allows PC-relative addressing, which facilitates position-independent code. A4-90 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 191. ARM Instructions Notes Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-91
- 192. ARM Instructions 4.1.46 STRBT 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I 0 U 1 1 0 Rn Rd addr_mode The STRBT (Store Register Byte with Translation) instruction stores a byte from the least significant byte of register <Rd> to the memory address calculated by <post_indexed_addressing_mode>. If the instruction is executed when the processor is in a privileged mode, the memory system is signaled to treat the access as if the processor were in User mode. Syntax STR{<cond>}BT <Rd>, <post_indexed_addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <post_indexed_addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W == 0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W == 1 instead, but the addressing mode is the same in all other respects. The syntax of all forms of <post_indexed_addressing_mode> includes a base register <Rn>. All forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then Memory[address,1] = Rd[7:0] A4-92 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 193. ARM Instructions Usage STRBT can be used by a (privileged) exception handler that is emulating a memory access instruction which would normally execute in User mode. The access is restricted as if it had User mode privilege. Notes User mode If this instruction is executed in User mode, an ordinary User mode access is performed. Operand restrictions If the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-93
- 194. ARM Instructions 4.1.47 STRH 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U I W 0 Rn Rd addr_mode 1 0 1 1 addr_mode The STRH (Store Register Halfword) instruction stores a halfword from the least significant halfword of register <Rd> to the memory address calculated by <addressing_mode>. If the address is not halfword-aligned, the result is UNPREDICTABLE. Syntax STR{<cond>}H <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <addressing_mode> Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It determines the P, U, I, W, Rn and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version Version 4 and above Exceptions Data Abort Operation if ConditionPassed(cond) then if address[0] == 0 data = Rd[15:0] else /* address[0] == 1 */ data = UNPREDICTABLE Memory[address,2] = data A4-94 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 195. ARM Instructions Usage Combined with a suitable addressing mode, STRH allows 16-bit data from a general-purpose register to be stored to memory. Using the PC as the base register allows PC-relative addressing, to facilitate position-independent code. Notes Operand restrictions If <addressing_mode> specifies base register writeback, and the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non halfword-aligned addresses If the store address is not halfword-aligned, the stored value is UNPREDICTABLE. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-95
- 196. ARM Instructions 4.1.48 STRT 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 I 0 U 0 1 0 Rn Rd addr_mode The STRT (Store Register with Translation) instruction stores a word from register <Rd> to the memory address calculated by <post_indexed_addressing_mode>. If the instruction is executed when the processor is in a privileged mode, the memory system is signaled to treat the access as if the processor was in User mode. Syntax STR{<cond>}T <Rd>, <post_indexed_addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the source register for the operation. If R15 is specified for <Rd>, the value stored is IMPLEMENTATION DEFINED. For more details, see Reading the program counter on page A2-7. <post_indexed_addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It determines the I, U, Rn and addr_mode bits of the instruction. Only post-indexed forms of Addressing Mode 2 are available for this instruction. These forms have P == 0 and W == 0, where P and W are bit[24] and bit[21] respectively. This instruction uses P == 0 and W == 1 instead, but the addressing mode is the same in all other respects. The syntax of all forms of <post_indexed_addressing_mode> includes a base register <Rn>. All forms also specify that the instruction modifies the base register value (this is known as base register writeback). Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then Memory[address,4] = Rd A4-96 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 197. ARM Instructions Usage STRT can be used by a (privileged) exception handler that is emulating a memory access instruction that would normally execute in User mode. The access is restricted as if it had User mode privilege. Notes User mode If this instruction is executed in User mode, an ordinary User mode access is performed. Operand restrictions If the same register is specified for <Rd> and <Rn>, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-97
- 198. ARM Instructions 4.1.49 SUB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 0 0 1 0 S Rn Rd shifter_operand The SUB (Subtract) instruction subtracts the value of <shifter_operand> from the value of register <Rn>, and stores the result in the destination register <Rd>. The condition code flags are optionally updated, based on the result. Syntax SUB{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Sets the S bit (bit[20]) in the instruction to 1 and specifies that the instruction updates the CPSR. If S is omitted, the S bit is set to 0 and the CPSR is not changed by the instruction. Two types of CPSR update can occur when S is specified: • If <Rd> is not R15, the N and Z flags are set according to the result of the subtraction, and the C and V flags are set according to whether the subtraction generated a borrow (unsigned underflow) and a signed overflow, respectively. The rest of the CPSR is unchanged. • If <Rd> is R15, the SPSR of the current mode is copied to the CPSR. This form of the instruction is UNPREDICTABLE if executed in User mode or System mode, because these modes do not have an SPSR. <Rd> Specifies the destination register of the instruction. <Rn> Specifies the register that contains the first operand for the subtraction. <shifter_operand> Specifies the second operand for the subtraction. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not SUB. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All A4-98 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 199. ARM Instructions Exceptions None Operation if ConditionPassed(cond) then Rd = Rn - shifter_operand if S == 1 and Rd == R15 then CPSR = SPSR else if S == 1 then N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - shifter_operand) V Flag = OverflowFrom(Rn - shifter_operand) Usage SUB is used to subtract one value from another to produce a third. To decrement a register value (in Rx) use: SUBS Ri, Ri, #1 SUBS is useful as a loop counter decrement, as the loop branch can test the flags for the appropriate termination condition, without the need for a compare instruction: CMP Rx, #0 This both decrements the loop counter in Ri and checks whether it has reached zero. The form of this instruction with the PC as its destination register and the S bit set can be used to return from interrupts and various other types of exception. See Exceptions on page A2-13 for more details. Notes C flag If S is specified, the C flag is set to: 1 if no borrow occurs 0 if a borrow does occur. In other words, the C flag is used as a NOT(borrow) flag. This inversion of the borrow condition is usually compensated for by subsequent instructions. For example: • The SBC and RSC instructions use the C flag as a NOT(borrow) operand, performing a normal subtraction if C == 1 and subtracting one more than usual if C == 0. • The HS (unsigned higher or same) and LO (unsigned lower) conditions are equivalent to CS (carry set) and CC (carry clear) respectively. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-99
- 200. ARM Instructions 4.1.50 SWI 31 28 27 26 25 24 23 0 cond 1 1 1 1 immed_24 The SWI (Software Interrupt) instruction causes a SWI exception (see Exceptions on page A2-13). Syntax SWI{<cond>} <immed_24> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <immed_24> Is a 24-bit immediate value that is put into bits[23:0] of the instruction. This value is ignored by the ARM processor, but can be used by an operating system SWI exception handler to determine what operating system service is being requested (see Usage on page A4-101 below for more details). Architecture version All Exceptions Software interrupt Operation if ConditionPassed(cond) then R14_svc = address of next instruction after the SWI instruction SPSR_svc = CPSR CPSR[4:0] = 0b10011 /* Enter Supervisor mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF0008 else PC = 0x00000008 A4-100 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 201. ARM Instructions Usage The SWI instruction is used as an operating system service call. The method used to select which operating system service is required is specified by the operating system, and the SWI exception handler for the operating system determines and provides the requested service. Two typical methods are: • The 24-bit immediate in the instruction specifies which service is required, and any parameters needed by the selected service are passed in general-purpose registers. • The 24-bit immediate in the instruction is ignored, general-purpose register R0 is used to select which service is wanted, and any parameters needed by the selected service are passed in other general-purpose registers. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-101
- 202. ARM Instructions 4.1.51 SWP 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 0 0 Rn Rd SBZ 1 0 0 1 Rm The SWP (Swap) instruction swaps a word between registers and memory. SWP loads a word from the memory address given by the value of register <Rn>. The value of register <Rm> is then stored to the memory address given by the value of <Rn>, and the original loaded value is written to register <Rd>. If the same register is specified for <Rd> and <Rm>, this instruction swaps the value of the register and the value at the memory address. Syntax SWP{<cond>} <Rd>, <Rm>, [<Rn>] where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the instruction. <Rm> Contains the value that is stored to memory. <Rn> Contains the memory address to load from. Architecture version Version 3 and above, plus version 2a Exceptions Data Abort Operation if ConditionPassed(cond) then if Rn[1:0] == 0b00 then temp = Memory[Rn,4] else if Rn[1:0] == 0b01 then temp = Memory[Rn,4] Rotate_Right 8 else if Rn[1:0] == 0b10 then temp = Memory[Rn,4] Rotate_Right 16 else /* Rn[1:0] == 0b11 */ temp = Memory[Rn,4] Rotate_Right 24 Memory[Rn,4] = Rm Rd = temp A4-102 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 203. ARM Instructions Usage The SWP instruction can be used to implement semaphores. For sample code, see Semaphore instructions on page A9-11. Notes Non word-aligned addresses If the address is not word-aligned, the loaded value is rotated right by 8 times the value of Rn[1:0]. The stored value is not rotated. Use of R15 If R15 is specified for <Rd>, <Rn>, or <Rm>, the result is UNPREDICTABLE. Operand restrictions If the same register is specified as <Rn> and <Rm>, or <Rn> and <Rd>, the result is UNPREDICTABLE. Data abort If a data abort is signaled on either the load access or the store access, the loaded value is not written to <Rd>. If a data abort is signaled on the load access, the store access does not occur. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-103
- 204. ARM Instructions 4.1.52 SWPB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 1 0 0 Rn Rd SBZ 1 0 0 1 Rm The SWPB (Swap Byte) instruction swaps a byte between registers and memory. SWPB loads a byte from the memory address given by the value of register <Rn>. The value of the least significant byte of register <Rm> is stored to the memory address given by <Rn>, the original loaded value is zero-extended to a 32-bit word, and the word is written to register <Rd>. If the same register is specified for <Rd> and <Rm>, this instruction swaps the value of the least significant byte of the register and the byte value at the memory address. Syntax SWP{<cond>}B <Rd>, <Rm>, [<Rn>] where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register for the instruction. <Rm> Contains the value that is stored to memory. <Rn> Contains the memory address to load from. Architecture version Version 3 and above, plus version 2a Exceptions Data Abort Operation if ConditionPassed(cond) then temp = Memory[Rn,1] Memory[Rn,1] = Rm[7:0] Rd = temp Usage The SWPB instruction can be used to implement semaphores, in a similar manner to that shown for SWP instructions in Semaphore instructions on page A9-11. A4-104 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 205. ARM Instructions Notes Use of R15 If R15 is specified for <Rd>, <Rn>, or <Rm>, the result is UNPREDICTABLE. Operand restrictions If the same register is specified as <Rn> and <Rm>, or <Rn> and <Rd>, the result is UNPREDICTABLE. Data abort If a data abort is signaled on either the load access or the store access, the loaded value is not written to <Rd>. If a data abort is signaled on the load access, the store access does not occur. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-105
- 206. ARM Instructions 4.1.53 TEQ 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 0 0 1 1 Rn SBZ shifter_operand The TEQ (Test Equivalence) instruction compares a register value with another arithmetic value. The condition flags are updated, based on the result of logically exclusive-ORing the two values, so that subsequent instructions can be conditionally executed. Syntax TEQ{<cond>} <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the register that contains the first operand for the comparison. <shifter_operand> Specifies the second operand for the comparison. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option sets the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not TEQ. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All Exceptions None Operation if ConditionPassed(cond) then alu_out = Rn EOR shifter_operand N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected Usage TEQ is used to test if two values are equal, without affecting the V flag (as CMP does). The C flag is also unaffected in many cases. TEQ is also useful for testing whether two values have the same sign. After the comparison, the N flag is the logical Exclusive OR of the sign bits of the two operands. A4-106 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 207. ARM Instructions 4.1.54 TST 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 0 0 0 1 Rn SBZ shifter_operand The TST (Test) instruction compares a register value with another arithmetic value. The condition flags are updated, based on the result of logically ANDing the two values, so that subsequent instructions can be conditionally executed. Syntax TST{<cond>} <Rn>, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the register that contains the first operand for the comparison. <shifter_operand> Specifies the second operand for the comparison. The options for this operand are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit[25]) and the shifter_operand bits (bits[11:0]) to be set in the instruction. If the I bit is 0 and both bit[7] and bit[4] of shifter_operand are 1, the instruction is not TST. Instead, see Extending the instruction set on page A3-27 to determine which instruction it is. Architecture version All Exceptions None Operation if ConditionPassed(cond) then alu_out = Rn AND shifter_operand N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = shifter_carry_out V Flag = unaffected ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-107
- 208. ARM Instructions Usage TST is used to determine whether a particular subset of register bits includes at least one set bit. A very common use for TST is to test whether a single bit is set or clear. A4-108 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 209. ARM Instructions 4.1.55 UMLAL 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 1 0 1 S RdHi RdLo Rs 1 0 0 1 Rm The UMLAL (Unsigned Multiply Accumulate Long) instruction multiplies the unsigned value of register <Rm> with the unsigned value of register <Rs> to produce a 64-bit product. This product is added to the 64-bit value held in <RdHi> and <RdLo>, and the sum is written back to <RdHi> and <RdLo>. The condition code flags are optionally updated, based on the result. Syntax UMLAL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR by setting the N and Z flags according to the result of the multiply-accumulate. If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by the instruction. <RdLo> Supplies the lower 32 bits of the value to be added to the product of <Rm> and <Rs>, and is the destination register for the lower 32 bits of the result. <RdHi> Supplies the upper 32 bits of the value to be added to the product of <Rm> and <Rs>, and is the destination register for the upper 32 bits of the result. <Rm> Holds the signed value to be multiplied with the value of <Rs>. <Rs> Holds the signed value to be multiplied with the value of <Rm>. Architecture version All M variants Exceptions None ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-109
- 210. ARM Instructions Operation if ConditionPassed(cond) then RdLo = (Rm * Rs)[31:0] + RdLo /* Unsigned multiplication */ RdHi = (Rm * Rs)[63:32] + RdHi + CarryFrom((Rm * Rs)[31:0] + RdLo) if S == 1 then N Flag = RdHi[31] Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0 C Flag = unaffected /* See "C and V flags" note */ V Flag = unaffected /* See "C and V flags" note */ Usage UMLAL multiplies unsigned variables to produce a 64-bit result, which is added to the 64-bit value in the two destination general-purpose registers. The result is written back to the two destination general-purpose registers. Notes Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has UNPREDICTABLE results. Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are UNPREDICTABLE. Early termination If the multiplier implementation supports early termination, it must be implemented on the value of the <Rs> operand. The type of early termination used (signed or unsigned) is IMPLEMENTATION DEFINED. C and V flags The UMLALS instruction is defined to leave the C and V flags unchanged in ARM architecture version 5 and above. In earlier versions of the architecture, the values of the C and V flags were UNPREDICTABLE after a UMLALS instruction. A4-110 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 211. ARM Instructions 4.1.56 UMULL 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 1 0 0 S RdHi RdLo Rs 1 0 0 1 Rm The UMULL (Unsigned Multiply Long) instruction multiplies the unsigned value of register <Rm> with the unsigned value of register <Rs> to produce a 64-bit result. The upper 32 bits of the result are stored in <RdHi>. The lower 32 bits are stored in <RdLo>. The condition code flags are optionally updated, based on the 64-bit result. Syntax UMULL{<cond>}{S} <RdLo>, <RdHi>, <Rm>, <Rs> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. S Causes the S bit (bit[20]) in the instruction to be set to 1 and specifies that the instruction updates the CPSR by setting the N and Z flags according to the result of the multiplication. If S is omitted, the S bit of the instruction is set to 0 and the entire CPSR is unaffected by the instruction. <RdLo> Stores the lower 32 bits of the result. <RdHi> Stores the upper 32 bits of the result. <Rm> Holds the signed value to be multiplied with the value of <Rs>. <Rs> Holds the signed value to be multiplied with the value of <Rm>. Architecture version All M variants Exceptions None ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-111
- 212. ARM Instructions Operation if ConditionPassed(cond) then RdHi = (Rm * Rs)[63:32] /* Unsigned multiplication */ RdLo = (Rm * Rs)[31:0] if S == 1 then N Flag = RdHi[31] Z Flag = if (RdHi == 0) and (RdLo == 0) then 1 else 0 C Flag = unaffected /* See "C and V flags" note */ V Flag = unaffected /* See "C and V flags" note */ Usage UMULL multiplies unsigned variables to produce a 64-bit result in two general-purpose registers. Notes Use of R15 Specifying R15 for register <RdHi>, <RdLo>, <Rm>, or <Rs> has UNPREDICTABLE results. Operand restriction <RdHi>, <RdLo>, and <Rm> must be three distinct registers, or the results are UNPREDICTABLE. Early termination If the multiplier implementation supports early termination, it must be implemented on the value of the <Rs> operand. The type of early termination used (signed or unsigned) is IMPLEMENTATION DEFINED. C and V flags The UMULLS instruction is defined to leave the C and V flags unchanged in ARM architecture version 5 and above. In earlier versions of the architecture, the values of the C and V flags were UNPREDICTABLE after a UMULLS instruction. A4-112 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 213. ARM Instructions 4.2 ARM instructions and architecture versions Table 4-1 shows which ARM instructions are present in each current ARM architecture version. Table 4-1 ARM instructions by architecture version Instruction v3, v3M v4, v4xM v4T, v4TxM v5, v5xM, v5T, v5TxM v5TE, v5TExP ADC Yes Yes Yes Yes Yes ADD Yes Yes Yes Yes Yes AND Yes Yes Yes Yes Yes B Yes Yes Yes Yes Yes BIC Yes Yes Yes Yes Yes BKPT No No No Yes Yes BL Yes Yes Yes Yes Yes BLX (both forms) No No No Yes Yes BX No No Yes Yes Yes CDP Yes Yes Yes Yes Yes CDP2 No No No Yes Yes CLZ No No No Yes Yes CMN Yes Yes Yes Yes Yes CMP Yes Yes Yes Yes Yes EOR Yes Yes Yes Yes Yes LDC Yes Yes Yes Yes Yes LDC2 No No No Yes Yes LDM (all forms) Yes Yes Yes Yes Yes LDR Yes Yes Yes Yes Yes LDRB Yes Yes Yes Yes Yes LDRD No No No No Only v5TE LDRBT Yes Yes Yes Yes Yes ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-113
- 214. ARM Instructions Table 4-1 ARM instructions by architecture version (Continued) Instruction v3, v3M v4, v4xM v4T, v4TxM v5, v5xM, v5T, v5TxM v5TE, v5TExP LDRH No Yes Yes Yes Yes LDRSB No Yes Yes Yes Yes LDRSH No Yes Yes Yes Yes LDRT Yes Yes Yes Yes Yes MCR Yes Yes Yes Yes Yes MCR2 No No No Yes Yes MCRR No No No No Only v5TE MLA Yes Yes Yes Yes Yes MOV Yes Yes Yes Yes Yes MRC Yes Yes Yes Yes Yes MRC2 No No No Yes Yes MRRC No No No No Only v5TE MRS Yes Yes Yes Yes Yes MSR Yes Yes Yes Yes Yes MUL Yes Yes Yes Yes Yes MVN Yes Yes Yes Yes Yes ORR Yes Yes Yes Yes Yes PLD No No No No Only v5TE QADD No No No No Yes QDADD No No No No Yes QDSUB No No No No Yes QSUB No No No No Yes RSB Yes Yes Yes Yes Yes RSC Yes Yes Yes Yes Yes A4-114 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 215. ARM Instructions Table 4-1 ARM instructions by architecture version (Continued) Instruction v3, v3M v4, v4xM v4T, v4TxM v5, v5xM, v5T, v5TxM v5TE, v5TExP SBC Yes Yes Yes Yes Yes SMLAL Only v3M Only v4 Only v4T Only v5/v5T Yes SMLA<x><y> No No No No Yes SMLAL<x><y> No No No No Yes SMLAW<y> No No No No Yes SMULL Only v3M Only v4 Only v4T Only v5/v5T Yes SMUL<x><y> No No No No Yes SMULW<y> No No No No Yes STC Yes Yes Yes Yes Yes STC2 No No No Yes Yes STM (both forms) Yes Yes Yes Yes Yes STR Yes Yes Yes Yes Yes STRB Yes Yes Yes Yes Yes STRBT Yes Yes Yes Yes Yes STRD No No No No Only v5TE STRH No Yes Yes Yes Yes STRT Yes Yes Yes Yes Yes SUB Yes Yes Yes Yes Yes SWI Yes Yes Yes Yes Yes SWP Yes Yes Yes Yes Yes SWPB Yes Yes Yes Yes Yes TEQ Yes Yes Yes Yes Yes ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A4-115
- 216. ARM Instructions Table 4-1 ARM instructions by architecture version (Continued) Instruction v3, v3M v4, v4xM v4T, v4TxM v5, v5xM, v5T, v5TxM v5TE, v5TExP TST Yes Yes Yes Yes Yes UMLAL Only v3M Only v4 Only v4T Only v5/v5T Yes UMULL Only v3M Only v4 Only v4T Only v5/v5T Yes A4-116 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 217. Chapter A5 ARM Addressing Modes This chapter describes each of the five addressing modes used with ARM instructions. The chapter contains the following sections: • Addressing Mode 1 - Data-processing operands on page A5-2 • Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18 • Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34 • Addressing Mode 4 - Load and Store Multiple on page A5-48 • Addressing Mode 5 - Load and Store Coprocessor on page A5-56. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-1
- 218. ARM Addressing Modes 5.1 Addressing Mode 1 - Data-processing operands There are 11 addressing modes used to calculate the <shifter_operand> in an ARM data-processing instruction. The general instruction syntax is: <opcode>{<cond>}{S} <Rd>, <Rn>, <shifter_operand> where <shifter_operand> is one of the following 11 options: 1. #<immediate> See Data-processing operands - Immediate on page A5-6. 2. <Rm> See Data-processing operands - Register on page A5-8. 3. <Rm>, LSL #<shift_imm> See Data-processing operands - Logical shift left by immediate on page A5-9. 4. <Rm>, LSL <Rs> See Data-processing operands - Logical shift left by register on page A5-10. 5. <Rm>, LSR #<shift_imm> See Data-processing operands - Logical shift right by immediate on page A5-11. 6. <Rm>, LSR <Rs> See Data-processing operands - Logical shift right by register on page A5-12. 7. <Rm>, ASR #<shift_imm> See Data-processing operands - Arithmetic shift right by immediate on page A5-13. 8. <Rm>, ASR <Rs> See Data-processing operands - Arithmetic shift right by register on page A5-14. 9. <Rm>, ROR #<shift_imm> See Data-processing operands - Rotate right by immediate on page A5-15. 10. <Rm>, ROR <Rs> See Data-processing operands - Rotate right by register on page A5-16. 11. <Rm>, RRX See Data-processing operands - Rotate right with extend on page A5-17. A5-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 219. ARM Addressing Modes 5.1.1 Encoding The following diagrams show the encodings for this addressing mode: 32-bit immediate 31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 0 cond 0 0 1 opcode S Rn Rd rotate_imm immed_8 Immediate shifts 31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd shift_imm shift 0 Rm Register shifts 31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd Rs 0 shift 1 Rm opcode Specifies the operation of the instruction. S bit Indicates that the instruction updates the condition codes. Rd Specifies the destination register. Rn Specifies the first source operand register. Bits[11:0] The fields within bits[11:0] are collectively called a shifter operand. This is described in The shifter operand on page A5-4. Bit[25] Is referred to as the I bit, and is used to distinguish between an immediate shifter operand and a register-based shifter operand. If all three of the following bits have the values shown, the instruction is not a data-processing instruction, but lies in the arithmetic or Load/Store instruction extension space: bit[25] == 0 bit[4] == 1 bit[7] == 1 See Extending the instruction set on page A3-27 for more information. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-3
- 220. ARM Addressing Modes 5.1.2 The shifter operand As well as producing the shifter operand, the shifter produces a carry-out which some instructions write into the Carry Flag. The default register operand (register Rm specified with no shift) uses the form register shift left by immediate, with the immediate set to zero. The shifter operand takes one of the following three basic formats. Immediate operand value An immediate operand value is formed by rotating an 8-bit constant (in a 32-bit word) by an even number of bits (0,2,4,8...26,28,30). Therefore, each instruction contains an 8-bit constant and a 4-bit rotate to be applied to that constant. Some valid constants are: 0xFF,0x104,0xFF0,0xFF00,0xFF000,0xFF000000,0xF000000F Some invalid constants are: 0x101,0x102,0xFF1,0xFF04,0xFF003,0xFFFFFFFF,0xF000001F For example: MOV R0, #0 ; Move zero to R0 ADD R3, R3, #1 ; Add one to the value of register 3 CMP R7, #1000 ; Compare value of R7 with 1000 BIC R9, R8, #0xFF00 ; Clear bits 8-15 of R8 and store in R9 Register operand value A register operand value is simply the value of a register. The value of the register is used directly as the operand to the data-processing instruction. For example: MOV R2, R0 ; Move the value of R0 to R2 ADD R4, R3, R2 ; Add R2 to R3, store result in R4 CMP R7, R8 ; Compare the value of R7 and R8 Shifted register operand value A shifted register operand value is the value of a register, shifted (or rotated) before it is used as the data-processing operand. There are five types of shift: ASR Arithmetic shift right LSL Logical shift left LSR Logical shift right ROR Rotate right RRX Rotate right with extend. A5-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 221. ARM Addressing Modes The number of bits to shift by is specified either as an immediate or as the value of a register. For example: MOV R2, R0, LSL #2 ; Shift R0 left by 2, write to R2, (R2=R0x4) ADD R9, R5, R5, LSL #3 ; R9 = R5 + R5 x 8 or R9 = R5 x 9 RSB R9, R5, R5, LSL #3 ; R9 = R5 x 8 - R5 or R9 = R5 x 7 SUB R10, R9, R8, LSR #4 ; R10 = R9 - R8 / 16 MOV R12, R4, ROR R3 ; R12 = R4 rotated right by value of R3 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-5
- 222. ARM Addressing Modes 5.1.3 Data-processing operands - Immediate 31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 0 cond 0 0 1 opcode S Rn Rd rotate_imm immed_8 This data-processing operand provides a constant (defined in the instruction) operand to a data-processing instruction. The shifter_operand value is formed by rotating (to the right) an 8-bit immediate value to any even bit position in a 32-bit word. If the rotate immediate is zero, the carry-out from the shifter is the value of the C flag, otherwise, it is set to bit[31] of the value of <shifter_operand>. Syntax #<immediate> where: <immediate> Specifies the immediate constant wanted. It is encoded in the instruction as an 8-bit immediate (immed_8) and a 4-bit immediate (rotate_imm), so that <immediate> is equal to the result of rotating immed_8 right by (2 * rotate_imm) bits. Architecture version All Operation shifter_operand = immed_8 Rotate_Right (rotate_imm * 2) if rotate_imm == 0 then shifter_carry_out = C flag else /* rotate_imm != 0 */ shifter_carry_out = shifter_operand[31] Notes Legitimate immediates Not all 32-bit immediates are legitimate. Only those that can be formed by rotating an 8-bit immediate right by an even amount are valid 32-bit immediates for this format. Encoding Some values of <immediate> have more than one possible encoding. For example, a value of 0x3F0 could be encoded as: immed_8 == 0x3F, rotate_imm == 0xE or as: immed_8 == 0xFC, rotate_imm == 0xF When more than one encoding is available, an assembler needs to choose the correct one to use, as follows: A5-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 223. ARM Addressing Modes • If <immediate> lies in the range 0 to 0xFF, an encoding with rotate_imm == 0 is available. The assembler must choose that encoding. (Choosing another encoding would affect how some instructions set the C flag.) • Otherwise, it is recommended that the encoding with the smallest value of rotate_imm is chosen. (This choice does not affect instruction functionality.) For more precise control of the encoding, the instruction fields can be specified directly by using the syntax: #<immed_8>, <rotate_amount> where <rotate_amount> = 2 * rotate_imm. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-7
- 224. ARM Addressing Modes 5.1.4 Data-processing operands - Register 31 28 27 26 25 24 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd 0 0 0 0 0 0 0 0 Rm This data-processing operand provides the value of a register directly. The carry-out from the shifter is the C flag. Syntax <Rm> where: <Rm> Specifies the register whose value is the instruction operand. Architecture version All Operation shifter_operand = Rm shifter_carry_out = C Flag Notes Encoding This instruction is encoded as a logical shift left by immediate (see Data-processing operands - Logical shift left by immediate on page A5-9) with a shift of zero (shift_imm == 0). Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current instruction plus 8. A5-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 225. ARM Addressing Modes 5.1.5 Data-processing operands - Logical shift left by immediate 31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd shift_imm 0 0 0 Rm This data-processing operand is used to provide either the value of a register directly (lone register operand, as described in Data-processing operands - Register on page A5-8), or the value of a register shifted left (multiplied by a constant power of two). This instruction operand is the value of register Rm, logically shifted left by an immediate value in the range 0 to 31. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last bit shifted out, or the C flag if no shift is specified. Syntax <Rm>, LSL #<shift_imm> where: <Rm> Specifies the register whose value is to be shifted. LSL Indicates a logical shift left. <shift_imm> Specifies the shift. This is a value between 0 and 31. Architecture version All Operation if shift_imm == 0 then /* Register Operand */ shifter_operand = Rm shifter_carry_out = C Flag else /* shift_imm > 0 */ shifter_operand = Rm Logical_Shift_Left shift_imm shifter_carry_out = Rm[32 - shift_imm] Notes Default shift If the value of <shift_imm> == 0, the operand can be written as just <Rm> (see Data-processing operands - Register on page A5-8). Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current instruction plus 8. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-9
- 226. ARM Addressing Modes 5.1.6 Data-processing operands - Logical shift left by register 31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd Rs 0 0 0 1 Rm This data-processing operand is used to provide the value of a register multiplied by a variable power of two. This instruction operand is the value of register Rm, logically shifted left by the value in the least significant byte of register Rs. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last bit shifted out, which is zero if the shift amount is more than 32, or the C flag if the shift amount is zero. Syntax <Rm>, LSL <Rs> where: <Rm> Specifies the register whose value is to be shifted. LSL Indicates a logical shift left. <Rs> Is the register containing the value of the shift. Architecture version All Operation if Rs[7:0] == 0 then shifter_operand = Rm shifter_carry_out = C Flag else if Rs[7:0] < 32 then shifter_operand = Rm Logical_Shift_Left Rs[7:0] shifter_carry_out = Rm[32 - Rs[7:0]] else if Rs[7:0] == 32 then shifter_operand = 0 shifter_carry_out = Rm[0] else /* Rs[7:0] > 32 */ shifter_operand = 0 shifter_carry_out = 0 Notes Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE results. A5-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 227. ARM Addressing Modes 5.1.7 Data-processing operands - Logical shift right by immediate 31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd shift_imm 0 1 0 Rm This data-processing operand is used to provide the unsigned value of a register shifted right (divided by a constant power of two). This instruction operand is the value of register Rm, logically shifted right by an immediate value in the range 1 to 32. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last bit shifted out. Syntax <Rm>, LSR #<shift_imm> where: <Rm> Specifies the register whose value is to be shifted. LSR Indicates a logical shift right. <shift_imm> Specifies the shift. This is an immediate value between 1 and 32. (A shift by 32 is encoded by shift_imm == 0.) Architecture version All Operation if shift_imm == 0 then shifter_operand = 0 shifter_carry_out = Rm[31] else /* shift_imm > 0 */ shifter_operand = Rm Logical_Shift_Right shift_imm shifter_carry_out = Rm[shift_imm - 1] Notes Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current instruction plus 8. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-11
- 228. ARM Addressing Modes 5.1.8 Data-processing operands - Logical shift right by register 31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd Rs 0 0 1 1 Rm This data-processing operand is used to provide the unsigned value of a register shifted right (divided by a variable power of two). It is produced by the value of register Rm, logically shifted right by the value in the least significant byte of register Rs. Zeros are inserted into the vacated bit positions. The carry-out from the shifter is the last bit shifted out, which is zero if the shift amount is more than 32, or the C flag if the shift amount is zero. Syntax <Rm>, LSR <Rs> where: <Rm> Specifies the register whose value is to be shifted. LSR Indicates a logical shift right. <Rs> Is the register containing the value of the shift. Architecture version All Operation if Rs[7:0] == 0 then shifter_operand = Rm shifter_carry_out = C Flag else if Rs[7:0] < 32 then shifter_operand = Rm Logical_Shift_Right Rs[7:0] shifter_carry_out = Rm[Rs[7:0] - 1] else if Rs[7:0] == 32 then shifter_operand = 0 shifter_carry_out = Rm[31] else /* Rs[7:0] > 32 */ shifter_operand = 0 shifter_carry_out = 0 Notes Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE results. A5-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 229. ARM Addressing Modes 5.1.9 Data-processing operands - Arithmetic shift right by immediate 31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd shift_imm 1 0 0 Rm This data-processing operand is used to provide the signed value of a register arithmetically shifted right (divided by a constant power of two). This instruction operand is the value of register Rm, arithmetically shifted right by an immediate value in the range 1 to 32. The sign bit of Rm (Rm[31]) is inserted into the vacated bit positions. The carry-out from the shifter is the last bit shifted out. Syntax <Rm>, ASR #<shift_imm> where: <Rm> Specifies the register whose value is to be shifted. ASR Indicates an arithmetic shift right. <shift_imm> Specifies the shift. This is an immediate value between 1 and 32. (A shift by 32 is encoded by shift_imm == 0.) Architecture version All Operation if shift_imm == 0 then if Rm[31] == 0 then shifter_operand = 0 shifter_carry_out = Rm[31] else /* Rm[31] == 1 */ shifter_operand = 0xFFFFFFFF shifter_carry_out = Rm[31] else /* shift_imm > 0 */ shifter_operand = Rm Arithmetic_Shift_Right <shift_imm> shifter_carry_out = Rm[shift_imm - 1] Notes Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current instruction plus 8. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-13
- 230. ARM Addressing Modes 5.1.10 Data-processing operands - Arithmetic shift right by register 31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd Rs 0 1 0 1 Rm This data-processing operand is used to provide the signed value of a register arithmetically shifted right (divided by a variable power of two). This instruction operand is the value of register Rm arithmetically shifted right by the value in the least significant byte of register Rs. The sign bit of Rm (Rm[31]) is inserted into the vacated bit positions. The carry-out from the shifter is the last bit shifted out, which is the sign bit of Rm if the shift amount is more than 32, or the C flag if the shift amount is zero. Syntax <Rm>, ASR <Rs> where: <Rm> Specifies the register whose value is to be shifted. ASR Indicates an arithmetic shift right. <Rs> Is the register containing the value of the shift. Architecture version All Operation if Rs[7:0] == 0 then shifter_operand = Rm shifter_carry_out = C Flag else if Rs[7:0] < 32 then shifter_operand = Rm Arithmetic_Shift_Right Rs[7:0] shifter_carry_out = Rm[Rs[7:0] - 1] else /* Rs[7:0] >= 32 */ if Rm[31] == 0 then shifter_operand = 0 shifter_carry_out = Rm[31] else /* Rm[31] == 1 */ shifter_operand = 0xFFFFFFFF shifter_carry_out = Rm[31] Notes Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE results. A5-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 231. ARM Addressing Modes 5.1.11 Data-processing operands - Rotate right by immediate 31 28 27 26 25 24 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd shift_imm 1 1 0 Rm This data-processing operand is used to provide the value of a register rotated by a constant value. This instruction operand is the value of register Rm rotated right by an immediate value in the range 1 to 31. As bits are rotated off the right end, they are inserted into the vacated bit positions on the left. The carry-out from the shifter is the last bit rotated off the right end. Syntax <Rm>, ROR #<shift_imm> where: <Rm> Specifies the register whose value is to be rotated. ROR Indicates a rotate right. <shift_imm> Specifies the rotation. This is an immediate value between 1 and 31. When shift_imm == 0, an RRX operation (rotate right with extend) is performed. This is described in Data-processing operands - Rotate right with extend on page A5-17. Architecture version All Operation if shift_imm == 0 then See “Data-processing operands - Rotate right with extend” on page A5-17 else /* shift_imm > 0 */ shifter_operand = Rm Rotate_Right shift_imm shifter_carry_out = Rm[shift_imm - 1] Notes Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current instruction plus 8. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-15
- 232. ARM Addressing Modes 5.1.12 Data-processing operands - Rotate right by register 31 28 27 26 25 24 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd Rs 0 1 1 1 Rm This data-processing operand is used to provide the value of a register rotated by a variable value. This instruction operand is produced by the value of register Rm rotated right by the value in the least significant byte of register Rs. As bits are rotated off the right end, they are inserted into the vacated bit positions on the left. The carry-out from the shifter is the last bit rotated off the right end, or the C flag if the shift amount is zero. Syntax <Rm>, ROR <Rs> where: <Rm> Specifies the register whose value is to be rotated. ROR Indicates a rotate right. <Rs> Is the register containing the value of the rotation. Architecture version All Operation if Rs[7:0] == 0 then shifter_operand = Rm shifter_carry_out = C Flag else if Rs[4:0] == 0 then shifter_operand = Rm shifter_carry_out = Rm[31] else /* Rs[4:0] > 0 */ shifter_operand = Rm Rotate_Right Rs[4:0] shifter_carry_out = Rm[Rs[4:0] - 1] Notes Use of R15 Specifying R15 as register Rd, register Rm, register Rn, or register Rs has UNPREDICTABLE results. A5-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 233. ARM Addressing Modes 5.1.13 Data-processing operands - Rotate right with extend 31 28 27 26 25 24 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 0 0 0 opcode S Rn Rd 0 0 0 0 0 1 1 0 Rm This data-processing operand can be used to perform a 33-bit rotate right using the Carry Flag as the 33rd bit. This instruction operand is the value of register Rm shifted right by one bit, with the Carry Flag replacing the vacated bit position. The carry-out from the shifter is the bit shifted off the right end. Syntax <Rm>, RRX where: <Rm> Specifies the register whose value is shifted right by one bit. RRX Indicates a rotate right with extend. Architecture version All Operation shifter_operand = (C Flag Logical_Shift_Left 31) OR (Rm Logical_Shift_Right 1) shifter_carry_out = Rm[0] Notes Encoding The instruction encoding is in the space that would be used for ROR #0. Use of R15 If R15 is specified as register Rm or Rn, the value used is the address of the current instruction plus 8. ADC instruction A rotate left with extend can be performed with an ADC instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-17
- 234. ARM Addressing Modes 5.2 Addressing Mode 2 - Load and Store Word or Unsigned Byte There are nine addressing modes used to calculate the address for a Load and Store Word or Unsigned Byte instruction. The general instruction syntax is: LDR|STR{<cond>}{B}{T} <Rd>, <addressing_mode> where <addressing_mode> is one of the nine options listed below. All nine of the following options are available for LDR, LDRB, STR and STRB. For LDRBT, LDRT, STRBT and STRBT, only the post-indexed options (the last three in the list) are available. For the PLD instruction described in PLD on page A10-14, only the offset options (the first three in the list) are available. 1. [<Rn>, #+/-<offset_12>] See Load and Store Word or Unsigned Byte - Immediate offset on page A5-20. 2. [<Rn>, +/-<Rm>] See Load and Store Word or Unsigned Byte - Register offset on page A5-21. 3. [<Rn>, +/-<Rm>, <shift> #<shift_imm>] See Load and Store Word or Unsigned Byte - Scaled register offset on page A5-22. 4. [<Rn>, #+/-<offset_12>]! See Load and Store Word or Unsigned Byte - Immediate pre-indexed on page A5-24. 5. [<Rn>, +/-<Rm>]! See Load and Store Word or Unsigned Byte - Register pre-indexed on page A5-25. 6. [<Rn>, +/-<Rm>, <shift> #<shift_imm>]! See Load and Store Word or Unsigned Byte - Scaled register pre-indexed on page A5-26. 7. [<Rn>], #+/-<offset_12> See Load and Store Word or Unsigned Byte - Immediate post-indexed on page A5-28. 8. [<Rn>], +/-<Rm> See Load and Store Word or Unsigned Byte - Register post-indexed on page A5-30. 9. [<Rn>], +/-<Rm>, <shift> #<shift_imm> See Load and Store Word or Unsigned Byte - Scaled register post-indexed on page A5-32. A5-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 235. ARM Addressing Modes 5.2.1 Encoding The following three diagrams show the encodings for this addressing mode: Immediate offset/index 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 0 P U B W L Rn Rd offset_12 Register offset/index 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 0 1 1 P U B W L Rn Rd 0 0 0 0 0 0 0 0 Rm Scaled register offset/index 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 1 1 P U B W L Rn Rd shift_imm shift 0 Rm The P bit Has two meanings: P == 0 Indicates the use of post-indexed addressing. The base register value is used for the memory address, and the offset is then applied to the base register value and written back to the base register. P == 1 Indicates the use of offset addressing or pre-indexed addressing (the W bit determines which). The memory address is generated by applying the offset to the base register value. The U bit Indicates whether the offset is added to the base (U == 1) or is subtracted from the base (U == 0). The B bit Distinguishes between an unsigned byte (B == 1) and a word (B == 0) access. The W bit Has two meanings: P == 0 If W == 0, the instruction is LDR, LDRB, STR or STRB and a normal memory access is performed. If W == 1, the instruction is LDRBT, LDRT, STRBT or STRT and an unprivileged (User mode) memory access is performed. P == 1 If W == 0, the base register is not updated (offset addressing). If W == 1, the calculated memory address is written back to the base register (pre-indexed addressing). The L bit Distinguishes between a Load (L == 1) and a Store (L == 0). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-19
- 236. ARM Addressing Modes 5.2.2 Load and Store Word or Unsigned Byte - Immediate offset 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 0 1 U B 0 L Rn Rd offset_12 This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or from the value of the base register Rn. Syntax [<Rn>, #+/-<offset_12>] where: <Rn> Specifies the register containing the base address. <offset_12> Specifies the immediate offset used with the value of Rn to form the address. Architecture version All Operation if U == 1 then address = Rn + offset_12 else /* U == 0 */ address = Rn - offset_12 Usage This addressing mode is useful for accessing structure (record) fields, and accessing parameters and local variables in a stack frame. With an offset of zero, the address produced is the unaltered value of the base register Rn. Notes Offset of zero The syntax [<Rn>] is treated as an abbreviation for [<Rn>, #0], unless the instruction is one that only allows post-indexed addressing modes (LDRBT, LDRT, STRBT or STRT). The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8. A5-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 237. ARM Addressing Modes 5.2.3 Load and Store Word or Unsigned Byte - Register offset 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 0 1 1 1 U B 0 L Rn Rd 0 0 0 0 0 0 0 0 Rm This addressing mode calculates an address by adding or subtracting the value of the index register Rm to or from the value of the base register Rn. Syntax [<Rn>, +/-<Rm>] where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the value to add to or subtract from Rn. Architecture version All Operation if U == 1 then address = Rn + Rm else /* U == 0 */ address = Rn - Rm Usage This addressing mode is used for pointer plus offset arithmetic, and accessing a single element of an array of bytes. Notes Encoding This addressing mode is encoded as an LSL scaled register offset, scaled by zero. The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8. Specifying R15 as register Rm has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-21
- 238. ARM Addressing Modes 5.2.4 Load and Store Word or Unsigned Byte - Scaled register offset 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 1 1 1 U B 0 L Rn Rd shift_imm shift 0 Rm These five addressing modes calculate an address by adding or subtracting the shifted or rotated value of the index register Rm to or from the value of the base register Rn. Syntax One of: [<Rn>, +/-<Rm>, LSL #<shift_imm>] [<Rn>, +/-<Rm>, LSR #<shift_imm>] [<Rn>, +/-<Rm>, ASR #<shift_imm>] [<Rn>, +/-<Rm>, ROR #<shift_imm>] [<Rn>, +/-<Rm>, RRX] where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. LSL Specifies a logical shift left. LSR Specifies a logical shift right. ASR Specifies an arithmetic shift right. ROR Specifies a rotate right. RRX Specifies a rotate right with extend. <shift_imm> Specifies the shift or rotation. LSL 0 to 31, encoded directly in the shift_imm field. LSR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift amounts are encoded directly. ASR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift amounts are encoded directly. ROR 1 to 31, encoded directly in the shift_imm field. (The shift_imm == 0 encoding is used to specify the RRX option.) Architecture version All A5-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 239. ARM Addressing Modes Operation case shift of 0b00 /* LSL */ index = Rm Logical_Shift_Left shift_imm 0b01 /* LSR */ if shift_imm == 0 then /* LSR #32 */ index = 0 else index = Rm Logical_Shift_Right shift_imm 0b10 /* ASR */ if shift_imm == 0 then /* ASR #32 */ if Rm[31] == 1 then index = 0xFFFFFFFF else index = 0 else index = Rm Arithmetic_Shift_Right shift_imm 0b11 /* ROR or RRX */ if shift_imm == 0 then /* RRX */ index = (C Flag Logical_Shift_Left 31) OR (Rm Logical_Shift_Right 1) else /* ROR */ index = Rm Rotate_Right shift_imm endcase if U == 1 then address = Rn + index else /* U == 0 */ address = Rn - index Usage These addressing modes are used for accessing a single element of an array of values larger than a byte. Notes The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8. Specifying R15 as register Rm has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-23
- 240. ARM Addressing Modes 5.2.5 Load and Store Word or Unsigned Byte - Immediate pre-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 0 1 U B 1 L Rn Rd offset_12 This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or from the value of the base register Rn. If the condition specified in the instruction matches the condition code status, the calculated address is written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>, #+/-<offset_12>]! where: <Rn> Specifies the register containing the base address. <offset_12> Specifies the immediate offset used with the value of Rn to form the address. ! Sets the W bit, causing base register update. Architecture version All Operation if U == 1 then address = Rn + offset_12 else /* if U == 0 */ address = Rn - offset_12 if ConditionPassed(cond) then Rn = address Usage This addressing mode is used for pointer access to arrays with automatic update of the pointer value. Notes Offset of zero The syntax [<Rn>] must never be treated as an abbreviation for [<Rn>, #0]!. The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results. A5-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 241. ARM Addressing Modes 5.2.6 Load and Store Word or Unsigned Byte - Register pre-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 0 1 1 1 U B 1 L Rn Rd 0 0 0 0 0 0 0 0 Rm This addressing mode calculates an address by adding or subtracting the value of an index register Rm to or from the value of the base register Rn. If the condition specified in the instruction matches the condition code status, the calculated address is written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>, +/-<Rm>]! where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. ! Sets the W bit, causing base register update. Architecture version All Operation if U == 1 then address = Rn + Rm else /* U == 0 */ address = Rn - Rm if ConditionPassed(cond) then Rn = address Notes Encoding This addressing mode is encoded as an LSL scaled register offset, scaled by zero. The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results. Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-25
- 242. ARM Addressing Modes 5.2.7 Load and Store Word or Unsigned Byte - Scaled register pre-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 1 1 1 U B 1 L Rn Rd shift_imm shift 0 Rm These five addressing modes calculate an address by adding or subtracting the shifted or rotated value of the index register Rm to or from the value of the base register Rn. If the condition specified in the instruction matches the condition code status, the calculated address is written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax One of: [<Rn>, +/-<Rm>, LSL #<shift_imm>]! [<Rn>, +/-<Rm>, LSR #<shift_imm>]! [<Rn>, +/-<Rm>, ASR #<shift_imm>]! [<Rn>, +/-<Rm>, ROR #<shift_imm>]! [<Rn>, +/-<Rm>, RRX]! where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. LSL Specifies a logical shift left. LSR Specifies a logical shift right. ASR Specifies an arithmetic shift right. ROR Specifies a rotate right. RRX Specifies a rotate right with extend. <shift_imm> Specifies the shift or rotation. LSL 0 to 31, encoded directly in the shift_imm field. LSR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift amounts are encoded directly. ASR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift amounts are encoded directly. ROR 1 to 31, encoded directly in the shift_imm field. (The shift_imm == 0 encoding is used to specify the RRX option.) ! Sets the W bit, causing base register update. A5-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 243. ARM Addressing Modes Architecture version All Operation case shift of 0b00 /* LSL */ index = Rm Logical_Shift_Left shift_imm 0b01 /* LSR */ if shift_imm == 0 then /* LSR #32 */ index = 0 else index = Rm Logical_Shift_Right shift_imm 0b10 /* ASR */ if shift_imm == 0 then /* ASR #32 */ if Rm[31] == 1 then index = 0xFFFFFFFF else index = 0 else index = Rm Arithmetic_Shift_Right shift_imm 0b11 /* ROR or RRX */ if shift_imm == 0 then /* RRX */ index = (C Flag Logical_Shift_Left 31) OR (Rm Logical_Shift_Right 1) else /* ROR */ index = Rm Rotate_Right shift_imm endcase if U == 1 then address = Rn + index else /* U == 0 */ address = Rn - index if ConditionPassed(cond) then Rn = address Notes The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results. Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-27
- 244. ARM Addressing Modes 5.2.8 Load and Store Word or Unsigned Byte - Immediate post-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 1 0 0 U B 0 L Rn Rd offset_12 This addressing mode uses the value of the base register Rn as the address for the memory access. If the condition specified in the instruction matches the condition code status, the value of the immediate offset is added to or subtracted from the value of the base register Rn and written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>], #+/-<offset_12> where: <Rn> Specifies the register containing the base address. <offset_12> Specifies the immediate offset used with the value of Rn to form the address. Architecture version All Operation address = Rn if ConditionPassed(cond) then if U == 1 then Rn = Rn + offset_12 else /* U == 0 */ Rn = Rn - offset_12 Usage This addressing mode is used for pointer access to arrays with automatic update of the pointer value. A5-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 245. ARM Addressing Modes Notes Post-indexed addressing modes LDRBT, LDRT, STRBT, and STRT only support post-indexed addressing modes. They use a minor modification of the above bit pattern, where bit[21] (the W bit) is 1, not 0 as shown. Offset of zero The syntax [<Rn>] is treated as an abbreviation for [<Rn>],#0 for instructions that only support post-indexed addressing modes (LDRBT, LDRT, STRBT, STRT), but not for other instructions. The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-29
- 246. ARM Addressing Modes 5.2.9 Load and Store Word or Unsigned Byte - Register post-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 0 1 1 0 U B 0 L Rn Rd 0 0 0 0 0 0 0 0 Rm This addressing mode uses the value of the base register Rn as the address for the memory access. If the condition specified in the instruction matches the condition code status, the value of the index register Rm is added to or subtracted from the value of the base register Rn and written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>], +/-<Rm> where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. Architecture version All Operation address = Rn if ConditionPassed(cond) then if U == 1 then Rn = Rn + Rm else /* U == 0 */ Rn = Rn - Rm A5-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 247. ARM Addressing Modes Notes Encoding This addressing mode is encoded as an LSL scaled register offset, scaled by zero. Post-indexed addressing modes LDRBT, LDRT, STRBT, and STRT only support post-indexed addressing modes. They use a minor modification of the above bit pattern, where bit[21] (the W bit) is 1, not 0 as shown. The B bit This bit distinguishes between an unsigned byte (B==1) and a word (B==0) access. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. Use of R15 Specifying R15 as register Rn or Rm has UNPREDICTABLE results. Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-31
- 248. ARM Addressing Modes 5.2.10 Load and Store Word or Unsigned Byte - Scaled register post-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0 cond 0 1 1 0 U B 0 L Rn Rd shift_imm shift 0 Rm This addressing mode uses the value of the base register Rn as the address for the memory access. If the condition specified in the instruction matches the condition code status, the shifted or rotated value of index register Rm is added to or subtracted from the value of the base register Rn and written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax One of: [<Rn>], +/-<Rm>, LSL #<shift_imm> [<Rn>], +/-<Rm>, LSR #<shift_imm> [<Rn>], +/-<Rm>, ASR #<shift_imm> [<Rn>], +/-<Rm>, ROR #<shift_imm> [<Rn>], +/-<Rm>, RRX where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. LSL Specifies a logical shift left. LSR Specifies a logical shift right. ASR Specifies an arithmetic shift right. ROR Specifies a rotate right. RRX Specifies a rotate right with extend. <shift_imm> Specifies the shift or rotation. LSL 0 to 31, encoded directly in the shift_imm field. LSR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift amounts are encoded directly. ASR 1 to 32. A shift amount of 32 is encoded as shift_imm == 0. Other shift amounts are encoded directly. ROR 1 to 31, encoded directly in the shift_imm field. (The shift_imm == 0 encoding is used to specify the RRX option.) A5-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 249. ARM Addressing Modes Architecture version All Operation address = Rn case shift of 0b00 /* LSL */ index = Rm Logical_Shift_Left shift_imm 0b01 /* LSR */ if shift_imm == 0 then /* LSR #32 */ index = 0 else index = Rm Logical_Shift_Right shift_imm 0b10 /* ASR */ if shift_imm == 0 then /* ASR #32 */ if Rm[31] == 1 then index = 0xFFFFFFFF else index = 0 else index = Rm Arithmetic_Shift_Right shift_imm 0b11 /* ROR or RRX */ if shift_imm == 0 then /* RRX */ index = (C Flag Logical_Shift_Left 31) OR (Rm Logical_Shift_Right 1) else /* ROR */ index = Rm Rotate_Right shift_imm endcase if ConditionPassed(cond) then if U == 1 then Rn = Rn + index else /* U == 0 */ Rn = Rn - index Notes Post-indexed addressing modes LDRBT, LDRT, STRBT, and STRT only support post-indexed addressing modes. They use a minor modification of the above bit pattern, where bit[21] (the W bit) is 1, not 0 as shown. The B bit This bit distinguishes between an unsigned byte (B == 1) and a word (B == 0) access. The L bit This bit distinguishes between a Load (L == 1) and a Store (L == 0) instruction. Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results. Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-33
- 250. ARM Addressing Modes 5.3 Addressing Mode 3 - Miscellaneous Loads and Stores There are six addressing modes used to calculate the address for load and store (signed or unsigned) halfword, load signed byte, or load and store doubleword instructions. The general instruction syntax is: LDR|STR{<cond>}H|SH|SB|D <Rd>, <addressing_mode> where <addressing_mode> is one of the following six options: 1. [<Rn>, #+/-<offset_8>] See Miscellaneous Loads and Stores - Immediate offset on page A5-36. 2. [<Rn>, +/-<Rm>] See Miscellaneous Loads and Stores - Register offset on page A5-38. 3. [<Rn>, #+/-<offset_8>]! See Miscellaneous Loads and Stores - Immediate pre-indexed on page A5-40. 4. [<Rn>, +/-<Rm>]! See Miscellaneous Loads and Stores - Register pre-indexed on page A5-42. 5. [<Rn>], #+/-<offset_8> See Miscellaneous Loads and Stores - Immediate post-indexed on page A5-44. 6. [<Rn>], +/-<Rm> See Miscellaneous Loads and Stores - Register post-indexed on page A5-46. 5.3.1 Encoding The following diagrams show the encodings for this addressing mode: Immediate offset/index 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U 1 W L Rn Rd immedH 1 S H 1 ImmedL Register offset/index 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U 0 W L Rn Rd SBZ 1 S H 1 Rm A5-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 251. ARM Addressing Modes The P bit Has two meanings: P == 0 Indicates the use of post-indexed addressing. The base register value is used for the memory address, and the offset is then applied to the base register value and written back to the base register. P == 1 Indicates the use of offset addressing or pre-indexed addressing (the W bit determines which). The memory address is generated by applying the offset to the base register value. The U bit Indicates whether the offset is added to the base (U == 1) or subtracted from the base (U == 0). The W bit Has two meanings: P == 0 The W bit must be 0 or the instruction is UNPREDICTABLE. P == 1 W == 1 indicates that the memory address is written back to the base register (pre-indexed addressing), and W == 0 that the base register is unchanged (offset addressing). The L bit This bit distinguishes between a Load (L == 1) and a Store (L == 0) instruction. The S bit This bit distinguishes between a signed (S == 1) and an unsigned (S == 0) halfword access. The H bit This bit distinguishes between a halfword (H == 1) and a byte (H == 0) access. Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or SWPB instruction, or an unallocated instruction in the arithmetic or load/store instruction extension space (see Extending the instruction set on page A3-27). Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions, which use addressing mode 2 rather than addressing mode 3. Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction is an unallocated instruction in the load/store extension space (see Extending the instruction set on page A3-27). Signed bytes and halfwords can be stored with the same STRB and STRH instructions as are used for unsigned quantities, so no separate signed store instructions are provided. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-35
- 252. ARM Addressing Modes 5.3.2 Miscellaneous Loads and Stores - Immediate offset 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 U 1 0 L Rn Rd immedH 1 S H 1 immedL This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or from the value of the base register Rn. Syntax [<Rn>, #+/-<offset_8>] where: <Rn> Specifies the register containing the base address. <offset_8> Specifies the immediate offset used with the value of Rn to form the address. The offset is encoded in immedH (top 4 bits) and immedL (bottom 4 bits). Architecture version Version 4 and above Operation offset_8 = (immedH << 4) OR immedL if U == 1 then address = Rn + offset_8 else /* U == 0 */ address = Rn - offset_8 Usage This addressing mode is used for accessing structure (record) fields, and accessing parameters and locals variable in a stack frame. With an offset of zero, the address produced is the unaltered value of the base register Rn. Notes Zero offset The syntax [<Rn>] is treated as an abbreviation for [<Rn>,#0]. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword access. The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access. Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8. A5-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 253. ARM Addressing Modes Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or SWPB instruction, or an unallocated instruction in the arithmetic or load/store instruction extension space (see Extending the instruction set on page A3-27). Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions, which use addressing mode 2 rather than addressing mode 3. Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction is an unallocated instruction in the load/store extension space (see Extending the instruction set on page A3-27). Signed bytes and halfwords can be stored with the same STRB and STRH instructions as are used for unsigned quantities, so no separate signed store instructions are provided. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-37
- 254. ARM Addressing Modes 5.3.3 Miscellaneous Loads and Stores - Register offset 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 U 0 0 L Rn Rd SBZ 1 S H 1 Rm This addressing mode calculates an address by adding or subtracting the value of the index register Rm to or from the value of the base register Rn. Syntax [<Rn>, +/-<Rm>] where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. Architecture version Version 4 and above Operation if U == 1 then address = Rn + Rm else /* U == 0 */ address = Rn - Rm Usage This addressing mode is useful for pointer plus offset arithmetic and for accessing a single element of an array. Notes The L bit Distinguishes between a Load (L==1) and a Store (L==0) instruction. The S bit Distinguishes between a signed (S==1) and an unsigned (S==0) halfword access. The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access. Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or SWPB instruction, or an unallocated instruction in the arithmetic or load/store instruction extension space (see Extending the instruction set on page A3-27). Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions, which use addressing mode 2 rather than addressing mode 3. A5-38 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 255. ARM Addressing Modes Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction is an unallocated instruction in the load/store extension space (see Extending the instruction set on page A3-27). Signed bytes and halfwords can be stored with the same STRB and STRH instructions as are used for unsigned quantities, so no separate signed store instructions are provided. Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8. Specifying R15 as register Rm has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-39
- 256. ARM Addressing Modes 5.3.4 Miscellaneous Loads and Stores - Immediate pre-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 U 1 1 L Rn Rd immedH 1 S H 1 ImmedL This addressing mode calculates an address by adding or subtracting the value of an immediate offset to or from the value of the base register Rn. If the condition specified in the instruction matches the condition code status, the calculated address is written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>, #+/-<offset_8>]! where: <Rn> Specifies the register containing the base address. <offset_8> Specifies the immediate offset used with the value of Rn to form the address. The offset is encoded in immedH (top 4 bits) and immedL (bottom 4 bits). ! Sets the W bit, causing base register update. Architecture version Version 4 and above Operation offset_8 = (immedH << 4) OR immedL if U == 1 then address = Rn + offset_8 else /* U == 0 */ address = Rn - offset_8 if ConditionPassed(cond) then Rn = address Usage This addressing mode gives pointer access to arrays, with automatic update of the pointer value. A5-40 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 257. ARM Addressing Modes Notes Offset of zero The syntax [<Rn>] must not be treated as an abbreviation for [<Rn>,#0]!. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword access. The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access. Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or SWPB instruction, or an unallocated instruction in the arithmetic or load/store instruction extension space (see Extending the instruction set on page A3-27). Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions, which use addressing mode 2 rather than addressing mode 3. Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction is an unallocated instruction in the load/store extension space (see Extending the instruction set on page A3-27). Signed bytes and halfwords can be stored with the same STRB and STRH instructions as are used for unsigned quantities, so no separate signed store instructions are provided. Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-41
- 258. ARM Addressing Modes 5.3.5 Miscellaneous Loads and Stores - Register pre-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 U 0 1 L Rn Rd SBZ 1 S H 1 Rm This addressing mode calculates an address by adding or subtracting the value of the index register Rm to or from the value of the base register Rn. If the condition specified in the instruction matches the condition code status, the calculated address is written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>, +/-<Rm>]! where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. ! Sets the W bit, causing base register update. Architecture version Version 4 and above Operation if U == 1 then address = Rn + Rm else /* U == 0 */ address = Rn - Rm if ConditionPassed(cond) then Rn = address A5-42 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 259. ARM Addressing Modes Notes The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword access. The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access. Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or SWPB instruction, or an unallocated instruction in the arithmetic or load/store instruction extension space (see Extending the instruction set on page A3-27). Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions, which use addressing mode 2 rather than addressing mode 3. Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction is an unallocated instruction in the load/store extension space (see Extending the instruction set on page A3-27). Signed bytes and halfwords can be stored with the same STRB and STRH instructions as are used for unsigned quantities, so no separate signed store instructions are provided. Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results. Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-43
- 260. ARM Addressing Modes 5.3.6 Miscellaneous Loads and Stores - Immediate post-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 U 1 0 L Rn Rd immedH 1 S H 1 ImmedL This addressing mode uses the value of the base register Rn as the address for the memory access. If the condition specified in the instruction matches the condition code status, the value of the immediate offset is added to or subtracted from the value of the base register Rn and written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>], #+/-<offset_8> where: <Rn> Specifies the register containing the base address. <offset_8> Specifies the immediate offset used with the value of Rn to form the address. The offset is encoded in immedH (top 4 bits) and immedL (bottom 4 bits). Architecture version Version 4 and above Operation address = Rn offset_8 = (immedH << 4) OR immedL if ConditionPassed(cond) then if U == 1 then Rn = Rn + offset_8 else /* U == 0 */ Rn = Rn - offset_8 Usage This addressing mode gives pointer access to arrays, with automatic update of the pointer value. A5-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 261. ARM Addressing Modes Notes Offset of zero The syntax [<Rn>] must not be treated as an abbreviation for [<Rn>],#0. The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword access. The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access. Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or SWPB instruction, or an unallocated instruction in the arithmetic or load/store instruction extension space (see Extending the instruction set on page A3-27). Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions, which use addressing mode 2 rather than addressing mode 3. Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction is an unallocated instruction in the load/store extension space (see Extending the instruction set on page A3-27). Signed bytes and halfwords can be stored with the same STRB and STRH instructions as are used for unsigned quantities, so no separate signed store instructions are provided. Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-45
- 262. ARM Addressing Modes 5.3.7 Miscellaneous Loads and Stores - Register post-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 0 U 0 0 L Rn Rd SBZ 1 S H 1 Rm This addressing mode uses the value of the base register Rn as the address for the memory access. If the condition specified in the instruction matches the condition code status, the value of the index register Rm is added to or subtracted from the value of the base register Rn and written back to the base register Rn. The conditions are defined in The condition field on page A3-5. Syntax [<Rn>], +/-<Rm> where: <Rn> Specifies the register containing the base address. <Rm> Specifies the register containing the offset to add to or subtract from Rn. Architecture version Version 4 and above Operation address = Rn if ConditionPassed(cond) then if U == 1 then Rn = Rn + Rm else /* U == 0 */ Rn = Rn - Rm A5-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 263. ARM Addressing Modes Notes The L bit This bit distinguishes between a Load (L==1) and a Store (L==0) instruction. The S bit This bit distinguishes between a signed (S==1) and an unsigned (S==0) halfword access. The H bit This bit distinguishes between a halfword (H==1) and a signed byte (H==0) access. Unsigned bytes If S == 0 and H == 0, apparently indicating an unsigned byte, the instruction is not one that uses this addressing mode. Instead, it is a multiply instruction, a SWP or SWPB instruction, or an unallocated instruction in the arithmetic or load/store instruction extension space (see Extending the instruction set on page A3-27). Unsigned bytes are accessed by the LDRB, LDRBT, STRB and STRBT instructions, which use addressing mode 2 rather than addressing mode 3. Signed stores If S == 1 and L == 0, apparently indicating a signed store instruction, the instruction is an unallocated instruction in the load/store extension space (see Extending the instruction set on page A3-27). Signed bytes and halfwords can be stored with the same STRB and STRH instructions as are used for unsigned quantities, so no separate signed store instructions are provided. Use of R15 Specifying R15 as register Rm or Rn has UNPREDICTABLE results. Operand restriction If the same register is specified for Rn and Rm, the result is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-47
- 264. ARM Addressing Modes 5.4 Addressing Mode 4 - Load and Store Multiple Load Multiple instructions load a subset (possibly all) of the general-purpose registers from memory. Store Multiple instructions store a subset (possibly all) of the general purpose registers to memory. Load and Store Multiple addressing modes produce a sequential range of addresses. The lowest-numbered register is stored at the lowest memory address and the highest-numbered register at the highest memory address. The general instruction syntax is: LDM|STM{<cond>}<addressing_mode> <Rn>{!}, <registers>{^} where <addressing_mode> is one of the following four addressing modes: 1. IA (Increment After) See Load and Store Multiple - Increment after on page A5-50. 2. IB (Increment Before) See Load and Store Multiple - Increment before on page A5-51. 3. DA (Decrement After) See Load and Store Multiple - Decrement after on page A5-52. 4. DB (Decrement Before) See Load and Store Multiple - Decrement before on page A5-53. 5.4.1 Encoding The following diagram shows the encoding for this addressing mode: 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 P U S W L Rn register list The P bit Has two meanings: P==1 indicates that the word addressed by Rn is included in the range of memory locations accessed, lying at the top (U==0) or bottom (U==1) of that range. P==0 indicates that the word addressed by Rn is excluded from the range of memory locations accessed, and lies one word beyond the top of the range (U==0) or one word below the bottom of the range (U==1). The U bit Indicates that the transfer is made upwards (U==1) or downwards (U==0) from the base register. A5-48 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 265. ARM Addressing Modes The S bit For LDMs that load the PC, the S bit indicates that the CPSR is loaded from the SPSR. For LDMs that do not load the PC and all STMs, the S bit indicates that when the processor is in a privileged mode, the User mode banked registers are transferred instead of the registers of the current mode. The W bit Indicates that the base register is updated after the transfer. The base register is incremented (U==1) or decremented (U==0) by four times the number of registers in the register list. The L bit Distinguishes between Load (L==1) and Store (L==0) instructions. Register list The register_list field of the instruction has one bit for each general-purpose register: bit[0] for register zero through to bit[15] for register 15 (the PC). If no bits are set, the result is UNPREDICTABLE. The instruction syntax specifies the registers to load or store in <registers>, which is a comma-separated list of registers, surrounded by { and }. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-49
- 266. ARM Addressing Modes 5.4.2 Load and Store Multiple - Increment after 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 0 1 S W L Rn register list This addressing mode is for Load and Store Multiple instructions, and forms a range of addresses. The first address formed is the <start_address>, and is the value of the base register Rn. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The last address produced is the <end_address>. Its value is four less than the sum of the value of the base register and four times the number of registers specified in <registers>. If the condition specified in the instruction matches the condition code status and the W bit is set, Rn is incremented by four times the number of registers in <registers>. The conditions are defined in The condition field on page A3-5. Syntax IA See also the alternative syntax described in Load and Store Multiple addressing modes (alternative names) on page A5-54. Architecture version All Operation start_address = Rn end_address = Rn + (Number_Of_Set_Bits_In(register_list) * 4) - 4 if ConditionPassed(cond) and W == 1 then Rn = Rn + (Number_Of_Set_Bits_In(register_list) * 4) Notes The L bit This bit distinguishes between a Load Multiple and a Store Multiple. The S bit For LDMs that load the PC, the S bit indicates that the CPSR is loaded from the SPSR. For LDMs that do not load the PC and all STMs, the S bit indicates that when the processor is in a privileged mode, the User mode banked registers are transferred instead of the registers of the current mode. A5-50 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 267. ARM Addressing Modes 5.4.3 Load and Store Multiple - Increment before 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 1 1 S W L Rn register list This addressing mode is for Load and Store Multiple instructions, and forms a range of addresses. The first address formed is the <start_address>, and is the value of the base register Rn plus four. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The last address produced is the <end_address>. Its value is the sum of the value of the base register and four times the number of registers specified in <registers>. If the condition specified in the instruction matches the condition code status and the W bit is set, Rn is incremented by four times the number of registers in <registers>. The conditions are defined in The condition field on page A3-5. Syntax IB See also the alternative syntax described in Load and Store Multiple addressing modes (alternative names) on page A5-54. Architecture version All Operation start_address = Rn + 4 end_address = Rn + (Number_Of_Set_Bits_In(register_list) * 4) if ConditionPassed(cond) and W == 1 then Rn = Rn + (Number_Of_Set_Bits_In(register_list) * 4) Notes The L bit This bit distinguishes between a Load Multiple and a Store Multiple. The S bit For LDMs that load the PC, the S bit indicates that the CPSR is loaded from the SPSR. For LDMs that do not load the PC and all STMs, the S bit indicates that when the processor is in a privileged mode, the User mode banked registers are transferred instead of the registers of the current mode. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-51
- 268. ARM Addressing Modes 5.4.4 Load and Store Multiple - Decrement after 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 0 0 S W L Rn register list This addressing mode is for Load and Store Multiple instructions, and forms a range of addresses. The first address formed is the <start_address>, and is the value of the base register minus four times the number of registers specified in <registers>, plus 4. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The last address produced is the <end_address>. Its value is the value of the base register Rn. If the condition specified in the instruction matches the condition code status and the W bit is set, Rn is decremented by four times the number of registers in <registers>. The conditions are defined in The condition field on page A3-5. Syntax DA See also the alternative syntax described in Load and Store Multiple addressing modes (alternative names) on page A5-54. Architecture version All Operation start_address = Rn - (Number_Of_Set_Bits_In(register_list) * 4) + 4 end_address = Rn if ConditionPassed(cond) and W == 1 then Rn = Rn - (Number_Of_Set_Bits_In(register_list) * 4) Notes The L bit This bit distinguishes between a Load Multiple and a Store Multiple. The S bit For LDMs that load the PC, the S bit indicates that the CPSR is loaded from the SPSR. For LDMs that do not load the PC and all STMs, the S bit indicates that when the processor is in a privileged mode, the User mode banked registers are transferred instead of the registers of the current mode. A5-52 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 269. ARM Addressing Modes 5.4.5 Load and Store Multiple - Decrement before 31 28 27 26 25 24 23 22 21 20 19 16 15 0 cond 1 0 0 1 0 S W L Rn register list This addressing mode is for Load and Store multiple instructions, and forms a range of addresses. The first address formed is the <start_address>, and is the value of the base register minus four times the number of registers specified in <registers>. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The last address produced is the <end_address>. Its value is the value of the base register Rn minus four. If the condition specified in the instruction matches the condition code status and the W bit is set, Rn is decremented by four times the number of registers in <registers>. The conditions are defined in The condition field on page A3-5. Syntax DB See also the alternative syntax described in Load and Store Multiple addressing modes (alternative names) on page A5-54. Architecture version All Operation start_address = Rn - (Number_Of_Set_Bits_In(register_list) * 4) end_address = Rn - 4 if ConditionPassed(cond) and W == 1 then Rn = Rn - (Number_Of_Set_Bits_In(register_list) * 4) Notes The L bit This bit distinguishes between a Load Multiple and a Store Multiple. The S bit For LDMs that load the PC, the S bit indicates that the CPSR is loaded from the SPSR. For LDMs that do not load the PC and all STMs, the S bit indicates that when the processor is in a privileged mode, the User mode banked registers are transferred instead of the registers of the current mode. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-53
- 270. ARM Addressing Modes 5.4.6 Load and Store Multiple addressing modes (alternative names) The four addressing mode names given in Addressing Mode 4 - Load and Store Multiple on page A5-48 (IA, IB, DA, DB) are most useful when a load and Store Multiple instruction is being used for block data transfer, as it is likely that the Load Multiple and Store Multiple have the same addressing mode, so that the data is stored in the same way that it was loaded. However, if Load Multiple and Store Multiple are being used to access a stack, the data is not loaded with the same addressing mode that was used to store the data, because the load (pop) and store (push) operations must adjust the stack in opposite directions. Stack operations Load Multiple and Store Multiple addressing modes can be specified with an alternative syntax, which is more applicable to stack operations: Full stacks Have stack pointers that point to the last used (full) location. Empty stacks Have stack pointers that point to the first unused (empty) location. Descending stacks Grow towards decreasing memory addresses (towards the bottom of memory). Ascending stacks Grow towards increasing memory addresses (towards the top of memory). Two attributes allow four types of stack to be defined: • Full Descending, with the syntax FD • Empty Descending, with the syntax ED • Full Ascending, with the syntax FA • Empty Ascending, with the syntax EA. Note When defining stacks on which coprocessor data is to be placed (or might be placed in the future), programmers are advised to use the FD or EA stack types. This is because coprocessor data can be pushed to these types of stack with a single STC instruction and popped from them with a single LDC instruction. Multi-instruction sequences are required for coprocessor access to FA or ED stacks. Table 5-1 and Table 5-2 on page A5-55 show the relationship between the four types of stack, the four types of addressing mode shown above, and the L, U, and P bits in the instruction format. A5-54 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 271. ARM Addressing Modes Table 5-1 shows the relationship for LDM instructions. Non-stack addressing mode Stack addressing mode L bit P bit U bit LDMDA (Decrement After) LDMFA (Full Ascending) 1 0 0 LDMIA (Increment After) LDMFD (Full Descending) 1 0 1 LDMDB (Decrement Before) LDMEA (Empty Ascending) 1 1 0 LDMIB (Increment Before) LDMED (Empty Descending) 1 1 1 Table 5-2 shows the relationship for STM instructions. Table 5-1 LDM addressing modes Table 5-2 STM addressing modes Non-stack addressing mode Stack addressing mode L bit P bit U bit STMDA (Decrement After) STMED (Empty Descending) 0 0 0 STMIA (Increment After) STMEA (Empty Ascending) 0 0 1 STMDB (Decrement Before) STMFD (Full Descending) 0 1 0 STMIB (Increment Before) STMFA (Full Ascending) 0 1 1 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-55
- 272. ARM Addressing Modes 5.5 Addressing Mode 5 - Load and Store Coprocessor There are four addressing modes which are used to calculate the address of a Load or Store Coprocessor instruction. The general instruction syntax is: <opcode>{<cond>}{L} <coproc>,<CRd>,<addressing_mode> where <addressing_mode> is one of the following four options: 1. [<Rn>,#+/-<offset_8>*4] See Load and Store Coprocessor - Immediate offset on page A5-58. 2. [<Rn>,#+/-<offset_8>*4]! See Load and Store Coprocessor - Immediate pre-indexed on page A5-60. 3. [<Rn>],#+/-<offset_8>*4 See Load and Store Coprocessor - Immediate post-indexed on page A5-62. 4. [<Rn>],<option> See Load and Store Coprocessor - Unindexed on page A5-64. 5.5.1 Encoding The following diagram shows the encoding for this addressing mode: 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 P U N W L Rn CRd cp# offset_8 The P bit Has two meanings: P == 1 Indicates the use of post-indexed addressing or unindexed addressing (the W bit determines which). The base register value is used for the memory address. P == 0 Indicates the use of offset addressing or pre-indexed addressing (the W bit determines which). The memory address is generated by applying the offset to the base register value. The U bit Has two meanings: U == 1 Indicates that the offset is added to the base. U == 0 Indicates that he offset is subtracted from the base The N bit The meaning of this bit is coprocessor-dependent. Its recommended use is to distinguish between different-sized values to be transferred. The W bit Has two meanings: W == 1 Indicates that the memory address is written back to the base register. W == 0 Indicates that the base register value is unchanged. A5-56 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 273. ARM Addressing Modes Also: • If P == 0, this distinguishes unindexed addressing (W == 0) from post-indexed addressing (W == 1). For unindexed addressing, U must equal 1 or the result is either UNDEFINED or UNPREDICTABLE (see Coprocessor instruction extension space on page A3-33). • If P == 1, this distinguishes offset addressing (W == 0) from pre-indexed addressing (W == 1). The L bit Distinguishes between Load (L == 1) and Store (L == 0) instructions. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-57
- 274. ARM Addressing Modes 5.5.2 Load and Store Coprocessor - Immediate offset 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 1 U N 0 L Rn CRd cp_num offset_8 This addressing mode produces a sequence of consecutive addresses. The first address is calculated by adding or subtracting four times the value of an immediate offset to or from the value of the base register Rn. The subsequent addresses in the sequence are produced by incrementing the previous address by four until the coprocessor signals the end of the instruction. This allows a coprocessor to access data whose size is coprocessor-defined. The coprocessor must not request a transfer of more than 16 words. Syntax [<Rn>, #+/-<offset_8>*4] where: <Rn> Specifies the register containing the base address. <offset_8> Specifies the immediate offset that is multiplied by 4, then added to or subtracted from the value of Rn to form the address. Architecture version Version 2 and above Operation if ConditionPassed(cond) then if U == 1 then address = Rn + offset_8 * 4 else /* U == 0 */ address = Rn - offset_8 * 4 start_address = address while (NotFinished(coprocessor[cp_num])) address = address + 4 end_address = address A5-58 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 275. ARM Addressing Modes Notes The N bit Is coprocessor-dependent. The L bit Distinguishes between Load (L==1) and Store (L==0) instructions. Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-59
- 276. ARM Addressing Modes 5.5.3 Load and Store Coprocessor - Immediate pre-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 1 U N 1 L Rn CRd cp_num offset_8 This addressing mode produces a sequence of consecutive addresses. The first address is calculated by adding or subtracting four times the value of an immediate offset to or from the value of the base register Rn. If the condition specified in the instruction matches the condition code status, the first address is written back to the base register Rn. The subsequent addresses in the sequence are produced by incrementing the previous address by four until the coprocessor signals the end of the instruction. This allows a coprocessor to access data whose size is coprocessor-defined. The coprocessor must not request a transfer of more than 16 words. Syntax [<Rn>, #+/-<offset_8>*4]! where: <Rn> Specifies the register containing the base address. <offset_8> Specifies the immediate offset that is multiplied by 4, then added to or subtracted from the value of Rn to form the address. ! Sets the W bit, causing base register update. Architecture version Version 2 and above Operation if ConditionPassed(cond) then if U == 1 then Rn = Rn + offset_8 * 4 else /* U == 0 */ Rn = Rn - offset_8 * 4 start_address = Rn address = start_address while (NotFinished(coprocessor[cp_num])) address = address + 4 end_address = address A5-60 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 277. ARM Addressing Modes Notes The N bit Is coprocessor-dependent. The L bit Distinguishes between Load (L==1) and Store (L==0) instructions. Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-61
- 278. ARM Addressing Modes 5.5.4 Load and Store Coprocessor - Immediate post-indexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 0 U N 1 L Rn CRd cp_num offset_8 This addressing mode produces a sequence of consecutive addresses. The first address is the value of the base register Rn. The subsequent addresses in the sequence are produced by incrementing the previous address by four until the coprocessor signals the end of the instruction. This allows a coprocessor to access data whose size is coprocessor-defined. If the condition specified in the instruction matches the condition code status, the base register Rn is updated by adding or subtracting four times the value of an immediate offset to or from the value of the base register Rn. The coprocessor must not request a transfer of more than 16 words. Syntax [<Rn>], #+/-<offset_8>*4 where: <Rn> Specifies the register containing the base address. <offset_8> Specifies the immediate offset that is multiplied by 4, then added to or subtracted from the value of Rn to form the address. Architecture version Version 2 and above Operation if ConditionPassed(cond) then start_address = Rn if U == 1 then Rn = Rn + offset_8 * 4 else /* U == 0 */ Rn = Rn - offset_8 * 4 address = start_address while (NotFinished(coprocessor[cp_num])) address = address + 4 end_address = address A5-62 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 279. ARM Addressing Modes Notes The N bit Is coprocessor-dependent. The L bit Distinguishes between Load (L==1) and Store (L==0) instructions. Use of R15 Specifying R15 as register Rn has UNPREDICTABLE results. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-63
- 280. ARM Addressing Modes 5.5.5 Load and Store Coprocessor - Unindexed 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 0 cond 1 1 0 0 U N 0 L Rn CRd cp_num option This addressing mode produces a sequence of consecutive addresses. The first address is the value of the base register Rn. The subsequent addresses in the sequence are produced by incrementing the previous address by four until the coprocessor signals the end of the instruction. This allows a coprocessor to access data whose size is coprocessor-defined. The base register Rn is not updated. Bits[7:0] of the instruction are therefore not used by the ARM, either for the address calculation or to calculate a new value for the base register, and so can be used to specify additional instruction options to the coprocessor. The coprocessor must not request a transfer of more than 16 words. Syntax [<Rn>], <option> where: <Rn> Specifies the register containing the base address. <option> Specifies additional instruction options to the coprocessor. The <option> is specified in the instruction syntax as an integer in the range 0-255, surrounded by { and }. Architecture version Version 2 and above Operation if ConditionPassed(cond) then start_address = Rn address = start_address while (NotFinished(coprocessor[cp_num])) address = address + 4 end_address = address A5-64 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 281. ARM Addressing Modes Notes The N bit Is coprocessor-dependent. The L bit Distinguishes between Load (L==1) and Store (L==0) instructions. Use of R15 If R15 is specified as register Rn, the value used is the address of the instruction plus 8. The U bit If bit[23] (the Up/down bit) is not set, the result is either UNDEFINED or UNPREDICTABLE (see Coprocessor instruction extension space on page A3-33). Option bits Are unused by the ARM in this addressing mode, and therefore can be used to request additional instruction options in a coprocessor-dependent fashion. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A5-65
- 282. ARM Addressing Modes A5-66 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 283. Chapter A6 The Thumb Instruction Set This chapter introduces the Thumb instruction set and describes how Thumb uses the ARM programmer’s model. It contains the following sections: • About the Thumb instruction set on page A6-2 • Instruction set encoding on page A6-4 • Branch instructions on page A6-6 • Data-processing instructions on page A6-8 • Load and Store Register instructions on page A6-15 • Load and Store Multiple instructions on page A6-18 • Exception-generating instructions on page A6-20 • Undefined instruction space on page A6-21. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-1
- 284. The Thumb Instruction Set 6.1 About the Thumb instruction set The Thumb instruction set is a re-encoded subset of the ARM instruction set. Thumb is designed to increase the performance of ARM implementations that use a 16-bit or narrower memory data bus and to allow better code density than ARM. T variants of the ARM architecture incorporate both a full 32-bit ARM instruction set and the 16-bit Thumb instruction set. Every Thumb instruction is encoded in 16 bits. Thumb does not alter the underlying programmer’s model of the ARM architecture. It merely presents restricted access to it. All Thumb data-processing instructions operate on full 32-bit values, and full 32-bit addresses are produced by both data-access instructions and instruction fetches. When the processor is executing Thumb instructions, eight general-purpose integer registers are available, R0 to R7, which are the same physical registers as R0 to R7 when executing ARM instructions. Some Thumb instructions also access the Program Counter (ARM Register 15), the Link Register (ARM Register 14) and the Stack Pointer (ARM Register 13). Further instructions allow limited access to ARM registers 8 to 15, which are know as the high registers. When R15 is read, bit[0] is zero and bits[31:1] contain the PC. When R15 is written, bit[0] is IGNORED and bits[31:1] are written to the PC. Depending on how it is used, the value of the PC is either the address of the instruction plus 4 or is UNPREDICTABLE. Thumb does not provide direct access to the CPSR or any SPSR (as the MSR and MRS instructions do in the ARM instruction set)). Thumb execution is flagged by the T bit (bit[5]) in the CPSR: T == 0 32-bit instructions are fetched (and the PC is incremented by four) and are executed as ARM instructions. T == 1 16-bit instructions are fetched (and the PC is incremented by two) and are executed as Thumb instructions. Note The Thumb instruction set is only compatible with the 32-bit ARM architectures. Thumb is not recommended for use with 26-bit architectures or with 26-bit compatibility options on 32-bit architectures. 6.1.1 Entering Thumb state Thumb execution is normally entered by executing an ARM BX instruction (Branch and Exchange). This instruction branches to the address held in a general-purpose register, and if bit[0] of that register is 1, Thumb execution begins at the branch target address. If bit[0] of the target register is 0, ARM execution continues from the branch target address. On architecture versions 5 and above, BLX instructions and LDR/LDM instructions that load the PC can be used similarly. Thumb execution can also be initiated by setting the T bit in the SPSR and executing an ARM instruction which restores the CPSR from the SPSR (a data-processing instruction with the S bit set and the PC as the destination, or a Load Multiple with Restore CPSR instruction). This allows an operating system to automatically restart a process independent of whether that process is executing Thumb code or ARM code. The result is UNPREDICTABLE if the T bit is altered directly by writing the CPSR. A6-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 285. The Thumb Instruction Set 6.1.2 Exceptions Exceptions generated during Thumb execution switch to ARM execution before executing the exception handler (whose first instruction is at the hardware vector). The state of the T bit is preserved in the SPSR, and the LR of the exception mode is set so that the normal return instruction performs correctly, regardless of whether the exception occurred during ARM or Thumb execution. Table 6-1 lists the values of the exception mode LR for exceptions generated during Thumb execution. Exception Exception link register value Return instruction Reset UNPREDICTABLE value - Undefined Address of undefined instruction + 2 MOVS PC, R14 SWI Address of SWI instruction + 2 MOVS PC, R14 Prefetch Abort Address of aborted instruction fetch + 4 SUBS PC, R14, #4 Data Abort Address of the instruction that generated the abort + 8 SUBS PC, R14, #8 IRQ Address of the next instruction to be executed + 4 SUBS PC, R14, #4 FIQ Address of the next instruction to be executed + 4 SUBS PC, R14, #4 Note Table 6-1 Exception return instructions For each exception, the return instruction indicated by Table 6-1 is the same as the return instruction required if the exception occurred during ARM execution, for the primary or only method of return from that instruction listed in Exceptions on page A2-13. However, the following two types of exception have a secondary return method, for which different return instructions are needed depending on whether the exception occurred during ARM or Thumb execution: • For the Data Abort exception, the primary method of return causes execution to resume at the aborted instruction, which causes it to be re-executed. As described in Data Abort (data access memory abort) on page A2-17, it is also possible to return to the next instruction after the aborted instruction, using a SUBS PC,R14,#4 instruction. If this type of return is required for a data abort caused by a Thumb instruction, use SUBS PC,R14,#6 for the return instruction. • For the Undefined Instruction exception, the primary method of return causes execution to resume at the next instruction after the undefined instruction. As described in Undefined Instruction exception on page A2-15, it is also possible to return to the undefined instruction itself, using the instruction SUBS PC,R14,#4. If this type of return is required for a Thumb undefined instruction, use SUBS PC,R14,#2 for the return instruction. However, the main use of this type of return is for some types of coprocessor instruction, and as the Thumb instruction set does not contain any coprocessor instructions, you are unlikely to need this secondary method of return for Thumb instructions. When these secondary methods of return are used, the exception handler code must test the SPSR T bit in order to determine which of the two return instructions to use. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-3
- 286. The Thumb Instruction Set 6.2 Instruction set encoding Figure 6-1 shows the Thumb instruction set encoding. An entry in square brackets, for example [1], indicates a note on the following page. 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0 0 0 0 opcode [1] immediate Rm Rd 1 1 0 1 1 1 opc Rm Rn Rd opc immediate Rn Rd opcode Rd / Rn immediate Rm / Rs Rd / Rn opcode opcode [1] Rd / Rn H1 H2 Rm Branch/exchange 0 1 0 0 0 1 1 1 L H2 Rm SBZ instruction set [3] PC-relative offset 0 1 0 0 0 1 1 1 Rm Rn Rd offset opcode B L Rn Rd offset 0 L Rn Rd SP-relative offset 0 0 0 0 Rd 1 0 0 1 L Rd 1 0 1 0 SP Rd immediate Miscellaneous: See Figure 6-2 1 0 1 1 x x x x x x x x x x x x 1 1 0 0 L Rn register list 1 cond [2] offset 1 1 0 1 1 1 1 0 x x x x x x x x 1 1 0 1 1 1 1 1 immediate 0 0 offset BLX suffix [4] 1 1 1 0 1 offset 0 1 1 1 0 1 x x x x x x x x x x 1 1 0 offset 1 1 1 1 1 offset Figure 6-1 Thumb instruction set overview Shift by immediate Add/subtract register Add/subtract immediate Add/subtract/compare/move immediate Data-processing register Special data processing Load from literal pool Load/store register offset Load/store word/byte immediate offset Load/store halfword immediate offset Load/store to/from stack Add to SP or PC 0 0 0 0 0 0 0 0 1 0 1 0 0 1 0 0 1 0 0 1 1 1 0 0 Load/store multiple Conditional branch Undefined instruction Software interrupt Unconditional branch Undefined instruction BL/BLX prefix BL suffix 1 1 0 1 1 1 1 1 1 A6-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 287. The Thumb Instruction Set 1. The opc field is not allowed to be 11 in this line. Other lines deal with the case that the opc field is 11. 2. The cond field is not allowed to be 1110 or 1111 in this line. Other lines deal with the cases where the cond field is 1110 or 1111. 3. The form with L==1 is UNPREDICTABLE prior to ARM architecture version 5. 4. This is an undefined instruction prior to ARM architecture version 5. 6.2.1 Miscellaneous instructions Figure 6-2 lists miscellaneous Thumb instructions. An entry in square brackets, for example [1], indicates a note below the figure. 1 5 1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0 1 0 1 1 0 0 0 0 opc immediate 1 0 1 1 L 1 0 R register list Software breakpoint [1] 1 0 1 1 1 1 1 0 immediate Figure 6-2 Miscellaneous Thumb instructions Adjust stack pointer Push/pop register list 1. This is an undefined instruction prior to ARM architecture version 5. Note Any instruction with bits[15:12] = 1011, and which is not shown in Figure 6-2, is an undefined instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-5
- 288. The Thumb Instruction Set 6.3 Branch instructions Thumb supports six types of branch instruction: • an unconditional branch that allows a forward or backward branch of up to 2KB • a conditional branch to allow forward and backward branches of up to 256 bytes • a Branch with Link (subroutine call) is supported with a pair of instructions that allow forward and backward branches of up to 4MB • a Branch and Exchange instruction branches to an address in a register and optionally switches to ARM code execution • a Branch with Link and Exchange instruction performs a subroutine call to an address in a register and optionally switches to ARM code execution • a second form of Branch with Link and Exchange uses a pair of instructions, similar to Branch with Link, but additionally switches to ARM code execution. The encoding for these instructions is given below. 6.3.1 Conditional branch B<cond> <target_address> 15 14 13 12 11 8 7 0 1 1 0 1 cond 8_bit_signed_offset 6.3.2 Unconditional branch B <target_address> BL <target_address> ; Produces two 16-bit instructions BLX <target_address> ; Produces two 16-bit instructions 15 14 13 12 11 10 0 1 1 1 H offset_11 6.3.3 Branch with exchange BX <Rm> BLX <Rm> 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 1 1 1 L H2 Rm SBZ A6-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 289. The Thumb Instruction Set 6.3.4 Examples B label ; unconditionally branch to label BCC label ; branch to label if carry flag is clear BEQ label ; branch to label if zero flag is set BL func ; subroutine call to function func ... ; Include body of function here ... MOV PC, LR ; R15=R14, return to instruction after the BL BX R12 ; branch to address in R12; begin ARM execution if ; bit 0 of R12 is zero; otherwise continue executing ; Thumb code 6.3.5 List of branch instructions The following instructions follow the formats shown above. B Conditional Branch. See B (1) on page A7-18. B Unconditional Branch. See B (2) on page A7-20. BL Branch with Link. See BL, BLX(1) on page A7-26. BX Branch and Exchange instruction set. See BX on page A7-32. BLX Branch with Link and Exchange instruction set. See BL, BLX(1) on page A7-26 and BLX(2) on page A7-30. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-7
- 290. The Thumb Instruction Set 6.4 Data-processing instructions Thumb data-processing instructions are a subset of the ARM data-processing instructions, as shown in Table 6-2. All Thumb data-processing instructions in this table set the condition codes. Table 6-2 Thumb data-processing instructions Mnemonic Operation Action ADC Rd, Rm Add with Carry Rd := Rd + Rm + Carry flag ADD Rd, Rn, Rm Add Rd := Rn + Rm ADD Rd, Rn, #0 to 7 Add Rd := Rn + 3-bit immediate ADD Rd, #0 to 255 Add Rd := Rd + 8-bit immediate AND Rd, Rm Logical AND Rd := Rd AND Rm ASR Rd, Rm, #1 to 32 Arithmetic Shift Right Rd := Rm ASR 5-bit immediate ASR Rd, Rs Arithmetic Shift Right Rd := Rd ASR Rs BIC Rd, Rm Bit Clear Rd := Rd AND NOT Rm CMN Rn, Rm Compare Negated Update flags after Rn + Rm CMP Rn, #0 to 255 Compare Update flags after Rn - 8-bit immediate CMP Rn, Rm Compare Update flags after Rn - Rm EOR Rd, Rm Logical Exclusive OR Rd := Rd EOR Rm LSL Rd, Rm, #0 to 31 Logical Shift Left Rd := Rm LSL 5-bit immediate LSL Rd, Rs Logical Shift Left Rd := Rd LSL Rs LSR Rd, Rm, #1 to 32 Logical Shift Right Rd := Rm LSR 5-bit immediate LSR Rd, Rs Logical Shift Right Rd := Rd LSR Rs MOV Rd, #0 to 255 Move Rd := 8-bit immediate MOV Rd, Rn Move Rd := Rn MUL Rd, Rm Multiply Rd := Rm x Rd MVN Rd, Rm Move Not Rd := NOT Rm NEG Rd, Rm Negate Rd := 0 - Rm ORR Rd, Rm Logical (inclusive) OR Rd := Rd OR Rm A6-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 291. The Thumb Instruction Set Table 6-2 Thumb data-processing instructions (Continued) Mnemonic Operation Action ROR Rd, Rs Rotate Right Rd := Rd ROR Rs SBC Rd, Rm Subtract with Carry Rd := Rd - Rm - NOT(Carry Flag) SUB Rd, Rn, Rm Subtract Rd := Rn - Rm SUB Rd, Rn, #0 to 7 Subtract Rd := Rn - 3-bit immediate SUB Rd, #0 to 255 Subtract Rd := Rd - 8-bit immediate TST Rn, Rm Test Update flags after Rn AND Rm For example: ADD R0, R4, R7 ; R0 = R4 + R7 SUB R6, R1, R2 ; R6 = R1 - R2 ADD R0, #255 ; R0 = R0 + 255 ADD R1, R4, #4 ; R1 = R4 + 4 NEG R3, R1 ; R3 = 0 - R1 AND R2, R5 ; R2 = R2 AND R5 EOR R1, R6 ; R1 = R1 EOR R6 CMP R2, R3 ; update flags after R2 - R3 CMP R7, #100 ; update flags after R7 - 100 MOV R0, #200 ; R0 = 200 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-9
- 292. The Thumb Instruction Set 6.4.1 High registers There are seven types of data-processing instruction which operate on ARM registers 8 to 14 and the PC as shown in Table 6-3. Apart from CMP, instructions in this table do not change the condition code flags. Mnemonic Operation Action MOV Rd, Rn Move Rd := Rn ADD Rd, Rm Add Rd := Rd + Rm CMP Rn, Rm Compare Update flags after Rn - Rm ADD SP, #0 to 508 Increment stack pointer R13 = R13 + 4* (7-bit immediate) SUB SP, #0 to 508 Decrement stack pointer R13 = R13 - 4* (7-bit immediate) ADD Rd, SP, #0 to 1020 Form Stack address Rd = R13 + 4* (8-bit immediate) ADD Rd, PC, #0 to 1020 Form PC address Rd = PC + 4* (8-bit immediate) For example: Table 6-3 High register data-processing instructions MOV R0, R12 ; R0 = R12 ADD R10, R1, R2 ; R6 = R1 - R2 MOV PC, LR ; PC = R14 CMP R10, R11 ; update flags after R10 - R11 SUB SP, #12 ; increase stack size by 12 bytes ADD SP, #16 ; decrease stack size by 16 bytes ADD R2, SP, #20 ; R2 = SP + 20 ADD R0, PC, #500 ; R0 = PC + 500 A6-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 293. The Thumb Instruction Set 6.4.2 Formats Data-processing instructions use the following eight instruction formats: Format 1 <opcode1> <Rd>, <Rn>, <Rm> <opcode1> := ADD | SUB 15 14 13 12 11 10 9 8 6 5 3 2 0 0 0 0 1 1 0 op_1 Rm Rn Rd Format 2 <opcode2> <Rd>, <Rn>, #<3_bit_immed> <opcode2> := ADD | SUB 15 14 13 12 11 10 9 8 6 5 3 2 0 0 0 0 1 1 1 op_2 3_bit_immediate Rn Rd Format 3 <opcode3> <Rd>|<Rn>, #<8_bit_immed> <opcode3> := ADD | SUB | MOV | CMP 15 14 13 12 11 10 8 7 0 0 0 1 op_3 Rd|Rn 8_bit_immediate Format 4 <opcode4> <Rd>, <Rm>, #<shift_imm> <opcode4> := LSL | LSR | ASR 15 14 13 12 11 10 6 5 3 2 0 0 0 0 op_4 shift_immediate Rm Rd ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-11
- 294. The Thumb Instruction Set Format 5 <opcode5> <Rd>|<Rn>, <Rm>|<Rs> <opcode5> := MVN | CMP | CMN | TST | ADC | SBC | NEG | MUL | LSL | LSR | ASR | ROR | AND | EOR | ORR | BIC 15 14 13 12 11 10 9 6 5 3 2 0 0 1 0 0 0 0 op_5 Rm|Rs Rd|Rn Format 6 ADD <Rd>, <reg>, #<8_bit_immed> <reg> := SP | PC 15 14 13 12 11 10 8 7 0 1 0 1 0 reg Rd 8_bit_immediate Format 7 <opcode6> SP, SP, #<7_bit_immed> <opcode6> := ADD | SUB 15 14 13 12 11 10 9 8 7 6 0 1 0 1 1 0 0 0 0 op_6 7_bit_immediate Format 8 <opcode7> <Rd>|<Rn>, <Rm> <opcode7> := MOV | ADD | CMP 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 1 opcode H1 H2 Rm Rd|Rn A6-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 295. The Thumb Instruction Set 6.4.3 List of data-processing instructions The following instructions follow the formats shown above. ADC Add with Carry. See ADC on page A7-4. ADD Add (immediate). See ADD (1) on page A7-5. ADD Add (large immediate). See ADD (2) on page A7-6. ADD Add (register). See ADD (3) on page A7-7. ADD Add high registers. See ADD (4) on page A7-8. ADD Add (immediate to program counter). See ADD (5) on page A7-10. ADD Add (immediate to stack pointer). See ADD (6) on page A7-11. ADD Increment stack pointer. See ADD (7) on page A7-12. AND Logical AND. See AND on page A7-13. ASR Arithmetic Shift Right (immediate). See ASR (1) on page A7-14. ASR Arithmetic Shift Right (register). See ASR (2) on page A7-16. BIC Bit Clear. See BIC on page A7-22. CMN Compare Negative (register). See CMN on page A7-34. CMP Compare (immediate). See CMP (1) on page A7-35. CMP Compare (register). See CMP (2) on page A7-36. CMP Compare high registers. See CMP (3) on page A7-37. EOR Exclusive OR. See EOR on page A7-39. LSL Logical Shift Left (immediate). See LSL (1) on page A7-59. LSL Logical Shift Left (register). See LSL (2) on page A7-60. LSR Logical Shift Right (immediate). See LSR (1) on page A7-62. LSR Logical Shift Right (register). See LSR (2) on page A7-64. MOV Move (immediate). See MOV (1) on page A7-66. MOV Move a low register to another low register. See MOV (2) on page A7-67. MOV Move high registers. See MOV (3) on page A7-68. MUL Multiply. See MUL on page A7-70. MVN Move NOT (register). See MVN on page A7-72. NEG Negate (register). See NEG on page A7-73. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-13
- 296. The Thumb Instruction Set ORR Logical OR. See ORR on page A7-74. ROR Rotate Right (register). See ROR on page A7-80. SBC Subtract with Carry (register). See SBC on page A7-82. SUB Subtract (immediate). See SUB (1) on page A7-98. SUB Subtract (large immediate). See SUB (2) on page A7-99. SUB Subtract (register). See SUB (3) on page A7-100. SUB Decrement stack pointer. See SUB (4) on page A7-101. TST Test (register). See TST on page A7-103. A6-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 297. The Thumb Instruction Set 6.5 Load and Store Register instructions Thumb supports eight types of Load and Store Register instructions. Two basic addressing modes are available. These allow the load and store of words, halfwords and bytes, and also the load of signed halfwords and bytes: • register plus register • register plus 5-bit immediate (not available for signed halfword and signed byte loads). If an immediate offset is used, it is scaled by 4 for word access and 2 for halfword accesses. In addition, three special instructions allow: • words to be loaded using the PC as a base with a 1KB (word-aligned) immediate offset • words to be loaded and stored with the stack pointer (R13) as the base and a 1KB (word-aligned) immediate offset. 6.5.1 Formats Load and Store Register instructions have the following formats: Format 1 <opcode1> <Rd>, [<Rn>, #<5_bit_offset>] <opcode1> := LDR|LDRH|LDRB|STR|STRH|STRB 15 11 10 6 5 3 2 0 Format 2 opcode1 5_bit_offset Rn Rd <opcode2> <Rd>, [<Rn>, <Rm>] <opcode2> := LDR|LDRH|LDRSH|LDRB|LDRSB|STR|STRH|STRB 15 9 8 6 5 3 2 0 Format 3 opcode2 Rm Rn Rd LDR <Rd>, [PC, #<8_bit_offset>] 15 14 13 12 11 10 8 7 0 0 1 0 0 1 Rd 8_bit_immediate ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-15
- 298. The Thumb Instruction Set Format 4 <opcode3> <Rd>, [SP, #<8_bit_offset>] <opcode3> := LDR | STR 15 14 13 12 11 10 8 7 0 1 0 0 1 L Rd 8_bit_immediate For example: LDR R4, [R2, #4] ; Load word into R4 from address R2 + 4 LDR R4, [R2, R1] ; Load word into R4 from address R2 + R1 STR R0, [R7, #0x7C] ; Store word from R0 to address R7 + 124 STRB R1, [R5, #31] ; Store byte from R1 to address R5 + 31 STRH R4, [R2, R3] ; Store halfword from R4 to R2 + R3 LDRH R3, [R6, R5] ; Load word into R3 from R6 + R5 LDRB R2, [R1, #5] ; Load byte into R2 from R1 + 5 LDR R6, [PC, #0x3FC] ; Load R6 from PC + 0x3FC LDR R5, [SP, #64] ; Load R5 from SP + 64 STR R4, [SP, #0x260] ; Load R5 from SP + 0x260 A6-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 299. The Thumb Instruction Set 6.5.2 List of Load and Store Register instructions The following instructions follow the formats shown above. LDR Load Word (immediate offset). See LDR (1) on page A7-42. LDR Load Word (register offset). See LDR (2) on page A7-44. LDR Load Word (PC-relative). See LDR (3) on page A7-46. LDR Load Word (SP-relative). See LDR (4) on page A7-48. LDRB Load Unsigned Byte (immediate offset). See LDRB (1) on page A7-50. LDRB Load Unsigned Byte (register offset). See LDRB (2) on page A7-51. LDRH Load Unsigned Halfword (immediate offset). See LDRH (1) on page A7-52. LDRH Load Unsigned Halfword (register offset). See LDRH (2) on page A7-54. LDRSB Load Signed Byte (register offset). See LDRSB on page A7-56. LDRSH Load Signed Halfword (register offset). See LDRSH on page A7-57. STR Store Word (immediate offset). See STR (1) on page A7-86. STR Store Word (register offset). See STR (2) on page A7-88. STR Store Word (SP-relative). See STR (3) on page A7-90. STRB Store Byte (immediate offset). See STRB (1) on page A7-92. STRB Store Byte (register offset). See STRB (2) on page A7-93. STRH Store Halfword (immediate offset). See STRH (1) on page A7-94. STRH Store Halfword (register offset). See STRH (2) on page A7-96. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-17
- 300. The Thumb Instruction Set 6.6 Load and Store Multiple instructions Thumb supports four types of Load and Store Multiple instructions: • Two instructions, LDMIA and STMIA, are designed to support block copy. They have a fixed Increment After addressing mode from a base register. • The other two instructions, PUSH and POP, also have a fixed addressing mode. They implement a full descending stack and the stack pointer (R13) is used as the base register. All four instructions update the base register after transfer and all can transfer any or all of the lower 8 registers. PUSH can also stack the return address, and POP can load the PC. 6.6.1 Formats Load and Store Multiple instructions have the following formats: Format 1 <opcode1> <Rn>!, <registers> <opcode1> := LDMIA | STMIA 15 14 13 12 11 10 8 7 0 1 1 0 0 L Rn register_list Format 2 PUSH {<registers>} POP {<registers>} 15 14 13 12 11 10 9 8 7 0 1 0 1 1 L 1 0 R register_list 6.6.2 Examples LDMIA R7!, {R0-R3, R5} ; Load R0 to R3-R5 from R7, add 20 to R7 STMIA R0!, {R3, R4, R5} ; Store R3-R5 to R0: add 12 to R0 function PUSH {R0-R7, LR} ; push onto the stack (R13) R0-R7 and ; the return address ... ; code of the function body ... POP {R0-R7, PC} ; restore R0-R7 from the stack ; and the program counter, and return A6-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 301. The Thumb Instruction Set 6.6.3 List of Load and Store Multiple instructions The following instructions follow the formats shown above. LDMIA Load Multiple. See LDMIA on page A7-40. POP Pop Multiple. See POP on page A7-75. PUSH Push Multiple. See PUSH on page A7-78. STMIA Store Multiple. See STMIA on page A7-84. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-19
- 302. The Thumb Instruction Set 6.7 Exception-generating instructions The Thumb instruction set provides two types of instruction whose main purpose is to cause a processor exception to occur: • The Software Interrupt (SWI) instruction is used to cause a SWI exception to occur (see Software Interrupt exception on page A2-16). This is the main mechanism in the Thumb instruction set by which User mode code can make calls to privileged Operating System code. • The Breakpoint (BKPT) instruction is used for software breakpoints in T variants of ARM architecture versions 5 and above. Its default behavior is to cause a Prefetch Abort exception to occur (see Prefetch Abort (instruction fetch memory abort) on page A2-16). A debug monitor program that has previously been installed on the Prefetch Abort vector can handle this exception. If debug hardware is present in the system, it is allowed to override this default behavior. Details of whether and how this happens are IMPLEMENTATION DEFINED. 6.7.1 Instruction encodings SWI <immed_8> 15 14 13 12 11 10 9 8 7 0 1 1 0 1 1 1 1 1 immed_8 BKPT <immed_8> 15 14 13 12 11 10 9 8 7 0 1 0 1 1 1 1 1 0 immed_8 In both SWI and BKPT, the immed_8 field of the instruction is ignored by the ARM processor. The SWI or Prefetch Abort handler can optionally be written to load the instruction that caused the exception and extract these fields. This allows them to be used to communicate extra information about the Operating System call or breakpoint to the handler. 6.7.2 List of exception-generating instructions BKPT Breakpoint. See BKPT on page A7-24. SWI Software Interrupt. See SWI on page A7-102. A6-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 303. The Thumb Instruction Set 6.8 Undefined instruction space The following instructions are UNDEFINED in the Thumb instruction set: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 0 1 1 0 0 0 1 x x x x x x x x 1 0 1 1 0 x 1 x x x x x x x x x 1 0 1 1 1 0 x x x x x x x x x x 1 0 1 1 1 1 1 1 x x x x x x x x 1 1 0 1 1 1 1 0 x x x x x x x x In general, these instructions can be used to extend the Thumb instruction set in the future. However, it is intended that the following group of instructions will not be used in this manner: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 0 1 1 1 1 0 x x x x x x x x Use one of these instructions if you want to use an undefined instruction for software purposes, with minimal risk that future hardware will treat it as a defined instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A6-21
- 304. The Thumb Instruction Set A6-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 305. Chapter A7 Thumb Instructions This chapter describes the syntax and usage of every Thumb instruction, in the section: • Alphabetical list of Thumb instructions on page A7-2 • Thumb instructions and architecture versions on page A7-104. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-1
- 306. Thumb Instructions 7.1 Alphabetical list of Thumb instructions Every Thumb instruction is listed on the following pages. Each instruction description shows: • the instruction encoding • the instruction syntax • the versions of the ARM architecture where the instruction is valid • any exceptions that might apply • a pseudo-code specification of how the instruction operates • notes on usage and special cases • the equivalent ARM instruction encoding. 7.1.1 General notes These notes explain the types of information and abbreviations used on the instruction pages. Syntax abbreviations The following abbreviations are used in the instruction pages: immed_<n> This is an <n>-bit immediate value. For example, an 8-bit immediate value is represented by: immed_8 signed_immed_<n> This is a signed immediate. For example, an 8-bit signed immediate is represented by: signed_immed_8 Architecture version For the convenience of the reader, this section describes the version of the ARM instruction set that the instruction is associated with, not the version of the Thumb instruction set. There have been two versions of the Thumb instruction set architecture to date: THUMBv1 This is used in T variants of version 4 of the ARM instruction set architecture. THUMBv2 This is used in T variants of version 5 and above of the ARM instruction set architecture. Instructions which are described as being in all T variants are therefore present in both THUMBv1 and THUMBv2, while those that are described as being in T variants of version 5 and above are in THUMBv2 only. A7-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 307. Thumb Instructions Equivalent ARM syntax and encoding This section shows the syntax and encoding of an equivalent ARM instruction. When no precise equivalent is available, a close equivalent is shown and the reasons why it is not a precise equivalent are explained. A common reason for the instruction not being a precise equivalent is that it reads the value of the PC. This produces the instruction’s own address plus N, where N is 8 for ARM instructions and 4 for Thumb instructions. This difference can often be compensated for by adjusting an immediate constant in the equivalent ARM instruction. In the equivalent instruction encodings, named fields and bits must be filled in with the corresponding fields and bits from the Thumb instruction, or in a few cases with values derived from the Thumb instruction as described in the text. The ARM instruction fields are normally the same length as the corresponding Thumb instruction fields, with one important exception. Thumb register fields are normally 3 bits long, whereas ARM register fields are normally 4 bits long. In these cases, the Thumb register field must be extended with a high-order 0 when substituted into the ARM register field, so that the ARM instruction refers to the correct one of R0 to R7. Information on usage Usage information is only given for Thumb instructions where it differs significantly from ARM instruction usage. If no Usage section appears for a Thumb instruction, see the equivalent ARM instruction page in Chapter A4 ARM Instructions for usage information. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-3
- 308. Thumb Instructions 7.1.2 ADC 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 0 1 0 1 Rm Rd The ADC (Add with Carry) instruction can be used to synthesize multi-word addition. The condition code flags are updated, based on the result. Syntax ADC <Rd>, <Rm> where: <Rd> Holds the first value for the addition, and is the destination register for the operation. <Rm> Specifies the register that contains the second operand for the addition. Architecture version All T variants Exceptions None Operation Rd = Rd + Rm + C Flag N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = CarryFrom(Rd + Rm + C Flag) V Flag = OverflowFrom(Rd + Rm + C Flag) Equivalent ARM syntax and encoding ADCS <Rd>, <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 0 1 0 1 1 Rd Rd 0 0 0 0 0 0 0 0 Rm A7-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 309. Thumb Instructions 7.1.3 ADD (1) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 0 0 1 1 1 0 immed_3 Rn Rd This form of ADD adds a small constant value to the value of a register and stores the result in a second register. The condition code flags are updated, based on the result. Syntax ADD <Rd>, <Rn>, #<immed_3> where: <Rd> Is the destination register for the completed operation. <Rn> Specifies the register that contains the operand for the addition. <immed_3> Specifies a 3-bit immediate value that is added to the value of <Rn>. Architecture version All T variants Exceptions None Operation Rd = Rn + immed_3 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = CarryFrom(Rn + immed_3) V Flag = OverflowFrom(Rn + immed_3) Equivalent ARM syntax and encoding ADDS <Rd>, <Rn>, #<immed_3> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 0 1 1 1 0 0 0 1 0 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 0 immed_3 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-5
- 310. Thumb Instructions 7.1.4 ADD (2) 15 14 13 12 11 10 8 7 0 0 0 1 1 0 Rd immed_8 This form of ADD adds a large immediate value to the value of a register and stores the result back in the same register. The condition code flags are updated, based on the result. Syntax ADD <Rd>, #<immed_8> where: <Rd> Holds the first operand for the addition, and is the destination register for the completed operation. <immed_8> Specifies an 8-bit immediate value that is added to the value of <Rd>. Architecture version All T variants Exceptions None Operation Rd = Rd + immed_8 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = CarryFrom(Rd + immed_8) V Flag = OverflowFrom(Rd + immed_8) Equivalent ARM syntax and encoding ADDS <Rd>, <Rd>, #<immed_8> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 1 1 1 0 0 0 1 0 1 0 0 1 Rd Rd 0 0 0 0 immed_8 A7-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 311. Thumb Instructions 7.1.5 ADD (3) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 0 0 1 1 0 0 Rm Rn Rd This form of ADD adds the value of one register to the value of a second register, and stores the result in a third register. The condition code flags are updated, based on the result. Syntax ADD <Rd>, <Rn>, <Rm> where: <Rd> Is the destination register for the completed operation. <Rn> Specifies the register containing the first value for the addition. <Rm> Specifies the register containing the second value for the addition. Architecture version All T variants Exceptions None Operation Rd = Rn + Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = CarryFrom(Rn + Rm) V Flag = OverflowFrom(Rn + Rm) Equivalent ARM syntax and encoding ADDS <Rd>, <Rn>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 0 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-7
- 312. Thumb Instructions 7.1.6 ADD (4) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 1 0 0 H1 H2 Rm Rd This form of ADD adds the values of two registers, one or both of which are high registers. Unlike the low-register ADD instruction (ADD (3) on page A7-7), this instruction does not change the flags. Syntax ADD <Rd>, <Rm> where: <Rd> Specifies the register containing the first value, and is also the destination register. It can be any of R0 to R15. The register number is encoded in the instruction in H1 (most significant bit) and Rd (remaining three bits). <Rm> Specifies the register containing the second value. It can be any of R0 to R15. Its number is encoded in the instruction in H2 (most significant bit) and Rm (remaining three bits). Architecture version All T variants Exceptions None Operation Rd = Rd + Rm Notes Operand restriction If a low register is specified for <Rd> and Rm (H1==0 and H2==0), the result is UNPREDICTABLE. A7-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 313. Thumb Instructions Equivalent ARM syntax and encoding A close equivalent is: ADD <Rd>, <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 18 16 15 14 12 11 10 9 8 7 6 5 4 3 2 0 1 1 1 0 0 0 0 0 1 0 0 0 H1 Rd H1 Rd 0 0 0 0 0 0 0 0 H2 Rm There are slight differences when the instruction accesses the PC, because of the different definitions of the PC when executing ARM and Thumb code. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-9
- 314. Thumb Instructions 7.1.7 ADD (5) 15 14 13 12 11 10 8 7 0 1 0 1 0 0 Rd immed_8 This form of ADD adds an immediate value to the PC and writes the resulting PC-relative address to a destination register. The immediate can be any multiple of 4 in the range 0 to 1020. The condition codes are not affected. Syntax ADD <Rd>, PC, #<immed_8> * 4 where: <Rd> Is the destination register for the completed operation. PC Indicates PC-relative addressing. <immed_8> Specifies an 8-bit immediate value that is quadrupled and added to the value of the PC. Architecture version All T variants Exceptions None Operation Rd = (PC AND 0xFFFFFFFC) + (immed_8 << 2) Equivalent ARM syntax and encoding A close equivalent is: ADD <Rd>, PC, #<immed_8> * 4 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 0 1 1 1 0 0 0 1 0 1 0 0 0 1 1 1 1 Rd 1 1 1 1 immed_8 The definitions of the PC differ between ARM and Thumb code. This makes a difference between the precise results of the instructions. A7-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 315. Thumb Instructions 7.1.8 ADD (6) 15 14 13 12 11 10 8 7 0 1 0 1 0 1 Rd immed_8 This form of ADD adds an immediate value to the SP and writes the resulting SP-relative address to a destination register. The immediate can be any multiple of 4 in the range 0 to 1020. The condition codes are not affected. Syntax ADD <Rd>, SP, #<immed_8> * 4 where: <Rd> Is the destination register for the completed operation. SP Indicates SP-relative addressing. <immed_8> Specifies an 8-bit immediate value that is quadrupled and added to the value of the SP. Architecture version All T variants Exceptions None Operation Rd = SP + (immed_8 << 2) Equivalent ARM syntax and encoding ADD <Rd>, SP, #<immed_8> * 4 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 0 1 1 1 0 0 0 1 0 1 0 0 0 1 1 0 1 Rd 1 1 1 1 immed_8 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-11
- 316. Thumb Instructions 7.1.9 ADD (7) 15 14 13 12 11 10 9 8 7 6 0 1 0 1 1 0 0 0 0 0 immed_7 This form of ADD increments the SP by four times a 7-bit immediate (that is, by a multiple of 4 in the range 0 to 508). The condition codes are not affected. Syntax ADD SP, #<immed_7> * 4 where: SP Contains the first operand for the addition. SP is also the destination register for the operation. <immed_7> Specifies the immediate value that is quadrupled and added to the value of the SP. Architecture version All T variants Exceptions None Operation SP = SP + (immed_7 << 2) Usage For the Full Descending stack which the Thumb instruction set is designed to use, incrementing the SP is used to discard data on the top of the stack. Notes Alternative syntax This instruction can also be written as ADD SP, SP, #(<immed_7> * 4). Equivalent ARM syntax and encoding ADD SP, SP, #<immed_7> * 4 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 0 1 1 1 0 0 0 1 0 1 0 0 0 1 1 0 1 1 1 0 1 1 1 1 1 0 immed_7 A7-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 317. Thumb Instructions 7.1.10 AND 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 0 0 0 0 Rm Rd The AND (Logical AND) instruction performs a bitwise AND of the values in two registers. The condition code flags are updated, based on the result. Syntax AND <Rd>, <Rm> where: <Rd> Specifies the register containing the first operand, and is also the destination register. <Rm> Specifies the register containing the second operand. Architecture version All T variants Exceptions None Operation Rd = Rd AND Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected V Flag = unaffected Equivalent ARM syntax and encoding ANDS <Rd>, <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 0 0 0 0 1 Rd Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-13
- 318. Thumb Instructions 7.1.11 ASR (1) 15 14 13 12 11 10 6 5 3 2 0 0 0 0 1 0 immed_5 Rm Rd This form of the ASR (Arithmetic Shift Right) instruction is used to provide the signed value of a register divided by a constant power of 2. The condition code flags are updated, based on the result. Syntax ASR <Rd>, <Rm>, #<immed_5> where: <Rd> Is the destination register for the completed operation. <Rm> Specifies the register that contains the value to be shifted. <immed_5> Specifies the shift amount, in the range 1 to 32. Shifts by 1 to 31 are encoded directly in immed_5. A shift by 32 is encoded as immed_5 == 0. Architecture version All T variants Exceptions None Operation if immed_5 == 0 C Flag = Rm[31] if Rm[31] == 0 then Rd = 0 else /* Rm[31] == 1 */] Rd = 0xFFFFFFFF else /* immed_5 > 0 */ C Flag = Rm[immed_5 - 1] Rd = Rm Arithmetic_Shift_Right immed_5 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 V Flag = unaffected A7-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 319. Thumb Instructions Equivalent ARM syntax and encoding MOVS <Rd>, <Rm>, ASR #<immed_5> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd immed_5 1 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-15
- 320. Thumb Instructions 7.1.12 ASR (2) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 0 1 0 0 Rs Rd This form of ASR is used to provide the signed value of a register divided by a variable power of 2. The condition code flags are updated, based on the result. Syntax ASR <Rd>, <Rs> where: <Rd> Contains the value to be shifted, and is also the destination register for the completed operation. <Rs> Specifies the register that contains the value of the shift. Architecture version All T variants Exceptions None Operation if Rs[7:0] == 0 then C Flag = unaffected Rd = unaffected else if Rs[7:0] < 32 then C Flag = Rd[Rs[7:0] - 1] Rd = Rd Arithmetic_Shift_Right Rs[7:0] else /* Rs[7:0] >= 32 */ C Flag = Rd[31] if Rd[31] == 0 then Rd = 0 else /* Rd[31] == 1 */ Rd = 0xFFFFFFFF N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 V Flag = unaffected A7-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 321. Thumb Instructions Equivalent ARM syntax and encoding MOVS <Rd>, <Rd>, ASR <Rs> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd Rs 0 1 0 1 Rd ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-17
- 322. Thumb Instructions 7.1.13 B (1) 15 14 13 12 11 8 7 0 1 1 0 1 cond signed_immed_8 This form of the B (Branch) instruction provides a conditional branch to a target address. Syntax B<cond> <target_address> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. <target_address> Specifies the address to branch to. The branch target address is calculated by: 1. Shifting the 8-bit signed offset field of the instruction left by one bit. 2. Sign-extending the result to 32 bits. 3. Adding this to the contents of the PC (which contains the address of the branch instruction plus 4). The instruction can therefore specify a branch of approximately ±256 bytes. Architecture version All T variants Exceptions None Operation if ConditionPassed(cond) then PC = PC + (SignExtend(signed_immed_8) << 1) A7-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 323. Thumb Instructions Usage To calculate the correct value of signed_immed_8, the assembler (or other toolkit component) needs to: 1. Form the base address for the branch. This is the address of the branch instruction, plus 4. In other words, the base address is equal to the PC value read by that instruction. 2. Subtract the base address from the target address to form a byte offset. This offset is always even, because all Thumb instructions are halfword-aligned. 3. If the byte offset is outside the range -256 to +254, use an alternative code-generation strategy or produce an error as appropriate. 4. Otherwise, set the signed_immed_8 field of the instruction to the byte offset divided by 2. Notes Memory bounds Branching backwards past location zero and forwards over the end of the 32-bit address space is UNPREDICTABLE. AL condition If the condition field indicates AL (0b1110), the instruction is instead UNDEFINED. When an unconditional branch is required, use the unconditional Branch instruction described in B (2) on page A7-20. NV condition If the condition field indicates NV (0b1111), the instruction is a SWI instead (see SWI on page A7-102). Equivalent ARM syntax and encoding A close equivalent is: B<cond> <target_address> 31 28 27 26 25 24 23 8 7 0 cond 1 0 1 0 sign extension of signed_immed_8 signed_immed_8 This differs from the Thumb instruction, because the offset in the ARM instruction is shifted left by 2 before being added to the PC, whereas the offset in the Thumb instruction is shifted left by 1. Also, the PC values read by the ARM and Thumb instructions are different. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-19
- 324. Thumb Instructions 7.1.14 B (2) 15 14 13 12 11 10 0 1 1 1 0 0 signed_immed_11 This form of B provides an unconditional branch to a target address. Syntax B <target_address> where: <target_address> Specifies the address to branch to. The branch target address is calculated by: 1. Shifting the 11-bit signed offset of the instruction left one bit. 2. Sign-extending the result to 32 bits. 3. Adding this to the contents of the PC (which contains the address of the branch instruction plus 4). The instruction can therefore specify a branch of approximately ±2048 bytes. Architecture version All T variants Exceptions None Operation PC = PC + (SignExtend(signed_immed_11) << 1) Usage To calculate the correct value of signed_immed_11, the assembler (or other toolkit component) needs to: 1. Form the base address for the branch. This is the address of the branch instruction, plus 4. In other words, the base address is equal to the PC value read by that instruction. 2. Subtract the base address from the target address to form a byte offset. This offset is always even, because all Thumb instructions are halfword-aligned. 3. If the byte offset is outside the range -2048 to +2046, use an alternative code-generation strategy or produce an error as appropriate. 4. Otherwise, set the signed_immed_11 field of the instruction to the byte offset divided by 2. A7-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 325. Thumb Instructions Notes Memory bounds Branching backwards past location zero and forwards over the end of the 32-bit address space is UNPREDICTABLE. Equivalent ARM syntax and encoding A close equivalent is: B <target_address> 31 28 27 26 25 24 23 11 10 0 1 1 1 0 1 0 1 0 sign extension of signed_immed_11 signed_immed_11 This differs from the Thumb instruction, because the offset in the ARM instruction is shifted left by 2 before being added to the PC, whereas the offset in the Thumb instruction is shifted left by 1. Also, the PC values read by the ARM and Thumb instructions are different. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-21
- 326. Thumb Instructions 7.1.15 BIC 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 1 1 0 Rm Rd The BIC (Bit Clear) instruction performs a bitwise AND of the value of one register and the bitwise inverse of the value of another register. The condition code flags are updated, based on the result. Syntax BIC <Rd>, <Rm> where: <Rd> Is the register containing the value to be ANDed, and is also the destination register for the completed operation. <Rm> Specifies the register that contains the value whose complement is ANDed with the value in <Rd>. Architecture version All T variants Exceptions None Operation Rd = Rd AND NOT Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected V Flag = unaffected A7-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 327. Thumb Instructions Equivalent ARM syntax and encoding BICS <Rd>, <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 1 0 1 Rd Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-23
- 328. Thumb Instructions 7.1.16 BKPT 15 14 13 12 11 10 9 8 7 0 1 0 1 1 1 1 1 0 immed_8 The BKPT (Breakpoint) instruction causes a software breakpoint to occur. This breakpoint can be handled by an exception handler installed on the prefetch abort vector. In implementations which also include debug hardware, the hardware can optionally override this behavior and handle the breakpoint itself. When this occurs, the prefetch abort vector is not entered. Syntax BKPT <immed_8> where: <immed_8> Is an 8-bit immediate value, which is placed in bits[7:0] of the instruction. This value is ignored by the ARM hardware, but can be used by a debugger to store additional information about the breakpoint. Architecture version T variants of version 5 and above Exceptions Prefetch Abort Operation if (not overridden by debug hardware) R14_abt = address of BKPT instruction + 4 SPSR_abt = CPSR CPSR[4:0] = 0b10111 /* Enter Abort mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF000C else PC = 0x0000000C A7-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 329. Thumb Instructions Usage The exact usage of the BKPT instruction depends on the debug system being used. A debug system can use the BKPT instruction in two ways: • Debug hardware (if present) does not override the normal behavior of the BKPT instruction, and so the prefetch abort vector is entered. If the system also allows real prefetch aborts to occur, the prefetch abort handler determines (in a system-dependent manner) whether the vector entry occurred as a result of a BKPT instruction or as a result of a real prefetch abort, and branches to debug code or prefetch abort code accordingly. Otherwise, the prefetch abort handler just branches straight to debug code. When used in this manner, the BKPT instruction must be avoided within abort handlers, as it corrupts R14_abt and SPSR_abt. For the same reason, it must also be avoided within FIQ handlers, as an FIQ interrupt can occur within an abort handler. • Debug hardware overrides the normal behavior of the BKPT instruction and handles the software breakpoint itself. When finished, it typically either resumes execution at the instruction following the BKPT, or replaces the BKPT in memory with another instruction and resumes execution at that instruction. When BKPT is used in this manner, R14_abt and SPSR_abt are not corrupted, and so the above restrictions about its use in abort and FIQ handlers do not apply. Notes Hardware override Debug hardware in an implementation is specifically permitted to override the normal behavior of the BKPT instruction. Because of this, software must not use this instruction for purposes other than those permitted by the debug system being used (if any). In particular, software cannot rely on the Prefetch Abort exception occurring, unless either there is guaranteed to be no debug hardware in the system or the debug system specifies that it will occur. For more information, consult the documentation for the debug system being used. Equivalent ARM syntax and encoding BKPT <immed_8> 31 30 29 28 27 26 25 24 23 22 21 10 19 18 17 6 15 14 13 12 11 8 7 4 3 0 1 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0 immed_8 [7:4] 0 1 1 1 immed_8 [3:0] ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-25
- 330. Thumb Instructions 7.1.17 BL, BLX(1) 15 14 13 12 11 10 0 1 1 1 H offset_11 This form of the BL (Branch with Link) instruction provides an unconditional subroutine call to another Thumb routine. The return from subroutine is typically performed by one of the following: • MOV PC,LR • BX LR • a POP instruction that loads the PC. This form of the BLX (Branch with Link and Exchange instruction set) instruction provides an unconditional subroutine call to an ARM routine. The return from subroutine is typically performed by a BX LR instruction, or an LDR or LDM instruction that loads the PC. To allow for a reasonably large offset to the target subroutine, each of these two instructions is automatically translated by the assembler into a sequence of two 16-bit Thumb instructions: • The first Thumb instruction has H == 10 and supplies the high part of the branch offset. This instruction sets up for the subroutine call and is shared between the BL and BLX forms. • The second Thumb instruction has H == 11 (for BL) or H == 01 (for BLX). It supplies the low part of the branch offset and causes the subroutine call to take place. Syntax BL <target_addr> BLX <target_addr> where: <target_addr> Specifies the address to branch to. The branch target address is calculated by: 1. Shifting the offset_11 field of the first instruction left twelve bits. 2. Sign-extending the result to 32 bits. 3. Adding this to the contents of the PC (which contains the address of the first instruction plus 4). 4. Adding twice the offset_11 field of the second instruction. For BLX, the resulting address is forced to be word-aligned by clearing bit[1]. The instruction can therefore specify a branch of approximately ±4MB. Architecture version BL (H == 10 and H == 11 forms) is in all T variants. BLX (H == 01 form) is in T variants of version 5 and above. A7-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 331. Thumb Instructions Exceptions None Operation if H == 10 then LR = PC + (SignExtend(offset_11) << 12) else if H == 11 then PC = LR + (offset_11 << 1) LR = (address of next instruction) | 1 else if H == 01 then PC = (LR + (offset_11 << 1)) AND 0xFFFFFFFC LR = (address of next instruction) | 1 T Flag = 0 Usage To generate the correct pair of instructions, the assembler (or other toolkit component) must first generate the branch offset, as follows: 1. Form the base address for the branch. This is the address of the first of the two Thumb instructions (the one with H == 10), plus 4. In other words, the base address is equal to the PC value read by that instruction. 2. If the instruction is BLX, set bit[1] of the target address to be equal to bit[1] of the base address. This is an exception to the normal rule that bits[1:0] of the address of an ARM instruction are 0b00. This adjustment is required to ensure that the restrictions associated with the H == 01 form of the instruction are obeyed. 3. Subtract the base address from the target address to form the offset. The resulting offset is always even. If the offset lies outside the range: -222 <= offset <= +222 - 2 the target address lies outside the addressing range of these instructions. This results in alternative code or an error, as appropriate. If the offset is in range, a sequence of two Thumb instructions must be generated, both using the above form: • The first with H == 10 and offset_11 = offset[22:12]. • The second with H == 11 (for BL) or H== 01 (for BLX) and offset_11 = offset[11:1]. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-27
- 332. Thumb Instructions Notes Encoding If H == 00, the instruction is an unconditional branch instruction instead (see the Thumb instruction B (2) on page A7-20). Bit[0] for BLX If H == 01, then bit[0] of the instruction must be zero, or the instruction is UNDEFINED. The offset calculation method described in Usage above ensures that the offset calculated for a BLX instruction is a multiple of four, and that this restriction is obeyed. Memory bounds Branching backwards past location zero and forwards over the end of the 32-bit address space is UNPREDICTABLE. Instruction pairs These Thumb instructions must always occur in the pairs described above. Specifically: • If a Thumb instruction at address A is the H==10 form of this instruction, the Thumb instruction at address A+2 must be either the H==01 or the H==11 form of this instruction. • If a Thumb instruction at address A is either the H==01 or the H==11 form of this instruction, the Thumb instruction at address A-2 must be the H==10 form of this instruction. Also, except as noted below under Exceptions, the second instruction of the pair must not be the target of any branch, whether as the result of a branch instruction or of some other instruction that changes the PC. Failure to adhere to any of these restrictions can result in UNPREDICTABLE behavior. Exceptions It is IMPLEMENTATION DEFINED whether processor exceptions can occur between the two instructions of a BL or BLX pair. If they can, the ARM instructions designed for use for exception returns must be capable of returning correctly to the second instruction of the pair. So, exception handlers need take no special precautions about returning to the second instruction of a BL or BLX pair. A7-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 333. Thumb Instructions Equivalent ARM syntax and encoding Close equivalents to these instruction pairs are as follows. To call a Thumb subroutine: BLX <target_addr> 31 30 29 28 27 26 25 24 23 22 21 20 0 1 1 1 1 1 0 1 L offset sign offset[22:2] where L == offset[1]. To call an ARM routine: BL <target_addr> 31 30 29 28 27 26 25 24 23 22 21 20 0 1 1 1 0 1 0 1 1 offset sign offset[22:2] These differ slightly from the Thumb instruction pairs because of the different values of the PC in ARM and Thumb code. This can be compensated for by adjusting the offset by 4. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-29
- 334. Thumb Instructions 7.1.18 BLX(2) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 1 1 1 1 H2 Rm SBZ This form of BLX is used to call an ARM or Thumb subroutine from the Thumb instruction set, at an address specified in a register. This instruction branches and selects the instruction decoder to use to decode the instructions at the branch destination. The T flag is updated with bit[0] of the value of register Rm. To return from the subroutine to the caller, use BX R14. Syntax BLX <Rm> where: <Rm> Is the register that contains the branch target address. It can be any of R0 to R14. The register number is encoded in the instruction in H2 (most significant bit) and Rm (remaining three bits). If R15 is specified for <Rm>, the results are UNPREDICTABLE. Architecture version T variants of version 5 and above Exceptions None Operation LR = (address of the instruction after this BLX) | 1 T Flag = Rm[0] PC = Rm[31:1] << 1 Notes Encoding Bit 7 is the H1 bit for some of the other instructions that access the high registers. If it is 0 for this instruction, rather than 1 as shown, the instruction is a BX instruction instead (see BX on page A7-32). ARM/Thumb state transfers If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned addresses are impossible in ARM state. A7-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 335. Thumb Instructions Equivalent ARM syntax and encoding BLX <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 2 0 1 1 1 0 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 1 1 H2 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-31
- 336. Thumb Instructions 7.1.19 BX 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 1 1 1 0 H2 Rm SBZ The BX (Branch and Exchange) instruction is used to branch between ARM code and Thumb code. Syntax BX <Rm> where: <Rm> Is the register that contains the branch target address. It can be any of R0 to R15. The register number is encoded in the instruction in H2 (most significant bit) and Rm (remaining three bits). Architecture version All T variants Exceptions None Operation T Flag = Rm[0] PC = Rm[31:1] << 1 Usage The normal subroutine return instruction in Thumb code is BX R14. The following subroutine call instructions leave a suitable return value in R14: • ARM BLX instructions (See BLX (1) on page A4-16 and BLX (2) on page A4-18) • Thumb BL and BLX instructions (see BL, BLX(1) on page A7-26 and BLX(2) on page A7-30). In T variants of ARM architecture version 4, a subroutine call to an ARM routine can be performed by a code sequence of the form: <Put address of routine to call in Ra> MOV LR,PC ; Return to second following instruction BX Ra In T variants of ARM architecture 5 and above, a subroutine call to an ARM routine can be performed more efficiently with a BLX instruction (see BL, BLX(1) on page A7-26 and BLX(2) on page A7-30). A7-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 337. Thumb Instructions Notes Encoding Bit 7 is the H1 bit for some of the other instructions that access the high registers. If it is 1 for this instruction, rather than 0 as shown, the instruction is: • a BLX instruction instead in ARM architecture version 5 and above (see BLX(2) on page A7-30) • UNPREDICTABLE prior to ARM architecture version 5. ARM/Thumb state transfers If Rm[1:0] == 0b10, the result is UNPREDICTABLE, as branches to non word-aligned addresses are impossible in ARM state. Use of R15 Register 15 can be specified for <Rm>. If this is done, R15 is read as normal for Thumb code, that is, it is the address of the BX instruction itself plus 4. If the BX instruction is at a word-aligned address, this results in a branch to the next word, executing in ARM state. However, if the BX instruction is not at a word-aligned address, this means that the results of the instruction are UNPREDICTABLE (because the value read for R15 has bits[1:0]==0b10). Equivalent ARM syntax and encoding A close equivalent is: BX <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 0 0 1 0 SBO SBO SBO 0 0 0 1 H2 Rm This ARM instruction is not quite equivalent to the Thumb instruction, because their specified behavior differs when <Rm> is R15. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-33
- 338. Thumb Instructions 7.1.20 CMN 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 0 1 1 Rm Rn The CMN (Compare Negative) instruction compares a register value with the negation of another register value. The condition flags are updated, based on the result of adding the two register values, so that subsequent instructions can be conditionally executed (using a conditional branch). Syntax CMN <Rn>, <Rm> where: <Rn> Is the register containing the first value for comparison. <Rm> Is the register containing the second value for comparison. Architecture version All T variants Exceptions None Operation alu_out = Rn + Rm N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn + Rm) V Flag = OverflowFrom(Rn + Rm) Equivalent ARM syntax and encoding CMN <Rn>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 0 1 1 1 Rn SBZ 0 0 0 0 0 0 0 0 Rm A7-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 339. Thumb Instructions 7.1.21 CMP (1) 15 14 13 12 11 10 8 7 0 0 0 1 0 1 Rn immed_8 This form of the CMP (Compare) instruction compares a register value with a large immediate value. The condition flags are updated, based on the result of subtracting the constant from the register value, so that subsequent instructions can be conditionally executed (using a conditional branch). Syntax CMP <Rn>, #<immed_8> where: <Rn> Is the register containing the first value for comparison. <immed_8> Is the 8-bit second value for comparison. Architecture version All T variants Exceptions None Operation alu_out = Rn - immed_8 N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - immed_8) V Flag = OverflowFrom(Rn - immed_8) Equivalent ARM syntax and encoding CMP <Rn>, #<immed_8> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 1 1 1 0 0 0 1 1 0 1 0 1 Rn SBZ 0 0 0 0 immed_8 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-35
- 340. Thumb Instructions 7.1.22 CMP (2) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 0 1 0 Rm Rn This form of CMP compares two register values. The condition code flags are updated, based on the result of subtracting the second register value from the first, so that subsequent instructions can be conditionally executed (using a conditional branch). Syntax CMP <Rn>, <Rm> where: <Rn> Is the register containing the first value for comparison. <Rm> Is the register containing the second value for comparison. Architecture version All T variants Exceptions None Operation alu_out = Rn - Rm N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - Rm) V Flag = OverflowFrom(Rn - Rm) Equivalent ARM syntax and encoding CMP <Rn>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 0 1 0 1 Rn SBZ 0 0 0 0 0 0 0 0 Rm A7-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 341. Thumb Instructions 7.1.23 CMP (3) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 1 0 1 H1 H2 Rm Rn This form of CMP compares the values of two registers, one or both of which are high registers. The condition flags are updated, based on the result of subtracting the second register value from the first, so that subsequent instructions can be conditionally executed (using a conditional branch). Syntax CMP <Rn>, <Rm> where: <Rn> Is the register containing the first value. It can be any of R0 to R15. Its number is encoded in the instruction in H1 (most significant bit) and Rn (remaining three bits). <Rm> Is the register containing the second value. It can be any of R0 to R15. Its number is encoded in the instruction in H2 (most significant bit) and Rm (remaining three bits). Architecture version All T variants Exceptions None Operation alu_out = Rn - Rm N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - Rm) V Flag = OverflowFrom(Rn - Rm) Notes Operand restriction If a low register is specified for both <Rn> and <Rm> (H1==0 and H2==0), the result is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-37
- 342. Thumb Instructions Equivalent ARM syntax and encoding A close equivalent is: CMP <Rn>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 18 16 15 12 11 10 9 8 7 6 5 4 3 2 0 1 1 1 0 0 0 0 1 0 1 0 1 H1 Rn SBZ 0 0 0 0 0 0 0 0 H2 Rm There are slight differences when the instruction accesses the PC, because of the different definitions of the PC when executing ARM and Thumb code. A7-38 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 343. Thumb Instructions 7.1.24 EOR 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 0 0 0 1 Rm Rd The EOR (Exclusive OR) instruction performs a bitwise EOR of the values in two registers. The condition code flags are updated, based on the result. Syntax EOR <Rd>, <Rm> where: <Rd> Specifies the register containing the first operand, and is also the destination register. <Rm> Specifies the register containing the second operand. Architecture version All T variants Exceptions None Operation Rd = Rd EOR Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected V Flag = unaffected Equivalent ARM syntax and encoding EORS <Rd>, <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 0 0 0 1 1 Rd Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-39
- 344. Thumb Instructions 7.1.25 LDMIA 15 14 13 12 11 10 8 7 0 1 1 0 0 1 Rn register_list The LDMIA (Load Multiple Increment After) instruction loads a non-empty subset, or possibly all, of the general-purpose registers from sequential memory locations. Syntax LDMIA <Rn>!, <registers> where: <Rn> Is the register containing the start address for the instruction. ! Causes base register writeback, and is not optional. <registers> Is a list of registers to be loaded, separated by commas and surrounded by { and }. The list is encoded in the register_list field of the instruction, by setting bit[i] to 1 if register Ri is included in the list and to 0 otherwise, for each of i=0 to 7. At least one register must be loaded. If bits[7:0] are all zero, the result is UNPREDICTABLE. The registers are loaded in sequence, the lowest-numbered register from the lowest memory address (start_address), through to the highest-numbered register from the highest memory address (end_address). The start_address is the value of the base register <Rn>. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The end_address value is four less than the sum of the value of the base register and four times the number of registers specified in <registers>. Finally, the base register <Rn> is incremented by four times the numbers of registers in <registers>. Architecture version All T variants Exceptions Data Abort A7-40 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 345. Thumb Instructions Operation start_address = Rn end_address = Rn + (Number_Of_Set_Bits_In(register_list) * 4) - 4 address = start_address for i = 0 to 7 if register_list[i] == 1 Ri = Memory[address,4] address = address + 4 assert end_address == address - 4 Rn = Rn + (Number_Of_Set_Bits_In(register_list) * 4) Usage The LDMIA instruction is useful as a block load instruction. Combined with STMIA (Store Multiple), it allows efficient block copy. Notes Operand restrictions If the base register <Rn> is specified in <registers>, the final value of <Rn> is the loaded value (not the written-back value). Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment Load Multiple instructions ignore the least significant two bits of address. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. Equivalent ARM syntax and encoding If <Rn> is not in the register list (W == 1): LDMIA <Rn>!, <registers> If <Rn> is in the register list (W == 0): LDMIA <Rn>, <registers> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 14 13 12 11 10 9 8 7 0 1 1 1 0 1 0 0 0 1 0 W 1 Rn 0 0 0 0 0 0 0 0 register_list ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-41
- 346. Thumb Instructions 7.1.26 LDR (1) 15 14 13 12 11 10 6 5 3 2 0 0 1 1 0 1 immed_5 Rn Rd This form of the LDR (Load Register) instruction allows 32-bit memory data to be loaded into a general-purpose register. The addressing mode is useful for accessing structure (record) fields. With an offset of zero, the address produced is the unaltered value of the base register <Rn>. Syntax LDR <Rd>, [<Rn>, #<immed_5> * 4] where: <Rd> Is the destination register for the word loaded from memory. <Rn> Is the register containing the base address for the instruction. <immed_5> Is a 5-bit value that is multiplied by 4 and added to the value of <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + (immed_5 * 4) if address[1:0] == 0b00 data = Memory[address,4] else data = UNPREDICTABLE Rd = data A7-42 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 347. Thumb Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not word-aligned and no data abort occurs, the value written to the destination register is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception (a type of data abort). Equivalent ARM syntax and encoding LDR <Rd>, [<Rn>, #<immed_5> * 4] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 2 1 0 1 1 1 0 0 1 0 1 1 0 0 1 Rn Rd 0 0 0 0 0 immed_5 0 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-43
- 348. Thumb Instructions 7.1.27 LDR (2) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 1 0 0 Rm Rn Rd This form of LDR allows 32-bit memory data to be loaded into a general-purpose register. The addressing mode is useful for pointer+large offset arithmetic and for accessing a single element of an array. Syntax LDR <Rd>, [<Rn>, <Rm>] where: <Rd> Is the destination register for the word loaded from memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register containing the second value used in forming the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm if address[1:0] == 0b00 data = Memory[address,4] else data = UNPREDICTABLE Rd = data A7-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 349. Thumb Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not word-aligned and no data abort occurs, the value written to the destination register is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception (a type of data abort). Equivalent ARM syntax and encoding LDR <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 1 1 1 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-45
- 350. Thumb Instructions 7.1.28 LDR (3) 15 14 13 12 11 10 8 7 0 0 1 0 0 1 Rd immed_8 This form of LDR allows 32-bit memory data to be loaded into a general-purpose register. The addressing mode is useful for accessing PC-relative data. Syntax LDR <Rd>, [PC, #<immed_8> * 4] where: <Rd> Is the destination register for the word loaded from memory. PC Is the program counter. Its value is used to calculate the memory address. Bit 1 of the PC value is forced to zero for the purpose of this calculation, so the address is always word-aligned. <immed_8> Is an 8-bit value that is multiplied by 4 and added to the value of the PC to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = (PC[31:2] << 2) + (immed_8 * 4) Rd = Memory[address, 4] Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. A7-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 351. Thumb Instructions Equivalent ARM syntax and encoding A close equivalent is: LDR <Rd>, [PC, #<immed_8> * 4] 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 2 1 0 1 1 1 0 0 1 0 1 1 0 0 1 1 1 1 1 Rd 0 0 immed_8 0 0 There are slight differences caused by the different definitions of the PC and the fact that the Thumb instruction ignores bit[1] of the PC. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-47
- 352. Thumb Instructions 7.1.29 LDR (4) 15 14 13 12 11 10 8 7 0 1 0 0 1 1 Rd immed_8 This form of LDR allows 32-bit memory data to be loaded into a general-purpose register. The addressing mode is useful for accessing stack data. Syntax LDR <Rd>, [SP, #<immed_8> * 4] where: <Rd> Is the destination register for the word loaded from memory. SP Is the stack pointer. Its value is used to calculate the memory address. <immed_8> Is an 8-bit value that is multiplied by 4 and added to the value of the SP to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = SP + (immed_8 * 4) if address[1:0] == 0b00 data = Memory[address,4] else data = UNPREDICTABLE Rd = data A7-48 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 353. Thumb Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not word-aligned and no data abort occurs, the value written to the destination register is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception (a type of data abort). Equivalent ARM syntax and encoding LDR <Rd>, [SP, #<immed_8> * 4] 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 2 1 0 1 1 1 0 0 1 0 1 1 0 0 1 1 1 0 1 Rd 0 0 immed_8 0 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-49
- 354. Thumb Instructions 7.1.30 LDRB (1) 15 14 13 12 11 10 6 5 3 2 0 0 1 1 1 1 immed_5 Rn Rd This form of the LDRB (Load Register Byte) instruction loads a byte from memory and zero-extends it to form a 32-bit word which is written to a general-purpose register. The addressing mode is useful for accessing structure (record) fields. With an offset of zero, the address produced is the unaltered value of the base register <Rn>. Syntax LDRB <Rd>, [<Rn>, #<immed_5>] where: <Rd> Is the destination register for the byte loaded from memory. <Rn> Is the register containing the base address for the instruction. <immed_5> Is a 5-bit value that is added to the value of <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + immed_5 Rd = Memory[address,1] Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Equivalent ARM syntax and encoding LDRB <Rd>, [<Rn>, #<immed_5>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 0 1 1 1 0 0 1 0 1 1 1 0 1 Rn Rd 0 0 0 0 0 0 0 immed_5 A7-50 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 355. Thumb Instructions 7.1.31 LDRB (2) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 1 1 0 Rm Rn Rd This form of LDRB loads a byte from memory and zero-extends it to form a 32-bit word, which is written to a general-purpose register. The addressing mode is useful for pointer+large offset arithmetic and for accessing a single element of an array. Syntax LDRB <Rd>, [<Rn>, <Rm>] where: <Rd> Is the destination register for the byte loaded from memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register containing the second value used in forming the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm Rd = Memory[address,1] Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Equivalent ARM syntax and encoding LDRB <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 1 1 1 1 1 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-51
- 356. Thumb Instructions 7.1.32 LDRH (1) 15 14 13 12 11 10 6 5 3 2 0 1 0 0 0 1 immed_5 Rn Rd This form of the LDRH (Load Register Halfword) instruction loads a halfword (16 bits) from memory and zero-extends it to form a 32-bit word, which is written to a general-purpose register. The addressing mode is useful for accessing structure (record) fields. With an offset of zero, the address produced is the unaltered value of the base register <Rn>. Syntax LDRH <Rd>, [<Rn>, #<immed_5> * 2] where: <Rd> Is the destination register for the halfword loaded from memory. <Rn> Is the register containing the base address for the instruction. <immed_5> Is a 5-bit value that is multiplied by 2, then added to the value of <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + (immed_5 * 2) if address[0] == 0 data = Memory[address,2] else data = UNPREDICTABLE Rd = data A7-52 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 357. Thumb Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written to the destination register is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception (a type of data abort). Equivalent ARM syntax and encoding LDRH <Rd>, [<Rn>, #<immed_5> * 2] 31 30 29 28 27 26 25 24 23 22 21 10 19 16 15 12 11 10 9 8 7 6 5 4 3 1 0 1 1 1 0 0 0 0 1 1 1 0 1 Rn Rd 0 0 immed [4:3] 1 0 1 1 immed [2:0] 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-53
- 358. Thumb Instructions 7.1.33 LDRH (2) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 1 0 1 Rm Rn Rd This form of LDRH loads a halfword (16 bits) from memory and zero-extends it to form a 32-bit word, which is written to a general-purpose register. The addressing mode is useful for pointer + large offset arithmetic and for accessing a single element of an array. Syntax LDRH <Rd>, [<Rn>, <Rm>] where: <Rd> Is the destination register for the halfword loaded from memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register containing the second value used in forming the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm if address[0] == 0 data = Memory[address,2] else data = UNPREDICTABLE Rd = data Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written to the destination register is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception (a type of data abort). A7-54 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 359. Thumb Instructions Equivalent ARM syntax and encoding LDRH <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 0 1 Rn Rd SBZ 1 0 1 1 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-55
- 360. Thumb Instructions 7.1.34 LDRSB 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 0 1 1 Rm Rn Rd The LDRSB (Load Register Signed Byte) instruction loads a byte from memory and sign-extends it to form a 32-bit word, which is written to a general-purpose register. Syntax LDRSB <Rd>, [<Rn>, <Rm>] where: <Rd> Is the destination register for the byte loaded from memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register containing the second value used in forming the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm Rd = SignExtend(Memory[address,1]) Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Equivalent ARM syntax and encoding LDRSB <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 0 1 Rn Rd SBZ 1 1 0 1 Rm A7-56 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 361. Thumb Instructions 7.1.35 LDRSH 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 1 1 1 Rm Rn Rd The LDRSH (Load Register Signed Halfword) instruction loads a halfword from memory and sign-extends it to form a 32-bit word, which is written to a general-purpose register. Syntax LDRSH <Rd>, [<Rn>, <Rm>] where: <Rd> Is the destination register for the halfword loaded from memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register containing the second value used in forming the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm if address[0] == 0 data = Memory[address,2] else data = UNPREDICTABLE Rd = SignExtend(data) Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written to the destination register is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception (a type of data abort). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-57
- 362. Thumb Instructions Equivalent ARM syntax and encoding LDRSH <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 0 1 Rn Rd SBZ 1 1 1 1 Rm A7-58 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 363. Thumb Instructions 7.1.36 LSL (1) 15 14 13 12 11 10 6 5 3 2 0 0 0 0 0 0 immed_5 Rm Rd This form of the LSL (Logical Shift Left) instruction is used to provide either the value of a register directly (LSL #0), or the value of a register multiplied by a constant power of two. Zeros are inserted into the bit positions vacated by the shift, and the condition code flags are updated, based on the result. Syntax LSL <Rd>, <Rm>, #<immed_5> where: <Rd> Is the register that stores the result of the operation. <Rm> Is the register containing the value to be shifted. <immed_5> Specifies the shift amount, in the range 0 to 31. Architecture version All T variants Exceptions None Operation if immed_5 == 0 C Flag = unaffected Rd = Rm else /* immed_5 > 0 */ C Flag = Rm[32 - immed_5] Rd = Rm Logical_Shift_Left immed_5 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 V Flag = unaffected Equivalent ARM syntax and encoding MOVS <Rd>, <Rm>, LSL #<immed_5> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd immed_5 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-59
- 364. Thumb Instructions 7.1.37 LSL (2) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 0 0 1 0 Rs Rd This form of LSL is used to provide the value of a register multiplied by a variable power of two. Zeros are inserted into the vacated bit positions. The condition code flags are updated, based on the result. Syntax LSL <Rd>, <Rs> where: <Rd> Contains the value to be shifted, and is the destination register for the result of the operation. <Rs> Is the register containing the shift value. The value is held in the least significant byte. Architecture version All T variants Exceptions None Operation if Rs[7:0] == 0 C Flag = unaffected Rd = unaffected else if Rs[7:0] < 32 then C Flag = Rd[32 - Rs[7:0]] Rd = Rd Logical_Shift_Left Rs[7:0] else if Rs[7:0] == 32 then C Flag = Rd[0] Rd = 0 else /* Rs[7:0] > 32 */ C Flag = 0 Rd = 0 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 V Flag = unaffected A7-60 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 365. Thumb Instructions Equivalent ARM syntax and encoding MOVS <Rd>, <Rd>, LSL <Rs> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd Rs 0 0 0 1 Rd ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-61
- 366. Thumb Instructions 7.1.38 LSR (1) 15 14 13 12 11 10 6 5 3 2 0 0 0 0 0 1 immed_5 Rm Rd This form of the LSR (Logical Shift Right) instruction is used to provide the unsigned value of a register, divided by a constant power of two. LSR performs a logical shift right of the value of register <Rm>, and zeros are inserted into the vacated bit positions. The condition code flags are updated, based on the result. Syntax LSR <Rd>, <Rm>, #<immed_5> where: <Rd> Is the destination register for the operation. <Rm> Is the register containing the value to be shifted. <immed_5> Specifies the shift amount, in the range 1 to 32. Shifts by 1 to 31 are encoded directly in immed_5. A shift by 32 is encoded as immed_5 == 0. Architecture version All T variants Exceptions None Operation if immed_5 == 0 C Flag = Rd[31] Rd = 0 else /* immed_5 > 0 */ C Flag = Rd[immed_5 - 1] Rd = Rm Logical_Shift_Right immed_5 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 V Flag = unaffected A7-62 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 367. Thumb Instructions Equivalent ARM syntax and encoding MOVS <Rd>, <Rm>, LSR #<immed_5> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd immed_5 0 1 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-63
- 368. Thumb Instructions 7.1.39 LSR (2) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 0 0 1 1 Rs Rd This form of LSR is used to provide the unsigned value of a register divided by a variable power of two. Zeros are inserted into the vacated bit positions. The condition code flags are updated, based on the result. Syntax LSR <Rd>, <Rs> where: <Rd> Contains the value to be shifted, and is the destination register for the result of the operation. <Rs> Is the register containing the shift value. The value is held in the least significant byte. Architecture version All T variants Exceptions None Operation if Rs[7:0] == 0 then C Flag = unaffected Rd = unaffected else if Rs[7:0] < 32 then C Flag = Rd[Rs[7:0] - 1] Rd = Rd Logical_Shift_Right Rs[7:0] else if Rs[7:0] == 32 then C Flag = Rd[31] Rd = 0 else /* Rs[7:0] > 32 */ C Flag = 0 Rd = 0 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 V Flag = unaffected A7-64 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 369. Thumb Instructions Equivalent ARM syntax and encoding MOVS <Rd>, <Rd>, LSR <Rs> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd Rs 0 0 1 1 Rd ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-65
- 370. Thumb Instructions 7.1.40 MOV (1) 15 14 13 12 11 10 8 7 0 0 0 1 0 0 Rd immed_8 This form of the MOV (Move) instruction moves a large immediate value to a register. The condition code flags are updated, based on the result. Syntax MOV <Rd>, #<immed_8> where: <Rd> Is the destination register for the operation. <immed_8> Is an 8-bit immediate value, in the range 0 to 255, to move into <Rd>. Architecture version All T variants Exceptions None Operation Rd = immed_8 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected V Flag = unaffected Equivalent ARM syntax and encoding MOVS <Rd>, #<immed_8> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 1 1 1 0 0 0 1 1 1 0 1 1 SBZ Rd 0 0 0 0 immed_8 A7-66 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 371. Thumb Instructions 7.1.41 MOV (2) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 0 0 1 1 1 0 0 0 0 Rn Rd This form of MOV is used to move a value from one low register to another, and the flags are set according to that value. Syntax MOV <Rd>, <Rn> where: <Rd> Is the destination register for the operation. <Rn> Is the register containing the value to be copied. Architecture Version All T variants Exceptions None Operation Rd = Rn N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = 0 V Flag = 0 Notes Encoding This instruction is encoded as ADD Rd, Rn, #0. See also ADD (1) on page A7-5. Equivalent ARM syntax and encoding ADDS <Rd>, <Rn>, #0 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 1 0 0 0 1 0 1 0 0 1 Rn Rd 0 0 0 0 0 0 0 0 0 0 0 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-67
- 372. Thumb Instructions 7.1.42 MOV (3) 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 1 1 0 H1 H2 Rm Rd This form of MOV is used to move a value to, from, or between high registers. Unlike the low register MOV instruction described in MOV (2) on page A7-67, this instruction does not change the flags. Syntax MOV <Rd>, <Rm> where: <Rd> Is the destination register for the operation. It can be any of R0 to R15, and its number is encoded in the instruction in H1 (most significant bit) and Rd (remaining three bits). <Rm> Is the register containing the value to be copied. It can be any of R0 to R15, and its number is encoded in the instruction in H2 (most significant bit) and Rm (remaining three bits). Architecture version All T variants Exceptions None Operation Rd = Rm Usage The instruction MOV PC,R14 can be used as a subroutine return instruction if it is known that the caller is also a Thumb routine. However, it is more usual to use BX R14 (see BX on page A7-32), which works regardless of whether the caller is an ARM routine or a Thumb routine. In T variants of ARM architecture 4, the instruction MOV R14,PC is often used to set up a return link value in R14 for a call to an ARM routine. In T variants of ARM architecture 5 and above, there are more efficient ways of doing this. See BX on page A7-32 for more details. Notes Operand restriction If a low register is specified for <Rd> and <Rm> (H1==0 and H2==0), the result is UNPREDICTABLE. A7-68 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 373. Thumb Instructions Equivalent ARM syntax and encoding A close equivalent is: MOV <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 14 12 11 10 9 8 7 6 5 4 3 2 0 1 1 1 0 0 0 0 1 1 0 1 0 SBZ H1 Rd 0 0 0 0 0 0 0 0 H2 Rm There are slight differences when the instruction accesses the PC, because of the different definitions of the PC when executing ARM and THUMB code. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-69
- 374. Thumb Instructions 7.1.43 MUL 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 1 0 1 Rm Rd The MUL (Multiply) instruction multiplies signed or unsigned variables to produce a 32-bit result. The condition code flags are updated, based on the result. Syntax MUL <Rd>, <Rm> where: <Rd> Contains the value to be multiplied with the value of <Rm>, and is also the destination register for the operation. <Rm> Is the register containing the value to be multiplied with the value of <Rd>. Architecture version All T variants Exceptions None Operation Rd = (Rm * Rd)[31:0] N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected /* See "C flag" note */ V Flag = unaffected A7-70 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 375. Thumb Instructions Notes Operand restriction Specifying the same register for <Rd> and <Rm> has UNPREDICTABLE results. Early termination If the multiplier implementation supports early termination, it must be implemented on the value of the <Rd> operand. The type of early termination used (signed or unsigned) is IMPLEMENTATION DEFINED. Signed and unsigned As the MUL instruction produces only the lower 32 bits of the 64-bit product, MUL gives the same answer for multiplication of both signed and unsigned numbers. C flag The MUL instruction is defined to leave the C flag unchanged in ARM architecture version 5 and above. In earlier versions of the architecture, the value of the C flag was UNPREDICTABLE after a MUL instruction. Equivalent ARM syntax and encoding MULS <Rd>, <Rm>, <Rd> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 0 0 0 0 1 Rd SBZ Rd 1 0 0 1 Rm Note The following instruction is not a suitable alternative, as it violates the operand restriction on the ARM instruction (see MUL on page A4-66) and might have the wrong early termination behavior: MULS <Rd>, <Rd>, <Rm> ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-71
- 376. Thumb Instructions 7.1.44 MVN 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 1 1 1 Rm Rd The MVN (Move NOT) instruction is used to complement a register value, often to form a bit mask. The condition code flags are updated, based on the result. Syntax MVN <Rd>, <Rm> where: <Rd> Is the destination register for the operation. <Rm> Is the register containing the value whose one’s complement is written to <Rd>. Architecture version All T variants Exceptions None Operation Rd = NOT Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected V Flag = unaffected Equivalent ARM syntax and encoding MVNS <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 1 1 1 SBZ Rd 0 0 0 0 0 0 0 0 Rm A7-72 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 377. Thumb Instructions 7.1.45 NEG 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 0 0 1 Rm Rd The NEG (Negate) instruction negates the value of one register and stores the result in a second register. The condition code flags are updated (based on the result). Syntax NEG <Rd>, <Rm> where: <Rd> Is the destination register for the operation. <Rm> Is the register containing the value that is subtracted from zero. Architecture version All T variants Exceptions None Operation Rd = 0 - Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(0 - Rm) V Flag = OverflowFrom(0 - Rm) Equivalent ARM syntax and encoding RSBS <Rd>, <Rm>, #0 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 1 0 0 0 1 0 0 1 1 1 Rm Rd 0 0 0 0 0 0 0 0 0 0 0 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-73
- 378. Thumb Instructions 7.1.46 ORR 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 1 0 0 Rm Rd The ORR (Logical OR) instruction performs a bitwise OR of the values in two registers. The condition code flags are updated, based on the result. Syntax ORR <Rd>, <Rm> where: <Rd> Is the destination register for the operation. <Rm> Is the register containing the value that is ORed with the value of <Rd>. The operation is a bitwise inclusive OR. Architecture version All T variants Exceptions None Operation Rd = Rd OR Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = unaffected V Flag = unaffected Operation ORRS <Rd>, <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 0 1 Rd Rd 0 0 0 0 0 0 0 0 Rm A7-74 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 379. Thumb Instructions 7.1.47 POP 15 14 13 12 11 10 9 8 7 0 1 0 1 1 1 1 0 R register_list The POP (Pop Multiple Registers) instruction loads a subset (or possibly all) of the general-purpose registers R0-R7 and the PC from the stack. The general-purpose registers loaded can include the PC. If they do, the word loaded for the PC is treated as an address and a branch occurs to that address. In ARM architecture version 5 and above, bit[0] of the loaded value determines whether execution continues after this branch in ARM state or in Thumb state, as though the following instruction had been executed: BX (loaded_value) In T variants of architecture version 4, bit[0] of the loaded value is ignored and execution continues in Thumb state, as though the following instruction had been executed: MOV PC,(loaded_value) Syntax POP <registers> where: <registers> Is the list of registers, separated by commas and surrounded by { and }. The list is encoded in the register_list field of the instruction, by setting bit[i] to 1 if register Ri is included in the list and to 0 otherwise, for each of i=0 to 7. The R bit (bit[8]) is set to 1 if the PC is in the list and to 0 otherwise. At least one register must be loaded. If bits[8:0] are all zero, the result is UNPREDICTABLE. The registers are loaded in sequence, the lowest-numbered register from the lowest memory address (start_address), through to the highest-numbered register from the highest memory address (end_address). If the PC is specified in the register list (opcode bit[8] is set), the instruction causes a branch to the address (data) loaded into the PC. The <start_address> is the value of the SP. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The end_address value is four less than the sum of the value of the SP and four times the number of registers specified in <registers>. The SP register is incremented by four times the numbers of registers in <registers>. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-75
- 380. Thumb Instructions Architecture version All T variants Exceptions Data Abort Operation start_address = SP end_address = SP + 4*(R + Number_Of_Set_Bits_In(register_list)) address = start_address for i = 0 to 7 if register_list[i] == 1 then Ri = Memory[address,4] address = address + 4 if R == 1 then value = Memory[address,4] PC = value AND 0xFFFFFFFE if (architecture version 5 or above) then T Bit = value[0] address = address + 4 assert end_address = address SP = end_address Usage The POP instruction is useful for stack operations. A POP instruction with the PC in the register list can be used for an efficient procedure exit, as it restores saved registers, loads the PC with the return address, and updates the stack pointer with a single instruction. Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. CPSR The CPSR is not updated when POP loads the PC. Alignment POP instructions ignore the least significant two bits of address. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. A7-76 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 381. Thumb Instructions ARM/Thumb state transfers In ARM architecture 5 and above, if bits[1:0] of a value loaded for R15 are 0b10, the result is UNPREDICTABLE, as branches to non word-aligned addresses are not possible in ARM state. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. Equivalent ARM syntax and encoding LDMIA SP!, <registers> 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 0 1 1 1 0 1 0 0 0 1 0 1 1 1 1 0 1 R 0 0 0 0 0 0 0 register_list ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-77
- 382. Thumb Instructions 7.1.48 PUSH 15 14 13 12 11 10 9 8 7 0 1 0 1 1 0 1 0 R register_list The PUSH (Push Multiple Registers) instruction stores a subset (or possibly all) of the general-purpose registers R0-R7 and the LR to the stack. Syntax PUSH <registers> where: <registers> Is the list of registers to be stored, separated by commas and surrounded by { and }. The list is encoded in the register_list field of the instruction, by setting bit[i] to 1 if register Ri is included in the list and to 0 otherwise, for each of i=0 to 7. The R bit (bit[8]) is set to 1 if the LR is in the list and to 0 otherwise. At least one register must be stored. If bits[8:0] are all zero, the result is UNPREDICTABLE. The registers are stored in sequence, the lowest-numbered register to the lowest memory address (start_address), through to the highest-numbered register to the highest memory address (end_address) The start_address is the value of the SP minus 4 times the number of registers to be stored. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The end_address value is four less than the original value of SP. The SP register is decremented by four times the numbers of registers in <registers>. Architecture version All T variants Exceptions Data Abort A7-78 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 383. Thumb Instructions Operation start_address = SP - 4*(R + Number_Of_Set_Bits_In(register_list)) end_address = SP - 4 address = start_address for i = 0 to 7 if register_list[i] == 1 Memory[address,4] = Ri address = address + 4 if R == 1 Memory[address,4] = LR address = address + 4 assert end_address == address - 4 SP = SP - 4*(R + Number_Of_Set_Bits_In(register_list)) Usage PUSH is useful for stack operations. A PUSH instruction with the LR in the register list can be used for an efficient procedure entry, as it saves registers (including the return address) on the stack and updates the stack pointer with a single instruction. A matching POP instruction can be used later to return from the procedure. Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment PUSH instructions ignore the least significant two bits of address. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. Equivalent ARM syntax and encoding STMDB SP!, <registers> 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 0 1 1 1 0 1 0 0 1 0 0 1 0 1 1 0 1 0 R 0 0 0 0 0 0 register_list ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-79
- 384. Thumb Instructions 7.1.49 ROR 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 0 1 1 1 Rs Rd The ROR (Rotate Right Register) instruction is used to provide the value of a register rotated by a variable value (from another register). The bits that are rotated off the right end are inserted into the vacated bit positions on the left. The condition code flags are updated, based on the result. Syntax ROR <Rd>, <Rs> where: <Rd> Contains the value to be rotated, and is also the destination register for the operation. <Rs> Is the register containing the rotation applied to the value of <Rd>. The value of the rotation is stored in the least significant byte. Architecture version All T variants Exceptions None Operation if Rs[7:0] == 0 then C Flag = unaffected Rd = unaffected else if Rs[4:0] == 0 then C Flag = Rd[31] Rd = unaffected else /* Rs[4:0] > 0 */ C Flag = Rd[Rs[4:0] - 1] Rd = Rd Rotate_Right Rs[4:0] N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 V Flag = unaffected A7-80 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 385. Thumb Instructions Equivalent ARM syntax and encoding MOVS <Rd>, <Rd>, ROR <Rs> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 1 0 1 1 SBZ Rd Rs 0 1 1 1 Rd ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-81
- 386. Thumb Instructions 7.1.50 SBC 15 14 13 12 11 10 9 9 8 6 5 3 2 0 0 1 0 0 0 0 0 1 1 0 Rm Rd The SBC (Subtract with Carry) instruction can be used to synthesize multi-word subtraction. It subtracts the value of register <Rm> and the value of NOT(Carry Flag) from the value of register <Rd>. The condition code flags are updated, based on the result. Syntax SBC <Rd>, <Rm> where: <Rd> Contains the first operand for the subtraction, and is also the destination register for the operation. <Rm> Contains the value to be subtracted from <Rd>. Architecture version All T variants Exceptions None Operation Rd = Rd - Rm - NOT(C Flag) N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rd - Rm - NOT(C Flag)) V Flag = OverflowFrom(Rd - Rm - NOT(C Flag)) A7-82 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 387. Thumb Instructions Equivalent ARM syntax and encoding SBCS <Rd>, <Rd>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 0 1 1 0 1 Rd Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-83
- 388. Thumb Instructions 7.1.51 STMIA 15 14 13 12 11 10 8 7 0 1 1 0 0 0 Rn register_list The STMIA (Store Multiple Increment After) instruction stores a non-empty subset, or possibly all, of the general-purpose registers to sequential memory locations. Syntax STMIA <Rn>!, <registers> where: <Rn> Is the register containing the start address for the instruction. ! Causes base register writeback, and is not optional. <registers> Is a list of registers to be stored, separated by commas and surrounded by { and }. The list is encoded in the register_list field of the instruction, by setting bit[i] to 1 if register Ri is included in the list and to 0 otherwise, for each of i=0 to 7. At least one register must be stored. If bits[7:0] are all zero, the result is UNPREDICTABLE. The registers are stored in sequence, the lowest-numbered register to the lowest memory address (start_address), through to the highest-numbered register to the highest memory address (end_address). The start_address is the value of the base register <Rn>. Subsequent addresses are formed by incrementing the previous address by four. One address is produced for each register that is specified in <registers>. The end_address value is four less than the sum of the value of the base register and four times the number of registers specified in <registers>. Finally, the base register <Rn> is incremented by 4 times the numbers of registers in <registers>. Architecture version All T variants Exceptions Data Abort A7-84 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 389. Thumb Instructions Operation start_address = Rn end_address = Rn + (Number_Of_Set_Bits_In(register_list) * 4) - 4 address = start_address for i = 0 to 7 if register_list[i] == 1 Memory[address,4] = Ri address = address + 4 assert end_address == address - 4 Rn = Rn + (Number_Of_Set_Bits_In(register_list) * 4) Usage STMIA is useful as a block store instruction. Combined with LDMIA (Load Multiple), it allows efficient block copy. Notes Operand restrictions If <Rn> is specified in <registers> and writeback is specified: • If <Rn> is the lowest-numbered register specified in <registers>, the original value of <Rn> is stored. • Otherwise, the stored value of <Rn> is UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment Store Multiple instructions ignore the least significant two bits of address. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception. Time order The time order of the accesses to individual words of memory generated by this instruction is only defined in some circumstances. See Data accesses to memory-mapped I/O on page A2-32 for details. Equivalent ARM syntax and encoding STMIA <Rn>!, <registers> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 14 13 12 11 10 9 8 7 0 1 1 1 0 1 0 0 0 1 0 1 0 Rn 0 0 0 0 0 0 0 0 register_list ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-85
- 390. Thumb Instructions 7.1.52 STR (1) 15 14 13 12 11 10 6 5 3 2 0 0 1 1 0 0 immed_5 Rn Rd This form of the STR (Store Register) instruction allows 32-bit data from a general-purpose register to be stored to memory. The addressing mode is useful for accessing structure (record) fields. With an offset of zero, the address produced is the unaltered value of the base register <Rn>. Syntax STR <Rd>, [<Rn>, #<immed_5> * 4] where: <Rd> Is the register that contains the word to be stored to memory. <Rn> Is the register containing the base address for the instruction. <immed_5> Is a 5-bit value that is multiplied by 4 and added to the value of <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + (immed_5 * 4) if address[1:0] == 0b00 Memory[address,4] = Rd else Memory[address,4] = UNPREDICTABLE A7-86 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 391. Thumb Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not word-aligned and no data abort occurs, the value written to memory is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception (a type of data abort). Equivalent ARM syntax and encoding STR <Rd>, [<Rn>, #<immed_5> * 4] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 2 1 0 1 1 1 0 0 1 0 1 1 0 0 0 Rn Rd 0 0 0 0 0 immed_5 0 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-87
- 392. Thumb Instructions 7.1.53 STR (2) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 0 0 0 Rm Rn Rd This form of STR allows 32-bit data from a general-purpose register to be stored to memory. The addressing mode is useful for pointer + large offset arithmetic, and for accessing a single element of an array. Syntax STR <Rd>, [<Rn>, <Rm>] where: <Rd> Is the register that contains the word to be stored to memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register containing the second value used in forming the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm if address[1:0] == 0b00 Memory[address,4] = Rd else Memory[address,4] = UNPREDICTABLE Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not word-aligned and no data abort occurs, the value written to memory is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception (a type of data abort). A7-88 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 393. Thumb Instructions Equivalent ARM syntax and encoding STR <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 1 1 1 1 0 0 0 Rn Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-89
- 394. Thumb Instructions 7.1.54 STR (3) 15 14 13 12 11 10 8 7 0 1 0 0 1 0 Rd immed_8 This form of STR allows 32-bit data from a general-purpose register to be stored to memory. The addressing mode is useful for accessing stack data. In this case, STR stores a word from register <Rd> to memory. Syntax STR <Rd>, [SP, #<immed_8> * 4] where: <Rd> Is the register that contains the word to be stored to memory. SP Is the stack pointer. Its value is used to calculate the memory address. <immed_8> Is an 8-bit value that is multiplied by 4 and added to the value of the SP to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = SP + (immed_8 * 4) if address[1:0] == 0b00 Memory[address,4] = Rd else Memory[address,4] = UNPREDICTABLE A7-90 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 395. Thumb Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not word-aligned and no data abort occurs, the value written to memory is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, an address with bits[1:0] != 0b00 causes an alignment exception (a type of data abort). Equivalent ARM syntax and encoding STR <Rd>, [SP, #<immed_8> * 4] 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 2 1 0 1 1 1 0 0 1 0 1 1 0 0 0 1 1 0 1 Rd 0 0 immed_8 0 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-91
- 396. Thumb Instructions 7.1.55 STRB (1) 15 14 13 12 11 10 6 5 3 2 0 0 1 1 1 0 immed_5 Rn Rd This form of the STRB (Store Register Byte) instruction allows 8-bit data from a general-purpose register to be stored to memory. The addressing mode is useful for accessing structure (record) fields. With an offset of zero, the address produced is the unaltered value of the base register <Rn>. Syntax STRB <Rd>, [<Rn>, #<immed_5>] where: <Rd> Is the register whose least significant byte is stored to memory. <Rn> Is the register containing the base address for the instruction. <immed_5> Is a 5-bit immediate value that is added to the value of <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + immed_5 Memory[address,1] = Rd[7:0] Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Equivalent ARM syntax and encoding STRB <Rd>, [<Rn>, #<immed_5>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 0 1 1 1 0 0 1 0 1 1 1 0 0 Rn Rd 0 0 0 0 0 0 0 immed_5 A7-92 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 397. Thumb Instructions 7.1.56 STRB (2) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 0 1 0 Rm Rn Rd This form of STRB allows 8-bit data from a general-purpose register to be stored to memory. The addressing mode is useful for pointer + large offset arithmetic, and for accessing a single element of an array. Syntax STRB <Rd>, [<Rn>, <Rm>] where: <Rd> Is the register whose least significant byte is stored to memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register whose value is added to <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm Memory[address,1] = Rd[7:0] Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Equivalent ARM syntax and encoding STRB <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 1 1 1 1 1 0 0 Rn Rd 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-93
- 398. Thumb Instructions 7.1.57 STRH (1) 15 14 13 12 11 10 6 5 3 2 0 1 0 0 0 0 immed_5 Rn Rd This form of the STRH (Store Register Halfword) instruction allows 16-bit data from a general-purpose register to be stored to memory. The addressing mode is useful for accessing structure (record) fields. With an offset of zero, the address produced is the unaltered value of the base register <Rn>. Syntax STRH <Rd>, [<Rn>, #<immed_5> * 2] where: <Rd> Is the register whose least significant halfword is stored to memory. <Rn> Is the register containing the base address for the instruction. <immed_5> Is a 5-bit immediate value that is multiplied by two and added to the value of <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + (immed_5 * 2) if address[1:0] == 0 Memory[address,2] = Rd[15:0] else Memory[address,2] = UNPREDICTABLE A7-94 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 399. Thumb Instructions Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written to memory is UNPREDICTABLE. If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception (a type of data abort). Equivalent ARM syntax and encoding STRH <Rd>, [<Rn>, #<immed_5> * 2] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 1 0 1 1 1 0 0 0 0 1 1 1 0 0 Rn Rd 0 0 immed [4:3] 1 0 1 1 immed [2:0] 0 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-95
- 400. Thumb Instructions 7.1.58 STRH (2) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 1 0 1 0 0 1 Rm Rn Rd This form of STRH allows 16-bit data from a general-purpose register to be stored to memory. The addressing mode is useful for pointer + large offset arithmetic and for accessing a single element of an array. Syntax STRH <Rd>, [<Rn>, <Rm>] where: <Rd> Is the register whose least significant halfword is stored to memory. <Rn> Is the register containing the first value used in forming the memory address. <Rm> Is the register whose value is added to <Rn> to form the memory address. Architecture version All T variants Exceptions Data Abort Operation address = Rn + Rm if address[1:0] == 0 Memory[address,2] = Rd[15:0] else Memory[address,2] = UNPREDICTABLE Notes Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Alignment If the memory address is not halfword-aligned and no data abort occurs, the value written to memory is UNPREDICTABLE If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor) and alignment checking is enabled, an address with bit[0] != 0 causes an alignment exception (a type of data abort). A7-96 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 401. Thumb Instructions Equivalent ARM syntax and encoding STRH <Rd>, [<Rn>, <Rm>] 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 2 1 0 1 1 1 0 0 0 0 1 1 0 0 0 Rn Rd SBZ 1 0 1 1 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-97
- 402. Thumb Instructions 7.1.59 SUB (1) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 0 0 1 1 1 1 immed_3 Rn Rd This form of the SUB (Subtract) instruction subtracts a small constant value from the value of a register and stores the result in a second register. The condition code flags are updated, based on the result. Syntax SUB <Rd>, <Rn>, #<immed_3> where: <Rd> Is the destination register for the operation. <Rn> Is the register containing the first operand for the subtraction. <immed_3> Is a 3-bit immediate value (values 0 to 7) that is subtracted from <Rn>. Architecture version All T variants Exceptions None Operation Rd = Rn - immed_3 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - immed_3) V Flag = OverflowFrom(Rn - immed_3) Equivalent ARM syntax and encoding SUBS <Rd>, <Rn>, #<immed_3> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 2 0 1 1 1 0 0 0 1 0 0 1 0 1 Rn Rd 0 0 0 0 0 0 0 0 0 immed_3 A7-98 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 403. Thumb Instructions 7.1.60 SUB (2) 15 14 13 12 11 10 8 7 0 0 0 1 1 1 Rd immed_8 This form of SUB subtracts a large immediate value from the value of a register and stores the result back in the same register. The condition code flags are updated, based on the result. Syntax SUB <Rd>, #<immed_8> where: <Rd> Is the register containing the first operand for the subtraction, and is also the destination register for the operation. <immed_8> Is an 8-bit immediate value (values 0 to 255) that is subtracted from <Rd>. Architecture version All T variants Exceptions None Operation Rd = Rd - immed_8 N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rd - immed_8) V Flag = OverflowFrom(Rd - immed_8) Equivalent ARM syntax and encoding SUBS <Rd, <Rd>, #<immed_8> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 1 1 1 0 0 0 1 0 0 1 0 1 Rd Rd 0 0 0 0 immed_8 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-99
- 404. Thumb Instructions 7.1.61 SUB (3) 15 14 13 12 11 10 9 8 6 5 3 2 0 0 0 0 1 1 0 1 Rm Rn Rd This form of SUB subtracts the value of one register from the value of a second register and stores the result in a third register. The condition code flags are updated, based on the result. Syntax SUB <Rd>, <Rn>, <Rm> where: <Rd> Is the destination register for the operation. <Rn> Is the register containing the first operand for the subtraction. <Rm> Is the register whose value is subtracted from <Rn>. Architecture version All T variants Exceptions None Operation Rd = Rn - Rm N Flag = Rd[31] Z Flag = if Rd == 0 then 1 else 0 C Flag = NOT BorrowFrom(Rn - Rm) V Flag = OverflowFrom(Rn - Rm) Equivalent ARM syntax and encoding SUBS <Rd>, <Rn>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 0 0 1 0 1 Rn Rd 0 0 0 0 0 0 0 0 Rm A7-100 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 405. Thumb Instructions 7.1.62 SUB (4) 15 14 13 12 11 10 9 8 7 6 0 1 0 1 1 0 0 0 0 1 immed_7 This form of SUB decrements the SP by four times a 7-bit immediate (that is, by a multiple of 4 in the range 0 to 508). The condition codes are not affected. Syntax SUB SP, #<immed_7> * 4 where: SP Indicates the stack pointer. The result of the operation is also stored in the SP. <immed_7> Is a 7-bit immediate value that is multiplied by 4 and then subtracted from the value of the stack pointer. Architecture version All T variants Exceptions None Operation SP = SP - (immed_7 << 2) Usage For the Full Descending stack which the Thumb instruction set is designed to use, decrementing the SP is used to allocate extra memory variables on the top of the stack. Notes Alternative syntax This instruction can also be written as SUB SP, SP, #<immed_7> * 4. Equivalent ARM syntax and encoding SUB SP, SP, #<immed_7> * 4 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 0 1 1 1 0 0 0 1 0 0 1 0 0 1 1 0 1 1 1 0 1 1 1 1 1 0 immed_7 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-101
- 406. Thumb Instructions 7.1.63 SWI 15 14 13 12 11 10 9 8 7 0 1 1 0 1 1 1 1 1 immed_8 The SWI (Software Interrupt) instruction is used as an operating system service call. A SWI exception is generated, which is handled by an operating system to provide the requested service. See Exceptions on page A2-13. Syntax SWI <immed_8> where: <immed_8> Is an 8-bit immediate value that is put into bits[7:0] of the instruction. This value is ignored by the processor, but can be used by an operating system’s SWI exception handler to determine which operating system service is being requested. Architecture version All T variants Exceptions Software Interrupt Operation R14_svc = address of next instruction after the SWI instruction SPSR_svc = CPSR CPSR[4:0] = 0b10011 /* Enter Supervisor mode */ CPSR[5] = 0 /* Execute in ARM state */ /* CPSR[6] is unchanged */ CPSR[7] = 1 /* Disable normal interrupts */ if high vectors configured then PC = 0xFFFF0008 else PC = 0x00000008 Equivalent ARM syntax and encoding SWI <immed_8> 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 0 1 1 1 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 immed_8 A7-102 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 407. Thumb Instructions 7.1.64 TST 15 14 13 12 11 10 9 8 7 6 5 3 2 0 0 1 0 0 0 0 1 0 0 0 Rm Rn The TST (Test) instruction is used to determine whether a particular subset of bits in a register includes at least one set bit. A very common use for TST is to test whether a single bit is set or clear. The condition code flags are updated, based on the result. Syntax TST <Rn>, <Rm> where: <Rn> Is the register containing the first operand for the instruction. <Rm> Is the register whose value is logically ANDed with the value of <Rn>. Architecture version All T variants Exceptions None Operation alu_out = Rn AND Rm N Flag = alu_out[31] Z Flag = if alu_out == 0 then 1 else 0 C Flag = unaffected V Flag = unaffected Equivalent ARM syntax and encoding TST <Rn>, <Rm> 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 1 1 1 0 0 0 0 1 0 0 0 1 Rn SBZ 0 0 0 0 0 0 0 0 Rm ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-103
- 408. Thumb Instructions 7.2 Thumb instructions and architecture versions Table 7-1 shows which Thumb instructions are present in each current ARM architecture version that supports Thumb. Table 7-1 Thumb instructions by architecture Instruction v4T, v4TxM v5T, v5TxM ADC Yes Yes ADD (all forms) Yes Yes AND Yes Yes ASR (both forms) Yes Yes B (both forms) Yes Yes BIC Yes Yes BKPT No Yes BL Yes Yes BLX (both forms) No Yes BX Yes Yes CMN Yes Yes CMP (all forms) Yes Yes EOR Yes Yes LDMIA Yes Yes LDR (all forms) Yes Yes LDRB (both forms) Yes Yes LDRH (both forms) Yes Yes LDRSB Yes Yes LDRSH Yes Yes LSL (both forms) Yes Yes LSR (both forms) Yes Yes MOV (all forms) Yes Yes A7-104 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 409. Thumb Instructions Table 7-1 Thumb instructions by architecture (Continued) Instruction v4T, v4TxM v5T, v5TxM MUL Yes Yes MVN Yes Yes NEG Yes Yes ORR Yes Yes POP Yes Yes PUSH Yes Yes ROR Yes Yes SBC Yes Yes STMIA Yes Yes STR (all forms) Yes Yes STRB (both forms) Yes Yes STRH (both forms) Yes Yes SUB (all forms) Yes Yes SWI Yes Yes TST Yes Yes ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A7-105
- 410. Thumb Instructions A7-106 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 411. Chapter A8 The 26-bit Architectures This chapter describes the differences between the 32-bit and 26-bit architectures. It contains the following sections: • Overview of the 26-bit architectures on page A8-2 • Format of register 15 on page A8-4 • 26-bit PSR update instructions on page A8-6 • Address exceptions on page A8-8 • Backwards compatibility from 32-bit architectures on page A8-9. Note The information in this chapter is to help with backwards-compatibility issues in existing systems. Programmers starting new projects are strongly encouraged to use only the 32-bit facilities. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-1
- 412. The 26-bit Architectures 8.1 Overview of the 26-bit architectures ARMv1, ARMv2, and ARMv2a are earlier versions of the ARM architecture which implement only a 26-bit address space, and are known as 26-bit architectures. ARM architecture version 3 and above implement a 32-bit address space and are known as 32-bit architectures. For backwards compatibility: • except for ARMv3G, all variants of ARM architecture version 3 implement the 26-bit address space • all non-T variants of ARM architecture version 4 and above can optionally implement the 26-bit address space. This chapter describes the differences between the 26-bit and 32-bit architectures, and how the backwards-compatibility features are used. There are several differences between the 26-bit and the 32-bit architectures: Program counter The 26-bit architectures implement only a 24-bit program counter in R15, which allows 64MB of program space. The 32-bit architectures have a 30-bit program counter in R15 which allows 4GB of program space on 32-bit architectures. Processor modes Only four processor modes are supported on 26-bit architectures: • User (0b00) • FIQ (0b01) • IRQ (0b10) • Supervisor (0b11). Register 15 In the 26-bit architectures, the following are also stored in register 15: • four condition flags (N, Z, C and V) • the interrupt disable flags (I and F) • two processor mode bits (M1 and M0). CPSR/SPSR The 26-bit architectures do not have a CPSR or any SPSRs. Exceptions An exception (called an address exception) is raised if a memory access instruction uses an address that is greater than 226–1 bytes. Branches In 26-bit architectures, there are no restrictions on branching backwards past location 0x0000000 or forwards past location 0x3FFFFFF. Such branches wrap around to the other end of the 26-bit address space, and so have a different target address than they would have had in a 32-bit architecture. As a result, the signed 24-bit word offset in the B and BL instructions allows any instruction in the 26-bit address space to be branched to. A8-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 413. The 26-bit Architectures Together, these differences make up the fundamental distinction between 26-bit and 32-bit architectures: 26-bit architectures All process status (namely the condition flags, interrupt status and processor mode) can be preserved across subroutine calls and nested exceptions without adding any instructions to the entry or exit sequence. 32-bit architectures This process status functionality is given up to allow 32-bit instruction addresses to be used. For exceptions, processor status is preserved in the SPSRs, and if nested exceptions using the same SPSR can occur, extra instructions are used to preserve this status in memory. For subroutine calls, processor status can be preserved across the subroutine call by using extra instructions, but this is not normally done. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-3
- 414. The 26-bit Architectures 8.2 Format of register 15 31 30 29 28 27 26 25 2 1 0 N Z C V I F Program Counter M1M0 Bits[25:2] are collectively known as the Program Counter (PC). Because the PC occupies only 24 bits of register 15, only 224 instructions (226 bytes) can be addressed, giving a maximum addressable program size of 64MB. Bits[31:26] and bits[1:0] are collectively known as the Program Status Register (PSR). The N, Z, C, V, I, and F bits have the same meaning in both 26-bit and 32-bit architectures. M[1:0] also have the same meaning in both architectures. Abort, Undefined, and System mode are not supported in 26-bit architectures. Aborts and Undefined Instruction exceptions have exactly the same actions in both modes, except that in 26-bit architectures, Supervisor mode is entered instead of Abort or Undefined mode. The precise effect of an exception on a 26-bit architecture can be derived from the rules in Exceptions on page A2-13, as follows: • The banked version of R14 has bits[25:2] set to the specified address, and bits[31:26, 1, 0] set to copies of the corresponding bits in R15. • The I, F, M1, and M0 bits are modified in the same way as CPSR[7], CPSR[6], CPSR[1], and CPSR[0] respectively, on a 32-bit architecture. The I, F and M[1:0] bits cannot be written directly when the processor is in User mode. In User mode they are only changed by an exception occurring. 8.2.1 Reading register 15 In 26-bit architectures, the value of R15 is read in five different ways: • Most importantly, if R15 has an UNPREDICTABLE value in the 32-bit architecture, it also has an UNPREDICTABLE value when used in the same way in the 26-bit architecture. • If R15 is specified in bits[19:16] of an instruction (and its value is not UNPREDICTABLE), only the PC (bits[25:2]) is used. All other bits read as zero. • If R15 is specified in bits[3:0] of an instruction (and its value is not UNPREDICTABLE), all 32 bits are used. • If R15 is stored using STR or STM, the value of the PC (bits[25:2]) is IMPLEMENTATION DEFINED, but all 32 bits of the register are stored. • All 32 bits are stored in the Link register (R14) after a Branch with Link instruction or an exception entry. A8-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 415. The 26-bit Architectures 8.2.2 Writing register 15 In 26-bit architectures, the value of R15 is written in three different ways: • The following instructions only write the PC part of R15, leaving the PSR part unchanged: — Data-processing instructions without the S bit set — LDR instructions — LDM instructions, other than Load Multiple with Restore CPSR. • The following instructions write both the PC and the PSR part of R15: — Data-processing instructions with the S bit set — Load Multiple with Restore CPSR. • Variants of the CMP, CMN, TST, and TEQ instructions write just the PSR part of R15 and leave the PC part unchanged. These instruction variants are described 26-bit PSR update instructions on page A8-6. These read/write rules mean that R15 is used in three basic ways: • When it is the Rn specifier in data-processing instructions, or the base address for load and store instructions, only the value of the program counter is used, to simplify PC-relative addressing and position-independent code. • When it is the Rm specifier in data-processing instructions, all 32 bits are used in order to allow all process status to be restored after a subroutine call or exception by subroutine-return instructions such as: MOVS PC, LR and LDM..., PC}^ These instructions are UNPREDICTABLE in User mode on 32-bit architectures, but are legal on 26-bit architectures, as they are used to restore the condition code flags on procedure exit. • All 32 bits are saved in the Link register, to preserve the PC and the PSR across subroutine calls and exceptions. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-5
- 416. The 26-bit Architectures 8.3 26-bit PSR update instructions 31 27 26 25 24 23 22 21 20 19 16 15 12 11 0 cond 0 0 I 1 0 opc 1 Rn 1 1 1 1 shifter_operand In 26-bit architectures, the MSR and MRS instructions are not supported. Instead, variants of the CMN, CMP, TEQ and TST instructions are used to write just the PSR part of R15 without affecting the PC part. These 26-bit-only variants are called CMNP, CMPP, TEQP and TSTP, and are distinguished by having instruction bits[15:12] equal to 0b1111. These bits are equal to 0b0000 in the normal variants of these instructions. These instructions write their ALU result directly to the PSR part of register 15 (only N, Z, C, and V are affected in User mode). When the processor is in User mode, only the condition codes are affected. All other modes allow all PSR bits to be altered. 8.3.1 Syntax CMN{<cond>}P Rn, <shifter_operand> CMP{<cond>}P Rn, <shifter_operand> TEQ{<cond>}P Rn, <shifter_operand> TST{<cond>}P Rn, <shifter_operand> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the register that contains the first operand for the instruction. <shifter_operand> Specifies the second operand for the instruction. Its options are described in Addressing Mode 1 - Data-processing operands on page A5-2, including how each option causes the I bit (bit 25) and the <shifter_operand> bits (bits 11:0) to be set in the instruction. 8.3.2 Exceptions None A8-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 417. The 26-bit Architectures 8.3.3 Operation if ConditionPassed(<cond>) then case <opc> of 0b00 /* TSTP */ <alu_out> = Rn AND <shifter_operand> 0b01 /* TEQP */ <alu_out> = Rn EOR <shifter_operand> 0b10 /* CMPP */ <alu_out> = Rn - <shifter_operand> 0b11 /* CMNP */ <alu_out> = Rn + <shifter_operand> endcase if R15[1:0] == 0b00 then /* M[1:0] == 0b00, User mode */ R15[31:28] = <alu_out>[31:28] /* update just NZCV */ else /* a privileged mode */ R15[31:26] = <alu_out>[31:26] /* update NZCVIF and ... */ R15[1:0] = <alu_out>[1:0] /* ... update M[1:0] */ ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-7
- 418. The 26-bit Architectures 8.4 Address exceptions In 26-bit architectures, all data addresses are checked to ensure that they are between 0 and 64MB. If a data address is produced with a 1 in any of the top 6 bits, an address exception is generated. When an address exception is generated, the following actions are performed: R14_svc[25:2] = address of instruction + 8 R14_svc[31:26,1,0] = R15[31:26,1,0] M[1:0] = 0b11 ; Supervisor mode F = unchanged I = 1 ; (normal) interrupts disabled PC = 0x14 The address of the instruction which caused the address exception is the value in R14 minus 8. 8.4.1 Returning from an address exception As this exception implies a programming error, it is not usual to return from address exceptions, but if a return is required, use: SUBS PC,R14,#8 This restores both the PC and PSR (from R14_svc) and returns to the instruction that generated the address exception. A8-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 419. The 26-bit Architectures 8.5 Backwards compatibility from 32-bit architectures As described in Overview of the 26-bit architectures on page A8-2, some 32-bit architectures include mandatory or optional backwards-compatibility support for 26-bit architectures. Architecture versions with this support implement the four 26-bit modes described in Overview of the 26-bit architectures on page A8-2. When the processor is in one of these modes, the PC behaves as explained in Format of register 15 on page A8-4 and the instructions described in 26-bit PSR update instructions on page A8-6 are available. This allows older 26-bit programs to be executed in a suitable 26-bit mode. In architecture versions without 26-bit backwards-compatibility support, CPSR bit[4] (M[4]) always reads as 1, and all writes to it are ignored. The complete list of processor modes is shown in Table 8-1. Table 8-1 32-bit and 26-bit modes M[4:0] Mode Accessible registers 0b00000 User_26 R0 to R14, PC, (CPSR) 0b00001 FIQ_26 R0 to R7, R8_fiq to R14_fiq, PC, (CPSR, SPSR_fiq) 0b00010 IRQ_26 R0 to R12, R13_irq, R14_irq, PC, (CPSR, SPSR_irq) 0b00011 SVC_26 R0 to R12, R13_svc, R14_svc, PC, (CPSR, SPSR_svc) 0b10000 User_32 R0 to R14, PC, CPSR 0b10001 FIQ_32 R0 to R7, R8_fiq to R14_fiq, PC, CPSR, SPSR_fiq 0b10010 IRQ_32 R0 to R12, R13_irq, R14_irq, PC, CPSR, SPSR_irq 0b10011 SVC_32 R0 to R12, R13_svc, R14_svc, PC, CPSR, SPSR_svc 0b10111 Abort_32 R0 to R12, R13_abt, R14_abt, PC, CPSR, SPSR_abt 0b11011 Undef_32 R0 to R12, R13_und, R14_und, PC, CPSR, SPSR_und 0b11111 System_32 R0 to R14, PC, CPSR (architecture version 4 and above) ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-9
- 420. The 26-bit Architectures Note The MRS and MSR instructions are supported in all cases, despite the fact that they do not exist in pure 26-bit architectures. This is to assist conversion to a 32-bit architecture, as the CMNP, CMPP, TEQP, and TSTP instructions cannot enter a 32-bit mode from a 26-bit mode. If an MSR instruction which lies outside the 26-bit address space is used to switch into a 26-bit mode, the results are UNPREDICTABLE. The results of an MSR instruction which switches between a 32-bit mode and a 26-bit mode (in either direction) are also UNPREDICTABLE if it is executed at any point where predictable subsequent execution can reach either of the following: • the end of the 26-bit address space • a branch instruction whose target addresses in the 26-bit and 32-bit address spaces are different. For this purpose, predictable subsequent execution is as defined in Other uses for IMBs on page A2-30. 8.5.1 32-bit and 26-bit configuration Architecture versions with 26-bit backwards compatibility support optionally incorporate two hardware signals that control 32-bit instruction accesses and 32-bit data accesses. The signals are mapped to two bits in register 1 of the system control coprocessor. These signals are: • PROG32 • DATA32. Note If an implementation does not support these signals but does provide 32-bit modes, it must behave as though both signals were 1. 32-bit configuration 1. If PROG32 is active and DATA32 is active, the processor switches to a 32-bit mode when processing exceptions (including Reset), using the _32 modes for handling all exceptions. This is called a 32-bit configuration. Abort_32 mode is used for handling memory aborts, and Undef_32 for handling Undefined Instruction exceptions. A 26-bit mode can be selected by putting a 26-bit mode number into the M[4:0] bits of the CPSR (either using MSR or an exception return sequence). A 32-bit mode can also be entered from a 26-bit mode using the MSR instruction. Once in a 26-bit mode, another 26-bit mode can be entered using one of the CMNP, CMPP, TEQP and TSTP instructions, or the MSR instruction. If an exception occurs when the processor is in a 26-bit mode, only the PC bits from R15[25:2] are copied to the link register. The remaining bits in the link register are zeroed. The PSR bits from R15[31:26] and R15[1:0] are copied into the SPSR, ready for a normal 32-bit return sequence. 2. If PROG32 is active and DATA32 is not active (32-bit programs with 26-bit data), the result is UNPREDICTABLE. A8-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 421. The 26-bit Architectures 26-bit configuration 1. If PROG32 is not active, the processor is locked into 26-bit modes (that is, cannot be placed into a 32-bit mode by any means) and handles exceptions in 26-bit modes. This is called a 26-bit configuration. In this configuration, CMNP, CMPP, TEQP and TSTP instructions, or the MSR instruction can be used to switch to 26-bit modes. Attempts to write CPSR bits[4:2] (M[4:2]) are ignored, stopping any attempts to switch to a 32-bit mode, and SVC_26 mode is used to handle memory aborts and Undefined Instruction exceptions. The PC is limited to 24 bits, limiting the addressable program memory to 64MB. 2. If PROG32 is not active, DATA32 has the following actions: • If DATA32 is not active, all data addresses are checked to ensure that they are between 0 and 64MB. If a data address is produced with a 1 in any of the top 6 bits, an address exception is generated. • If DATA32 is active, full 32-bit addresses can be produced and are not checked for address exceptions. This allows 26-bit programs to access data in the full 32-bit address space. 8.5.2 Vector exceptions When the processor is in a 32-bit configuration (PROG32 is active) and in a 26-bit mode (CPSR[4] == 0), data access (but not instruction fetches) to the exception vectors (address 0x0 to 0x1F) causes a data abort. This is known as a vector exception. Vector exceptions are always produced if the exception vectors are written in a 32-bit configuration and a 26-bit mode. It is IMPLEMENTATION DEFINED whether reading the exception vectors in a 32-bit configuration and a 26-bit mode also causes a vector exception. Vector exceptions are provided to support 26-bit backwards compatibility. When a vector exception is generated, it indicates that a 26-bit mode process is trying to install a (26-bit) vector handler. Because the processor is in a 32-bit configuration, exceptions are handled in a 32-bit mode, so a veneer must be used to change from the 32-bit exception mode to a 26-bit mode before calling the 26-bit exception handler. This veneer can be installed on each vector and can switch to a 26-bit mode before calling any 26-bit handlers. The return from the 26-bit exception handler might also need to be veneered. Some SWI handlers return status information in the processor flags, and this information needs to be transferred from the link register to the SPSR with a return veneer for the SWI handler. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A8-11
- 422. The 26-bit Architectures A8-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 423. Chapter A9 ARM Code Sequences The ARM instruction set is a powerful tool for generating high-performance microprocessor systems. Used to its full extent, the ARM instruction set allows algorithms to be coded in a very compact and efficient way. This chapter describes some sample routines that provide insight into the ARM instruction set. It contains the following sections: • Arithmetic instructions on page A9-2 • Branch instructions on page A9-5 • Load and Store instructions on page A9-7 • Load and Store Multiple instructions on page A9-10 • Semaphore instructions on page A9-11 • Other code examples on page A9-12. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-1
- 424. ARM Code Sequences 9.1 Arithmetic instructions The following subsections illustrate some ways of using ARM data-processing instructions. The examples illustrate: • Bit field manipulation • Multiplication by constant • Multi-precision arithmetic on page A9-3 • Swapping endianness on page A9-4. 9.1.1 Bit field manipulation The ARM shift and logical instructions can be used for bit field manipulation: ; Extract 8 bits from the top of R2 and insert them into ; the bottom of R3, shifting up the data in R3 ; R0 is a temporary value MOV R0, R2, LSR #24 ; extract top bits from R2 into R0 ORR R3, R0, R3, LSL #8 ; shift up R3 and insert R0 9.1.2 Multiplication by constant Combinations of shifts, add with shifts, and reverse subtract with shift can be used to perform multiplications by constants: ; multiplication of R0 by 2^n MOV R0, R0, LSL #n ; R0 = R0 << n ; multiplication of R0 by 2^n + 1 ADD R0, R0, R0, LSL #n ; R0 = R0 + (R0 << n) ; multiplication of R0 by 2^n - 1 RSB R0, R0, R0, LSL #n ; R0 = (R0 << n) - R0 ; R0 = R0 * 10 + R1 ADD R0, R0, R0, LSL #2 ; R0 = R0 * 5 ADD R0, R1, R0, LSL #1 ; R0 = R1 + R0 * 2 ; R0 = R0 * 100 + R1 ADD R0, R0, R0, LSL #2 ; R0 = R0 * 5 ADD R0, R0, R0, LSL #2 ; R0 = R0 * 5 (R0 = R0 * 25) ADD R0, R1, R0, LSL #2 ; R0 = R1 + R0 * 4 A9-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 425. ARM Code Sequences 9.1.3 Multi-precision arithmetic Arithmetic instructions allow efficient arithmetic on 64-bit or larger objects: • Add, and Add with Carry perform multi-precision addition • Subtract, and Subtract with Carry perform subtraction • Compare can be used for comparison. ; On entry : R0 and R1 hold a 64-bit number ; : (R0 is least significant) ; : R2 and R3 hold a second 64-bit number ; On exit : R0 and R1 hold 64-bit sum (or difference) of the 2 numbers add64 ADDS R0, R0, R2 ; add lower halves and update Carry flag ADC R1, R1, R3 ; add the high halves and Carry flag sub64 SUBS R0, R0, R2 ; subtract lower halves, update Carry SBC R1, R1, R3 ; subtract high halves and Carry ; This routine compares two 64-bit numbers ; On entry : As above ; On exit : N, Z, and C flags updated correctly cmp64 CMP R1, R3 ; compare high halves, if they are CMPEQ R0, R2 ; equal, then compare lower halves Be aware that in the above example, the V flag is not updated correctly. For example: R1 = 0x00000001, R0 = 0x80000000 R3 = 0x00000001, R2 = 0x7FFFFFFF R0 – R2 overflows as a 32-bit signed number, so the CMPEQ instruction sets the V flag. But (R1, R0) – (R3, R2) does not overflow as a 64-bit number. An alternative routine exists which updates the V flag correctly, but not the Z flag: ; This routine compares two 64-bit numbers ; On entry: as above ; On exit: N, V and C set correctly ; R4 is destroyed cmp64 SUBS R4, R0, R2 SBCS R4, R1, R3 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-3
- 426. ARM Code Sequences 9.1.4 Swapping endianness Swapping the order of bytes in a word (the endianness) can be performed in two ways: • This method is best for single words: ; On entry : R0 holds the word to be swapped ; On exit : R0 holds the swapped word, R1 is destroyed byteswap ; R0 = A , B , C , D EOR R1, R0, R0, ROR #16 ; R1 = A^C,B^D,C^A,D^B BIC R1, R1, #0xFF0000 ; R1 = A^C, 0 ,C^A,D^B MOV R0, R0, ROR #8 ; R0 = D , A , B , C EOR R0, R0, R1, LSR #8 ; R0 = D , C , B , A • This method is best for swapping the endianness of a large number of words: ; On entry : R0 holds the word to be swapped ; On exit : R0 holds the swapped word, ; : R1, R2 and R3 are destroyed byteswap ; first the two-instruction initialization MOV R2, #0xFF ; R2 = 0xFF ORR R2, R2, #0xFF0000 ; R2 = 0x00FF00FF ; repeat the following code for each word to swap ; R0 = A B C D AND R1, R2, R0 ; R1 = 0 B 0 D AND R0, R2, R0, ROR #24 ; R0 = 0 C 0 A ORR R0, R0, R1, ROR #8 ; R0 = D C B A A9-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 427. ARM Code Sequences 9.2 Branch instructions The following subsections show some different ways of controlling the flow of execution in ARM code. 9.2.1 Procedure call and return The BL (Branch and Link) instruction makes a procedure call by preserving the address of the instruction after the BL in R14 (the link register, LR), and then branching to the target address. Returning from a procedure is achieved by moving R14 to the PC: .... BL function ; call ‘function’ .... ; procedure returns to here .... function ; function body .... .... MOV PC, LR ; Put R14 into PC to return Another method to return from a called procedure is given in Procedure entry and exit on page A9-10. 9.2.2 Conditional execution Conditional execution allows if-then-else statements to be collapsed into sequences that do not require forward branches: /* C code for Euclid’s Greatest Common Divisor (GCD)*/ /* Returns the GCD of its two parameters */ int gcd(int a, int b) { while (a != b) if (a > b ) a = a - b ; else b = b - a ; return a ; } ; ARM assembler code for Euclid’s Greatest Common Divisor ; On entry: R0 holds ‘a’, R1 holds ‘b’ ; On exit : R0 hold GCD of A and B gcd CMP R0, R1 ; compare ‘a’ and ‘b’ SUBGT R0, R0, R1 ; if (a>b) a=a-b (if a==b do nothing) SUBLT R1, R1, R0 ; if (b>a) b=b-a (if a==b do nothing) BNE gcd ; if (a!=b) then keep going MOV PC, LR ; return to caller ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-5
- 428. ARM Code Sequences 9.2.3 Conditional compare instructions Compare instructions can be conditionally executed to implement more complicated expressions: if (a==0 || b==1) c = d + e ; CMP R0, #0 ; compare a with 0 CMPNE R1, #1 ; if a is not 0, compare b to 1 ADDEQ R2, R3, R4 ; if either was true c = d + e 9.2.4 Loop variables The Subtract instruction can be used to both decrement a loop counter and set the condition codes to test for a zero: MOV R0, #loopcount ; initialize the loop counter loop ; loop body .... SUBS R0, R0, #1 ; subtract 1 from counter ; and set condition codes BNE loop ; if not zero, continue looping .... 9.2.5 Multi-way branch A very simple multi-way branch can be implemented with a single instruction. The following code dispatches the control of execution to any number of routines, with the restriction that the code to handle each case of the multi-way branch is the same size, and that size is a power of two bytes: ; Multi-way branch ; On entry: R0 holds the branch index CMP R0, #maxindex ; checks the index is in range ADDLO PC, PC, R0, LSL #RoutineSizeLog2 ; scale index by the log of the size of ; each handler, add to the PC, which points ; 2 instructions beyond this one ; (at Index0Handler), then jump there B IndexOutOfRange ; jump to the error handler Index0Handler .... .... Index1Handler .... .... Index2Handler .... A9-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 429. ARM Code Sequences 9.3 Load and Store instructions Load and Store instructions are the best way to load or store a single word. They are also the only instructions that can load or store a byte or halfword. 9.3.1 Linked lists The following code searches for an element in a linked list that has two elements (a single byte value and a pointer to the next record) in each record. A null next pointer indicates this is the last element in the list: ; Linked list search ; On entry : R0 holds a pointer to the first record in the list ; : R1 holds the byte we are searching for ; : Call this code with a BL ; On exit : R0 holds the address of the first record matched ; : or a null pointer if no match was found ; : R2 is destroyed llsearch CMP R0, #0 ; null pointer? LDRNEB R2, [R0] ; load the byte value from this record CMPNE R1, R2 ; compare with the looked-for value LDRNE R0, [R0, #4] ; if not found, follow the link to the BNE llsearch ; next record and then keep looking MOV PC, LR ; return with pointer in R0 9.3.2 Simple string compare The following code performs a very simple string compare on two zero-terminated strings: ; String compare ; On entry : R0 points to the first string ; : R1 points to the second string ; : Call this code with a BL ; On exit : R0 is < 0 if the first string is less than the second ; : R0 is = 0 if the first string is equal to the second ; : R0 is > 0 if the first string is greater than the second ; : R1, R2 and R3 are destroyed strcmp LDRB R2, [R0], #1 ; Get a byte from the first string LDRB R3, [R1], #1 ; Get a byte from the second string CMP R2, #0 ; Have we reached the end of either CMPNE R3, #0 ; string? BEQ return ; Go to return code if so CMP R2, R3 ; Are the strings the same so far? BEQ strcmp ; Repeat for next character if so return SUB R0, R2, R3 ; Calculate result value and return MOV PC, LR ; by copying R14 (LR) into the PC ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-7
- 430. ARM Code Sequences The following code performs a more optimized string compare: int strcmp(char *s1, char *s2) { unsigned int ch1, ch2; do { ch1 = *s1++; ch2 = *s2++; } while (ch1 >= 1 && ch1 == ch2); return ch1 - ch2; } This code uses an unsigned comparison with 1 to test for a null character, rather than the normal comparison with 0. The corresponding ARM code is: strcmp LDRB R2,[R0],#1 LDRB R3,[R1],#1 CMP R2,#1 CMPCS R2,R3 BEQ strcmp SUB R0,R2,R3 MOV PC,LR The change in the way that null characters are detected allows the condition tests to be combined: • If R2 == 0, the CMP instruction sets Z = 0, C = 0. Neither the CMPCS instruction nor the BEQ instruction is executed, and the loop terminates. • If R2 != 0 and R3 == 0, the CMP instruction sets C = 1, then the CMPCS instruction is executed and sets Z = 0. So, the BEQ instruction is not executed and the loop terminates. • If R2 != 0 and R3 != 0, the CMP instruction sets C = 1, then the CMPCS instruction is executed and sets Z according to whether R2 == R3. So, the BEQ instruction is executed if R2 == R3 and the loop terminates if R2 != R3. Much faster string comparison routines are possible by loading one word of each string at a time and comparing all four bytes. A9-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 431. ARM Code Sequences 9.3.3 Long branch A Load instruction can be used to generate a branch to anywhere in the 4GB address space. By manually setting the value of the link register (R14), a subroutine call can be made to anywhere in the address space. ; Long branch (and link) ADD LR, PC, #4 ; set the return address to be 8 bytes ; after the next instruction LDR PC, [PC, #-4] ; get the address from the next word DCD function ; store the address of the function ; (DCD is an assembler directive) return_here ; return to here This code uses the location after the load to hold the address of the function to call. In practice, this location can be anywhere as long as it is within 4KB of the load instruction. Notice also that this code is position-independent except for the address of the function to call. Full position-independence can be achieved by storing the offset of the branch target after the load, and using an ADD instruction to add it to the PC. 9.3.4 Multi-way branches The following code improves on the multi-way branch code shown above by using a table of addresses of functions to call: ; Multi-way branch ; On entry: R0 holds the branch index CMP R0, #maxindex ; checks the index is in the range ; by using an unsigned compare. LDRLO PC, [PC, R0, LSL #2] ; convert the index to a word offset ; do a look up in the table put the loaded ; value into the PC and jump there B IndexOutOfRange ; jump to the error handler DCD Handler0 ; DCD is an assembler directive to DCD Handler1 ; store a word (in this case an DCD Handler2 ; address in memory). DCD Handler3 .... ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-9
- 432. ARM Code Sequences 9.4 Load and Store Multiple instructions Load and Store Multiple instructions are the most efficient way to manipulate blocks of data. 9.4.1 Simple block copy This code performs a very simple block copy, 48 bytes at a time, and approaches the maximum throughput for a particular machine: ; Simple block copy function ; R12 points to the start of the source block ; R13 points to the start of the destination block ; R14 points to the end of the source block loop LDMIA R12!, (R0-R11} ; load 48 bytes STMIA R13!, {R0-R11} ; store 48 bytes CMP R12, R14 ; reached the end yet? BLO loop ; branch to the top of the loop The source and destination must be word-aligned, and if the object to be copied is not a multiple of 48 bytes long, extra bytes are copied to bring the total to the next multiple of 48 bytes. A more sophisticated routine is needed if this extra copying is to be avoided. 9.4.2 Procedure entry and exit This code uses Load and Store Multiple to preserve and restore the processor state during a procedure. The code assumes that registers R0 to R3 are argument registers, preserved by the caller of the function, so do not need to be preserved. R13 is also assumed to point to a full descending stack. function STMFD R13!, {R4 - R12, R14} ; preserve all the local registers ; and the return address, and ; update the stack pointer. .... Insert the function body here .... LDMFD R13!, {R4 - R12, PC} ; restore the local register, load ; the PC from the saved return ; update the stack pointer. Notice that this code restores all saved registers, updates the stack pointer, and returns the caller (by loading the PC value) in a single instruction. This allows very efficient conditional return for exceptional cases from a procedure (by checking the condition with a compare instruction and then conditionally executing the Load Multiple). A9-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 433. ARM Code Sequences 9.5 Semaphore instructions This code controls the entry and exit from a critical section of code. The semaphore instruction (SWP) does not provide a compare and conditional write facility, so this must be done explicitly. The following code achieves this by using a semaphore value to indicate that the lock is being inspected. The code below causes the calling process to busy-wait until the lock is free. To ensure progress, three OS calls need to be made (one before each loop branch) to sleep the process if the lock cannot be accessed. ; Critical section entry and exit ; The code uses a process ID to identify the lock owner ; An ID of zero indicates the lock is free ; An ID of -1 indicates the lock is being inspected ; On entry: R0 holds the address of the semaphore ; R1 holds the ID of the process requesting the lock MVN R2, #0 ; load the ‘looking’ value (-1) in R2 spinin SWP R3, R2, [R0] ; look at the lock, and lock others out CMN R3, #1 ; anyone else trying to look? .... Insert conditional OS call to sleep process here .... BEQ spinin ; yes, so wait our turn CMP R3, #0 ; no-one looking, is the lock free? STRNE R3, [R0] ; no, then restore the previous owner .... Insert conditional OS call to sleep process here .... BNE spinin ; and wait again STR R1, [R0] ; otherwise grab the lock ..... Insert critical code here ..... spinout SWP R3, R2, [R0] ; look at the lock, and lock others out CMN R3, #1 ; anyone else trying to look ? .... Insert conditional OS call to sleep process here .... BEQ spinout ; yes, so wait our turn CMP R3, R1 ; check we own it BNE CorruptSemaphore ; we should have been the owner! MOV R2, #0 ; load the ‘free’ value STR R2, [R0] ; and open the lock ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-11
- 434. ARM Code Sequences 9.6 Other code examples The following sequences illustrate some other applications of ARM assembly language. 9.6.1 Software interrupt dispatch This code segment dispatches software interrupts (SWIs) to individual handlers. For it to work, the instruction at the software interrupt vector (memory location 0x00000008) must branch to the first instruction of this code. The SWI instruction has a 24-bit field that can be used for specific SWI functions. This code also handles the 16-bit Thumb SWI instruction, which has an 8-bit SWI number field rather than a 24-bit field. This example assumes that the code to handle each of the individual SWIs only modifies r0-r3, r12, lr and the PC. If more registers are needed, the example should be modified to include the extra registers needed in the register lists of the STMFD and LDMFD instructions. This makes the extra registers available to all of the SWI handlers, but the code will typically take longer to execute because of the extra memory accesses. Alternatively, if only a few of the individual SWI handlers require extra registers, use extra STMFD and LDMFD instructions within those handlers. This ensures that SWIs which do not require the extra registers are not slowed down. SWIHandler STMFD sp!, {r0-r3,r12,lr} ; Store the registers MRS r0, spsr ; Move SPSR into general purpose ; register TST r0, #0x20 ; Test the SPSR T bit to discover ; ARM/Thumb state when SWI occurred LDRNEH r0, [lr, #-2] ; T bit set so load halfword (Thumb) BICNE r0, r0, #0xff00 ; and clear top 8 bits of halfword ; (LDRH clears top 16 bits of word) LDREQ r0, [lr, #-4] ; T bit clear so load word (ARM) BICEQ r0, r0, #0xff000000 ; and clear top 8 bits of word CMP r0, #MaxSWI ; Check the SWI number is in range LDRLS pc, [pc, r0, LSL #2] ; If so, jump to the correct routine B SWIOutOfRange switable DCD do_swi_0 DCD do_swi_1 : : do_swi_0 ..... Insert code to handle SWI 0 here ..... LDMFD sp!, {r0-r3,r12,pc}^ ; Restore the registers and return. do_swi_1 : A9-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 435. ARM Code Sequences 9.6.2 Single-channel DMA transfer The following code is an interrupt handler to perform interrupt driven input/output to memory transfers (soft DMA). The code is written as an FIQ handler, and uses the banked FIQ registers to maintain state between interrupts. Therefore this code is best situated at location 0x1C. The entire sequence to handle a normal transfer is just four instructions. Code situated after the conditional return is used to signal that the transfer is complete. LDR r11, [r8, #IOData] ; load port data from the I/O device STR r11, [r9], #4 ; store it to memory: update the pointer CMP r9, r10 ; reached the end? SUBLTS pc, lr, #4 ; no, so return ; Insert transfer complete code here where: R8 Points to the base address of the input/output device that data is read from. IOData Is the offset from the base address to the 32-bit data register that is read. Reading this register disables the interrupt. R9 Points to the memory location where data is being transferred. R10 Points to the last address to transfer to. Of course, byte transfers can be made by replacing the load and store instructions with Load and Store byte instructions, and changing the offset in the store instruction from 4 to 1. Transfers from memory to an input/output device are made by swapping the addressing modes between the Load instruction and the Store instruction. 9.6.3 Dual-channel DMA transfer This code is similar to the example in Single-channel DMA transfer on page A9-13, except that it handles two channels (which can be the input and output side of the same channel). Again, this code is written as an FIQ handler, and uses the banked FIQ registers to maintain state between interrupts. Therefore this code is best situated at location 0x1C. The entire sequence to handle a normal transfer is just nine instructions. Code situated after the conditional return is used to signal that the transfer is complete. LDR r13, [r8, #IOStat] ; load status register to find .... TST r13, #IOPort1Active ; .... which port caused the interrupt? LDREQ r13, [r8, #IOPort1] ; load port 1 data LDRNE r13, [r8, #IOPort2] ; load port 2 data STREQ r13, [r9], #4 ; store to buffer 1 STRNE r13, [r10], #4 ; store to buffer 2 CMP r9, r11 ; reached the end? CMPNE r10, r12 ; on either channel? SUBNES pc, lr, #4 ; return ; Insert transfer complete code here where: ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-13
- 436. ARM Code Sequences R8 Points to the base address of the input/output device that data is read from. IOStat Is the offset from the base address to a register indicating which of two ports caused the interrupt. IOPort1Active Is a bit mask indicating if the first port caused the interrupt (otherwise it is assumed that the second port caused the interrupt). IOPort1,IOPort2 Are offsets to the two data registers to be read. Reading a data register disables the interrupt for that port. R9 Points to the memory location that data from the first port is being transferred to. R10 Points to the memory location that data from the second port is being transferred to. R11,R12 Point to the last address to transfer to (R11 for the first port, R12 for the second). Again, byte transfers can be made by suitably replacing the load and store instructions. Transfers from memory to an input/output device are made by swapping the addressing modes between the conditional load instructions and the conditional store instructions. A9-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 437. ARM Code Sequences 9.6.4 Interrupt prioritization This code dispatches up to 32 interrupt sources to their appropriate handler routines. This code is intended to use the normal interrupt vector, so memory location 0x00000018 must contain an instruction that branches to the first instruction of this code. External hardware is used to prioritize the interrupt and present the number of the highest-priority active interrupt in an input register. Interrupts are re-enabled after 10 instructions (including the branch to this code). ; first save the critical state ; SUB r14, r14, #4 ; adjust return address before saving it STMFD r13!, {r12, r14} ; stack return address and working register MRS r12, SPSR ; get the SPSR ... STMFD r13!, {r12} ; ... and stack that too ; ; now get the priority level of the highest priority active interrupt MOV r12, #IntBase ; get interrupt controller’s base address LDR r12, [r12, #IntLevel] ; get the interrupt level (0 to 31) ; ; now read-modify-write the CPSR to enable interrupts MRS r14, CPSR ; read the status register BIC r14, r14, #0x80 ; clear the I bit (use 0x40 for the F bit) MSR CPSR_c, r14 ; write it back to re-enable interrupts ; jump to the correct handler LDR PC, [PC, r12, LSL #2] ; and jump to the correct handler. PC base ; address points to this instruction + 8 NOP ; pad so the PC indexes this table ; ; table of handler start addresses ; DCD Priority0Handler DCD Priority1Handler ........ Priority0Handler STMFD r13!, {r0 - r11} ; save working registers ; ; insert handler code here ; ........ MRS r12, CPSR ; Read-modify-write the CPSR to disable ORR r12, r12, #0x80 ; interrupts (use 0x40 instead for FIQs) MSR CPSR_c, r12 ; Note: Do not use r14 instead of r12. It ; will be corrupted if an interrupt occurs LDMFD r13!, {r0-r12} ; Recover the working registers and SPSR MSR SPSR_cxsf, r12 ; Put the SPSR back LDMFD r13!, {r12, PC}^ ; Restore last working register and return Priority1Handler ........ ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-15
- 438. ARM Code Sequences where: R13 Is assumed to point to a small Full Descending stack. The stack space required is 60 bytes times the maximum level to which interrupts can possibly be nested. IntBase Holds the base address of the interrupt handler. IntLevel Holds the offset (from IntBase) of the register containing the highest priority active interrupt. 9.6.5 Context switch This section gives a very simple example of how to perform context switches between User mode processes, in order to illustrate some of the instructions used for this purpose. It makes the following assumptions about the system design: • Context switches are performed by an IRQ handler. This handler first performs normal interrupt processing to identify the source of the interrupt and deal with it. The details of this are system-specific and are not described here. At the end of normal interrupt processing, the interrupt handler can choose either to return to the interrupted process, or to switch to another process. • Only User mode context switches are to be supported. If an IRQ is allowed to occur in a privileged process, the IRQ handler always returns to the interrupted process. • The normal interrupt processing code requires registers R0-R3, R12 and R14_irq to be preserved around it. It leaves R4-R11 unchanged, and uses R13_irq as a Full Descending stack pointer. (These assumptions basically mean that it can call subroutines that adhere to the standard ARM Procedure Calling Standard.) • The normal interrupt processing code does not re-enable interrupts, change SPSR_irq or change to another processor mode, and FIQ handlers also do not re-enable interrupts. As a result, neither SPSR_irq nor the banked versions of R13, R14 and the SPSR belonging to the interrupted process are changed by execution of the normal interrupt processing code. • Each User mode process has an associated Process Control Block (PCB), which stores its register values while it is not running. The format of a PCB is shown in Figure 9-1. A9-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 439. ARM Code Sequences CPSR Restart address R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 Figure 9-1 PCB layout Increasing addresses On entry to the IRQ handler, the following code is used to calculate the correct return address and to preserve the registers required by the normal interrupt processing code: SUB R14, R14, #4 STMFD R13!, {R0-R3, R12, R14} This is followed by the normal interrupt processing code. If this code decides to return to the interrupted process, it executes the instruction: LDMFD R13!, {R0-R3, R12, PC}^ This instruction is the form of LDM described in LDM (3) on page A4-34, and causes: • Registers R0-R3 and R12 to be reloaded with their values on entry to the IRQ handler, which were stored by the STMFD instruction. • The PC to be reloaded with the R14 value stored by the STMFD instruction, which is 4 less than the value of R14_irq on entry to the IRQ handler and so is the address of the next instruction to be executed in the interrupted process (see Interrupt request (IRQ) exception on page A2-19). • The CPSR to be reloaded from SPSR_irq, which was set to the CPSR of the interrupted process on interrupt entry and has remained unchanged since. The values of all other registers belonging to the interrupted process were left unchanged by interrupt entry and by execution of the normal interrupt processing code, so this fully restores the context of the interrupted process. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A9-17
- 440. ARM Code Sequences If the normal interrupt processing code instead switches to another User mode process, it puts pointers to the PCBs of the old and new processes in R0 and R1 respectively and branches to the following code: ; First store the old process’s User mode state to the PCB pointed to by R0. MRS R12, SPSR ; Get CPSR of interrupted process STR R12, [R0], #8 ; Store CPSR to PCB, point R0 at ; PCB location for R0 value LDMFD R13!, {R2, R3} ; Reload R0/R1 of interrupted ; process from stack STMIA R0!, {R2, R3} ; Store R0/R1 values to PCB, point ; R0 at PCB location for R2 value LDMFD R13!, {R2, R3, R12, R14} ; Reload remaining stacked values STR R14, [R0, #-12] ; Store R14_irq, the interrupted ; process’s restart address STMIA R0, {R2-R14}^ ; Store user R2-R14 - see Note 1 ; Then load the new process’s User mode state and return to it. LDMIA R1!, {R12, R14} ; Put interrupted process’s CPSR MSR SPSR_fsxc, R12 ; and restart address in SPSR_irq ; and R14_irq LDMIA R1, {R0-R14}^ ; Load user R0-R14 - see Note 2 NOP ; Note: Cannot use banked register ; immediately after User mode LDM MOVS PC, R14 ; Return to address in R14_irq, ; with SPSR_irq -> CPSR transfer Note 1. This instruction is an example of the form of STM described in STM (2) on page A4-86. It stores the registers R2, R3, ..., R12, R13_usr, R14_usr to the correct places in the PCB. 2. This instruction is an example of the form of LDM described in LDM (2) on page A4-32. It loads the registers R0, R1, ..., R12, R13_usr, R14_usr from the correct places in the PCB. A9-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 441. Chapter A10 Enhanced DSP Extension This chapter describes the enhanced DSP additions to the ARM programmer’s model and instruction set, included in E variants of ARM architecture versions 5 and above. It contains the following sections: • About the enhanced DSP instructions on page A10-2 • Saturated integer arithmetic on page A10-3 • Saturated Q15 and Q31 arithmetic on page A10-4 • The Q flag on page A10-5 • Enhanced DSP instructions on page A10-6 • Alphabetical list of enhanced DSP instructions on page A10-8. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-1
- 442. Enhanced DSP Extension 10.1 About the enhanced DSP instructions Many digital signal processing (DSP) algorithms operate on arrays of 16-bit data, where the 16-bit value is to be interpreted as a signed fixed-point number with 15 binary places. Such values are sometimes called Q15 numbers, and represent numeric values ranging from –1 up to +1 – 2–15. To preserve accuracy, intermediate values in these algorithms are often calculated as Q31 numbers, which are similar but have 32 bits and 31 binary places. Also, in order to avoid spikes in the output from the algorithm if numeric overflow occurs, arithmetic on Q15 and Q31 values is normally saturated. This means that if overflow occurs, the result is set to the most positive or most negative possible value depending on the direction of overflow. In contrast, normal integer arithmetic will wrap around modulo 232. Performing saturated arithmetic on Q15 and Q31 numbers is possible using the standard ARM instruction set, but takes roughly 5-10 instructions per arithmetic operation. The enhanced DSP instructions described in this chapter include instructions to perform such arithmetic considerably more quickly, using 1-2 instructions per arithmetic operation. In order to maximize the performance of DSP algorithms, it is also important that data and coefficient values should be loaded and stored efficiently. The enhanced DSP instructions therefore also include instructions to assist with this loading and storing. Finally, the enhanced DSP instructions include coprocessor instructions which transfer 64 bits of data directly between the ARM processor and the coprocessor, in order to assist the design of coprocessors which will further improve the performance of DSP algorithms. A10-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 443. Enhanced DSP Extension 10.2 Saturated integer arithmetic When viewed as a signed number, the value of a general-purpose register lies in the range from –231 (or 0x80000000) to +231 – 1 (or 0x7FFFFFFF). If an addition or subtraction is performed on such numbers and the correct mathematical result lies outside this range, it would require more than 32 bits to represent. In these circumstances, the surplus bits are normally discarded, which has the effect that the result obtained is equal to the correct mathematical result reduced modulo 232. For example, 0x60000000 could be used to represent +3 ´ 229 as a signed integer. If you add this number to itself, you get +3 ´ 230, which lies outside the representable range, but could be represented as the 33-bit signed number 0x0C0000000. The actual result obtained will be the rightmost 32 bits of this, which are 0xC0000000. This represents –230, which is smaller than the correct mathematical result by 232, and does not even have the same sign as the correct result. This kind of inaccuracy is unacceptable in many DSP applications. For example, if it occurred while processing an audio signal, the abrupt change of sign would be likely to result in a loud click. To avoid this sort of effect, many DSP algorithms use saturated signed arithmetic. This modifies the way normal integer arithmetic behaves as follows: • If the correct mathematical result lies within the available range from –231 to +231 – 1, the result of the operation is equal to the correct mathematical result. • If the correct mathematical result is greater than +231 – 1 and so overflows the upper end of the representable range, the result of the operation is equal to +231 – 1. • If the correct mathematical result is less than –231 and so overflows the lower end of the representable range, the result of the operation is equal to –231. Put another way, the result of a saturated arithmetic operation is the closest representable number to the correct mathematical result of the operation. The enhanced DSP instructions support saturated signed 32-bit integer additions and subtractions, by use of the QADD and QSUB instructions. Variants of these instructions (QDADD and QDSUB) perform a saturated doubling of one of the operands before the saturated addition or subtraction. Saturated integer multiplications are not supported, because the product of two values of widths A and B bits never overflows an (A+B)-bit destination. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-3
- 444. Enhanced DSP Extension 10.3 Saturated Q15 and Q31 arithmetic A 32-bit signed value can be treated as having a binary point immediately after its sign bit. This is equivalent to dividing its signed integer value by 231, so that it can now represent numbers from –1 to +1 – 2–31. When a 32-bit value is used to represent a fractional number in this fashion, it is known as a Q31 number. Saturated additions, subtractions, and doublings can be performed on Q31 numbers using the same instructions as are used for saturated integer arithmetic, since everything is simply scaled down by a factor of 2–31. Similarly, a 16-bit value can be treated as having a binary point immediately after its sign bit, which effectively divides its signed integer value by 215. When a 16-bit value is used in this fashion, it can represent numbers from –1 to +1 – 2–15 and is known as a Q15 number. If two Q15 numbers are multiplied together as integers, the resulting integer needs to be scaled down by a factor of 2–15 ´ 2–15 == 2–30. For example, multiplying the Q15 number 0x8000 (representing –1) by itself using an integer multiplication instruction yields the value 0x40000000, which is 230 times the desired result of +1. This means that the result of the integer multiplication instruction is not quite in Q31 form. To get it into Q31 form, it must be doubled, so that the required scaling factor becomes 2–31. Furthermore, it is possible that the doubling will cause integer overflow, so the result should in fact be doubled with saturation. In particular, the result 0x40000000 from the multiplication of 0x8000 by itself should be doubled with saturation to produce 0x7FFFFFFF (the closest possible Q31 number to the correct mathematical result of –1 ´ –1 == +1). If it were doubled without saturation, it would instead produce 0x80000000, which is the Q31 representation of –1. To implement a saturated Q15 ´ Q15 ® Q31 multiplication, therefore, an integer multiply instruction should be followed by a saturated integer doubling. The latter can be performed by a QADD instruction adding the multiply result to itself. Similarly, a saturated Q15 ´ Q15 + Q31 ® Q31 multiply-accumulate can be performed using an integer multiply instruction followed by the use of a QDADD instruction. Some other examples of arithmetic on Q15 and Q31 numbers are described in the Usage sections for the individual instructions. A10-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 445. Enhanced DSP Extension 10.4 The Q flag The enhanced DSP extension incorporates a mechanism to determine whether saturation or overflow has occurred in the course of a calculation. Bit[27] of the CPSR is a sticky overflow flag, also known as the Q flag. This flag is set to 1 if any of the following occurs: • Saturation of the addition result in a QADD or QDADD instruction • Saturation of the subtraction result in a QSUB or QDSUB instruction • Saturation of the doubling intermediate result in a QDADD or QDSUB instruction • Signed overflow during an SMLA<x><y> or SMLAW<y> instruction. Note The Q flag is not affected by overflow during any other arithmetic instruction, such as ADD, SUB, or MLA. The Q flag is sticky in that once it has been set to 1, it is not affected by whether subsequent calculations saturate and/or overflow. Its intended usage is: 1. Use an MSR CPSR_f,#0 instruction to clear the Q flag (this also clears the condition code flags). 2. Perform a sequence of calculations. 3. Use an MRS Rn,CPSR instruction to read the CPSR, then test the value of the Q flag. If it is still 0, none of the above types of saturation or overflow occurred during step 2. Otherwise, at least one instance of saturation or overflow occurred. Each SPSR also has a Q flag into which the CPSR Q flag is copied when an exception occurs as part of the general CPSR ® SPSR copy performed on exception entry. Similarly, the exception return instructions which atomically return to the correct address and perform an SPSR ® CPSR transfer will copy the SPSR Q flag back to the CPSR. Between them, these ensure that the value of the Q flag is not changed by an interrupt or other exception. For more details of this, see Exceptions on page A2-13. Except as described above, the only instructions that affect or are affected by the Q flags are MSR instructions which write to the flags byte of the destination PSR, and MRS instructions. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-5
- 446. Enhanced DSP Extension 10.5 Enhanced DSP instructions The enhanced DSP instructions can be divided into the following groups: • Integer multiply and multiply-accumulate instructions on page A10-6 • Saturated addition and subtraction instructions on page A10-7 • Two-word load and store instructions on page A10-7 • Cache preload instruction on page A10-7 • Two-word coprocessor register transfer instructions on page A10-7. 10.5.1 Integer multiply and multiply-accumulate instructions The enhanced DSP instructions augment the normal integer multiply and multiply-accumulate instructions with versions designed to work on 16-bit signed data. These form one or both of the operands to the multiplication by taking either the bottom half (bits[15:0]) or the top half (bits[31:16]) of a source register and sign-extending it, while ignoring the other half of the source register. These instructions allow multiplications to be performed on 16-bit data values that are held packed in registers, without the need to unpack them. The data values can be loaded in this format by LDR or LDM instructions, which generally results in more efficient use of memory bandwidth than would be obtained by the use of LDRH or LDRSH instructions. The instructions available are: • 16 ´ 16 ® 32 bit signed multiply instructions SMUL<x><y>. These instructions are very similar to the normal MUL instruction, apart from the fact that their operands are generated as described above. • 16 ´ 16 + 32 ® 32 bit signed multiply-accumulate instructions SMLA<x><y>. These instructions are very similar to the normal MLA instruction, apart from the fact that their operands are generated as described above. • 32 ´ 16 ® 32 bit signed multiply instructions SMULW<y>. These instructions produce the upper 32 bits of the 48-bit product. • 32 ´ 16 + 32 ® 32 bit signed multiply-accumulate instructions SMLAW<y>. The multiplication in these instructions produces the upper 32 bits of the 48-bit product, and these 32 bits are then added to the accumulate value. • 16 ´ 16 + 64 ® 64 bit signed multiply instructions SMLAL<x><y>. These instructions are very similar to the normal SMLAL instruction, apart from the fact that their operands are generated as described above. In all cases, <y> is either the letter B or T, depending on whether the bottom or top half of the second operand register should be used. Where the first operand is a sign-extended 16-bit value, <x> specifies whether the bottom or top half of the first operand register should be used in a similar fashion. A10-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 447. Enhanced DSP Extension 10.5.2 Saturated addition and subtraction instructions Instructions are provided to perform saturated integer addition and subtraction, with an optional saturated doubling of one operand. These instructions can be used to perform saturated integer, Q15, and Q31 arithmetic, as described in Saturated integer arithmetic on page A10-3 and Saturated Q15 and Q31 arithmetic on page A10-4. The instructions are: QADD Performs a saturated integer addition. QDADD Performs a saturated integer doubling of one operand followed by a saturated integer addition with the other operand. QSUB Performs a saturated integer subtraction. QDSUB Performs a saturated integer doubling of one operand followed by a saturated integer subtraction from the other operand. 10.5.3 Two-word load and store instructions Load and store instructions are provided which can be used to transfer two adjacent words of memory to or from any of the register pairs {R0,R1}, {R2,R3}, {R4,R5}, {R6,R7}, {R8,R9}, {R10,R11}, or {R12,R13}. These instructions use the same addressing modes as LDRH, LDRSB, LDRSH, and STRH, and thus allow a larger addressing range than is possible with the LDM and STM instructions. A restriction is that the doubleword formed by the two adjacent words of memory is required to be doubleword-aligned. The instructions are: LDRD Loads two words. STRD Stores two words. 10.5.4 Cache preload instruction The PLD instruction can be used to tell the memory system that an access to the data at a specified memory address is likely to occur soon. A memory system that recognizes this request will bring the data into cache, so that when the access actually occurs, any delay caused by a cache miss is reduced or eliminated. 10.5.5 Two-word coprocessor register transfer instructions Two additional generic coprocessor instructions are defined. These are similar to the normal MCR and MRC generic coprocessor instructions, but transfer two ARM register values to or from the coprocessor rather than just one. They can be used when designing coprocessors which require more ARM-coprocessor bandwidth than is available from the MCR and MRC instructions. The instructions are: MCRR Transfers two ARM register values to a coprocessor. MRRC Transfers values from a coprocessor to two ARM registers. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-7
- 448. Enhanced DSP Extension 10.6 Alphabetical list of enhanced DSP instructions This section describes each enhanced DSP instruction in alphabetical order, using the same format and conventions as are used in the descriptions of normal ARM instructions (see Alphabetical list of ARM instructions on page A4-2). 10.6.1 LDRD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U I W 0 Rn Rd addr_mode 1 1 0 1 addr_mode The LDRD instruction loads a pair of ARM registers from two consecutive words of memory. The pair of registers is restricted to being an even-numbered register and the odd-numbered register that immediately follows it (for example, R10 and R11). A greater variety of addressing modes is available than for a 2-register LDM. However, the address of the first of the two words is required to be doubleword-aligned (that is, the address must be divisible by 8). Syntax LDR{<cond>}D <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the even-numbered destination register for the memory word addressed by <addressing_mode>. The immediately following odd-numbered register is the destination register for the next memory word. If <Rd> is R14, which would specify R15 as the second destination register, the instruction is UNPREDICTABLE. If <Rd> specifies an odd-numbered register, the instruction is UNDEFINED. <addressing_mode> Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It determines the P, U, I, W, Rn, and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). The address generated by <addressing_mode> is the address of the lower of the two words loaded by the LDRD instruction. The address of the higher word is generated by adding 4 to this address. Architecture version E variants of version 5 and above, excluding ARMv5TExP A10-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 449. Enhanced DSP Extension Exceptions Data abort Operation if ConditionPassed(cond) then if (Rd is even-numbered) then if (address[2:0] == 0b000) and (Rd is not R14) then Rd = Memory[address,4] R(d+1) = Memory[address+4,4] else UNPREDICTABLE else UNDEFINED Notes Operand restrictions If <addressing_mode> performs base register writeback and the base register <Rn> is one of the two destination registers of the instruction, the results are UNPREDICTABLE. If <addressing_mode> specifies an index register <Rm> and <Rm> is one of the two destination registers of the instruction, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non doubleword-aligned addresses If the load address is not doubleword-aligned, the results are UNPREDICTABLE. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, then: • an address with bits[1:0] != 0b00 causes an alignment exception • an address with bits[2:0] == 0b000 does not cause an alignment exception • it is IMPLEMENTATION DEFINED whether an address with bits[2:0] == 0b100 causes an alignment exception. Time order The time order of the accesses to the two memory words is not architecturally defined. In particular, an implementation is allowed to perform the two 32-bit memory accesses in either order, or to combine them into a single 64-bit memory access. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-9
- 450. Enhanced DSP Extension 10.6.2 MCRR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 4 3 0 cond 1 1 0 0 0 1 0 0 Rn Rd cp_num opcode CRm The MCRR instruction passes the values of ARM registers <Rd> and <Rn> to the coprocessor whose number is cp_num. If no coprocessors indicate that they can execute the instruction, an undefined instruction exception is generated. Syntax MCRR{<cond>} <coproc>, <opcode>, <Rd>, <Rn>, <CRm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to be placed in the cp_num field of the instruction. The standard generic coprocessor names are p0, p1, …, p15. <opcode> Is a coprocessor-specific opcode. <Rd> Is the first ARM register whose value is transferred to the coprocessor. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <Rn> Is the second ARM register whose value is transferred to the coprocessor. If R15 is specified for <Rn>, the result is UNPREDICTABLE. <CRm> Is the destination coprocessor register. Architecture version E variants of version 5 and above, excluding ARMv5TExP Exceptions Undefined instruction Operation if ConditionPassed(cond) then send Rd value to Coprocessor[cp_num] send Rn value to Coprocessor[cp_num] A10-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 451. Enhanced DSP Extension Usage MCRR is used to initiate coprocessor operations that depend on values in two ARM registers. An example for a floating-point coprocessor is an instruction to transfer a double-precision floating-point number held in two ARM registers to a floating-point register. Notes Coprocessor fields Only instruction bits[31:8] are defined by the ARM architecture. The remaining fields are recommendations, for compatibility with ARM Development Systems. Unimplemented coprocessor instructions Hardware coprocessor support is optional, regardless of the architecture version. An implementation may choose to implement a subset of the coprocessor instructions, or no coprocessor instructions at all. Any coprocessor instructions that are not implemented instead cause an undefined instruction trap. Order of transfers If a coprocessor uses these instructions, it will define how each of the values of <Rd> and <Rn> is used. There is no architectural requirement for the two register transfers to occur in any particular time order. It is IMPLEMENTATION DEFINED whether Rd is transferred before Rn, after Rn, or at the same time as Rn. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-11
- 452. Enhanced DSP Extension 10.6.3 MRRC 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 4 3 0 cond 1 1 0 0 0 1 0 1 Rn Rd cp_num opcode CRm The MRRC instruction causes the coprocessor whose number is cp_num to transfer values to two ARM registers <Rd> and <Rn>. If no coprocessors indicate that they can execute the instruction, an undefined instruction exception is generated. Syntax MRRC{<cond>} <coproc>, <opcode>, <Rd>, <Rn>, <CRm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <coproc> Specifies the name of the coprocessor, and causes the corresponding coprocessor number to be placed in the cp_num field of the instruction. The standard generic coprocessor names are p0, p1, …, p15. <opcode> Is a coprocessor-specific opcode. <Rd> Is the first destination ARM register. If R15 is specified for <Rd>, the result is UNPREDICTABLE. <Rn> Is the second destination ARM register. If R15 is specified for <Rn>, the result is UNPREDICTABLE. <CRm> Is the coprocessor register which supplies the data to be transferred. Architecture version E variants of version 5 and above, excluding ARMv5TExP Exceptions Undefined instruction Operation if ConditionPassed(cond) then Rd = first value from Coprocessor[cp_num] Rn = second value from Coprocessor[cp_num] A10-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 453. Enhanced DSP Extension Usage MRRC is used to initiate coprocessor operations that write values to two ARM registers. An example for a floating-point coprocessor is an instruction to transfer a double-precision floating-point number held in a floating-point register to two ARM registers. Notes Operand restrictions Specifying the same register for <Rd> and <Rn> has UNPREDICTABLE results. Coprocessor fields Only instruction bits[31:8] are defined by the ARM architecture. The remaining fields are recommendations, for compatibility with ARM Development Systems. Unimplemented coprocessor instructions Hardware coprocessor support is optional, regardless of the architecture version. An implementation may choose to implement a subset of the coprocessor instructions, or no coprocessor instructions at all. Any coprocessor instructions that are not implemented instead cause an undefined instruction trap. Order of transfers If a coprocessor uses these instructions, it will define which value is written to <Rd> and which value to <Rn>. There is no architectural requirement for the two register transfers to occur in any particular time order. It is IMPLEMENTATION DEFINED whether Rd is transferred before Rn, after Rn, or at the same time as Rn. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-13
- 454. Enhanced DSP Extension 10.6.4 PLD 31 30 29 28 27 26 25 24 23 22 21 20 19 16 15 14 13 12 11 0 1 1 1 1 0 1 I 1 U 1 0 1 Rn 1 1 1 1 addr_mode The PLD instruction signals the memory system that memory accesses from a specified address are likely in the near future. The memory system can respond by taking actions which are expected to speed up the memory accesses when they do occur, such as pre-loading the cache line containing the specified address into the cache. PLD is a hint instruction, aimed at optimizing memory system performance. It has no architecturally defined effect, and memory systems that do not support this optimization can ignore it. On such memory systems, PLD acts as a NOP. Syntax PLD <addressing_mode> where: <addressing_mode> Is described in Addressing Mode 2 - Load and Store Word or Unsigned Byte on page A5-18. It specifies the I, U, Rn, and addr_mode bits of the instruction. Only addressing modes with P == 1 and W == 0 are available for this instruction. Pre-indexed and post-indexed addressing modes have P == 0 or W == 1 and so are not available. Architecture version E variants of version 5 and above, excluding ARMv5TExP Exceptions None Operation /* No change occurs to programmer’s model state, but where * appropriate, the memory system is signalled that memory accesses * to the specified address are likely in the near future. */ A10-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 455. Enhanced DSP Extension Notes Condition Unlike most other ARM instructions, this instruction cannot be executed conditionally. Writeback Clearing bit[24] (the P bit) or setting bit[21] (the W bit) has UNPREDICTABLE results. Data aborts This instruction never generates a data abort, nor does it signal any sort of memory system exception detected for the address generated by <addressing_mode> in any other way. All such memory system exceptions must be ignored by the memory system. Typically, the memory system does this by treating the PLD instruction as a NOP if any exceptional case is encountered while handling it. Alignment There are no alignment restrictions on the address generated by <addressing_mode>. If an implementation contains a System Control coprocessor (see Chapter B2 The System Control Coprocessor), it will not generate an alignment exception for any PLD instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-15
- 456. Enhanced DSP Extension 10.6.5 QADD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 0 0 Rn Rd SBZ 0 1 0 1 Rm The QADD instruction performs integer addition, saturating the result to the 32-bit signed integer range –231 £ x £ 231 – 1. If saturation actually occurs, the instruction sets the Q flag in the CPSR. Syntax QADD{<cond>} <Rd>, <Rm>, <Rn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the register that contains the first operand for the saturated addition. <Rn> Specifies the register that contains the second operand for the saturated addition. Architecture version E variants of version 5 and above Exceptions None Operation if ConditionPassed(cond) then Rd = SignedSat(Rm + Rn, 32) if SignedDoesSat(Rm + Rn, 32) then Q Flag = 1 A10-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 457. Enhanced DSP Extension Usage As well as performing saturated integer and Q31 additions, this instruction can be used in combination with an SMUL<x><y>, SMULW<y>, or SMULL instruction to produce multiplications of Q15 and Q31 numbers. Three examples are: • To multiply the Q15 numbers in the bottom halves of R0 and R1 and place the Q31 result in R2, use: SMULBB R2, R0, R1 QADD R2, R2, R2 • To multiply the Q31 number in R0 by the Q15 number in the top half of R1 and place the Q31 result in R2, use: SMULWT R2, R0, R1 QADD R2, R2, R2 • To multiply the Q31 numbers in R0 and R1 and place the Q31 result in R2, use: SMULL R3, R2, R0, R1 QADD R2, R2, R2 Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rn> has UNPREDICTABLE results. Condition flags The QADD instruction does not affect the N, Z, C, or V flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-17
- 458. Enhanced DSP Extension 10.6.6 QDADD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 1 0 0 Rn Rd SBZ 0 1 0 1 Rm The QDADD instruction doubles its second operand, then adds the result to its first operand. Both the doubling and the addition have their results saturated to the 32-bit signed integer range –231 £ x £ 231 – 1. If saturation actually occurs in either operation, the instruction sets the Q flag in the CPSR. Syntax QDADD{<cond>} <Rd>, <Rm>, <Rn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the register that contains the first operand for the saturated addition. <Rn> Specifies the register whose value is to be doubled, saturated, and used as the second operand for the saturated addition. Architecture version E variants of version 5 and above Exceptions None Operation if ConditionPassed(cond) then Rd = SignedSat(Rm + SignedSat(Rn*2, 32), 32) if SignedDoesSat(Rm + SignedSat(Rn*2, 32), 32) or SignedDoesSat(Rn*2, 32) then Q Flag = 1 A10-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 459. Enhanced DSP Extension Usage The primary use for this instruction is to generate multiply-accumulate operations on Q15 and Q31 numbers, by placing it after an integer multiply instruction. Three examples are: • To multiply the Q15 numbers in the top halves of R4 and R5 and add the product to the Q31 number in R6, use: SMULTT R0, R4, R5 QDADD R6, R6, R0 • To multiply the Q15 number in the bottom half of R2 by the Q31 number in R3 and add the product to the Q31 number in R7, use: SMULWB R0, R3, R2 QDADD R7, R7, R0 • To multiply the Q31 numbers in R2 and R3 and add the product to the Q31 number in R4, use: SMULL R0, R1, R2, R3 QDADD R4, R4, R1 Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rn> has UNPREDICTABLE results. Condition flags The QDADD instruction does not affect the N, Z, C, or V flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-19
- 460. Enhanced DSP Extension 10.6.7 QDSUB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 1 1 0 Rn Rd SBZ 0 1 0 1 Rm The QDSUB instruction doubles its second operand, then subtracts the result from its first operand. Both the doubling and the subtraction have their results saturated to the 32-bit signed integer range –231 £ x £ 231 – 1. If saturation actually occurs in either operation, the instruction sets the Q flag in the CPSR. Syntax QDSUB{<cond>} <Rd>, <Rm>, <Rn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the register that contains the first operand for the saturated subtraction. <Rn> Specifies the register whose value is to be doubled, saturated, and used as the second operand for the saturated subtraction. Architecture version E variants of version 5 and above Exceptions None Operation if ConditionPassed(cond) then Rd = SignedSat(Rm - SignedSat(Rn*2, 32), 32) if SignedDoesSat(Rm - SignedSat(Rn*2, 32), 32) or SignedDoesSat(Rn*2, 32) then Q Flag = 1 A10-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 461. Enhanced DSP Extension Usage The primary use for this instruction is to generate multiply-subtract operations on Q15 and Q31 numbers, by placing it after an integer multiply instruction. Three examples are: • To multiply the Q15 numbers in the top half of R4 and the bottom half of R5, and subtract the product from the Q31 number in R6, use: SMULTB R0, R4, R5 QDSUB R6, R6, R0 • To multiply the Q15 number in the bottom half of R2 by the Q31 number in R3 and subtract the product from the Q31 number in R7, use: SMULWB R0, R3, R2 QDSUB R7, R7, R0 • To multiply the Q31 numbers in R2 and R3 and subtract the product from the Q31 number in R4, use: SMULL R0, R1, R2, R3 QDSUB R4, R4, R1 Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rn> has UNPREDICTABLE results. Condition flags The QDSUB instruction does not affect the N, Z, C, or V flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-21
- 462. Enhanced DSP Extension 10.6.8 QSUB 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 1 0 Rn Rd SBZ 0 1 0 1 Rm The QSUB instruction performs integer subtraction, saturating the result to the 32-bit signed integer range –231 £ x £ 231 – 1. If saturation actually occurs, the instruction sets the Q flag in the CPSR. Syntax QSUB{<cond>} <Rd>, <Rm>, <Rn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the register that contains the first operand for the saturated subtraction. <Rn> Specifies the register that contains the second operand for the saturated subtraction. Architecture version E variants of version 5 and above Exceptions None Operation if ConditionPassed(cond) then Rd = SignedSat(Rm - Rn, 32) if SignedDoesSat(Rm - Rn, 32) then Q Flag = 1 Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rn> has UNPREDICTABLE results. Condition flags The QSUB instruction does not affect the N, Z, C, or V flags. A10-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 463. Enhanced DSP Extension 10.6.9 SMLA<x><y> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 0 0 Rd Rn Rs 1 y x 0 Rm The SMLA<x><y> instructions (SMLABB, SMLABT, SMLATB and SMLATT) perform a signed multiply-accumulate operation. The multiply acts on two signed 16-bit quantities, taken from either the bottom or the top half of their respective source registers. The other halves of these source registers are ignored. The 32-bit product is added to a 32-bit accumulate value and the result is written to the destination register. If overflow occurs during the addition of the accumulate value, the instruction sets the Q flag in the CPSR. It is not possible for overflow to occur during the multiplication. Syntax SMLA<x><y>{<cond>} <Rd>, <Rm>, <Rs>, <Rn> where: <x> Specifies which half of the source register <Rm> is used as the first multiply operand. If <x> is B, then x == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rm> is used. If <x> is T, then x == 1 in the instruction encoding and the top half (bits[31:16]) of <Rm> is used. <y> Specifies which half of the source register <Rs> is used as the second multiply operand. If <y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs> is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of <Rs> is used. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the source register whose bottom or top half (selected by <x>) is the first multiply operand. <Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second multiply operand. <Rn> Specifies the register which contains the accumulate value. Architecture version E variants of version 5 and above ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-23
- 464. Enhanced DSP Extension Exceptions None Operation if ConditionPassed(cond) then if (x == 0) then operand1 = SignExtend(Rm[15:0]) else /* x == 1 */ operand1 = SignExtend(Rm[31:16]) if (y == 0) then operand2 = SignExtend(Rs[15:0]) else /* y == 1 */ operand2 = SignExtend(Rs[31:16]) Rd = (operand1 * operand2) + Rn if OverflowFrom((operand1 * operand2) + Rn) then Q Flag = 1 Usage In addition to its straightforward uses for integer multiply-accumulates, these instructions sometimes provide a faster alternative to Q15 ´ Q15 + Q31 ® Q31 multiply-accumulates synthesized from SMUL<x><y> and QDADD instructions. The main circumstances under which this is possible are: • if it is known that saturation and/or overflow cannot occur during the calculation • if saturation and/or overflow can occur during the calculation but the Q flag is going to be used to detect this and take remedial action if it does occur. For example, the following code produces the dot product of the four Q15 numbers in R0 and R1 by the four Q15 numbers in R2 and R3: SMULBB R4, R0, R2 QADD R4, R4, R4 SMULTT R5, R0, R2 QDADD R4, R4, R5 SMULBB R5, R1, R3 QDADD R4, R4, R5 SMULTT R5, R1, R3 QDADD R4, R4, R5 In the absence of saturation, the following code provides a faster alternative: A10-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 465. Enhanced DSP Extension SMULBB R4, R0, R2 SMLATT R4, R0, R2, R4 SMLABB R4, R1, R3, R4 SMLATT R4, R1, R3, R4 QADD R4, R4, R4 Furthermore, if saturation and/or overflow occurs in this second sequence, it will set the Q flag. This allows remedial action to be taken, such as scaling down the data values and repeating the calculation. Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, <Rs>, or <Rn> has UNPREDICTABLE results. Condition flags The SMLA<x><y> instructions do not affect the N, Z, C, or V flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-25
- 466. Enhanced DSP Extension 10.6.10 SMLAL<x><y> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 1 0 0 RdHi RdLo Rs 1 y x 0 Rm The SMLAL<x><y> instructions (SMLALBB, SMLALBT, SMLALTB and SMLALTT) perform a signed multiply-accumulate operation. The multiply acts on two signed 16-bit quantities, taken from either the bottom or the top half of their respective source registers. The other halves of these source registers are ignored. The 32-bit product is sign-extended and added to the 64-bit accumulate value held in <RdHi> and <RdLo>, and the result is written back to <RdHi> and <RdLo>. Overflow is possible during this instruction, but only as a result of the 64-bit addition. This overflow is not detected if it occurs. Instead, the result wraps around modulo 264. Syntax SMLAL<x><y>{<cond>} <RdLo>, <RdHi>, <Rm>, <Rs> where: <x> Specifies which half of the source register <Rm> is used as the first multiply operand. If <x> is B, then x == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rm> is used. If <x> is T, then x == 1 in the instruction encoding and the top half (bits[31:16]) of <Rm> is used. <y> Specifies which half of the source register <Rs> is used as the second multiply operand. If <y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs> is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of <Rs> is used. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <RdLo> Supplies the lower 32 bits of the 64-bit accumulate value to be added to the product, and is the destination register for the lower 32 bits of the 64-bit result. <RdHi> Supplies the upper 32 bits of the 64-bit accumulate value to be added to the product, and is the destination register for the upper 32 bits of the 64-bit result. <Rm> Specifies the source register whose bottom or top half (selected by <x>) is the first multiply operand. <Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second multiply operand. Architecture version E variants of version 5 and above A10-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 467. Enhanced DSP Extension Exceptions None Operation if ConditionPassed(cond) then if (x == 0) then operand1 = SignExtend(Rm[15:0]) else /* x == 1 */ operand1 = SignExtend(Rm[31:16]) if (y == 0) then operand2 = SignExtend(Rs[15:0]) else /* y == 1 */ operand2 = SignExtend(Rs[31:16]) RdLo = RdLo + (operand1 * operand2) RdHi = RdHi + (if (operand1*operand2) < 0 then 0xFFFFFFFF else 0) + CarryFrom(RdLo + (operand1 * operand2)) Usage These instructions allow a long sequence of multiply-accumulates of signed 16-bit integers or Q15 numbers to be performed, with sufficient guard bits to ensure that the result cannot overflow the 64-bit destination in practice. It would take more than 233 consecutive multiply-accumulates to cause such overflow. If the overall calculation does not overflow a signed 32-bit number, then <RdLo> will hold the result of the calculation. A simple test to determine whether such a calculation has overflowed <RdLo> is to execute the instruction: CMP <RdHi>, <RdLo>, ASR #31 at the end of the calculation. If the Z flag is set, <RdLo> holds an accurate final result. If the Z flag is clear, the final result has overflowed a signed 32-bit destination. Notes Use of R15 Specifying R15 for register <RdLo>, <RdHi>, <Rm>, or <Rs> has UNPREDICTABLE results. Operand restriction If <RdLo> and <RdHi> are the same register, the results are UNPREDICTABLE. Condition flags The SMLAL<x><y> instructions do not affect the N, Z, C, V, or Q flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-27
- 468. Enhanced DSP Extension 10.6.11 SMLAW<y> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 1 0 Rd Rn Rs 1 y 0 0 Rm The SMLAW<y> instructions (SMLAWB and SMLAWT) perform a signed multiply-accumulate operation. The multiply acts on a signed 32-bit quantity and a signed 16-bit quantity, with the latter being taken from either the bottom or the top half of its source register. The other half of the second source register is ignored. The top 32 bits of the 48-bit product are added to a 32-bit accumulate value and the result is written to the destination register. The bottom 16 bits of the 48-bit product are ignored. If overflow occurs during the addition of the accumulate value, the instruction sets the Q flag in the CPSR. No overflow can occur during the multiplication, because of the use of the top 32 bits of the 48-bit product. Syntax SMLAW<y>{<cond>} <Rd>, <Rm>, <Rs>, <Rn> where: <y> Specifies which half of the source register <Rs> is used as the second multiply operand. If <y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs> is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of <Rs> is used. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the source register which contains the 32-bit first multiply operand. <Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second multiply operand. <Rn> Specifies the register which contains the accumulate value. Architecture version E variants of version 5 and above Exceptions None A10-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 469. Enhanced DSP Extension Operation if ConditionPassed(cond) then if (y == 0) then operand2 = SignExtend(Rs[15:0]) else /* y == 1 */ operand2 = SignExtend(Rs[31:16]) Rd = (Rm * operand2)[47:16] + Rn /* Signed multiplication */ if OverflowFrom((Rm * operand2)[47:16] + Rn) then Q Flag = 1 Usage In addition to its straightforward uses for integer multiply-accumulates, these instructions sometimes provide a faster alternative to Q31 ´ Q15 + Q31 ® Q31 multiply-accumulates synthesized from SMULW<y> and QDADD instructions. The circumstances under which this is possible and the benefits it provides are very similar to those for the SMLA<x><y> instructions. See Usage on page A10-24 for more details. Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, <Rs>, or <Rn> has UNPREDICTABLE results. Condition flags The SMLAW<y> instructions do not affect the N, Z, C, or V flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-29
- 470. Enhanced DSP Extension 10.6.12 SMUL<x><y> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 1 1 0 Rd SBZ Rs 1 y x 0 Rm The SMUL<x><y> instructions (SMULBB, SMULBT, SMULTB, and SMULTT) perform a signed multiply operation. The multiply acts on two signed 16-bit quantities, taken from either the bottom or the top half of their respective source registers. The other halves of these source registers are ignored. No overflow is possible during this instruction. Syntax SMUL<x><y>{<cond>} <Rd>, <Rm>, <Rs> where: <x> Specifies which half of the source register <Rm> is used as the first multiply operand. If <x> is B, then x == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rm> is used. If <x> is T, then x == 1 in the instruction encoding and the top half (bits[31:16]) of <Rm> is used. <y> Specifies which half of the source register <Rs> is used as the second multiply operand. If <y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs> is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of <Rs> is used. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the source register whose bottom or top half (selected by <x>) is the first multiply operand. <Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second multiply operand. Architecture version E variants of version 5 and above Exceptions None A10-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 471. Enhanced DSP Extension Operation if ConditionPassed(cond) then if (x == 0) then operand1 = SignExtend(Rm[15:0]) else /* x == 1 */ operand1 = SignExtend(Rm[31:16]) if (y == 0) then operand2 = SignExtend(Rs[15:0]) else /* y == 1 */ operand2 = SignExtend(Rs[31:16]) Rd = operand1 * operand2 Usage In addition to its straightforward uses for integer multiplies, this instruction can be used in combination with the QADD, QDADD, and QDSUB instructions to perform multiplies, multiply-accumulates, and multiply-subtracts on Q15 numbers. See the Usage sections on page A10-17, page A10-19, and page A10-21 for examples. Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rs> has UNPREDICTABLE results. Condition flags The SMUL<x><y> instructions do not affect the N, Z, C, V, or Q flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-31
- 472. Enhanced DSP Extension 10.6.13 SMULW<y> 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 1 0 0 1 0 Rd SBZ Rs 1 y 1 0 Rm The SMULW<y> instructions (SMULWB and SMULWT) perform a signed multiply operation. The multiply acts on a signed 32-bit quantity and a signed 16-bit quantity, with the latter being taken from either the bottom or the top half of its source register. The other half of the second source register is ignored. The top 32 bits of the 48-bit product are written to the destination register. The bottom 16 bits of the 48-bit product are ignored. No overflow is possible during this instruction. Syntax SMULW<y>{<cond>} <Rd>, <Rm>, <Rs> where: <y> Specifies which half of the source register <Rs> is used as the second multiply operand. If <y> is B, then y == 0 in the instruction encoding and the bottom half (bits[15:0]) of <Rs> is used. If <y> is T, then y == 1 in the instruction encoding and the top half (bits[31:16]) of <Rs> is used. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination register of the instruction. <Rm> Specifies the source register which contains the 32-bit first operand. <Rs> Specifies the source register whose bottom or top half (selected by <y>) is the second operand. Architecture version E variants of version 5 and above Exceptions None A10-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 473. Enhanced DSP Extension Operation if ConditionPassed(cond) then if (y == 0) then operand2 = SignExtend(Rs[15:0]) else /* y == 1 */ operand2 = SignExtend(Rs[31:16]) Rd = (Rm * operand2)[47:16] /* Signed multiplication */ Usage In addition to its straightforward uses for integer multiplies, this instruction can be used in combination with the QADD, QDADD, and QDSUB instructions to perform multiplies, multiply-accumulates and multiply-subtracts between Q31 and Q15 numbers. See the Usage sections on page A10-17, page A10-19, and page A10-21 for examples. Notes Use of R15 Specifying R15 for register <Rd>, <Rm>, or <Rs> has UNPREDICTABLE results. Condition flags The SMULW<y> instructions do not affect the N, Z, C, V, or Q flags. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-33
- 474. Enhanced DSP Extension 10.6.14 STRD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 0 0 0 P U I W 0 Rn Rd addr_mode 1 1 1 1 addr_mode The STRD instruction stores a pair of ARM registers to two consecutive words of memory. The pair of registers is restricted to being an even-numbered register and the odd-numbered register that immediately follows it (for example, R10 and R11). A greater variety of addressing modes is available than for a 2-register STM. However, the address of the first of the two words is required to be doubleword-aligned (that is, the address must be divisible by 8). Syntax STR{<cond>}D <Rd>, <addressing_mode> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the even-numbered register which will be stored to the memory word addressed by <addressing_mode>. The immediately following odd-numbered register will be stored to the next memory word. If <Rd> is R14, which would specify R15 as the second source register, the instruction is UNPREDICTABLE. If <Rd> specifies an odd-numbered register, the instruction is UNDEFINED. <addressing_mode> Is described in Addressing Mode 3 - Miscellaneous Loads and Stores on page A5-34. It determines the P, U, I, W, Rn, and addr_mode bits of the instruction. The syntax of all forms of <addressing_mode> includes a base register <Rn>. Some forms also specify that the instruction modifies the base register value (this is known as base register writeback). The address generated by <addressing_mode> is the address of the lower of the two words stored by the STRD instruction. The address of the higher word is generated by adding 4 to this address. Architecture version E variants of version 5 and above, excluding ARMv5TExP Exceptions Data abort A10-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 475. Enhanced DSP Extension Operation if ConditionPassed(cond) then if (Rd is even-numbered) then if (address[2:0] == 0b000) and (Rd is not R14) then Memory[address,4] = Rd Memory[address+4,4] = R(d+1) else UNPREDICTABLE else UNDEFINED Notes Operand restrictions If <addressing_mode> performs base register writeback and the base register <Rn> is one of the two source registers of the instruction, the results are UNPREDICTABLE. Data abort For details of the effects of the instruction if a data abort occurs, see Effects of data-aborted instructions on page A2-17. Non doubleword-aligned addresses If the store address is not doubleword-aligned, the results are UNPREDICTABLE. Alignment If an implementation includes a System Control coprocessor (see Chapter B2 The System Control Coprocessor), and alignment checking is enabled, then: • an address with bits[1:0] != 0b00 causes an alignment exception • an address with bits[2:0] == 0b000 does not cause an alignment exception • it is IMPLEMENTATION DEFINED whether an address with bits[2:0] == 0b100 causes an alignment exception. Time order The time order of the accesses to the two memory words is not architecturally defined. In particular, an implementation is allowed to perform the two 32-bit memory accesses in either order, or to combine them into a single 64-bit memory access. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. A10-35
- 476. Enhanced DSP Extension A10-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 477. Part B Memory and System Architectures
- 479. Chapter B1 Introduction to Memory and System Architectures This chapter provides a high-level overview of memory and system architectures. It contains the following sections: • About the memory system on page B1-2 • System-level issues on page B1-4. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B1-1
- 480. Introduction to Memory and System Architectures 1.1 About the memory system ARM processors are used in a wide range of embedded systems and other applications. The memory system requirements of these applications vary considerably, from simple memory blocks with a flat address map, to systems using any or all of the following to optimize their use of memory resources: • multiple types of memory • caches • write buffers • virtual memory and other memory remapping techniques. The range of memory-mapped I/O devices that can be used adds further to the variety of systems based on ARM processors. Most systems need to initialize and control their memory system facilities in various ways, such as: • Enabling a cache, to ensure that its performance benefits are realized. • Setting up the virtual-to-physical address mapping for a virtual memory system. • Restricting access to memory regions. • Ensuring that the correct accesses to memory-mapped I/O devices occur, and that they occur at the correct times. (This usually happens automatically in the simplest memory systems, but caches and other facilities in more complex systems can interfere with it.) The standard way to perform memory system control in ARM-based systems is to use coprocessor 15 (CP15), which is also known as the System Control coprocessor. Chapter B2 The System Control Coprocessor provides a top-level overview of this coprocessor. The rest of Part B (Chapter B3 through Chapter B6) describes standardized ways to perform a variety of memory system control operations using CP15: Chapter B3 Memory Management Unit Describes a sophisticated system to control virtual-to-physical address mapping, access permissions to memory, and other memory attributes, based on the use of a Memory Management Unit (MMU). Chapter B4 Protection Unit Describes a simpler Protection Unit system which is suitable for many applications which do not require the full facilities provided by the MMU memory system. Chapter B5 Caches and Write Buffers Describes facilities to control caches and write buffers. These are common to the MMU and Protection Unit systems Chapter B6 Fast Context Switch Extension Describes the Fast Context Switch Extension, which facilitates fast switching between up to 128 processes executing in separate process blocks, each of size up to 32MB. B1-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 481. Introduction to Memory and System Architectures Note Because of the wide variety of systems based on ARM processors, all functionality described in Part B might be inappropriate to any given system. Furthermore, some ARM processors have implemented functions in a different manner to the one described here. Because of this, the datasheet or Technical Reference Manual for a particular ARM processor is the definitive source for its memory and system control facilities. Part B therefore does not attempt to specify absolute requirements on the functionality of the System Control coprocessor or other memory system components. Instead, it contains guidelines which, if followed: • mean that the system is more likely to be compatible with existing and future ARM software. • probably make it easier to port incompatible software to the system. In order to provide an adequate description of the range of memory and system facilities on existing ARM implementations, Part B describes a number of options that will not be used on new ARM implementations. For information on the rules that must be followed by new implementations of the memory and system architectures, contact ARM Ltd. The fact that Part B describes a broad range of facilities, many of which are used only on some existing ARM implementations, also means that architecture version numbers for the memory and system architectures would not be helpful or descriptive. They are therefore not used. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B1-3
- 482. Introduction to Memory and System Architectures 1.2 System-level issues This section lists a number of general and operating-system issues that the system designer needs to address when using an ARM processor. 1.2.1 Memory systems, write buffers and caches ARM processors and software are designed to be connected to a byte-addressed memory. Word and halfword accesses to the memory ignore the alignment of the address and access the naturally-aligned value that is addressed (so a memory access ignores address bits 0 and 1 for word access, and ignores bit 0 for halfword accesses). The endianness of the ARM processor should normally match that of the memory system, or be configured to match it before any non-word accesses occur (when the endianness is configurable and CP15 is implemented, bit[7] of CP15 register 1 controls the endianness). Memory that is used to hold programs and data should be marked as follows: • Main (RAM) memory is normally set as cachable and bufferable. • ROM memory is normally set as cachable, and should be marked as read only, so the bufferable attribute is not used and should be 1. Write buffers Some ARM implementations incorporate a merging write buffer that subsumes multiple writes to the same location into a single write to main memory. Furthermore, some write buffers re-order writes, so that writes are issued to memory in a different order to the order in which they are issued by the processor. Therefore, I/O locations should not normally be marked as bufferable, to ensure all writes are issued to the I/O device in the correct order. For writes to bufferable areas of memory, memory aborts can only be signaled to the processor as a result of conditions that are detectable at the time the data is placed in the write buffer. Conditions that can only be detected when the data is later written to main memory (such as a parity error from main memory) must be handled by other methods (typically by raising an interrupt). Caches Frame buffers can be cachable, but frame buffers on writeback cache implementations must be copied back to memory after the frame buffer has been updated. Frame buffers can be bufferable, but again the write buffer must be written back to memory after the frame buffer has been updated. ARM processors do not normally support cache coherence between the ARM and other system bus masters. Bus snooping is not supported. If memory data is to be shared between multiple bus masters without taking special software measures to ensure coherency, then the data must be mapped as: • uncachable to ensure that all reads access main memory • unbufferable to ensure that all write access main memory. B1-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 483. Introduction to Memory and System Architectures Alternatively, using software, you can manage the coherence of data buffers that are read or written by another bus master by: • cleaning data from writeback caches and write buffers to memory when the processor has written to the data buffer and before the other bus master reads the buffer • flushing relevant data from caches when the buffer is being read after the other bus master has written the buffer. You can use an uncached, unbuffered semaphore to maintain synchronization between multiple bus masters (see Semaphores on page B1-6). For implementations with writeback caches, all dirty cache data must be written back before any alterations are made to the MMU page tables, to ensure that cache line write back can use the page tables to form the correct physical address for the transfer. You can index caches using either virtual or physical addresses. Physical pages must only be mapped into a single virtual page, otherwise the result is UNPREDICTABLE. ARM processors do not normally provide coherence between multiple virtual copies of a single physical page. Some ARM implementations support separate instruction and data caches. Coherence between the data and instruction caches is not necessarily maintained in hardware, so if the instruction stream is written, the instruction cache and data cache must be made coherent. This can entail: • cleaning the data cache (storing dirty data to memory) • draining the write buffer (completing all buffered writes) • flushing the instruction cache. Instruction and data memory incoherence occurs after a program has been loaded (and therefore treated as data) and is about to be executed. It also occurs if self-modifying code is used or generated. 1.2.2 Interrupts ARM processors implement fast and normal levels of interrupt. Both interrupts are signaled externally, and many implementations synchronize interrupts before an exception is raised. Fast interrupt request (FIQ) Disables subsequent normal and fast interrupts by setting the I and F bits in the CPSR. Normal interrupt request (IRQ) Disables subsequent normal interrupts by setting the I bit in the CPSR. For more information, see Exceptions on page A2-13. Canceling interrupts It is the responsibility of software (the interrupt handler) to ensure that the cause of an interrupt is canceled (no longer signaled to the processor) before interrupts are re-enabled (by clearing the I and/or F bit in the CPSR). Interrupts can be canceled with any instruction that might make an external data bus access, meaning any load or store, a swap, or any coprocessor instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B1-5
- 484. Introduction to Memory and System Architectures Canceling an interrupt via an instruction fetch is UNPREDICTABLE. Canceling an interrupt with a load multiple that restores the CPSR and re-enables interrupts is UNPREDICTABLE. Devices that do not instantaneously cancel an interrupt (that is, they do not cancel the interrupt before letting the access complete) must be probed by software to ensure that interrupts have been canceled before interrupts are re-enabled. This allows a device connected to a remote I/O bus to operate correctly. 1.2.3 Semaphores The Swap and Swap Byte instructions have predictable behavior when used in two ways: • Systems with multiple bus masters that use the Swap instructions to implement semaphores to control interaction between different bus masters. In this case, the semaphores must be placed in an uncached and unbufferable region of memory. The Swap instruction then causes a (locked) read-write bus transaction. This type of semaphore can be externally aborted. • Systems with multiple threads running on a uniprocessor that use the Swap instructions to implement semaphores to control interaction of the threads. In this case, the semaphores can be placed in a cached and bufferable region of memory, and a (locked) read-write bus transaction might or might not occur. The Swap and Swap Byte instructions are likely to have better performance on such a system than they do on a system with multiple bus masters (as described above). This type of semaphore has UNPREDICTABLE behavior if it is externally aborted. Semaphores placed in uncachable/bufferable memory regions have UNPREDICTABLE results. Semaphores placed in cachable/unbufferable memory regions have UNPREDICTABLE results. B1-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 485. Chapter B2 The System Control Coprocessor This chapter describes coprocessor 15, the System Control coprocessor. It contains the following sections: • About the System Control coprocessor on page B2-2 • Registers on page B2-3 • Register 0: ID codes on page B2-6 • Register 1: Control register on page B2-13 • Registers 2-15 on page B2-17. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-1
- 486. The System Control Coprocessor 2.1 About the System Control coprocessor All of the standard memory and system facilities are controlled by coprocessor 15 (CP15), which is therefore called the System Control coprocessor. Some also use other methods of control, which are described in the chapters describing the facilities concerned. For example, the Memory Management Unit described in Chapter B3 Memory Management Unit is also controlled by page tables in memory. If none of the standard memory and system facilities are implemented in a system, the System Control coprocessor might not be present. In this case, no coprocessor accepts CP15 instructions, and so all such instructions are UNDEFINED. However, new implementations of the memory and system architectures must implement the System Control coprocessor, and must follow some additional rules about which facilities are implemented. For details of these rules, contact ARM Ltd. This chapter describes the overall design of the System Control coprocessor and how its registers are accessed. Detailed information is given on some of its registers. Other registers are allocated to facilities described in detail in other chapters and are only summarized in this chapter. B2-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 487. The System Control Coprocessor 2.2 Registers The System Control coprocessor can contain up to 16 primary registers, each of which is 32 bits long. For some of these, additional bits in the register access instructions are used to identify a specific version of the register and/or specific types of access to the register, so the number of physical 32-bit registers in CP15 can be more than 16. However, the 4-bit primary register number is used to identify registers in descriptions of the System Control coprocessor, because it is the primary factor determining the function of the register. CP15 registers can be read-only, write-only or read/write. The detailed descriptions of the registers specify: • what types of access are allowed • what functionality is invoked by each type of access • whether a primary register identifies more than one physical register, and if so, how they are distinguished • any other details that are relevant to the use of the register. 2.2.1 Register access instructions The only defined System Control coprocessor instructions are: • MCR instructions to write an ARM register to a CP15 register • MRC instructions to read the value of a CP15 register into an ARM register. All CP15 CDP, LDC and STC instructions are UNDEFINED. 31 28 27 26 25 24 23 21 20 19 16 15 12 11 8 7 5 4 3 0 cond 1 1 1 0 SBZ L CRn Rd 1 1 1 1 opcode2 1 CRm The MCR and MRC instructions to access the CP15 registers use the generic syntax for those instructions: MCR{<cond>} p15, 0, <Rd>, <CRn>, <CRm>{, <opcode2>} MRC{<cond>} p15, 0, <Rd>, <CRn>, <CRm>{, <opcode2>} where: <cond> This is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. Bits[23:21] These bits of the instruction, which are the <opcode1> field in generic MRC and MCR instructions, are always 0b000 in valid CP15 instructions. If they are not 0b000, the instruction is UNPREDICTABLE. <Rd> This is the ARM register involved in the transfer (the source register for MCR and the destination register for MRC). This register must not be R15, even though MCR instructions normally allow it to be R15. If R15 is specified for <Rd> in a CP15 MRC or MCR instruction, the instruction is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-3
- 488. The System Control Coprocessor <CRn> This is the primary CP15 register involved in the transfer (the destination register for MCR and the source register for MRC). The standard generic coprocessor register names are c0, c1, ..., c15. <CRm> This is an additional coprocessor register name which is used for accesses to some primary registers to specify additional information about the version of the register and/or the type of access. When the description of a primary register does not specify <CRm>, c0 must be specified. If another register is specified, the instruction is UNPREDICTABLE. <opcode2> This is an optional 3-bit number which is used for accesses to some primary registers to specify additional information about the version of the register and/or the type of access. If it is omitted, 0 is used. When the description of a primary register does not specify <opcode2>, it must be omitted or 0 must be specified. If another value is specified, the instruction is UNPREDICTABLE. These MCR and MRC instructions can only be used while the processor is in a privileged mode. If they are executed while the processor is in User mode, an Undefined Instruction exception occurs. Note If access to some System Control coprocessor functionality by User mode programs is required, the usual solution is that the operating system defines one or more SWIs to supply it. As the precise set of memory and system facilities available on different processors can vary considerably, it is recommended that all such SWIs are implemented in an easily replaceable module and that the SWI interface of this module is defined to be as independent of processor details as possible. The IMB and IMB_Range SWIs described in Instruction Memory Barriers (IMBs) on page A2-28 are examples of such SWIs. B2-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 489. The System Control Coprocessor 2.2.2 Primary register allocation Table 2-1 shows the allocation of the primary registers of the System Control coprocessor. Table 2-1 Primary register allocation Reg Generic use Specific uses Details in 0 ID codes (read-only) ID and Cache type Register 0: ID codes on page B2-6 1 Control bits (read/write) Miscellaneous control bits Register 1: Control register on page B2-13 2 Memory protection and control MMU: Translation table base PU: Cachability bits Register 2: Translation table base on page B3-23 Register 2: Cachability bits on page B4-6 3 Memory protection and control MMU: Domain access control PU: Bufferability bits Register 3: Domain access control on page B3-24 Register 3: Bufferability bits on page B4-6 4 Memory protection and control MMU: Reserved PU: Reserved Register 4: Reserved on page B3-24 Registers 4, 8, 10: Reserved on page B4-7 5 Memory protection and control MMU: Fault status PU: Access permission bits Register 5: Fault status on page B3-24 Register 5: Access permission bits on page B4-7 6 Memory protection and control MMU: Fault address PU: Protection area control Register 6: Fault address on page B3-25 Register 6: Protection area control on page B4-8 7 Cache and write buffer Cache/write buffer control Register 7: Cache functions on page B5-15 8 Memory protection and control MMU: TLB control PU: Reserved Register 8: TLB functions on page B3-25 Registers 4, 8, 10: Reserved on page B4-7 9 Cache and write buffer Cache lockdown Register 9: Cache lockdown on page B5-18 10 Memory protection and control MMU: TLB lockdown PU: Reserved Register 10: TLB lockdown on page B3-27 Registers 4, 8, 10: Reserved on page B4-73 11 Reserved - - 12 Reserved - - 13 Process ID Process ID Register 13: Process ID on page B6-6 14 Reserved - - 15 IMPLEMENTATION DEFINED IMPLEMENTATION DEFINED Implementation documents ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-5
- 490. The System Control Coprocessor 2.3 Register 0: ID codes CP15 register 0 contains one or more identification codes for the ARM and system implementation. When this register is read, the opcode2 field of the MRC instruction selects which identification code is wanted, as shown in Table 2-2, and the CRm field must be specified as c0 (if it is not, the instruction is UNPREDICTABLE). Writing to CP15 register 0 is UNPREDICTABLE. Table 2-2 System Control coprocessor ID registers opcode2 Register Details in 0b000 Main ID register Main ID register 0b001 Cache Type register Cache Type register on page B2-9 other Reserved (see main text) - It is recommended that all the ID registers in Table 2-2 are implemented, but only the main ID register (<opcode2> == 0) is mandatory. Whether or not other ID registers are implemented is IMPLEMENTATION DEFINED. If an <opcode2> value corresponding to an unimplemented or reserved ID register is encountered, the System Control coprocessor returns the value of the main ID register. ID registers other than the main ID register are defined so that when implemented, their value cannot be equal to that of the main ID register. Software can therefore determine whether they exist by reading both the main ID register and the desired register and comparing their values. If the two values are not equal, the desired register exists. 2.3.1 Main ID register When CP15 register 0 is read with <opcode2> == 0, an identification code is returned from which, among other things, the ARM architecture version number can be determined, as well as whether or not the Thumb instruction set has been implemented. Note Only some of the fields in CP15 register 0 are architecturally defined. The rest are IMPLEMENTATION DEFINED and provide more detailed information about the exact processor variant. Consult individual datasheets for the precise identification codes used for each processor. For historical reasons, there are three distinct ways in which the CP15 register 0 ID code might need to be interpreted. To determine which to use, look at bits[15:12] of the ID code: • if they are 0x0, this indicates a pre-ARM7 processor • if they are 0x7, this indicates that the processor is in the ARM7 family • otherwise, a more recent processor family than ARM7 is involved. B2-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 491. The System Control Coprocessor Post-ARM7 processors If bits[15:12] of the ID code are neither 0x0 nor 0x7, the ID code is interpreted as follows: 31 24 23 20 19 16 15 4 3 0 Implementor Variant Architecture Primary part number Revision Bits[3:0] Contain the IMPLEMENTATION DEFINED revision number for the processor. Bits[15:4] Contain an IMPLEMENTATION DEFINED representation of the primary part number for the processor. The top four bits of this number are not allowed to be 0x0 or 0x7. Bits[19:16] Contain an architecture code. The following architecture codes are defined (all other values of the architecture code are reserved by ARM Ltd): 0x1 Architecture 4 0x2 Architecture 4T 0x3 Architecture 5 0x4 Architecture 5T 0x5 Architecture 5TE. Bits[23:20] Contain an IMPLEMENTATION DEFINED variant number. This is typically used to distinguish two variants of the same primary part, for example, two different cache size variants. Bits[31:24] Contain an implementor code. The following codes are defined (all other values of the architecture code are reserved by ARM Ltd): 0x41 = A (ARM Ltd) 0x44 = D (Digital Equipment Corporation). 0x69 = i (Intel Corporation). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-7
- 492. The System Control Coprocessor ARM7 family processors If bits[15:12] of the ID code are 0x7, the ID code is interpreted as follows: 31 24 23 22 16 15 4 3 0 Implementor A Variant Primary part number Revision Bits[3:0] Contain the IMPLEMENTATION DEFINED revision number for the processor. Bits[15:4] Contain an IMPLEMENTATION DEFINED representation of the primary part number for the processor. The top four bits of this number are 0x7. Bits[22:16] Contain an IMPLEMENTATION DEFINED variant number. Bit[23] Indicates which of the two possible architectures for an ARM7-based processor is involved: 0 Architecture 3 1 Architecture 4T. Bits[31:24] Contain an implementor code. See Post-ARM7 processors for these codes. Pre-ARM7 processors Four processors prior to ARM7 use ID codes in which bits[15:12] are 0x0, and no further processors will be allocated such ID codes. They are interpreted as a 28-bit processor ID and a 4-bit revision number: 31 4 3 0 Processor ID Revision The processor ID values are as follows: 0x4156030 ARM3 (Architecture 2) 0x4156060 ARM600 (Architecture 3) 0x4156061 ARM610 (Architecture 3) 0x4156062 ARM620 (Architecture 3). B2-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 493. The System Control Coprocessor 2.3.2 Cache Type register If present, the Cache Type register supplies the following details about the cache: • whether it is a unified cache or separate instruction and data caches • its size, line length and associativity • whether it is a write-through cache or a write-back cache • how it can be cleaned efficiently (in the case of a write-back cache) • whether cache lock-down is supported. See Types of cache on page B5-5 for a discussion of these details. The format of the Cache Type register is: 31 29 28 25 24 23 12 11 0 0 0 0 ctype S Dsize Isize ctype Specifies details of the cache not specified by the S bit and the Dsize and Isize fields. See Table 2-3 on page B2-9 for details of the encoding. All values not specified in the table are reserved for future expansion. S bit Specifies whether the cache is a unified cache (S == 0), or separate instruction and data caches (S == 1). If S == 0, the Isize and Dsize fields both describe the unified cache, and must be identical. Dsize Specifies the size, line length and associativity of the data cache, or of the unified cache if S == 0. See Cache size fields on page B2-10 for details of the encoding. Isize Specifies the size, line length and associativity of the instruction cache, or of the unified cache if S == 0. See Cache size fields on page B2-10 for details of the encoding. Table 2-3 Cache type values ctype field Method Cache cleaning Cache lock-down 0b0000 Write-through Not needed Not supported 0b0001 Write-back Read data block Not supported 0b0010 Write-back Register 7 operations Not supported 0b0110 Write-back Register 7 operations Supported, format A 0b0111 Write-back Register 7 operations Supported, format B ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-9
- 494. The System Control Coprocessor The Read data block method of cleaning write-back caches encoded by ctype == 0b0001 consists of loading a sequential block of data with size equal to that of the cache, and which is known not to be in the cache already. It is only suitable for use when the cache organization guarantees that this causes the entire cache to be reloaded. (For example, direct-mapped caches normally have this property, as do caches using some types of round-robin replacement.) Note This method of cache cleaning must only be used if the Cache Type register has ctype == 0b0001, or if implementation documentation states that it is a valid method for the implementation. Register 7: Cache functions on page B5-15 gives details of the register 7 operations used for cleaning other write-back caches. For an explanation of cache lockdown and of the formats referred to in Table 2-3, see Register 9: Cache lockdown on page B5-18. 2.3.3 Cache size fields The Dsize and Isize fields in the Cache Type register have the same format, as follows: 11 9 8 6 5 3 2 1 0 0 0 0 size assoc M len Bits[11:9] are reserved for future expansion. The size of the cache is determined by the size field and M bit, as shown in Table 2-4. Table 2-4 Cache sizes size field Size if M == 0 Size if M == 1 0b000 0.5KB 0.75KB 0b001 1KB 1.5KB 0b010 2KB 3KB 0b011 4KB 6KB 0b100 8KB 12KB 0b101 16KB 24KB 0b110 32KB 48KB 0b111 64KB 96KB B2-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 495. The System Control Coprocessor The line length of the cache is determined by the len field, as shown in Table 2-5. Table 2-5 Cache line lengths len field Cache line length 0b00 2 words (8 bytes) 0b01 4 words (16 bytes) 0b10 8 words (32 bytes) 0b11 16 words (64 bytes) The associativity of the cache is determined by the assoc field and M bit, as shown in Table 2-6. Table 2-6 Cache associativity assoc field Associativity if M == 0 Associativity if M == 1 0b000 1-way (direct-mapped) cache absent 0b001 2-way 3-way 0b010 4-way 6-way 0b011 8-way 12-way 0b100 16-way 24-way 0b101 32-way 48-way 0b110 64-way 96-way 0b111 128-way 192-way The cache absent encoding overrides all other data in the cache size field. Alternatively, the following formulae can be used to determine the values LINELEN, ASSOCIATIVITY and NSETS, defined in Cache size on page B5-4, once the cache absent case (assoc == 0b000, M == 1) has been checked for and eliminated: LINELEN = 1 << (len+3) /* In bytes */ MULTIPLIER = 2 + M ASSOCIATIVITY = MULTIPLIER << (assoc-1) NSETS = 1 << (size + 6 - assoc - len) ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-11
- 496. The System Control Coprocessor Multiplying these together gives the overall cache size as: CACHE_SIZE = MULTIPLIER << (size+8) /* In bytes */ Note Cache length fields with (size + 6 - assoc - len) < 0 are invalid, as they correspond to impossible combinations of line length, associativity and overall cache size. So the formula for NSETS never involves a negative shift amount. B2-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 497. The System Control Coprocessor 2.4 Register 1: Control register This register contains: • Enable/disable bits for the caches, MMUs, and other memory system blocks that are primarily controlled by other CP15 registers. This allows these memory system blocks to be programmed correctly before they are enabled. • Various configuration bits for memory system blocks and for the ARM processor itself. Note Extra bits of both varieties might be added in the future. Because of this, this register should normally be updated using read/modify/write techniques, to ensure that currently unallocated bits are not needlessly modified. Failure to observe this rule might result in code which has unexpected side-effects on future processors. One exception that may be useful in some circumstances is that 0 can be written to this register to restore it to its reset state. 31 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 UNP/SBZP L4RRV I Z F R S B L D P W C A M Reading from CP15 register 1 reads the control bits. The <CRm> and <opcode2> fields are ignored when reading CP15 register 1, and should be zero. Writing to CP15 register 1 sets the control bits. The <CRm> and <opcode2> fields are not used when writing CP15 register 1, and should be zero. When a control bit in CP15 register 1 is not applicable to a particular implementation, it reads as the value that most closely reflects that implementation, and ignores writes. (Specific examples of this general rule are documented in the individual bit descriptions below.) Apart from bits that read as 1 according to this rule, all bits in CP15 register 1 are set to 0 on reset. M (bit[0]) This is the enable/disable bit for the MMU or Protection Unit: 0 = MMU or Protection Unit disabled 1 = MMU or Protection Unit enabled. On systems without an MMU or Protection Unit, this bit reads as 0 and ignores writes. A (bit[1]) For memory systems which optionally allow the alignment of data memory accesses to be checked, this bit enables and disables alignment fault checking: 0 = Alignment fault checking disabled 1 = Alignment fault checking enabled. For other memory systems, this bit ignores writes, and reads as 1 or 0 according to whether the memory system does or does not check the alignment of data memory accesses. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-13
- 498. The System Control Coprocessor C (bit[2]) If a unified cache is used, this is the enable/disable bit for the unified cache. If separate caches are used, this is the enable/disable bit for the data cache. In either case: 0 = Cache disabled 1 = Cache enabled. If the cache is not implemented, this bit reads as 0 and ignores writes. If the cache cannot be disabled, this bit reads as 1 and ignores writes. W (bit[3]) This is the enable/disable bit for the write buffer: 0 = Write buffer disabled 1 = Write buffer enabled. If the write buffer is not implemented, this bit reads as 0 and ignores writes. If the write buffer cannot be disabled, this bit reads as 1 and ignores writes. P (bit[4]) On ARM processors which support 26-bit backwards-compatibility configurations, this bit controls the PROG32 configuration signal: 0 = Exception handlers entered in 26-bit modes 1 = Exception handlers entered in 32-bit modes. For details, see 32-bit and 26-bit configuration on page A8-10. On ARM processors which do not support 26-bit backwards-compatibility configurations, this bit reads as 1 and ignores writes. D (bit[5]) On ARM processors which support 26-bit backwards-compatibility configurations, this bit controls the DATA32 configuration signal: 0 = 26-bit address exception checking enabled 1 = 26-bit address exception checking disabled. For details, see 32-bit and 26-bit configuration on page A8-10. On ARM processors which do not support 26-bit backwards-compatibility configurations, this bit reads as 1 and ignores writes. L (bit[6]) In some ARMv3 and earlier implementations, the abort model of the processor could be configured: 0 = Early Abort Model selected (now obsolete) 1 = Late Abort Model selected (same as Base Updated Abort Model). For details, see Effects of data-aborted instructions on page A2-17. On later processors, this bit reads as 1 and ignores writes. B (bit[7]) On ARM processors which support both little-endian and big-endian memory systems, this bit is used to configure the ARM processor to the endianness of the memory system: 0 = Configured for little-endian memory system 1 = Configured for big-endian memory system. For details, see Endianness on page A2-23. B2-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 499. The System Control Coprocessor On ARM processors that support only little-endian memory systems, this bit reads as 0 and ignores writes. On ARM processors that support only big-endian memory systems, this bit reads as 1 and ignores writes. S (bit[8]) System protection bit (in an MMU-based memory system). The effect of this bit is described in Access permissions on page B3-16. R (bit[9]) ROM protection bit (in an MMU-based memory system). The effect of this bit is described in Access permissions on page B3-16. F (bit[10]) The meaning of this bit is IMPLEMENTATION DEFINED. Z (bit[11]) On ARM processors which support branch prediction, this is the enable/disable bit for branch prediction: 0 = Branch prediction disabled 1 = Branch prediction enabled. If branch prediction cannot be disabled, this bit reads as 1 and ignores writes. On ARM processors that do not support branch prediction, this bit reads as 0 and ignores writes. I (bit[12]) If separate caches are used, this is the enable/disable bit for the instruction cache: 0 = Instruction cache disabled 1 = Instruction cache enabled. If a unified cache is used or the instruction cache is not implemented, this bit reads as 0 and ignores writes. If the instruction cache cannot be disabled, this bit reads as 1 and ignores writes. V (bit[13]) On ARM processors which support the alternative high vectors described in High vectors on page A2-21, this bit is used to select the location of the exception vector: 0 = Normal exception vectors selected (address range 0x00000000 - 0x0000001C) 1 = High exception vectors selected (address range 0xFFFF0000 - 0xFFFF001C). On ARM processors that do not support high vectors, this bit reads as 0 and ignores writes. RR (bit[14]) If the cache allows an alternative replacement strategy to be used which has more easily predictable worst-case performance, this bit selects it: 0 = Normal replacement strategy (for example, random replacement) 1 = Predictable strategy (for example, round-robin replacement). If the cache does not allow a choice of replacement strategy this bit should ignore writes, and read as 0 or 1 according to whether the replacement strategy has a reasonably easily predictable worst-case performance. L4 (bit[15]) For some ARM processors that support architecture version 5 or above, this bit controls a backwards-compatibility feature with previous versions of the architecture. 0 = normal behaviour for the architecture ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-15
- 500. The System Control Coprocessor 1 = load instructions that load the PC and use bit[0] of the loaded value to determine whether to enter ARM or Thumb state (that is, to set the new value of the CPSR T bit) instead ignore bit[0] of the T bit and stay in the current execution state. The instructions affected by this are: • LDM (1) on page A4-30 • LDR on page A4-37 • POP on page A7-75 For ARM processors that support architecture versions prior to version 5, this bit should be treated as UNP/SBZP. For ARM processors that support architecture versions 5 or above, but which do not support this backwards-compatibility feature, this bit reads as zero and ignores writes. Bits[31:16] When these bits are read, they return an UNPREDICTABLE value. When they are written, the value written must either be zero or a value previously read from the same bits. B2-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 501. The System Control Coprocessor 2.5 Registers 2-15 System Control coprocessor registers other than registers 0 and 1 are allocated to specific areas as follows: • CP15 registers 2-6, 8 and 10 are allocated to the memory protection system. See Chapter B3 Memory Management Unit and Chapter B4 Protection Unit for details of these registers. • CP15 registers 7 and 9 are allocated to the control of caches and write buffers. See Chapter B5 Caches and Write Buffers for details of these registers. • CP15 register 13 is allocated to the process ID of the Fast Context Switch Extension. See Chapter B6 Fast Context Switch Extension for details of this register. • CP15 register 15 is reserved for IMPLEMENTATION DEFINED purposes. See the technical reference manual for the implementation or other implementation-specific documentation for details of the facilities available through this register. • CP15 registers 11, 12 and 14 are reserved for future expansion. Accessing (reading or writing) any of these registers is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B2-17
- 502. The System Control Coprocessor B2-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 503. Chapter B3 Memory Management Unit This chapter describes the memory system architecture based on a Memory Management Unit (MMU). It contains the following sections: • About the MMU architecture on page B3-2 • Memory access sequence on page B3-4 • Translation process on page B3-6 • Access permissions on page B3-16 • Domains on page B3-17 • Aborts on page B3-18 • CP15 registers on page B3-23. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-1
- 504. Memory Management Unit 3.1 About the MMU architecture The Memory Management Unit (MMU) memory system architecture allows fine-grained control of a memory system. Most of the detailed control is provided through translation tables held in memory. Entries in these tables define the properties of memory areas of various sizes from 1KB to 1MB. The properties include: Virtual-to-physical address mapping An address generated by the ARM processor is called a virtual address. The MMU allows this address to be mapped to a different physical address. This physical address identifies which main memory location is being accessed. This can be used to manage the allocation of physical memory in many ways. For example, it can be used to allocate memory to different processes with potentially conflicting address maps, or to allow an application with a sparse address map to use a contiguous region of physical memory. Note If the Fast Context Switch Extension is being used (see Chapter B6 Fast Context Switch Extension), all references to virtual addresses in this chapter mean the modified virtual address that it generates. Memory access permissions These control whether a program has no access, read-only access or read/write access to the memory area. When an access is not permitted, a memory abort is signaled to the ARM processor. The level of access allowed can also be affected by whether the program is running in User mode or a privileged mode, and by the use of domains (see Domains on page B3-17). Cachability and bufferability bits (C and B) These are described in Cachability and bufferability on page B5-8. System Control coprocessor registers allow high-level control of this system, such as the location of the translation tables. They are also used to provide status information about memory aborts to the ARM. The process of doing a full translation table lookup is known as a translation table walk. It is performed automatically by hardware, and has a significant execution time cost (at least one main memory access, and often two). To reduce the average cost of a memory access, the results of translation table walks are cached in one or more structures known as Translation Lookaside Buffers (TLBs). Usually, there is a TLB for each memory interface of the ARM implementation: • a system with a single memory interface normally has a single unified TLB • a system with separate instruction and data memory interfaces normally has separate instruction and data TLBs. If the system also contains a cache or caches, the number of caches is usually determined in the same way (see Unified or separate caches on page B5-5). So in cached systems, there is normally one TLB per cache. B3-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 505. Memory Management Unit When translation tables in memory are changed or a different translation table is selected (by writing to CP15 register 2), previously cached translation table walk results in the TLBs are likely to cease to be valid. The MMU architecture therefore supplies operations to flush TLBs. The MMU architecture also allows specific translation table walk results to be locked down in a TLB. This ensures that accesses to the associated memory areas never cause a translation table walk, and has similar benefits for real-time code to those of locking down instructions and data in the cache (see Register 9: Cache lockdown on page B5-18). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-3
- 506. Memory Management Unit 3.2 Memory access sequence When the ARM generates a memory access, the MMU first looks up the virtual address of the access in the TLB. If the implementation has separate data and instruction TLBs, it uses: • the instruction TLB for an instruction fetch • the data TLB for all other types of access. If the TLB does not contain an entry for the virtual address, translation table walk hardware is invoked to retrieve the translation and access permission information from the translation table held in main memory. Once retrieved, the information is placed in the TLB, either in a currently unused entry or overwriting an existing entry. See Translation process on page B3-6 for a description of translation tables and of how the translation table walk is performed. Once the TLB entry for a memory access is obtained, the information it contains is used as follows: 1. The C (cachable) and B (bufferable) bits are used to control the cache and write buffer and to determine whether the access is cached or uncached, as described in Cachability and bufferability on page B5-8. (If the system does not contain a cache and/or a write buffer, the relevant bits are ignored.) 2. The access permission bits and domain are used to determine whether the access is permitted. If the access is not permitted, the MMU signals a memory abort to the ARM processor. Otherwise, the access is allowed to proceed. The sections Access permissions on page B3-16, Domains on page B3-17 and Aborts on page B3-18 describe in more detail how this is done. 3. For an uncached memory access (including all memory accesses in an uncached system), the physical address is used as the address for the main memory access. For a cached memory access, the physical address is used as the address for the line fetch in the event of a cache miss. If a cache hit occurs, the physical address is ignored. Figure 3-1 illustrates this for a cached system: Translation table walk hardware Physical address (PA) C, B bits Cache and write buffer Cache line fetch hardware Main memory Figure 3-1 Cached MMU memory system overview TLB Access control hardware Access bits, domain Abort ARM Virtual address (VA) B3-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 507. Memory Management Unit 3.2.1 Enabling and disabling the MMU The MMU can be enabled and disabled by writing bit[0] of register 1 of the System Control coprocessor. On reset, this bit is cleared to 0, so the MMU is disabled after a reset. When the MMU is disabled, memory accesses are treated as follows: • It is IMPLEMENTATION DEFINED whether caches and write buffers are allowed to be enabled while the MMU is disabled. — For caches and write buffers which are not allowed to be enabled while the MMU is disabled, the values of the C and B bits for a memory access are irrelevant. — For caches and write buffers which are allowed to be enabled while the MMU is disabled, a data access is treated as uncachable and unbufferable (C == 0, B == 0). An instruction fetch is treated as uncachable (C == 0) in a system with a unified TLB, and as cachable (C == 1) in a system with a separate instruction TLB. • No memory access permission checks are performed, and no aborts are generated by the MMU. • The physical address for every access is equal to its virtual address (this is known as a flat address mapping). Before the MMU is enabled, suitable translation tables must be set up in memory and all relevant CP15 registers must be programmed. Note Enabling or disabling the MMU effectively changes the virtual-to-physical address mapping (unless the translation tables are set up to implement a flat address mapping). It is therefore likely that any caches, for example, that are enabled at the time will need to be flushed (see Address mapping changes on page B5-10). In addition, if the physical address of the code that enables or disables the MMU differs from its virtual address, instruction prefetching can cause complications (see Prefetching and self-modifying code on page A2-27). It is therefore strongly recommended that code which enables or disables the MMU has identical virtual and physical addresses. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-5
- 508. Memory Management Unit 3.3 Translation process The MMU supports memory accesses based on sections or pages: Sections Are comprised of 1MB blocks of memory. Three different page sizes are supported: Tiny pages Consist of 1KB blocks of memory. Small pages Consist of 4KB blocks of memory. Large pages Consist of 64KB blocks of memory. Sections and large pages are supported to allow mapping of a large region of memory while using only a single entry in the TLB. Additional access control mechanisms are extended within small pages to 1KB subpages, and within large pages to 16KB subpages. There are no similar subpages for tiny pages, so access controls for tiny pages apply to the entire page. The translation table held in main memory has two levels: First-level table Holds both section translations and pointers to second-level tables. Second-level tables Hold both large and small page translations. One type of second-level table can also hold tiny page translations. The MMU translates virtual addresses generated by the CPU into physical addresses to access external memory, and also derives and checks the access permission. There are four routes by which the address translation (and therefore permission check) takes place. The route taken depends on whether the address in question has been marked as a section-mapped access or a page-mapped access. A page-mapped access can be a large, small, or tiny page access. However, the translation process always starts out in the same way, as described below, with a first-level fetch. A section-mapped access only requires a first level fetch, but a page-mapped access also requires a second-level fetch. 3.3.1 Translation table base The translation process is initiated when the on-chip TLB does not contain an entry for the requested virtual address. The Translation Table Base Register (CP15 register 2) holds the physical address of the base of the first-level table. Only bits[31:14] of the Translation Table Base Register are significant, and bits[13:0] should be zero. Therefore, the first-level page table must reside on a 16KB boundary. 3.3.2 First-level fetch Bits[31:14] of the Translation Table Base register are concatenated with bits[31:20] of the virtual address and two zero bits to produce a 32-bit physical address as illustrated in Figure 3-2. This address selects a four-byte translation table entry which is a first-level descriptor for a section or a pointer to a second-level page table. B3-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 509. Memory Management Unit 31 14 13 Translation base SBZ 31 20 19 Table index 31 14 13 2 1 Translation base Table index 0 0 0 0 0 Figure 3-2 Accessing the translation table first-level descriptors 3.3.3 First-level descriptors Each entry in the first-level table is a descriptor of how its associated 1MB virtual address range is mapped. There are the following possibilities, selected by bits[1:0] of the descriptor as shown in Table 3-1: • If bits[1:0] == 0b00, the associated virtual addresses are unmapped, and attempts to access them generate a translation fault (see Aborts on page B3-18). Software can use bits[31:2] for its own purposes in such a descriptor, as they are ignored by the hardware. Where appropriate, it is suggested that bits[31:2] continue to hold valid access permissions for the descriptor. • If bits[1:0] == 0b10, the entry is a section descriptor for its associated virtual addresses. See Section descriptor and translating section references on page B3-8 for details of how it is interpreted. • If bit[0] == 1, the entry gives the physical address of a coarse second-level table (if bit[1] == 0) or of a fine second-level table (if bit[1] == 1). Each type of table specifies how the associated 1MB virtual address range is mapped. Coarse tables are smaller, at 1KB per table (compared with 4KB per table for fine tables). However, coarse tables can only map large pages and small pages, while fine tables can map large, small and tiny pages. The two types of table are described in Coarse page table descriptor on page B3-9 and Fine page table descriptor on page B3-10. Table 3-1 First-level descriptor format 31 20 19 12 11 10 9 8 5 4 3 2 1 0 Fault IGN 0 0 Coarse page table Coarse page table base address S B Z Domain IMP 0 1 Section Section base address SBZ AP S B Z Domain I M P C B 1 0 Fine page table Fine page table base address SBZ Domain IMP 1 1 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-7
- 510. Memory Management Unit 3.3.4 Section descriptor and translating section references If the first-level descriptor is a section descriptor, the fields have the following meanings: Bits[1:0] Identify the type of descriptor (0b10 marks a section descriptor). Bits[3:2] The cachable and bufferable bits (see Cachability and bufferability on page B5-8). Bit[4] The meaning of this bit is IMPLEMENTATION DEFINED. Bits[8:5] The domain field specifies one of the sixteen possible domains for the section controlled by this descriptor. Bit[9] This bit is not currently used, and should be zero. Bits[11:10] Access permissions. These bits control the access to the section. See Table 3-3 on page B3-16 for the interpretation of these bits. Bits[19:12] These bits are not currently used, and should be zero. Bits[31:20] The Section Base Address forms the top 12 bits of the physical address. Figure 3-3 illustrates the complete section translation sequence. Note The access permissions in the first-level descriptor must be checked before the physical address is generated. The sequence for checking access permissions is given in Access permissions on page B3-16. Translation base SBZ 31 20 19 Table index Virtual address 31 14 13 2 1 0 0 0 0 0 Section index 31 20 19 12 11 10 9 8 5 4 3 2 1 0 Section base address SBZ AP SBZ Domain I MP C B 1 0 31 20 19 0 Section base address Section index Figure 3-3 Section translation 31 14 13 Translation base Table index Translation table base Physical address Address of first-level descriptor First-level fetch First-level descriptor B3-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 511. Memory Management Unit 3.3.5 Coarse page table descriptor If the first-level descriptor is a coarse page table descriptor, the fields have the following meanings: Bits[1:0] Identify the type of descriptor (0b01 marks a coarse page table descriptor). Bits[4:2] The meaning of these bits is IMPLEMENTATION DEFINED. Bits[8:5] The domain field specifies one of the sixteen possible domains for all the pages controlled by this descriptor. Bit[9] This bit is not currently used, and should be zero. Bits[31:10] The Page Table Base Address is a pointer to a coarse second-level page table, giving the base address for a second-level fetch to be performed. Coarse second-level page tables must be aligned on a 1KB boundary. If a coarse page table descriptor is returned from the first-level fetch, a second-level fetch is initiated to retrieve a second-level descriptor, as shown in Figure 3-4. 31 14 13 0 Translation base SBZ Virtual address 31 20 19 12 11 0 First-level table index Second-level table index 31 14 13 2 1 0 First-level table index Translation base 0 0 First-level fetch First-level descriptor 10 31 9 8 5 4 2 1 0 S Page table base address Domain IMP 0 1 BZ 31 9 2 1 0 10 Page table base address Second-level 0 0 table index Address of second-level descriptor Figure 3-4 Accessing coarse page table second-level descriptors Translation table base Address of first-level descriptor ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-9
- 512. Memory Management Unit 3.3.6 Fine page table descriptor If the first-level descriptor is a fine page table descriptor, the fields have the following meanings: Bits[1:0] Identify the type of descriptor (0b11 marks a fine page table descriptor). Bits[4:2] The meaning of these bits is IMPLEMENTATION DEFINED. Bits[8:5] The domain field specifies one of the sixteen possible domains for all the pages controlled by this descriptor. Bit[11:9] These bits are not currently used, and should be zero. Bits[31:12] The Page Table Base Address is a pointer to a fine second-level page table, giving the base address for a second-level fetch to be performed. Fine second-level page tables must be aligned on a 4KB boundary. If a fine page table descriptor is returned from the first-level fetch, a second-level fetch is initiated to retrieve a second-level descriptor, as shown in Figure 3-5. 31 14 13 0 Translation base SBZ Virtual address 31 20 19 10 9 0 First-level table index 31 14 13 2 1 0 First-level table index Translation base 0 0 First-level fetch First-level descriptor Second-level table index 31 12 11 9 8 5 4 2 1 0 Page table base address SBZ Domain IMP 1 1 31 12 11 2 1 0 Page table base address Second-level 0 0 table index Address of second-level descriptor Figure 3-5 Accessing fine page table second-level descriptors Translation table base Address of first-level descriptor B3-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 513. Memory Management Unit 3.3.7 Second-level descriptor Each entry in a coarse second-level table corresponds to how a 4KB virtual address range is mapped, and each entry in a fine second-level table corresponds to how a 1KB virtual address range is mapped. The entries are page descriptors, and can describe a page which is larger than 4KB or 1KB respectively. When this happens, the descriptor must be repeated enough times in the second-level table to ensure that the same descriptor is always used for the page, regardless of which virtual address is being accessed within the page. There are four possibilities for a second-level descriptor, selected by bits[1:0] of the descriptor as shown in Table 3-2: • If bits[1:0] == 0b00, the associated virtual addresses are unmapped, and attempts to access them generate a translation fault (see Aborts on page B3-18). Software can use bits[31:2] for its own purposes in such a descriptor, as they are ignored by the hardware. Where appropriate, it is suggested that bits[31:2] continue to hold valid access permissions for the descriptor. • If bits[1:0] == 0b01, the entry is a large page descriptor, describing 64KB of virtual addresses. See Translating large page references on page B3-13 for details. A large page descriptor must be repeated 64 times in a fine second-level table or 16 times in a coarse second-level table to ensure that all of its virtual addresses are described. • If bits[1:0] == 0b10, the entry is a small page descriptor, describing 4KB of virtual addresses. See Translating small page references on page B3-14 for details. A small page descriptor must be repeated four times in a fine second-level table to ensure that all of its virtual addresses are described. In a coarse second-level table, only one instance of each small page descriptor is needed. • If bits[1:0] == 0b11, the entry is a tiny page descriptor, describing 1KB of virtual addresses. See Translating tiny page references on page B3-15 for details. Only one instance of each tiny page descriptor is needed in a fine second-level table. Tiny page descriptors must not appear in coarse second-level tables. If they do, the results are UNPREDICTABLE. Table 3-2 Second-level descriptor format 31 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 Fault IGN 0 0 Large page Large table base address SBZ AP3 AP2 AP1 AP0 C B 0 1 Small page Small page base address AP3 AP2 AP1 AP0 C B 1 0 Tiny page Tiny page base address SBZ AP C B 1 1 ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-11
- 514. Memory Management Unit Large page descriptor fields The fields in a large page descriptor have the following meanings: Bits[1:0] Identify the type of descriptor. Bits[3:2] Are the cachable and bufferable bits (see Cachability and bufferability on page B5-8). Bits[11:4] Are the access permission bits. These bits control access to the page. See Table 3-3 on page B3-16 for the interpretation of these bits. Large pages are split into four subpages: AP0 Encodes the access permissions for the first subpages. AP1 Encodes the access permissions for the second subpages. AP2 Encodes the access permissions for the third subpages. AP3 Encodes the access permissions for the fourth (last) subpages. Bits[15:12] Are not currently used, and must be zero. Bits[31:16] Are used to form the corresponding bits of the physical address. Small page descriptor fields The fields in a small page descriptor have the following meanings: Bits[1:0] Identify the type of descriptor. Bits[3:2] Are the cachable and bufferable bits (see Cachability and bufferability on page B5-8). Bits[11:4] Are the access permission bits. These bits control access to the page. See Table 3-3 on page B3-16 for the interpretation of these bits. Small pages are split into four subpages: AP0 Encodes the access permissions for the first subpages. AP1 Encodes the access permissions for the second subpages. AP2 Encodes the access permissions for the third subpages. AP3 Encodes the access permissions for the fourth (last) subpages. Bits[31:12] Are used to form the corresponding bits of the physical address. Tiny page descriptor fields The fields in a tiny page descriptor have the following meanings: Bits[1:0] Identify the type of descriptor. Bits[3:2] Are the cachable and bufferable bits (see Cachability and bufferability on page B5-8). Bits[5:4] Are the access permission bits. See Table 3-3 on page B3-16 for the interpretation of these bits, which apply to the entire tiny page. Bits[9:6] Are not currently used, and must be zero. Bits[31:10] Are used to form the corresponding bits of the physical address. B3-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 515. Memory Management Unit 3.3.8 Translating large page references Figure 3-6 shows the complete translation sequence for a 64KB large page in a coarse second-level table. The translation sequence for a large page in a fine second-level table is similar, but with the address of the second-level descriptor being determined as shown in Fine page table descriptor on page B3-10. Note As the upper four bits of the Page Index and low-order four bits of the Second-level Table Index overlap, each page table entry for a large page must be repeated 16 times (in consecutive memory locations) in a coarse page table. Similarly, each page table entry for a large page must be repeated 64 times in a fine page table. 31 14 13 0 Translation base SBZ Virtual address 31 20 19 12 11 0 16 15 First-level Second- table index level table index 31 14 13 2 1 0 First-level table index Translation base 0 0 10 9 8 5 4 2 1 0 SBZ Page table base address Domain IMP 0 1 31 10 9 2 1 0 Page table base address Second-level 0 0 Second-level fetch table index Page index 31 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 Large page base address AP3 AP2 AP1 AP0 C B 0 1 First-level fetch First-level descriptor Address of second-level descriptor 31 31 16 15 0 Large page base address SBZ Page index Second-level descriptor Physical address Figure 3-6 Large page translation in a coarse second-level table Translation table base Address of first-level descriptor ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-13
- 516. Memory Management Unit 3.3.9 Translating small page references Figure 3-7 shows the complete translation sequence for a 4KB small page in a coarse second-level table. The translation sequence for a small page in a fine second-level table is similar, but with the address of the second-level descriptor being determined as shown in Fine page table descriptor on page B3-10. Note When a small page appears in a fine second-level table, the upper two bits of the Page Index and the low-order two bits of the Second-level Table Index overlap. So each page table entry for a small page must be repeated four times (in consecutive memory locations) in a fine page table. 31 14 13 0 Translation base SBZ Virtual address 31 20 19 12 11 0 First-level table index Second-level table index 31 14 13 2 1 0 First-level table index Translation base 0 0 31 10 9 8 5 4 2 1 0 SBZ Page table base address Domain IMP 0 1 31 10 9 2 1 0 Page table base address Second-level 0 0 Second-level fetch table index Page index 31 12 11 10 9 8 7 6 5 4 3 2 1 0 Small page base address AP2 AP1 AP0 B 1 0 First-level fetch First-level descriptor AP3 C 31 12 11 0 Small page base address Page index Second-level descriptor Physical address Figure 3-7 Small page translation in a coarse second-level table Translation table base Address of first-level descriptor Address of second-level descriptor B3-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 517. Memory Management Unit 3.3.10 Translating tiny page references Figure 3-8 shows the complete translation sequence for a 1KB tiny page in a fine second-level table. Note Tiny pages can not appear in a coarse second-level table. 31 14 13 0 Translation base SBZ Virtual address 31 20 19 10 9 0 First-level table index 31 14 13 2 1 0 First-level table index Translation base 0 0 Second-level table index 31 8 5 4 2 1 0 12 11 9 Page table base address SBZ Domain IMP 1 1 31 2 1 0 12 11 Page table base address Second-level 0 0 Second-level fetch table index Page index 31 10 9 6 5 4 3 2 1 0 Tiny page base address SBZ AP C B 1 1 31 0 Tiny page base address Second-level descriptor Physical address 10 9 Page index Figure 3-8 Tiny page translation in a fine second-level table Translation table base Address of first-level descriptor First-level fetch First-level descriptor Address of second-level descriptor ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-15
- 518. Memory Management Unit 3.4 Access permissions The access permission bits in section and page descriptors control access to the corresponding section or page. The access permissions are modified by the System (S) and ROM (R) control bits in CP15 register 1. Table 3-3 describes the meaning of the access permission bits in conjunction with the S and R bits. If an access is made to an area of memory without the required permission, a Permission Fault is raised (see Aborts on page B3-18). Table 3-3 MMU access permissions AP S R Privileged permissions User permissions 0b00 0 0 No access No access 0b00 1 0 Read only No access 0b00 0 1 Read only Read only 0b00 1 1 UNPREDICTABLE UNPREDICTABLE 0b01 x x Read/write No access 0b10 x x Read/write Read only 0b11 x x Read/write Read/write B3-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 519. Memory Management Unit 3.5 Domains A domain is a collection of sections, large pages and small pages. The ARM architecture supports 16 domains. Access to each domain is controlled by a 2-bit field in the Domain Access Control Register. Each field allows the access to an entire domain to be enabled and disabled very quickly, so that whole memory areas can be swapped in and out of virtual memory very efficiently. Two kinds of domain access are supported: Clients Are users of domains (execute programs, access data), and are guarded by the access permissions of the individual sections and pages that make up the domain. Managers Control the behavior of the domain (the current sections and pages in the domain, and the domain access), and are not guarded by the access permissions of individual sections and pages in the domain. One program can be a client of some domains, and a manager of some other domains, and have no access to the remaining domains. This allows very flexible memory protection for programs that access different memory resources. Table 3-4 illustrates the encoding of the bits in the Domain Access Control Register. Table 3-4 Domain Access Values Value Access types Description 0b00 No access Any access generates a domain fault 0b01 Client Accesses are checked against the access permission bits in the section or page descriptor 0b10 Reserved Using this value has UNPREDICTABLE results 0b11 Manager Accesses are not checked against the access permission bits in the section or page descriptor, so a permission fault cannot be generated ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-17
- 520. Memory Management Unit 3.6 Aborts Mechanisms that can cause the ARM processor to halt execution because of memory access restrictions are: MMU fault The MMU detects the restriction and signals the processor. External abort The external memory system signals an illegal memory access. Collectively, MMU faults and external aborts are just called aborts. Accesses that cause aborts are said to be aborted. If the memory request that is aborted is an instruction fetch, then a Prefetch Abort exception is raised if and when the processor attempts to execute the instruction corresponding to the illegal access. If the aborted access is a data access, a Data Abort exception is raised. See Exceptions on page A2-13 for more information about Prefetch and Data Aborts. 3.6.1 MMU faults The MMU generates four types of fault: • alignment fault • translation fault • domain fault • permission fault. The memory system can abort three types of access: • line fetches • memory accesses (uncached or unbuffered accesses) • translation table accesses. Aborts that are detected by the MMU are stopped before any external memory access takes place. It is the responsibility of the external system to stop external accesses that cause external aborts. The System Control coprocessor contains two registers which are updated when a data access is aborted. These registers are not updated for Prefetch Aborts, as the aborted instruction might not be executed due to changes in program flow. Fault Address Register (FAR) and Fault Status Register (FSR) Aborts resulting from data accesses (Data Aborts) are immediately acted upon by the CPU. The Fault Status Register (FSR) is updated with a 4-bit Fault Status (FS[3:0]) and the domain number of the access. In addition, the virtual address which caused the Data Abort is written into the Fault Address Register (FAR). If a data access simultaneously generates more than one type of Data Abort, they are prioritized in the order given in Table 3-5 on page B3-19. Aborts arising from instruction fetches are flagged as the instruction enters the instruction pipeline. Only when (and if) the instruction is executed does it cause a Prefetch Abort exception. An abort resulting from an instruction fetch is not acted upon if the instruction is not used (for example, if it is branched around). Normally, the fault address associated with a Prefetch Abort exception is determined from the value saved in R14_abt when the Prefetch Abort exception vector is entered. It is IMPLEMENTATION DEFINED whether B3-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 521. Memory Management Unit the FSR and FAR are updated for an abort arising from an instruction fetch, and if so, what useful information they contain about the fault. However, an abort arising from an instruction fetch will never update the FSR and the FAR between the time that an abort arising from a data access updates them and the time of the corresponding entry into the Data Abort exception vector. In other words, a Data Abort handler can rely upon its FAR and FSR values not being corrupted by an abort arising from an instruction fetch that was not acted upon. Priority Sources Highest Terminal Exception 0b0010 Invalid IMPLEMENTATION Vector Exception 0b0000 Invalid Valid Alignment 0b00x1 Invalid Valid External Abort on Translation First level Translation Section Domain Section Permission Section External Abort on Linefetch Section Lowest External Abort on Non-linefetch Note Table 3-5 Priority encoding of fault status FS [3:0] Domain [3:0] FAR DEFINED Second level 0b1100 0b1110 Invalid Valid Valid Valid Page 0b0101 0b0111 Invalid Valid Valid Valid Page 0b1001 0b1011 Valid Valid Valid Valid Page 0b1101 0b1111 Valid Valid Valid Valid Page 0b0100 0b0110 Valid Valid Valid Valid Section Page 0b1000 0b1010 Valid Valid Valid Valid Alignment faults can write either 0b0001 or 0b0011 into FS[3:0]. Invalid values in Domain[3:0] occur because the fault is raised before a valid domain field has been loaded. Any abort masked by the priority encoding can be regenerated by fixing the primary abort and restarting the instruction. The FS[3:0] encoding for Vector Exception breaks from the pattern that FS[0] is zero for all external aborts. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-19
- 522. Memory Management Unit Fault-checking sequence The sequence by which the MMU checks for access faults is slightly different for Sections and Pages. Figure 3-9 illustrates the sequence for both types of access. Check address alignment Yes Yes No access Misaligned? Alignment fault Yes No Get second-level descriptor External abort No Invalid descriptor? No Check domain Access type Client Check access permissions Violation? No Yes Yes Virtual address Checking alignment? No Get first-level descriptor External abort No Descriptor fault Translation external abort Section translation fault Section Page Section or page? Manager Physical address Translation external abort Page translation fault Page domain fault Sub-page permission fault Figure 3-9 Sequence for checking faults No Yes No access Yes Check domain Access type Client Check access permissions Violation? Section domain fault Section permission fault No B3-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 523. Memory Management Unit Terminal exception This indicates that an irrecoverable fault has occurred. The circumstances under which this can happen (if at all) are IMPLEMENTATION DEFINED. Vector exception When the processor is in a 32-bit configuration (PROG32 is active) and in a 26-bit mode (CPSR[4] == 0), data access (but not instruction fetches) to the normal exception vectors (address 0x0 to 0x1F) cause a Data Abort, known as a vector exception. It is IMPLEMENTATION DEFINED if vector exceptions are generated when the MMU is not enabled. For more information, see Vector exceptions on page A8-11. Alignment fault If alignment faults are enabled, an alignment fault is generated on any data word access whose address is not word-aligned (virtual address bits [1:0] != 0b00), or any halfword access that is not halfword-aligned (virtual address bit[0] != 0). Alignment faults are not generated on any instruction fetch, or on any byte access. Note If the access generates an alignment fault, it is aborted without reference to further permission checks. It is IMPLEMENTATION DEFINED if alignment exceptions are generated when the MMU is not enabled. Translation fault There are two types of translation fault: Section This is generated if the first-level descriptor is marked as invalid. This happens if bits[1:0] of the descriptor are both 0. Page This is generated if the second-level descriptor is marked as invalid. This happens if bits[1:0] of the descriptor are both 0. Domain fault There are two types of domain fault: • Section • Page. In both cases, the first descriptor holds the 4-bit Domain field which selects one of the sixteen 2-bit domains in the Domain Access Control Register. The two bits of the specified domain are then checked for access permissions as detailed in Table 3-4 on page B3-17: • In the case of a section, the domain is checked when the first-level descriptor is returned. • In the case of a page, the domain is checked when the second-level descriptor is returned. If the specified access is marked as No Access in the Domain Access Control Register, either a Section Domain Fault or Page Domain Fault occurs. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-21
- 524. Memory Management Unit Permission fault There are section permission faults and subpage permission faults. Permission faults are checked at the same time as domain faults. If the 2-bit domain field returns client (01), the permission access check is invoked as follows: Section If the first-level descriptor defines a section access, the AP bits of the descriptor define whether or not the access is allowed, according to Table 3-3 on page B3-16. If the access is not allowed, a section permission fault is generated. Subpages If the first-level descriptor defines a page-mapped access, the second-level descriptor can define a tiny page, a small page or a large page. If the second-level descriptor defines a tiny page, it contains a single access permission field (AP), which applies to the whole page. Otherwise, the second-level descriptor contains four access permission fields (AP3, AP2, AP1, AP0), each corresponding to one quarter of the page. For small pages, AP3 is selected by the top 1KB of the page, and AP0 is selected by the bottom 1KB of the page. For large pages, AP3 is selected by the top 16KB of the page, and AP0 is selected by the bottom 16KB of the page. The selected AP bits are then interpreted in exactly the same way as for a section, (see Table 3-3 on page B3-16) the only difference being that the fault generated is a subpage permission fault. 3.6.2 External aborts In addition to the MMU faults, the ARM architecture defines an external abort pin which can be used to flag an error on an external memory access. However, not all accesses can be aborted in this way, so this pin must be used with great care. The following accesses can be externally aborted and restarted safely: • reads • unbuffered writes • first-level descriptor fetch • second-level descriptor fetch • semaphores in uncachable/unbufferable memory areas. A linefetch can be safely aborted on any word in the line transfer. If the abort happens on data that has been requested by the processor (rather than data that is being fetched as the remainder of a cache line), the access is aborted. Any data transferred that is not immediately accessed (the remainder of the cache line) is only aborted when it is accessed. It is IMPLEMENTATION DEFINED if the FAR points to the start address of the cache line, or the address that generated the abort. Buffered writes cannot be externally aborted. Therefore, the system must be configured so that it does not do buffered writes to areas of memory which are capable of flagging an external abort, or a different mechanism needs to be used to signal the abort (an interrupt for example). The value of a memory location that causes an abort is UNPREDICTABLE after the abort. B3-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 525. Memory Management Unit 3.7 CP15 registers The MMU is controlled with the System Control coprocessor registers 2, 3, 4, 5, 6, 8 and 10, and some bits of register 1. 3.7.1 MMU control bits in register 1 The following bits in the System Control coprocessor register 1 are used to control the MMU: M (bit[0]) This is the enable/disable bit for the MMU: 0 = MMU disabled 1 = MMU enabled. On systems without an MMU or Protection Unit, this bit must read as zero and ignore writes. A (bit[1]) This bit enables and disables alignment fault checking (see Alignment fault on page B3-21): 0 = Alignment fault checking disabled 1 = Alignment fault checking enabled. S (bit[8]) This is the system protection bit. The effect of this bit is described in Access permissions on page B3-16. R (bit[9]) This is the ROM protection bit. The effect of this bit is described in Access permissions on page B3-16. 3.7.2 Register 2: Translation table base 31 14 13 0 Translation Table Base UNP/SBZP Reading from CP15 register 2 returns the physical address of the currently active first-level translation table in bits[31:14] and an UNPREDICTABLE value in bits[13:0]. The CRm and opcode2 fields are ignored when reading CP15 register 2 and should be zero. Writing to CP15 register 2 updates the physical address of the currently active first-level translation table from the value in bits[31:14] of the written value. Bits[13:0] must be written as zero or as a value previously read from bits[13:0] of this register. The CRm and opcode2 fields are ignored when writing CP15 register 2 and should be zero. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-23
- 526. Memory Management Unit 3.7.3 Register 3: Domain access control 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 Reading from CP15 register 3 returns the value of the Domain Access Control Register. The CRm and opcode2 fields are ignored when reading CP15 register 3 and should be zero. Writing to CP15 register 3 writes the value of the Domain Access Control Register. The CRm and opcode2 fields are ignored when writing CP15 register 3 and should be zero. The Domain Access Control Register consists of sixteen 2-bit fields (see Domains on page B3-17), each defining the access permissions for one of the sixteen Domains (D15–D0). 3.7.4 Register 4: Reserved Reading and writing CP15 register 4 is UNPREDICTABLE. 3.7.5 Register 5: Fault status 31 9 8 7 4 3 0 UNP/SBZP 0 Domain Status Reading CP15 register 5 returns the value of the Fault Status Register (FSR). The FSR contains the source of the last data fault. Only the bottom nine bits are returned. The upper 23 bits are UNPREDICTABLE. The FSR indicates the domain and type of access being attempted when an abort occurred. Bit[8] Returns zero. Bits[7:4] Specify which of the sixteen domains (D15–D0) was being accessed when a fault occurred. Bits[3:0] Indicate the type of access being attempted. The encoding of these bits is shown in Table 3-5 on page B3-19. The FSR is updated for data faults. It is IMPLEMENTATION DEFINED whether the FSR is updated for prefetch faults (for details, see Fault Address Register (FAR) and Fault Status Register (FSR) on page B3-18). The CRm and opcode2 fields are ignored when reading CP15 register 5 and should be zero. Writing CP15 register 5 sets the Fault Status Register to the value of the data written. This is useful for a debugger to restore the value of the FSR. The upper 24 bits written should be zero or a value previously read from the FSR. The CRm and opcode2 fields are ignored when writing CP15 register 5 and should be zero. B3-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 527. Memory Management Unit 3.7.6 Register 6: Fault address 31 0 Fault address Reading CP15 register 6 returns the value of the FAR. The FAR holds the virtual address of the access which was attempted when a fault occurred. The FAR is updated for data faults. It is IMPLEMENTATION DEFINED whether the FAR is updated for prefetch faults (for details see Fault Address Register (FAR) and Fault Status Register (FSR) on page B3-18). The CRm and opcode2 fields are ignored when reading CP15 register 6 and should be zero. Writing CP15 register 6 sets the Fault Address Register to the value of the data written. This is useful for a debugger to restore the value of the FAR. The CRm and opcode2 fields are ignored when writing CP15 register 6 and should be zero. Note If the Fast Context Switch Extension (FCSE) described in Chapter B6 is being used, then: • When the FAR is updated for a memory fault, the modified virtual address is written to the FAR. • When the FAR is read by an MRC instruction or written by an MCR instruction, its value is being treated as data and so no address modification is performed by the FCSE. 3.7.7 Register 8: TLB functions CP15 register 8 is a write-only register which is used to control TLBs. Table 3-6 on page B3-26 shows the defined TLB operations and the values of <CRm> and <opcode2> used in the MCR instruction for each of them. The results of using any combination of <CRm> and <opcode2> not specified in the table are UNPREDICTABLE. If any of the following operations are used on an implementation with a unified TLB, the equivalent function is performed on the unified TLB: • invalidate instruction TLB • invalidate instruction single entry • invalidate entire data TLB • invalidate data single entry. Otherwise, if a function not relevant to a particular implementation is performed, the results are UNPREDICTABLE. Attempting to read CP15 register 8 with an MRC instruction is UNPREDICTABLE. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-25
- 528. Memory Management Unit The invalidate single entry operations can be used to improve performance on some implementations when only a small amount of memory is being remapped. For each remapped memory area (section, small page or large page), the invalidate single entry operation needs to be performed on a virtual address in the memory area. The performance improvement comes from not having to subsequently reload TLB entries relating to memory areas that were not remapped. Note The performance improvement for the invalidate single entry operations is not guaranteed. Implementations are free to invalidate more than the single requested entry, up to and including invalidating the entire TLB. Caution When memory is remapped, all TLB entries relating to the old mapping must be invalidated. If this is not done, it is possible to get into a state where two TLB entries relate to overlapping ranges of virtual addresses. Any attempt to address the overlapped virtual memory addresses results in UNPREDICTABLE behavior at best, and might even physically damage the MMU on some implementations. It is therefore strongly recommended that great care is taken to invalidate the TLB appropriately whenever memory is remapped. Function <opcode2> <CRm> Data Instruction Invalidate entire unified TLB or both instruction and data TLBs Invalidate unified single entry 0b001 0b0111 Virtual Invalidate entire instruction TLB Invalidate instruction single entry Invalidate entire data TLB 0b000 0b0110 SBZ MCR p15, 0, Rd, c8, c6, 0 Invalidate data single entry 0b001 0b0110 Virtual Note Table 3-6 TLB functions 0b000 0b0111 SBZ MCR p15, 0, Rd, c8, c7, 0 address MCR p15, 0, Rd, c8, c7, 1 0b000 0b0101 SBZ MCR p15, 0, Rd, c8, c5, 0 0b001 0b0101 Virtual address MCR p15, 0, Rd, c8, c5, 1 address MCR p15, 0, Rd, c8, c6, 1 If the Fast Context Switch Extension (FCSE) described in Chapter B6 is being used, the virtual addresses passed to CP15 by some of the functions in Table 3-6 are passed as data. This means that no address modification is performed for them by the FCSE. B3-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 529. Memory Management Unit 3.7.8 Register 10: TLB lockdown Translation table walks can take a considerable amount of time, especially as they involve potentially slow main memory accesses. In real-time interrupt handlers, translation table walks caused by the TLB not containing translations for the handler and/or the data it accesses can increase interrupt latency significantly. TLB lockdown is a feature of some ARM memory systems which allows the results of specified translation table walks to be loaded into the TLB, in such a way that they are not overwritten by the results of subsequent translation table walks. It is programmed via CP15 register 10. If W is the logarithm base 2 of the number of TLB entries, rounded up to an integer if necessary, then the format of CP15 register 10 is: 31 32-W 31-W 32-2W 31-2W 1 0 base victim UNP/SBZP P If the implementation has separate instruction and data TLBs, there are two variants of this register, selected by the <opcode2> field of the MCR or MRC instruction used to access register 10: <opcode2> == 0 Selects the data TLB lockdown register. <opcode2> == 1 Selects the instruction TLB lockdown register. If the implementation has a unified TLB, only one variant of this register exists, and <opcode2> should be zero. <CRm> must always be c0 for MCR and MRC instructions that access register 10. Writing register 10 has the following effects: • The victim field specifies which TLB entry is replaced by the translation table walk result generated by the next TLB miss. • The base field constrains the TLB replacement strategy to only use the TLB entries numbered from (base) to (number of TLB entries)-1, provided the victim field is already in that range. • Any translation table walk results written to TLB entries while P == 1 are protected from being invalidated by the register 8 invalidate entire TLB operations. Ones written while P == 0 are invalidated normally by these operations. Note If the number of TLB entries is not a power of two, writing a value to either the base or victim fields which is greater than or equal to the number of TLB entries has UNPREDICTABLE results. Reading register 10 returns the last values written to the base field and the P bit, and the number of the next TLB entry to be replaced in the victim field. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-27
- 530. Memory Management Unit TLB lockdown procedure The normal procedure to lock down N TLB entries is as follows: 1. Ensure that no processor exceptions can occur during the execution of this procedure, by disabling interrupts, and so on. 2. If an instruction TLB or unified TLB is being locked down, write the appropriate version of register 10 with base == N, index == N, and P == 0. If appropriate, also turn off facilities like branch prediction that make instruction prefetching harder to understand. 3. Invalidate the entire TLB to be locked down. 4. If an instruction TLB is being locked down, ensure that all TLB entries are loaded which relate to any instruction that could be prefetched by the rest of the lockdown procedure. (Provided care is taken about where the lockdown procedure starts, it is normally possible for one TLB entry to cover all of these, in which case the first instruction prefetch after the TLB is invalidated can do this job.) If a data TLB is being locked down, ensure that all TLB entries are loaded which relate to any data accessed by the rest of the lockdown procedure, including any inline literals used by its code. (This is usually best done by avoiding the use of inline literals in the lockdown procedure and by putting all other data used by it in an area covered by a single TLB entry, then loading one data item.) If a unified TLB is being locked down, do both of the above. 5. For each of i = 0 to N-1: a. Write to register 10 with base == i, victim == i, and P == 1. b. Force a translation table walk to occur for the area of memory whose translation table walk result is to be locked into TLB entry i, by: • If a data TLB or unified TLB is being locked down, loading an item of data from the area of memory. • If an instruction TLB is being locked down, using the register 7 prefetch instruction cache line operation defined in Register 7: Cache functions on page B5-15 to cause an instruction to be prefetched from the area of memory. 6. Write to register 10 with base == N, victim == N, and P == 0. Note If you are using the Fast Context Switch Extension (FCSE) (see Chapter B6), take care in step 5b, because: • If a data TLB or a unified TLB is being locked down, the address used for the load instruction is subject to modification by the FCSE. • If an instruction TLB is being locked down, the address used for the register 7 operation is being treated as data and so is not subject to modification by the FCSE. To minimise the possible confusion caused by this, it is recommended that the lockdown procedure should: • start by disabling the FCSE (by setting the PID to zero) • where appropriate, generate modified virtual addresses itself by ORing the appropriate PID value into the top 7 bits of the virtual addresses it uses. B3-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 531. Memory Management Unit TLB unlock procedure To unlock the locked-down portion of the TLB after it has been locked down using the above procedure: 1. Use register 8 operations to invalidate each single entry that was locked down. 2. Write to register 10 with base == 0, victim == 0, and P == 0. Note Step 1 is used in order to ensure that P == 1 entries are not left in the TLB. If they were left in the TLB, the entire TLB invalidation step (step 3) of a subsequent TLB lockdown procedure would not have the desired effect. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B3-29
- 532. Memory Management Unit B3-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 533. Chapter B4 Protection Unit This chapter describes the ARM Protection Unit. It contains the following sections: • About the Protection Unit on page B4-2 • Overlapping regions on page B4-5 • CP15 registers on page B4-6. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B4-1
- 534. Protection Unit 4.1 About the Protection Unit The Protection Unit memory system architecture provides a considerably simpler alternative to the MMU memory system architecture described in Chapter B3 Memory Management Unit. This allows both hardware and software to be simplified in systems which do not require all facilities of the MMU. The main simplification is that the Protection Unit does not use translation tables. Instead, up to eight protection regions can be defined by writing to Protection Unit registers. This eliminates the need for hardware to do translation table walks, and for software to set up and maintain the translation tables. The cost of this simplification is that the level of control over memory behavior is considerably less fine-grained. MMU translation tables can define many thousands of memory regions with different attributes, considerably more than are possible with the Protection Unit. A second simplification is that virtual-to-physical address mapping is not supported. The physical memory address is always the same as the virtual address generated by the ARM processor (except that if the Fast Context Switch Extension is used, as described in Chapter B6 Fast Context Switch Extension, the physical address is the same as the modified virtual address that it generates). 4.1.1 Protection regions The sizes and starting addresses of the eight protection regions are defined using CP15 register 6. The size of a region must be a power of two, and can range from 4KB up to 4GB. The starting address of a region must be a multiple of its size. For example, a 4KB region (size 0x1000) can start at address 0x12345000, but an 8KB region (size 0x2000) or larger can not. The cachable bits (C), bufferable bits (B) and Access Permission (AP) bits of the eight protection regions are defined by CP15 registers 2, 3 and 5 respectively. These behave similarly to the corresponding bits in the MMU memory system architecture, except that the interpretation of the AP bits is not modified by the System (S) and ROM (R) bits in CP15 register 1. (These bits are therefore not normally implemented in Protection Unit systems.) Protection regions can legitimately overlap each other. If an address lies within more than one protection region, the attributes of the highest-numbered protection region apply to it. For more details, see Overlapping regions on page B4-5. Some implementations (typically those with separate instruction and data caches) provide two separate sets of eight protection regions, with one set being used for instruction fetches and the other set for data accesses. Note It is possible to program the instruction and data protection regions differently in these implementations, but some care needs to be taken. In particular, most ARM and Thumb code areas include inline literals, which are constant addresses and other data items held in the same area as the code itself. Inline literals are automatically generated by compilers, and can also appear in assembler code. (For ARM Development Systems, they are typically generated by the use of DCD, DCB and related assembler directives, or by the use of the construction LDR Rn,=expression together with an LTORG directive.) Inline literals are typically accessed by LDR instructions with base register R15. Such instructions generate a data access to the code area, so it does not normally make sense to program an instruction protection region without programming a corresponding data protection region to allow the code to access its inline literals. B4-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 535. Protection Unit 4.1.2 Memory access sequence When the ARM processor generates a memory access, the Protection Unit checks whether the address lies within the range of each of the eight protection regions (using the appropriate set of eight regions if the implementation has separate sets for instruction and data accesses): • If the address does not lie in any protection region, the memory access is aborted. • If the address lies in one or more protection regions, the access is controlled by the highest-numbered of those regions. The C, B and AP bits associated with that region are used by the access as follows: 1. The C (cachable) and B (bufferable) bits are used to control the cache and write buffer and to determine whether the access is cached or uncached, as described in Cachability and bufferability on page B5-8. (If the system does not contain a cache and/or a write buffer, the relevant bits are ignored.) 2. The AP (access permission) bits are used to determine whether the access is permitted. If the access is not permitted, the Protection Unit signals a memory abort to the ARM processor. Otherwise, the access is allowed to proceed. Register 5: Access permission bits on page B4-7 and Exceptions on page A2-13 describe this in more detail. Figure 4-1 illustrates the memory access sequence for a cached system: Registers 2, 3 Cache and write buffer Cache line fetch hardware Main memory C, B bits Selected region Priority encoder Region address comparators Virtual/physical address Figure 4-1 Cached Protection Unit memory system overview Register 5 AP bits Access control hardware Abort ARM ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B4-3
- 536. Protection Unit 4.1.3 Enabling the Protection Unit The Protection Unit can be enabled by writing 1 to CP15 register 1 bit[0], and can be disabled by writing 0 to the same bit. On reset, this bit is cleared and all protection regions are invalidated. Before the Protection Unit is enabled, at least one valid protection region must be specified and its C, B and AP bits must be set. In the case of a Protection Unit with separate sets of protection regions for instruction and data accesses, at least one valid protection region in each of the two sets must be programmed in this manner. Furthermore, the code that enables the Protection Unit must lie within a valid (instruction) protection region. Enabling the Protection Unit without adhering to these rules can result in behavior which is at best UNPREDICTABLE, and can even result in a hung state which can only be recovered from by resetting the system. While the Protection Unit is disabled, all memory accesses are treated as uncachable, unbufferable and non-aborting. B4-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 537. Protection Unit 4.2 Overlapping regions The Protection Unit can be programmed with two or more overlapping regions. When overlapping regions are programmed, a fixed priority scheme is applied to determine the region whose attributes are applied to the memory access. Attributes for region 7 take highest priority and those for region 0 take lowest priority. For example: • Data region 2 is programmed to be 4KB in size, starting from address 0x3000 with AP == 0b10 (Privileged mode full access, User mode read only). • Data region 1 is programmed to be 16KB in size, starting from address 0x0 with AP == 0b01 (Privileged mode access only). When the processor performs a data load from address 0x3010 while in User mode, the address falls into both region 1 and region 2, as shown in Figure 4-2. Because there is a clash, the attributes associated with region 2 are applied. In this case, the load would not abort. 0x4000 0x3000 0x3010 0x0 Region 2 Region 1 Figure 4-2 Overlapping memory regions 4.2.1 Background regions Overlapping regions increase the flexibility of how the eight regions can be mapped onto physical memory devices in the system. The overlapping properties can also be used to specify a background region. For example, there might be a number of physical memory areas sparsely distributed across the 4GB address space. If a programming error occurs therefore, it might be possible for the processor to issue an address which does not fall into any defined region. If the address issued by the processor falls outside of any of the defined regions, the Protection Unit is hard-wired to abort the access. This behavior can be overridden by programming region 0 to be a 4GB background region. In this way, if the address does not fall into any of the other seven regions, the access is controlled by the attributes the user has specified for region 0. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B4-5
- 538. Protection Unit 4.3 CP15 registers The Protection Unit is controlled with the System Control coprocessor registers 2, 3, 5, and 6, and one or two bits of register 1. 4.3.1 Protection Unit control bits in register 1 M(bit[0]) in the System Control coprocessor register 1 is the enable/disable bit for the Protection Unit: 0 = Protection Unit disabled 1 = Protection Unit enabled. On systems without a Protection Unit or MMU, this bit must read as zero and ignore writes. A Protection Unit memory system can optionally implement alignment checking, as described in Alignment fault on page B3-21. If it does, the A bit (bit[1]) of register 1 behaves as described there and in MMU control bits in register 1 on page B3-23. Otherwise, the A bit should be zero. 4.3.2 Register 2: Cachability bits 31 8 7 6 5 4 3 2 1 0 UNP/SBZP C7C6C5C4C3C2C1C0 Reading from CP15 register 2 returns the cachable (C) bits for the eight protection regions in bits[7:0], with bit[n] corresponding to region n, and an UNPREDICTABLE value in bits[31:8]. Writing to CP15 register 2 updates the cachable (C) bits of the eight protection regions, with the C bit of region n being set to bit[n] of the value written. Bits[31:8] must be written as zero or as a value previously read from bits[31:8] of this register. In each case, the <CRm> field of the MRC or MCR instruction is ignored and must be c0. If the implementation only has one set of eight protection regions, the <opcode2> field should be zero. If it has separate sets of protection regions for instruction and data accesses, <opcode2> must be specified as 0 to select the data protection regions and 1 to select the instruction protection regions. For details of how the C bit affects cache and write buffer behavior, see Cachability and bufferability on page B5-8. 4.3.3 Register 3: Bufferability bits 31 8 7 6 5 4 3 2 1 0 UNP/SBZP B7B6B5B4B3B2B1B0 Reading from CP15 register 3 returns the bufferable (B) bits for the eight protection regions in bits[7:0], with bit[n] corresponding to region n, and an UNPREDICTABLE value in bits[31:8]. B4-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 539. Protection Unit Writing to CP15 register 3 updates the bufferable (B) bits of the eight regions, with the B bit of region n being set to bit[n] of the value written. Bits[31:8] must be written as zero or as a value previously read from bits[31:8] of this register. In each case, the <CRm> and <opcode2> fields of the MRC or MCR instruction are ignored and must be c0 and zero respectively. For details of how the B bit affects cache and write buffer behavior, see Cachability and bufferability on page B5-8. 4.3.4 Registers 4, 8, 10: Reserved Accessing (reading or writing) any of these registers is UNPREDICTABLE. 4.3.5 Register 5: Access permission bits 31 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 UNP/SBZP AP7 AP6 AP5 AP4 AP3 AP2 AP1 AP0 Reading from CP15 register 3 returns the AP bits for the eight protection regions in bits[15:0], with bits[2n+1:2n] corresponding to region n, and an UNPREDICTABLE value in bits[31:16]. Writing to CP15 register 3 updates the AP bits of the eight regions, with the AP bits of region n being set to bits[2n+1:2n] of the value written. Bits[31:16] must be written as zero or as a value previously read from bits[31:16] of this register. In each case, the <CRm> field of the MRC or MCR instruction is ignored and must be c0. If the implementation only has one set of eight protection regions, the <opcode2> field should be zero. If it has separate sets of protection regions for instruction and data accesses, <opcode2> must be specified as 0 to select the data protection regions and 1 to select the instruction protection regions. The interpretation of each set of AP bits is as shown in Table 4-1 on page B4-7. If the requested type of access is not permitted, an abort is signaled to the ARM processor. Table 4-1 Protection Unit access permissions AP Privileged permissions User permissions 0b00 No access No access 0b01 Read/write No access 0b10 Read/write Read only 0b11 Read/write Read/write ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B4-7
- 540. Protection Unit 4.3.6 Register 6: Protection area control 31 12 11 6 5 1 0 Base address UNP/SBZP Size E Reading from CP15 register 6 returns the current base address, size and enabled/disabled status of a protection region, in the format shown in the above diagram. The value read for bits[11:6] is UNPREDICTABLE. Writing to CP15 register 6 sets the base address, size and enabled/disabled status of a protection region, in the format shown in the above diagram. The value written to bits[11:6] must either be zero or a value previously read from bits[11:6] of CP15 register 6. There is one version of register 6 for each protection region in the Protection Unit. The version used (and therefore the protection region affected) is selected by the <CRm> and <opcode2> fields of the MCR or MRC instruction used to access the register: • <CRm> is used to select the number of the protection region, by specifying c0 to select protection region 0, c1 to select protection region 1, and so on, through to c7 to select protection region 7. • If the implementation only has one set of eight protection regions, the <opcode2> field should be zero. • If the implementation has separate sets of protection regions for instruction and data accesses, <opcode2> must be specified as 0 to select a data protection region and 1 to select an instruction protection region. The meaning of the fields in the value read from or written to register 6 is as follows: • The E bit enables or disables the associated protection region: 0 = protection region disabled 1 = protection region enabled. A disabled protection region never matches any addresses, and therefore does not affect the memory access sequence in any way. All protection regions are disabled on reset. • The Size field selects the associated protection region's size, which can vary from 4KB to 4GB. The encoding is shown in Table 4-2 on page B4-9. • The Base address field specifies bits[31:12] of the address of the first byte in the associated protection region. The address of this first byte is required to be a multiple of the region size, so its bits[11:0] are always zero and do not need to be programmed. Also, because of this requirement for the region to be aligned to a multiple of its size, one or more further bits of the base address may also be constrained to be zero. These additional constraints are shown in Table 4-2 on page B4-9. If they are broken, the protection region is misaligned, and UNPREDICTABLE behavior might result. B4-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 541. Protection Unit Table 4-2 Region size encoding Size field Area size Base area constraints 0b00000 to 0b01010 UNPREDICTABLE - 0b01011 4KB None 0b01100 8KB Bit[12] must be zero 0b01101 16KB Bits[13:12] must be zero 0b01110 32KB Bits[14:12] must be zero 0b01111 64KB Bits[15:12] must be zero 0b10000 128KB Bits[16:12] must be zero 0b10001 256KB Bits[17:12] must be zero 0b10010 512KB Bits[18:12] must be zero 0b10011 1MB Bits[19:12] must be zero 0b10100 2MB Bits[20:12] must be zero 0b10101 4MB Bits[21:12] must be zero 0b10110 8MB Bits[22:12] must be zero 0b10111 16MB Bits[23:12] must be zero 0b11000 32MB Bits[24:12] must be zero 0b11001 64MB Bits[25:12] must be zero 0b11010 128MB Bits[26:12] must be zero 0b11011 256MB Bits[27:12] must be zero 0b11100 512MB Bits[28:12] must be zero 0b11101 1GB Bits[29:12] must be zero 0b11110 2GB Bits[30:12] must be zero 0b11111 4GB Bits[31:12] must be zero ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B4-9
- 542. Protection Unit B4-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 543. Chapter B5 Caches and Write Buffers This chapter describes cache and write buffer control functions that are common to both the MMU-based memory system and the Protection Unit-based memory system. It contains the following sections: • About caches and write buffers on page B5-2 • Cache organization on page B5-3 • Types of cache on page B5-5 • Cachability and bufferability on page B5-8 • Memory coherency on page B5-10 • CP15 registers on page B5-14. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-1
- 544. Caches and Write Buffers 5.1 About caches and write buffers Caches and write buffers can be used in ARM memory systems to improve their average performance. A cache is a block of high-speed memory locations whose addresses can be changed, and whose purpose is to increase the average speed of a memory access. Each memory location of a cache is known as a cache line. Normally, changes to the address of a cache line occur automatically. Whenever the processor loads data from a memory address and no cache line currently holds that data, a cache line is allocated to that address and the data is read into the cache line. If data at the same address is accessed again before the cache line is re-allocated to another address, the cache can process the memory access at high speed. So a cache typically speeds up the second and subsequent accesses to the data. In practice, these second and subsequent accesses are common enough for this is to produce a significant performance gain. This effect is known as temporal locality. To reduce the percentage overhead of storing the current addresses of the cache lines, each cache line normally consists of several memory words. This increases the cost of the first access to a cache line, since several words need to be loaded from main memory to satisfy a request for just one word. However, it also means that a subsequent access to another word in the same cache line can be processed by the cache at high speed. This sort of access is also common enough to increase performance significantly. This effect is known as spatial locality. A memory access which can be processed at high speed because the data it addresses is already in the cache is known as a cache hit. Other memory accesses are called cache misses. A write buffer is a block of high-speed memory whose purpose is to optimize stores to main memory. When a store occurs, its data, address and other details (such as data size) are written to the write buffer at high speed. The write buffer then completes the store at main memory speed, which is typically much slower than the speed of the ARM processor. In the meantime, the ARM processor can proceed to execute further instructions at full speed. Write buffers and caches introduce a number of potential problems, mainly due to: • memory accesses occurring at times other than when the programmer would normally expect them • there being multiple physical locations where a data item can be held. This chapter discusses these problems, and describes cache and write buffer control facilities that can be used to work around them. They are common to the Memory Management Unit system architecture described in Chapter B3 Memory Management Unit and the Protection Unit system architecture described in Chapter B4 Protection Unit. Note The caches described in this chapter are accessed using the virtual address of the memory access. This implies that they will need to be invalidated and/or cleaned when the virtual-to-physical address mapping changes or in certain other circumstances, as described in Memory coherency on page B5-10. If the Fast Context Switch Extension (FCSE) described in Chapter B6 is being used, all references to virtual addresses in this chapter mean the modified virtual address that it generates. B5-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 545. Caches and Write Buffers 5.2 Cache organization The basic unit of storage in a cache is the cache line. A cache line is said to be valid when it contains cached data or instructions, and invalid when it does not. All cache lines in a cache are invalidated on reset. A cache line becomes valid when data or instructions are loaded into it from memory. When a cache line is valid, it contains up-to-date values for a block of consecutive main memory locations. The length of this block (and therefore the length of the cache line) is always a power of two, and is typically 16 bytes (4 words) or 32 bytes (8 words). If the cache line length is 2L bytes, the block of main memory locations is always 2L-byte aligned. Such blocks of main memory locations are called memory cache lines or (loosely) just cache lines. Because of this alignment requirement, virtual address bits[31:L] are identical for all bytes in a cache line. A cache hit occurs when bits[31:L] of the virtual address supplied by the ARM processor match the same bits of the virtual address associated with a valid cache line. To simplify and speed up the process of determining whether a cache hit occurs, a cache is usually divided into a number of cache sets. The number of cache sets is always a power of two. If the cache line length is 2L bytes and there are 2S cache sets, bits[L+S-1:L] of the virtual address supplied by the ARM processor are used to select a cache set. Only the cache lines in that set are allowed to hold the data or instructions at the address. The remaining bits of the virtual address (bits[31:L+S]) are known as its tag bits. A cache hit occurs if the tag bits of the virtual address supplied by the ARM processor match the tag bits associated with a valid line in the selected cache set. Figure 5-1 illustrates how the virtual address is used to look up data or instructions in the cache. 31 L+S L+S-1 L L-1 0 tag set pos S Select one of 2 cache sets Look for cache line with tag in selected cache set if not found if found Cache hit Return data at position pos in cache line Figure 5-1 Cache look-up Virtual address Cache miss Get data from main memory ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-3
- 546. Caches and Write Buffers 5.2.1 Set-associativity The set-associativity of a cache is the number of cache lines in each of its cache sets. It can be any number ³ 1, and is not restricted to being a power of two. Low set-associativity generally simplifies cache look-up. However, if the number of frequently-used memory cache lines that use a particular cache set exceeds the set-associativity, main memory activity goes up and performance drops. This is known as cache contention, and becomes more likely as the set associativity is decreased. The two extreme cases are fully associative caches and direct-mapped caches: • A fully associative cache has just one cache set, which consists of the entire cache. It is N-way set-associative, where N is the total number of cache lines in the cache. Any cache look-up in a fully associative cache needs to check every cache line. • A direct-mapped cache is a one-way set-associative cache. Each cache set consists of a single cache line, so cache look-up just needs to select and check one cache line. However, cache contention is particularly likely to occur in direct-mapped caches. Within each cache set, the cache lines are numbered from 0 to (set associativity)-1. The number associated with each cache line is known as its index. Some cache operations take a cache line index as a parameter, to allow a software loop to work systematically through a cache set. 5.2.2 Cache size Generally, as the size of a cache increases, a higher percentage of memory accesses are cache hits. This reduces the average time per memory access and so improves performance. However, a large cache typically uses a significant amount of silicon area and power. Different sizes of cache can therefore be used in an ARM memory system, depending on the relative importance of performance, silicon area, and power consumption. The cache size can be broken down into a product of three factors: • The cache line length LINELEN, measured in bytes. • The set-associativity ASSOCIATIVITY. A cache set consists of ASSOCIATIVITY cache lines, so the size of a cache set is ASSOCIATIVITY ´ LINELEN. • The number NSETS of cache sets making up the cache. If separate data and instruction caches are used, different values of these parameters can be used for each, and the resulting cache sizes can be different. If the System Control coprocessor supports the Cache Type register, it can be used to determine these cache size parameters (see Cache Type register on page B2-9). B5-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 547. Caches and Write Buffers 5.3 Types of cache There are many different possible types of cache, which can be distinguished by implementation choices such as: • how big they are • how they handle instruction fetches • how they handle data writes • how much of the cache is eligible to hold any particular item of data. A number of these implementation choices are detailed in the subsections below. Also see Cache Type register on page B2-9 for details of how most of these choices can be determined for implementations which include a Cache Type register. Note A high-performance memory system can contain more than one level of cache, with the first level being small and very high speed, the next level being bigger and slower, and so on, out to main memory, which is the largest and slowest component of the memory system. Furthermore, different cache levels can be of different types. This chapter only describes the first (or only) level of cache, as does the Cache Type register. If a memory system implementation provides facilities to control second or higher level caches, details of those facilities are IMPLEMENTATION DEFINED. Accordingly, all references to main memory in the rest of this chapter refer to all of the memory system beyond the first level cache, including any further levels of cache. 5.3.1 Unified or separate caches A memory system can use the same cache when processing instruction fetches as it does when processing data loads and stores. Such a cache is known as a unified cache. Alternatively, a memory system can use a different cache to process instruction fetches to the cache it uses to process data loads and stores. In this case, the two caches are known collectively as separate caches and individually as the instruction cache and data cache respectively. The use of separate caches has the advantage that the memory system can often process both an instruction fetch and a data load/store in the same clock cycle, without a need for the cache memory to be multi-ported. The main disadvantage is that care must be taken to avoid problems caused by the instruction cache becoming out-of-date with respect to the data cache and/or main memory (see Memory coherency on page B5-10). It is also possible for a memory system to have an instruction cache but no data cache, or vice versa. For the purpose of the memory system architectures, such a system is treated as having separate caches, where one cache is not present or has zero size. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-5
- 548. Caches and Write Buffers 5.3.2 Write-through or write-back caches When a cache hit occurs for a data store access, the cache line containing the data is updated to contain its new value. As this cache line will eventually be re-allocated to another address, the main memory location for the data also needs to have the new value written to it. There are two common techniques for handling this: • In a write-through cache, the new data is also immediately written to the main memory location. (This is usually done though a write buffer, to avoid slowing down the processor.) • In a write-back cache, the cache line is marked as dirty, which means that it contains data values which are more up-to-date than those in main memory. Whenever a dirty cache line is selected to be re-allocated to another address, the data currently in the cache line is written back to main memory. Writing back the contents of the cache line in this manner is known as cleaning the cache line. Another common term for a write-back cache is a copy-back cache. The main disadvantage of write-through caches is that if the processor speed becomes high enough relative to that of main memory, it generates data stores faster than they can be processed by the write buffer. The result is that the processor is slowed down by having to wait for the write buffer to be able to accept more data. Because a write-back cache only stores to main memory once when a cache line is re-allocated, even if many stores have occurred to the cache line, write-back caches normally generate fewer stores to main memory than write-through caches. This helps to alleviate the problem described above for write-through caches. However, write-back caches have a number of drawbacks, including: • longer-lasting discrepancies between cache and main memory contents (see Memory coherency on page B5-10) • a longer worst-case sequence of main memory operations before a data load can be completed, which can increase the system's worst-case interrupt latency • increased complexity of implementation. Some write-back caches allow a choice to be made between write-back and write-through behavior (see Cachability and bufferability on page B5-8). 5.3.3 Read-allocate or write-allocate caches There are two common techniques to deal with a cache miss on a data store access: • In a read-allocate cache, the data is simply stored to main memory. Cache lines are only allocated to memory locations when data is read/loaded, not when it is written/stored. • In a write-allocate cache, a cache line is allocated to the data and the current contents of main memory are read into it, then the data is written to the cache line. (It can also be written to main memory, depending on whether the cache is write-through or write-back.) B5-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 549. Caches and Write Buffers The main advantages and disadvantages of these techniques are performance-related. Compared with a read-allocate cache, a write-allocate cache can generate extra main memory read accesses that would not have otherwise occurred and/or save main memory accesses on subsequent stores because the data is now in the cache. The balance between these depends mainly on the number and type of the load/store accesses to the data concerned, and on whether the cache is write-through or write-back. Whether write-allocate or read-allocate caches are used in an ARM memory system is IMPLEMENTATION DEFINED. 5.3.4 Replacement strategies If a cache is not direct-mapped, a cache miss for a memory address requires one of the cache lines in the cache set associated with the address to be re-allocated. The way in which this cache line is chosen is known as the replacement strategy of the cache. Two typical replacement strategies are: Random replacement The cache control logic contains a pseudo-random number generator, the output of which is used to select the cache line to be re-allocated. Round-robin replacement The cache control logic contains a counter which is used to select the cache line to be re-allocated. Each time this is done, the counter is incremented, so that a different choice is made next time. Some caches allow a choice of the replacement strategy in use. Typically, one choice is a simple, easily predictable strategy like round-robin replacement, which allows the worst-case cache performance for a code sequence to be determined reasonably easily. The main drawback of such strategies is that their average performance can change abruptly when comparatively minor details of the program change. For example, suppose a program is accessing data items D1, D2, ..., Dn cyclically and that all of these data items happen to use the same cache set. With round-robin replacement in an m-way set-associative cache, the program is liable to get: • nearly 100% cache hits on these data items when n £ m • 0% cache hits as soon as n becomes m+1 or greater. In other words, a minor increase in the amount of data being processed can lead to a major change in how effective the cache is. When a cache allows a choice of replacement strategies, the second choice is normally a strategy like random replacement which has less easily predictable behavior. This makes the worst-case behavior harder to determine, but also makes the average performance of the cache vary more smoothly with parameters like working set size. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-7
- 550. Caches and Write Buffers 5.4 Cachability and bufferability Because caches and write buffers change the number, type and timing of accesses to main memory, they are not suitable for some types of memory location. In particular, caches rely on normal memory characteristics such as: • A load from a memory location returns the last value stored to the location, with no side-effects. • A store to a memory location has no side-effects other than to change the memory location value. • Two consecutive loads from a memory location both get the same value. • Two consecutive stores to a memory location result in its value becoming the second value stored, and the first value stored is discarded. Memory-mapped I/O locations usually lack one or more of these characteristics, and so are unsuitable for caching. Also, write buffers and write-back caches rely on it being possible to delay a store to main memory so that it actually occurs at a later time than the store instruction was executed by the ARM processor. Again, this might not be valid for memory-mapped I/O locations. A typical example is an ARM interrupt handler which stores to an I/O device to acknowledge an interrupt it is generating, and then re-enables interrupts (either explicitly or as a result of the SPSR ® CPSR transfer performed on return from the interrupt handler). If the actual store to the I/O device occurs when the ARM store instruction is executed, the I/O device is no longer requesting an interrupt by the time that interrupts are re-enabled. But if a write buffer or write-back cache delays the store, the I/O device might still be requesting the interrupt. If so, this results in a spurious extra call to the interrupt handler. Because of problems like these, both the Memory Management Unit and the Protection Unit architectures allow a memory area to be designated as uncachable, unbufferable or both. This is done by using the memory address to generate two bits (C and B) for each memory access. Details of how the C and B bits are produced for each architecture can be found in Chapter B3 Memory Management Unit and Chapter B4 Protection Unit. Table 5-1 shows how the C and B bits are interpreted for write-through caches, write-back caches without selectable write-through behavior, and write-back caches with selectable write-through behavior. Table 5-1 Interpretation of Cachable and Bufferable bits C B Write-through cache Write-back only cache Write-back/write-through cache 0 0 Uncached/unbuffered Uncached/unbuffered Uncached/unbuffered 0 1 Uncached/buffered Uncached/buffered Uncached/buffered 1 0 Cached/unbuffered UNPREDICTABLE Write-through cached/buffered 1 1 Cached/buffered Cached/buffered Write-back cached/buffered B5-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 551. Caches and Write Buffers The purpose of making a memory area unbufferable is to prevent stores to it being delayed. However, if the area is cachable and a write-back cache is in use, stores can be delayed anyway. This means that the obvious interpretation of C == 1, B == 0 as cached/unbuffered is not useful for write-back caches. It therefore only has this interpretation in write-through caches. In write-back caches, it instead results in UNPREDICTABLE behavior or selects write-through caching, as shown in Table 5-1 on page B5-8. Note • The reason that a memory-mapped I/O location generally needs to be marked as uncachable is effectively to prevent the memory system hardware from incorrectly optimizing away loads and stores to the location. If the I/O system is being programmed in a high-level language, this is not enough. The compiler also needs to be told not to optimize away these loads and stores. In C and related languages, the way to do this is to use the volatile qualifier in the declaration of the memory-mapped I/O location. • It can also be desirable to mark a memory area as uncachable for performance reasons. This typically occurs for large arrays which are used frequently, but whose access pattern contains little temporal or spatial locality. Making such arrays uncachable avoids the cost of loading a whole cache line when only a single access is typically going to occur. It also means that other data items are evicted from the cache less frequently, which increases the effectiveness of the cache on the rest of the data. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-9
- 552. Caches and Write Buffers 5.5 Memory coherency When a cache and/or a write buffer is used, the system can hold multiple versions of the value of a memory location. Possible physical locations for these values are main memory, the write buffer and the cache. If separate caches are used, either or both of the instruction cache and the data cache can contain a value for the memory location. Not all of these physical locations necessarily contain the value most recently written to the memory location. The memory coherency problem is to ensure that when a memory location is read (either by a data read or an instruction fetch), the value actually obtained is always the value that was most recently written to the location. In the ARM memory system architectures, some aspects of memory system coherency are required to be provided automatically by the system. Other aspects are dealt with by memory coherency rules, which are limitations on how programs must behave if memory coherency is to be maintained. The behavior of a program that breaks a memory coherency rule is UNPREDICTABLE. The following subsections discuss particular aspects of memory coherency in more detail: • Address mapping changes • Instruction cache coherency on page B5-11 • Direct Memory Access (DMA) operations on page B5-12 • Other memory coherency issues on page B5-13. 5.5.1 Address mapping changes In an ARM memory system that implements virtual-to-physical address mapping (such as the MMU-based memory system described in Chapter B3 Memory Management Unit), there are two implementation choices for the address associated with a cache line: • It can be the virtual address of the data in the cache line. This is the more usual choice, because it allows cache line look-up to proceed in parallel with address translation. • It can be the physical address of the data in the cache line. If an implementation is designed to use the virtual address, a change to the virtual-to-physical address mapping can cause major memory coherency problems, as any data in the remapped address range which is in the cache ceases to be associated with the correct physical memory location. Similarly, the data in a write buffer can have virtual or physical addresses associated with it, depending on whether the address mapping is done when data is placed in the write buffer or when it is stored from the write buffer to main memory. If a write buffer is designed to use the virtual address, a change to the virtual-to-physical address mapping can again cause memory coherency problems. These problems can be avoided by performing an IMPLEMENTATION DEFINED sequence of cache and/or write buffer operations before a change of virtual-to-physical address mapping. Typically, this sequence contains one or more of the following: B5-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 553. Caches and Write Buffers • cleaning the data cache if it is a write-back cache • invalidating the data cache • invalidating the instruction cache • draining the write buffer. There might also be requirements for the code that performs the change of address mapping and any data it accesses to be uncachable, unbufferable or both. 5.5.2 Instruction cache coherency A memory system is permitted to satisfy an instruction fetch request from a separate instruction cache. An instruction cache line fetch can be satisfied from main memory, and there is no requirement for data stores to update a separate instruction cache. This means that the following sequence of events causes a potential memory coherency problem: 1. An instruction is fetched from an address A1, causing the cache line containing that address to be loaded into the instruction cache. 2. A data store occurs to an address A2 in the same cache line as A1, causing an update to one or more of the data cache, the write buffer and main memory, but not to the instruction cache. (A2 might be the same address as A1, or a different address in the same cache line. The same considerations apply in both cases.) 3. An instruction is executed from the address A2. This could result in either the old contents or the new contents of the memory location being executed, depending on whether the cache line is still present in the instruction cache or needs to be reloaded. This problem can be avoided by performing an IMPLEMENTATION DEFINED sequence of cache control operations between steps 2 and 3. Typically, this sequence consists of: • nothing at all for an implementation with a unified cache • invalidating the instruction cache for an implementation with separate caches and a write-through data cache • cleaning the data cache followed by invalidating the instruction cache for an implementation with separate caches and a write-back data cache. Therefore, the memory coherency rule to maintain instruction cache coherency is that: if a data store writes an instruction to memory, this IMPLEMENTATION DEFINED sequence must be executed before the instruction is executed. A typical case where this needs to be done is when an executable file is loaded into memory. After loading the file and before branching to the entry point of the newly loaded code, the IMPLEMENTATION DEFINED sequence must be executed to ensure that the newly loaded program executes correctly. The performance cost of the cache cleaning and invalidating required when this happens can be large, both as a direct result of executing the cache control operations and indirectly because the instruction cache needs to be reloaded. This means that programming techniques that involve frequent real-time generation and execution of small code fragments must be treated with care, as porting them to a new memory system can require careful redesign and re-optimization to avoid excessive performance loss. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-11
- 554. Caches and Write Buffers Note • The sequence required to maintain instruction cache coherency is part of the sequence executed by an Instruction Memory Barrier, but not necessarily all of it. See Instruction Memory Barriers (IMBs) on page A2-28 for more details. • On some implementations, it is possible to exploit knowledge of the range of addresses occupied by newly stored instructions to reduce the cost of the required cache operations. For example, it might be possible to restrict the cache cleaning and invalidating to that address range. Whether this is possible is IMPLEMENTATION DEFINED. • If it is known that none of the range of addresses containing newly stored instructions is in the instruction cache, the memory coherency problem described above cannot occur. However, it is difficult to be certain of this across all ARM implementations because: — A fetch of any instruction in a cache line causes all of the instructions in that cache line to be loaded into the instruction cache. — Typically, some instructions are fetched but never executed, so it is possible for an instruction cache line to have been loaded but not to contain any executed instructions. Also, although instructions that are fetched but not executed are typically close to instructions that have been executed, this need not be the case in implementations that use branch prediction or similar techniques. As a result, code that uses this technique to avoid the instruction cache coherency problem is not fully implementation-independent. 5.5.3 Direct Memory Access (DMA) operations I/O devices can perform Direct Memory Access (DMA) operations, in which they access main memory directly, without the processor performing any accesses to the data concerned. If a DMA operation stores to main memory without updating the cache and/or write buffer, some rules normally relied upon to simplify memory coherency issues might be violated. For example, it is normally the case that if a data item is in the cache, the copy of it in main memory is not newer than the copy in the cache. This allows the value in the cache to be returned for a data load without explicitly checking whether there is a more recently written version in main memory. However, a DMA store to main memory can cause the main memory value to be more recently written than the cache value. Similarly, if a DMA operation loads data from main memory without also checking the cache and/or write buffer to see whether they contain more recent versions, it might get an out-of-date version of the data. In both cases, a possible solution would be for DMA to also access the cache and write buffer. However, this would significantly complicate the memory system. So, a memory system implementation can have IMPLEMENTATION DEFINED memory coherency rules for handling DMA operations. Typically, these involve one or more of the following: • marking the memory areas involved in the DMA operation as uncachable and/or unbufferable B5-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 555. Caches and Write Buffers • cleaning and/or invalidating the data cache, at least with respect to the address range involved in the DMA operation • draining the write buffer • restrictions on processor accesses to the address range involved in the DMA operation until it is known that the DMA operation is complete. 5.5.4 Other memory coherency issues Memory coherency issues not covered above are those involving the data cache, main memory and/or the write buffer, and which do not involve a change of virtual-to-physical address mapping or a DMA operation. All such issues must be dealt with automatically by the memory system, so that the value returned to the ARM processor is the most up-to-date of the values in the possible physical locations. Note This requirement applies to a single processor only. If a system contains multiple ARM processors, all issues relating to memory coherency between the separate processors are system-dependent. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-13
- 556. Caches and Write Buffers 5.6 CP15 registers Caches and write buffers are controlled with System Control coprocessor registers 7 and 9, and some bits of register 1. 5.6.1 Cache and write buffer control bits in register 1 The following bits in register 1 of the System Control Coprocessor control caches and write buffers: C (bit[2]) If a unified cache is used, this is the enable/disable bit for the unified cache. If separate caches are used, this is the enable/disable bit for the data cache. In either case: 0 = Cache disabled 1 = Cache enabled. If the cache is not implemented, this bit reads as 0 and ignores writes. If the cache cannot be disabled, this bit reads as 1 and ignores writes. W (bit[3]) This is the enable/disable bit for the write buffer: 0 = Write buffer disabled 1 = Write buffer enabled. If the write buffer is not implemented, this bit reads as 0 and ignore writes. If the write buffer cannot be disabled, this bit reads as 1 and ignores writes. I (bit[12]) If separate caches are used, this is the enable/disable bit for the instruction cache: 0 = Cache disabled 1 = Cache enabled. If a unified cache is used or the instruction cache is not implemented, this bit reads as 0 and ignores writes. If the instruction cache cannot be disabled, this bit reads as 1 and ignores writes. RR (bit[14]) If the cache allows an alternative replacement strategy to be used which has a more easily predictable worst-case performance, this bit selects it: 0 = Normal replacement strategy (for example, random replacement) 1 = Predictable strategy (for example, round-robin replacement). B5-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 557. Caches and Write Buffers 5.6.2 Register 7: Cache functions The System Control coprocessor register 7 is a write-only register which is used to control caches and write buffers. It is also used to implement some similar functions on prefetch buffers and branch target caches, if they exist, and to implement the wait for interrupt clock control function. Table 5-2 lists the functions available by using an MCR p15,0,<Rd>,c7,<CRm>,<opcode2> instruction to write to register 7. Writing to register 7 with a combination of <CRm> and <opcode2> which is not listed in Table 5-2 has UNPREDICTABLE results. Not all functions listed in Table 5-2 are necessarily relevant to or supported by an implementation. Attempting to perform a function which is not supported by the implementation has UNPREDICTABLE results. Reading register 7 with an MRC instruction has UNPREDICTABLE results. In Table 5-2, the following terms apply: Clean Applies to write-back data caches, and means that if the cache line contains stored data that has not yet been written out to main memory, it is written to main memory now. Invalidate Means that the cache line (or all the lines in the cache) is marked as invalid, so that no cache hits occur for that line until it is re-allocated to an address. For write-back data caches, this does not include cleaning the cache line unless that is also stated. Prefetch Means the memory cache line at the specified virtual address (which must be cache line aligned) is loaded into the cache. Drain write buffer Stops the ARM from executing further until all data in the write buffer has been stored to main memory. It can be used instead of unbufferable memory when the timing of specific main memory stores needs to be controlled (for example, when a store to an interrupt acknowledge location needs to complete before interrupts are enabled). Wait for interrupt Puts the ARM into a low power state and stops it executing further until an interrupt occurs. When an interrupt does occur, the MCR instruction completes and the IRQ or FIQ handler is entered as normal. The return link in R14_irq or R14_fiq contains the address of the MCR instruction plus 8, so that the normal instruction used for interrupt return (SUBS PC,R14,#4) returns to the instruction following the MCR. Prefetch buffer Operations are IMPLEMENTATION DEFINED. Branch target cache Operations are IMPLEMENTATION DEFINED. Data Is the value that is written to register 7, which is the value in the register <Rd> specified in the MCR instruction. If the data is stated to be a virtual address, it must be cache line aligned. This means that if the cache line length is 2L bytes, bits[L-1:0] of the address must be zero. This address is looked up in the cache. If a cache hit occurs, the specified operation ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-15
- 558. Caches and Write Buffers occurs on the cache line it identifies. If a cache miss occurs, nothing more is done. A loop of operations of this type can be used to clean and/or invalidate all cache lines relating to a specified range of addresses. Note If the Fast Context Switch Extension (FCSE) described in Chapter B6 is being used, the virtual addresses passed to CP15 by some of the functions in Table 5-2 on page B5-16 are passed as data. This means that no address modification is performed for them by the FCSE. If the data is stated to be set/index, it identifies the cache line that the operation is to be applied to by specifying which cache set it belongs to and what its index is within the set. A loop of operations of this type can be used to clean and/or invalidate an entire cache, or all of the non-locked down portion of the cache (see Register 9: Cache lockdown on page B5-18). If L, A, and S are the logarithms base 2 of the cache size parameters LINELEN, ASSOCIATIVITY and NSETS, rounded up to an integer in the case of A, the format of set/index data is: 31 32-A 31-A L+S L+S-1 L L-1 0 index SBZ set SBZ For the definitions of the cache size parameters, see Cache size on page B5-4. If the implementation includes a Cache Type register, these parameters can be determined from it (see Cache Type register on page B2-9). Table 5-2 Cache and similar functions <CRm> <opcode2> Function Data c0 4 Wait for interrupt SBZ c5 0 Invalidate entire instruction cache SBZ c5 1 Invalidate instruction cache line Virtual address c5 2 Invalidate instruction cache line Set/index c5 4 Flush prefetch buffer SBZ c5 6 Flush entire branch target cache SBZ c5 7 Flush branch target cache entry IMP c6 0 Invalidate entire data cache SBZ c6 1 Invalidate data cache line Virtual address B5-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 559. Caches and Write Buffers Table 5-2 Cache and similar functions (Continued) <CRm> <opcode2> Function Data c6 2 Invalidate data cache line Set/index c7 0 Invalidate entire unified cache or both instruction and data caches SBZ c7 1 Invalidate unified cache line Virtual address c7 2 Invalidate unified cache line Set/index c8 2 Wait for interrupt (alternative, deprecated encoding) SBZ c10 1 Clean data cache line Virtual address c10 2 Clean data cache line Set/index c10 4 Drain write buffer SBZ c11 1 Clean unified cache line Virtual address c11 2 Clean unified cache line Set/index c13 1 Prefetch instruction cache line Virtual address c14 1 Clean and invalidate data cache line Virtual address c14 2 Clean and invalidate data cache line Set/index c15 1 Clean and invalidate unified cache line Virtual address c15 2 Clean and invalidate unified cache line Set/index ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-17
- 560. Caches and Write Buffers 5.6.3 Register 9: Cache lockdown One problem with caches is that while they normally improve average access time to data and instructions, they usually make the worst-case access time worse. This occurs for a number of reasons, including: • There is a delay before the system determines that a cache miss has occurred and starts the main memory access. • If a write-back cache is being used, there might be a further delay due to the need to store the contents of the cache line that is being re-allocated. • A whole cache line is loaded from main memory, not just the data requested by the ARM processor. In real-time applications, this increase in the worst-case access time can be very significant. Cache lockdown is a feature of some ARM memory systems designed to alleviate this. It allows critical code and data (such as high-priority interrupt routines and the data they access) to be loaded into the cache in such a way that the cache lines containing them are not subsequently re-allocated. This ensures that all subsequent accesses to the code and data concerned are cache hits and therefore complete quickly. In the description that follows, LINELEN, ASSOCIATIVITY and NSETS are the cache size parameters described in Cache size on page B5-4. For lockdown purposes, the cache is divided into lockdown blocks, each of which consists of one line from each cache set. The lockdown blocks are indexed from 0 to ASSOCIATIVITY-1. The cache lines in a lockdown block are chosen to have the same index as the lockdown block (see Set-associativity on page B5-4). So lockdown block 0 consists of the cache line with index 0 from each cache set, etc. Cache lockdown can be performed with a granularity of a lockdown block. Any number of lockdown blocks from 1 to ASSOCIATIVITY-1 can be chosen. If N lockdown blocks are locked down, they have indices 0 to N-1, and lockdown blocks N to ASSOCIATIVITY-1 are available for normal cache operation. Note You cannot lock down the entire cache. At least one lockdown block must be left for normal cache operation. Each lockdown block can hold NSETS memory cache lines, provided each of the memory cache lines is associated with a different cache set. It is recommended that systems are designed so that each lockdown block contains a set of NSETS consecutive memory cache lines. This is NSETS ´ LINELEN consecutive memory locations, starting at a cache line boundary. (Such sets are easily identified and are guaranteed to consist of one cache line associated with each cache set.) The System Control coprocessor primary register 9 is used to control cache lockdown. If separate caches are used, there are two variants of this register, selected by the <opcode2> field of the MCR or MRC instruction used to access register 9: <opcode2> == 0 Selects the data cache lockdown register. <opcode2> == 1 Selects the instruction cache lockdown register. If a unified cache is used, there is only one variant of this register and <opcode2> should be 0. <CRm> must always be c0 for MCR and MRC instructions that access register 9. B5-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 561. Caches and Write Buffers Formats for the cache lockdown register There are two possible formats for the cache lockdown register. These are shown in the following text as Format A and Format B, and it is IMPLEMENTATION DEFINED which one an implementation uses. Both formats include an index field, which is chosen to be wide enough to hold a lockdown block index. So, its width W is the logarithm base 2 of ASSOCIATIVITY, rounded up to an integer if necessary. If the implementation includes a Cache Type register, it can be used to determine the register 9 format (see Cache Type register on page B2-9). Format A: 31 32-W 31-W 0 index UNP/SBZ Reading a Format A register returns the value last written to it. Writing a Format A register has the following effects: • The next cache miss in each cache set replaces the cache line with the specified index in that cache set. • The replacement strategy for the cache is constrained so that it can only select cache lines with the specified index and higher, until the register is written again. Format B: 31 30 W W-1 0 L UNP/SBZ index Reading a Format B register returns the value last written to it. Writing a Format B register has the following effects: • If L == 1, all cache misses replace the cache line with the specified index in the relevant cache set until the register is written again. • If L == 0, then: — If the previous value of L was 0 and the previous value of index is smaller than the new value, the behavior is UNPREDICTABLE. — Otherwise, the replacement strategy for the cache is constrained so that it can only select cache lines with the specified index and higher, until the register is written again. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-19
- 562. Caches and Write Buffers Cache lockdown procedure The procedure to lock down N lockdown blocks is as follows: 1. Ensure that no processor exceptions can occur during the execution of this procedure, by disabling interrupts, for example. If for some reason this is not possible, all code and data used by any exception handlers that can get called must be treated as code and data used by this procedure for the purpose of steps 2 and 3. 2. If an instruction cache or a unified cache is being locked down, ensure that all the code executed by this procedure is in an uncachable area of memory. 3. If a data cache or a unified cache is being locked down, ensure that all data used by the following code is in an uncachable area of memory, apart from the data which is to be locked down. 4. Ensure that the data/instructions that are to be locked down are in a cachable area of memory. 5. Ensure that the data/instructions that are to be locked down are not already in the cache, using cache clean and/or invalidate instructions as appropriate. 6. For each of i = 0 to N-1: a. Write to register 9 with index == i (for Formats A and B), and L == 1 (for Format B). b. For each of the cache lines to be locked down in lockdown block i: If a data cache or a unified cache is being locked down, use an LDR instruction to load a word from the memory cache line, which ensures that the memory cache line is loaded into the cache. If an instruction cache is being locked down, use the register 7 prefetch instruction cache line operation (<CRm> == c13, <opcode2> == 1) to fetch the memory cache line into the cache. 7. Write to register 9 with index == N (for Formats A and B), and L == 0 (for Format B). Note If the Fast Context Switch Extension (FCSE) described in Chapter B6 is being used, care needs to be taken in step 6b. This is because: • If a data cache or a unified cache is being locked down, the address used for the LDR instruction is subject to modification by the FCSE. • If an instruction cache is being locked down, the address used for the register 7 operation is being treated as data and so is not subject to modification by the FCSE. To minimise the possible confusion caused by this, it is recommended that the lockdown procedure should: • start by disabling the FCSE (by setting the PID to zero) • where appropriate, generate modified virtual addresses itself by ORing the appropriate PID value into the top 7 bits of the virtual addresses it uses. Cache unlock procedure To unlock the locked-down portion of the cache, write to register 9 with index == 0 (for Formats A and B), and L == 0 (for Format B). B5-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 563. Caches and Write Buffers Interactions with register 7 operations Cache lockdown only prevents the normal replacement strategy used on cache misses from choosing to re-allocate cache lines in the locked-down region. Register 7 operations that invalidate cache contents affect locked-down cache lines as normal. If such operations are used, ensure that they do not use virtual addresses or cache set/index combinations that affect the locked-down cache lines. (Or if it is difficult to avoid affecting the locked-down cache lines, repeat the cache lockdown procedure afterwards.) Also, on some implementations and under some circumstances, the clean single Dcache line and clean and invalidate single Dcache line register 7 operations (see Table 5-2 on page B5-16) which identify a cache line by set/index can disrupt the constraints on the normal replacement strategy. To avoid this affecting the locked-down area of the cache, read register 9 immediately before code that uses these operations, and write the value read back to register 9 immediately afterwards. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B5-21
- 564. Caches and Write Buffers B5-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 565. Chapter B6 Fast Context Switch Extension This chapter describes the Fast Context Switch Extension (FCSE). It contains the following sections: • About the FCSE on page B6-2 • Modified virtual addresses on page B6-3 • Enabling the FCSE on page B6-5 • CP15 registers on page B6-6. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B6-1
- 566. Fast Context Switch Extension 6.1 About the FCSE The Fast Context Switch Extension (FCSE) modifies the behavior of an ARM memory system. This modification allows multiple programs running on the ARM processor to use identical address ranges, while ensuring that the addresses they present to the rest of the memory system differ. Normally, a swap between two software processes whose address ranges overlap requires changes to be made to the virtual-to-physical address mapping defined by the MMU’s page tables (see Chapter B3 Memory Management Unit). It also typically causes cache and TLB contents to become invalid (because they relate to the old virtual-to-physical address mapping), and so requires caches and TLBs to be flushed. As a result, each process swap has a considerable overhead, both directly because of the cost of changing the page tables and indirectly because of the cost of subsequently reloading caches and TLBs. By presenting different addresses to the rest of the memory system for different software processes even when they are using identical addresses, the FCSE avoids this overhead. It also allows software processes to use identical address ranges even when the rest of the memory system does not support virtual-to-physical address mapping. B6-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 567. Fast Context Switch Extension 6.2 Modified virtual addresses The 4GB virtual address space is divided into 128 process blocks, each of size 32MB. Each process block can contain a program which has been compiled to use the address range 0x00000000 to 0x01FFFFFF. For each of i=0 to 127, process block i runs from address (i ´ 0x02000000) to address (i ´ 0x02000000 + 0x01FFFFFF). The FCSE processes each virtual address for a memory access generated by the ARM processor to produce a modified virtual address, which is sent to the rest of the memory system to be used in place of the normal virtual address. For an MMU-based memory system, the process is illustrated in Figure 6-1: Virtual address (VA) Modified virtual address (MVA) Physical address (PA) ARM FCSE MMU Main memory Cache Figure 6-1 Address flow in MMU memory system with FCSE When the ARM processor generates a memory access, the relationship between the Virtual Address (VA) and Modified Virtual Address (MVA) is: if (VA[31:25] == 0b0000000) then MVA = VA | (PID << 25) else MVA = VA where PID is a 7-bit number that identifies which process block the current process is loaded into. This is also known as the process ID of the current process. Note Virtual addresses are sometimes passed to the memory system as data, as for example in some of the cache control operations described in Register 7: Cache functions on page B5-15. For these operations, no address modification occurs, and MVA = VA. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B6-3
- 568. Fast Context Switch Extension Each process is compiled to use the address range 0x00000000 to 0x01FFFFFF. When referring to its own instructions and data, therefore, the program generates VAs whose top seven bits are all zero. The resulting MVAs have their top seven bits replaced by PID, and so lie in the process block of the current process. The program is also allowed to generate VAs whose top 7 bits are not all zero. When this happens, the MVA is equal to the VA. This allows the program to address the process block of another process, provided the other process does not have process ID 0. Provided access permissions are set correctly, this can be used for inter-process communication. Note It is recommended that only process IDs 1 and above are used for general-purpose processes, because the process with process ID 0 cannot be communicated with in this fashion. Use of the FCSE therefore allows the cost of a process swap to be reduced to: • The cost of a write of the PID. • The cost of changing access permissions if they need changing for the new process. In an MMU-based system, this might require a TLB flush. However, this is usually significantly cheaper than the cache flush that would have been required without the FCSE. Furthermore, even the TLB flush can often be avoided by the use of domains. This reduces the cost to that of a write to the Domain Access Control Register (see Domains on page B3-17). B6-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 569. Fast Context Switch Extension 6.3 Enabling the FCSE When PID == 0b0000000, the rules for processing a VA always result in MVA == VA, as if the FCSE were not present. There is therefore no specific FCSE enable bit. Instead, the PID is initialized to 0b0000000 on reset, resulting in the FCSE being effectively disabled. The FCSE can then be enabled by writing a non-zero value to the PID, and disabled by writing 0b0000000 to the PID. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. B6-5
- 570. Fast Context Switch Extension 6.4 CP15 registers The FCSE only uses one coprocessor 15 register, namely register 13. 6.4.1 Register 13: Process ID 31 25 24 0 PID UNP/SBZP Reading register 13 returns the PID in bits[31:25]. Bits[24:0] of the value read are UNPREDICTABLE. Writing register 13 sets the PID to bits[31:25] of the value written. Bits[24:0] of the value written should be zero or bits[24:0] of a value previously read from register 13. The results of writing any other value to bits[24:0] are UNPREDICTABLE. In MCR and MRC instructions used to write and read register 13, <CRm> should be c0 and <opcode2> should be 0 (or omitted). If they have other values, the instruction is UNPREDICTABLE. Note When the PID is written, the overall virtual-to-physical address mapping changes. Because of this, care must be taken to ensure that instructions which might have already been prefetched are not affected by the address mapping change. Formally, this means that an IMB sequence is required, as described in Instruction Memory Barriers (IMBs) on page A2-28. However, the mapping from the modified virtual address to the physical address is unchanged, so typically there is no need to flush caches, etc. This means that an implementation can usually specify a restricted form of the IMB sequence to be used after using an MCR to write to register 13. Typically, this means that all instructions reached by predictable subsequent execution from the MCR instruction must be fetched with addresses whose top seven bits are not 0b0000000. For the definition of predictable subsequent execution and further details, see Other uses for IMBs on page A2-30. B6-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 571. Part C Vector Floating-point Architecture
- 573. Chapter C1 Introduction to the Vector Floating-point Architecture This chapter gives an introduction to the Vector Floating-Point (VFP) architecture, and its compliance with the IEEE 754 standard. It contains the following sections: • About the Vector Floating-point architecture on page C1-2 • Overview of the VFP architecture on page C1-3 • Compliance with the IEEE 754 standard on page C1-7 • IEEE 754 implementation choices on page C1-8. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C1-1
- 574. Introduction to the Vector Floating-point Architecture 1.1 About the Vector Floating-point architecture The Vector Floating-Point (VFP) architecture is a coprocessor extension to the ARM architecture. It provides single-precision and double-precision floating-point arithmetic, as defined by ANSI/IEEE Std. 754-1985 IEEE Standard for Binary Floating-Point Arithmetic. This document is referred to as the IEEE 754 standard in the following text. Short vectors of up to 8 single-precision or 4 double-precision numbers are handled particularly efficiently by the VFP architecture. Most arithmetic instructions can be used on these vectors, allowing single-instruction, multiple-data (SIMD) parallelism. Furthermore, the floating-point load and store instructions have multiple register forms, allowing vectors to be transferred to and from memory efficiently. To date, there has only been one major version of the VFP architecture (Version 1, or VFPv1). Double-precision support is optional, with its presence being indicated by the variant letter D. So the VFPv1D variant has both single precision and double precision, while VFPv1xD supports single precision only. By default, double-precision support is present. C1-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 575. Introduction to the Vector Floating-point Architecture 1.2 Overview of the VFP architecture This section provides a brief overview of the VFP architecture. More extensive and detailed information on the architecture is given in Chapter C2 VFP Programmer’s Model. 1.2.1 Registers VFP has 32 general-purpose registers, each capable of holding a single-precision floating-point number or a 32-bit integer. In D variants of the architecture, these registers can also be used in pairs to hold up to 16 double-precision floating-point numbers. There are also three or more system registers: FPSID Is read-only. It can be read to determine which implementation of the VFP architecture is being used. FPSCR Supplies all user-level status and control. Status bits hold comparison results and cumulative flags for floating-point exceptions. Control bits are provided to select rounding options and vector length/stride, and to enable floating-point exception traps. FPEXC Contains a few bits for system-level status and control. The remaining bits of the FPEXC register and any further system registers are IMPLEMENTATION DEFINED, and are typically used for internal communication between the hardware and software components of a VFP implementation (see Hardware and software implementations on page C1-4). 1.2.2 Instructions Instructions are provided to: • Load floating-point values into registers from memory, and store floating-point values in registers to memory. Some of these instructions allow multiple register values to be transferred, providing floating-point equivalents to ARM LDM and STM instructions. Among other purposes, such instructions can be used to load and store short vectors of floating-point values. • Transfer 32-bit values directly between VFP and ARM general-purpose registers. • Transfer 32-bit values directly between VFP system registers and ARM general-purpose registers. • Add, subtract, multiply, divide, and take the square root of floating-point register values. These instructions can be used on short vectors as well as on individual floating-point values. • Copy floating-point values between registers. In the process, the sign bit can be inverted or cleared (or left unchanged), providing negation and absolute value instructions as well as straightforward copies. All of these instructions can also be used on short vectors. • Perform combined multiply-accumulate operations on floating-point values and short vectors, providing space-efficient equivalents for common sequences of multiply, negate, add, and subtract. • Perform conversions between single-precision values, double-precision values, unsigned 32-bit integers and two's complement signed 32-bit integers. • Compare floating-point values in registers with each other or with zero. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C1-3
- 576. Introduction to the Vector Floating-point Architecture 1.2.3 Floating-point exceptions The VFP architecture supports all five of the floating-point exceptions defined in the IEEE 754 standard: • Invalid Operation • Division by Zero • Overflow • Underflow • Inexact. These are supported in both untrapped and trapped forms: Untrapped handling of an exception This causes the appropriate cumulative flag in the FPSCR to be set to 1, and any result registers of the exception-generating instruction to be set to the result values specified by the standard. Execution of the program containing the exception-generating instruction then continues. Trapped handling of an exception This is selected by setting the appropriate control bit in the FPSCR. When the exception occurs, a trap handler software routine is called. Details of how trap handler routines are called and of the facilities available to them are IMPLEMENTATION DEFINED. 1.2.4 Hardware and software implementations Because of the existence of trapped floating-point exceptions, any implementation of the VFP architecture must include a software component. This is typically installed on the ARM undefined instruction vector, and has the job of catching a trapped exception and converting it into a trap handler call. The software component of a VFP implementation can perform other tasks in addition to trap handler calls. The division of labour between the hardware and software components of a VFP implementation is IMPLEMENTATION DEFINED. VFP implementations can be classified according to whether they also include a hardware component: Software implementation This implementation consists of software only, with all floating-point arithmetic being emulated by ARM routines. A software implementation is also sometimes called a VFP emulator. Hardware implementation This implementation contains both hardware and software components. Typically, the hardware is designed to handle all common cases, to optimize performance. When a case where the hardware cannot handle on its own is encountered, the software component (also known as support code for the hardware) is called to deal with it. Details of how the hardware and its support code interact are IMPLEMENTATION DEPENDENT. C1-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 577. Introduction to the Vector Floating-point Architecture 1.2.5 Interactions with the ARM architecture The VFP architecture has been designed to conform fully with the ARM coprocessor architecture. All VFP instructions are special cases of the ARM’s generic coprocessor instructions (CDP, LDC, MCR, MRC, and STC), using coprocessor numbers 10 and 11. As a general rule, coprocessor 10 is used for single-precision instructions and coprocessor 11 for double-precision instructions. All coprocessor 10 and 11 instructions that have not been allocated meanings as VFP instructions are reserved for future expansion of the VFP architecture, and must be treated as UNDEFINED. Hardware coprocessor implementations of the VFP architecture will fail to respond to these instructions, causing the ARM’s Undefined Instruction exception to occur. For more details, see Undefined Instruction exception on page A2-15. The recommended way for a VFP coprocessor to invoke its support code uses the same mechanism: 1. Before the VFP hardware is enabled, the support code is installed on the ARM’s undefined instruction vector. 2. When the hardware needs assistance from the support code, it fails to respond to a VFP instruction. 3. This results in an Undefined Instruction exception, causing the support code to be executed. In such a system, the support code is responsible for distinguishing these Undefined Instruction exceptions from those caused by the reserved instructions and taking different actions accordingly. The ARM tests whether a coprocessor instruction satisfies its condition (as described in The condition field on page A3-5), using the CPSR flags, and treats it as a NOP if the condition fails. If this happens, the ARM signals coprocessors not to execute the instruction, so they also treat the instruction as a NOP. This implies that all VFP instructions are treated as NOPs if their condition check fails. The condition code check is based on the ARM processor’s CPSR flags, not on the similarly named flags in the VFP FPSCR register. To use the FPSCR flags for conditional execution, they must first be transferred to the CPSR by an FMSTAT instruction. VFP load and store instructions are allowed to produce data aborts, and so VFP implementations are able to cope with a data abort on any memory access caused by such instructions. Interrupts As described above, hardware VFP implementations typically use the Undefined Instruction exception to communicate between their hardware and software components. Software VFP implementations also use the Undefined Instruction exception, since all coprocessor instructions that are not claimed by a hardware coprocessor are treated as undefined instructions. Entry to the Undefined Instruction exception causes IRQs to be disabled (see Undefined Instruction exception on page A2-15), and they will not normally be re-enabled until the exception handler returns. Straightforward use of VFP in a system therefore increases worst case IRQ latency considerably. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C1-5
- 578. Introduction to the Vector Floating-point Architecture It is possible to reduce this IRQ latency penalty considerably by explicitly re-enabling interrupts soon after entry to the Undefined Instruction handler. This requires careful integration of the Undefined Instruction handler into the rest of the operating system. Details of how this should be done are highly system-specific and go beyond the scope of this manual. In a hardware implementation, if the IRQ handler is going to use the VFP coprocessor itself, there is a second potential cause of increased IRQ latency. This is that a long latency VFP operation initiated by the interrupted program will deny the use of the VFP hardware to the IRQ handler for a significant number of cycles. If a system contains IRQ handlers which require both low interrupt latency and the use of VFP instructions, therefore, it is recommended that the use of the highest latency VFP instructions is avoided. In particular, the use of vector division instructions and vector square root instructions is not recommended in such systems, because these instructions typically have very long latencies. Note FIQs are not disabled by entry to the Undefined Instruction handler, and so FIQ latency is not affected by the way that a VFP implementation uses the Undefined Instruction exception. However, this also means that an FIQ can occur at any point during the execution of a VFP implementation’s software component, including during the entry and exit sequences of the Undefined Instruction handler. If a FIQ handler is going to do anything other than leave the VFP implementation’s state entirely unchanged, great care must be taken to ensure that it handles every case correctly. This is usually incompatible with the intention that FIQs should provide fast interrupt processing, and so it is recommended that FIQ handlers should not use VFP. C1-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 579. Introduction to the Vector Floating-point Architecture 1.3 Compliance with the IEEE 754 standard The VFP architecture supplies a subset of IEEE 754 functionality. The following operations are mandatory under the standard, but not supplied by the VFP architecture: • the remainder operation • the binary « decimal conversions • the Round Floating-Point Number to Integer Value operation • in D variants of the VFP architecture, comparisons directly between single-precision and double-precision values without first converting the single-precision value to double precision. To obtain a fully compliant implementation of the standard, the VFP architecture must be augmented with these operations (typically in the form of software library routines). Note In some environments, not all of these operations are required. For example, the C language specifies that if a float and a double are compared, the first argument must be converted to a double by the usual binary conversions before the comparison is performed. So, C code never specifies a direct comparison of a single-precision value and a double-precision value. Also, when the Flush to Zero (FZ) bit in the FPSCR is set to 1, the way the VFP architecture handles denormalized numbers and underflow exceptions does not comply with the standard. To obtain fully compliant behavior from the VFP architecture, the FZ bit must be set to 0 (see Flush-to-zero mode on page C2-13 for more details). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C1-7
- 580. Introduction to the Vector Floating-point Architecture 1.4 IEEE 754 implementation choices Many design choices about a compliant floating-point system are left as an implementation option by the IEEE 754 standard. The VFP architecture specifies how many of these choices are to be made. The rest of this section briefly describes these implementation choices. 1.4.1 Supported formats The VFP architecture supports the basic single floating-point format from the standard, and D variants also support the basic double floating-point format. These are known as single precision and double precision in this manual. The standard’s extended formats are not supported. Supported integer formats are unsigned 32-bit integers and two’s complement signed 32-bit integers. 1.4.2 NaNs The IEEE 754 standard only specifies that there must be at least one signaling NaN and at least one quiet NaN, and partly specifies what the representation of NaNs should be (for any NaN, the exponent field should be maximum, and the fraction field non-zero). The VFP architecture specifies its NaNs more fully: • In each format, all values with the exponent field maximum and the fraction field non-zero are valid NaNs. Two such values represent distinct NaNs if their sign bits and/or fraction fields are different. • Copying a signaling NaN with a change of format does not generate an Invalid Operation exception. • Signaling NaNs are distinguished from quiet NaNs by the most significant fraction bit. The NaN is signaling if this bit is 0, and quiet if it is 1. • There are precise rules in the VFP architecture about which NaN is produced for each operation with a NaN result. These rules are described in NaNs on page C2-5. Note The fact that NaNs whose sign or fraction bits differ are treated as distinct NaNs in the VFP architecture does not mean that the floating-point comparison instructions can be used to distinguish them from each other. The IEEE 754 standard requires all NaNs to compare as unordered with every value, including themselves. What it does mean is that the distinct NaNs can be distinguished by using ARM code that looks at their precise bit patterns, and that the NaN handling rules are designed not to change bits in NaN values except where this is required by the standard. C1-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 581. Introduction to the Vector Floating-point Architecture 1.4.3 Comparison results The results of comparison instructions are delivered as condition codes. In particular, they are flag combinations (N,Z,C,V), compatible with those used by the ARM program status registers. To assist with the alternative approach of testing predicates, each comparison instruction is supplied in two variants whose behavior differs with respect to NaNs, and the flag combinations (N,Z,C,V) for the four possible comparison results are chosen to maximize the number of predicates that can be tested with a single ARM condition check. See Testing the IEEE 754 predicates on page C3-8 for more details. 1.4.4 Underflow exception Underflow is detected using the after rounding form of tininess and the denormalization loss form of loss of accuracy, as defined in the IEEE 754 standard. 1.4.5 Exception traps The FPSCR contains bits to specify whether exception traps are enabled, and the VFP implementation determines whether a trapped exception as defined by the IEEE 754 standard does in fact occur. All further details of trapped exception handling are IMPLEMENTATION DEFINED. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C1-9
- 582. Introduction to the Vector Floating-point Architecture C1-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 583. Chapter C2 VFP Programmer’s Model This chapter gives details of the VFP programmer’s model. It contains the following sections: • Floating-point formats on page C2-2 • Rounding on page C2-9 • Floating-point exceptions on page C2-10 • Flush-to-zero mode on page C2-13 • Floating-point general-purpose registers on page C2-14 • System registers on page C2-19 • Reset behavior and initialization on page C2-26. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-1
- 584. VFP Programmer’s Model 2.1 Floating-point formats This section outlines the basic single-precision and double-precision floating-point formats, as defined by the IEEE 754 standard and used by the VFP architecture. In addition, it describes VFP-specific details of these formats that are left open by the standard. All versions and variants of the VFP architecture support the single-precision format. D variants also support the double-precision format. The VFP architecture does not support either of the extended formats described in the IEEE 754 standard. This section is only intended as an introduction to these formats and to the various types of value they can contain, not as comprehensive reference material on them. For full details, especially of the handling of infinities, NaNs and signed zeros, see the IEEE 754 standard. 2.1.1 Single-precision format A single-precision value is a 32-bit word, and must be word-aligned when held in memory. It has the following format: 31 30 23 22 0 S exponent fraction The value represented depends primarily on the exponent field: • If 0 < exponent < 0xFF, the value is a normalized number and is equal to: -1S ´ 2exponent-127 ´ (1.fraction) The mantissa of the value is the number 1.fraction, consisting of: — 1 — a binary point — the 23 fraction bits. The mantissa therefore lies in the range 1 £ mantissa < 2 and is a multiple of 2-23. The unbiased exponent of the value is the power to which 2 is raised in this formula. In this case, it is (exponent-127). The minimum positive normalized number is 2-126, or approximately 1.175 ´ 10-38. The maximum positive normalized number is (2-2-23) ´ 2127, or approximately 3.403 ´ 1038. • If exponent == 0, the value is either a zero or a denormalized number, depending on the fraction bits: — If fraction == 0, the value is a zero. There are two distinct zeros: +0 with S==0 -0 with S==1. C2-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 585. VFP Programmer’s Model These behave identically in most circumstances, including getting an equal result if +0 and -0 are compared as floating-point numbers. However, they yield different results in some exceptional circumstances (for example, they affect the sign of the infinity produced as the default result for a Division by Zero exception). They can also be distinguished from each other by performing an integer comparison of the two words. — If fraction != 0, the value is a denormalized number and is equal to: -1S ´ 2-126 ´ (0.fraction) In this case, the mantissa of the value has a zero before the binary point, rather than the one used by a normalized number. It lies in the range 0 < mantissa < 1 and is a multiple of 2-23. The value's unbiased exponent is -126. The minimum positive denormalized number is 2-149, or approximately 1.401 ´ 10-45. • If exponent == 0xFF, the value is either an infinity or a Not a Number (NaN), depending on the fraction bits. If fraction == 0, the value is an infinity. There are two infinities: +¥ Has S==0 and represents all positive numbers which are too big to be represented accurately as a normalized number. -¥ Has S==1 and represents all negative numbers which are too big to be represented accurately as a normalized number. If fraction != 0, the value is a NaN, and can be either a quiet NaN or a signaling NaN (see NaNs on page C2-5 for details of these types of NaN). In the VFP architecture, the two types of NaN are distinguished on the basis of their most significant fraction bit (bit[22]): — If bit[22] == 0, the NaN is a signaling NaN. The sign bit can take any value, and the remaining fraction bits can take any value except all zeros, so there are 2 ´ (222-1) = 8388606 possible signaling NaNs. — If bit[22] == 1, the NaN is a quiet NaN. The sign bit and remaining fraction bits can take any value, so there are 2 ´ 222 = 8388608 possible quiet NaNs. Two NaNs are treated as being different values in the VFP architecture if their sign bits and/or any of their fraction bits differ. This implies that all 232 possible word values are treated as distinct from each other by the VFP architecture. Note The fact that NaNs with different sign and/or fraction bits are distinct NaNs does not mean that floating-point comparison instructions can be used to distinguish them. This is because the IEEE 754 standard specifies that a NaN compares as unordered with everything, including itself. However, different NaNs can be distinguished by using integer comparisons. Also, the rules for handling NaNs are designed not to arbitrarily change one NaN into another (see NaNs on page C2-5). These rules about NaNs also ensure that single-precision registers can be used to hold integer values without any risk of corrupting them (see Holding integers in single-precision registers on page C2-18). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-3
- 586. VFP Programmer’s Model 2.1.2 Double-precision format A double-precision value consists of two 32-bit words, with the following formats: Most significant word: 31 30 20 19 0 S exponent fraction[51:32] Least significant word: 31 0 fraction[31:0] When held in memory, the two words must appear consecutively and must both be word-aligned. The order of the two words depends on the endianness of the memory system: • In a little-endian memory system, the least significant word appears at the lower memory address and the most significant word at the higher memory address. • In a big-endian memory system, the most significant word appears at the lower memory address and the least significant word at the lower memory address. A VFP implementation must use the same endianness as the ARM implementation it is attached to. If the ARM implementation has configurable endianness, double-precision values must not be loaded or stored before the ARM processor endianness has been set to match that of the memory system (see Endianness on page A2-23 for more details). Note The word order defined here for the VFP architecture differs from that of the earlier FPA floating-point architecture. In the FPA architecture, the most significant word always appeared at the lower memory address, with the least significant word at the higher, regardless of the memory system endianness. Double-precision values represent numbers, infinities and NaNs analogously to single-precision values: • If 0 < exponent < 0x7FF, the value is a normalized number and is equal to: -1S ´ 2exponent-1023 ´ (1.fraction) The mantissa of the value is the number 1.fraction, consisting of a one, followed by a binary point, followed by the 52 fraction bits. The mantissa therefore lies in the range 1 £ mantissa < 2 and is a multiple of 2-52. The unbiased exponent of the value is (exponent-1023). The minimum positive normalized number is 2-1022, or approximately 2.225 ´ 10-308. The maximum positive normalized number is (2-2-52) ´ 21023, or approximately 1.798 ´ 10308. C2-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 587. VFP Programmer’s Model • If exponent == 0, the value is either a zero or a denormalized number, depending on the fraction bits. If fraction == 0, the value is a zero. There are two distinct zeros which behave analogously to the two single-precision zeros: +0 with S==0 -0 with S==1. If fraction != 0, the value is a denormalized number and is equal to: -1S ´ 2-1022 ´ (0.fraction) In this case, the mantissa of the value has a zero before the binary point, rather than the one used by a normalized number. It lies in the range 0 < mantissa < 1 and is a multiple of 2-52. The unbiased exponent of the value is -1022. The minimum positive denormalized number is 2-1074, or approximately 4.941 ´ 10-324. • If exponent == 0x7FF, the value is either an infinity or a NaN, depending on the fraction bits. — If fraction == 0, the value is an infinity. As for single precision, there are two infinities: +¥ Plus infinity with S==0 +¥ Minus infinity with S==1. — If fraction != 0, the value is a NaN. The rules about interpreting NaN values are analogous to those for single precision: — The type of a NaN is determined by the most significant fraction bit (bit[19] of the most significant word). A 0 indicates a signaling NaN and a 1 indicates a quiet NaN. — Two NaNs whose sign bits and/or fractions differ are different NaNs. 2.1.3 NaNs NaNs are special floating-point values which can be used when neither a numeric value nor an infinity is appropriate. There are two types of NaN, each of which can be used for a variety of purposes: Quiet NaNs These propagate unchanged through most floating-point operations. They can be generated by floating-point arithmetic operations in some rare circumstances when there is no other sensible result. Any further calculations which depend on the result of such an operation then also produce a quiet NaN result. (Quiet NaNs can only be generated in this way if the associated Invalid Operation exception is untrapped. If it is trapped, a trap handler is called instead.) Another typical use for quiet NaNs is to represent missing or unavailable data values. The results of any calculations that depend on the missing values are then also quiet NaNs. Signaling NaNs These cause an Invalid Operation exception whenever any floating-point operation receives a signaling NaN as an operand. One possible use for signaling NaNs is in debugging, to track down some uses of uninitialized variables. To do this, pre-load memory with copies of a signaling NaN, then load and run the program with Invalid Operation traps enabled. Any floating-point operation whose operand has been loaded from uninitialized memory then calls the Invalid Operation trap handler. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-5
- 588. VFP Programmer’s Model The IEEE 754 standard does not specify how the two types of NaN are distinguished or how many different NaNs of each type can exist in a floating-point system. However, these details are specified by the VFP architecture, as described in Single-precision format on page C2-2 and Double-precision format on page C2-4. The following subsections describe the main requirements of the IEEE 754 standard about how floating-point operations involving NaNs behave, and additional requirements on such operations imposed by the VFP architecture. Instructions with non floating-point results The VFP architecture contains instructions to convert floating-point values to integers. In accordance with the IEEE 754 standard, these instructions always generate an Invalid Operation exception if their operand is a NaN, regardless of whether it is a signaling NaN or a quiet NaN. If this exception is untrapped, the VFP architecture specifies that the integer result must be 0. The VFP architecture also contains comparison instructions, which deliver condition code results. These instructions generate Invalid Operation exceptions for signaling NaN operands. For quiet NaN operands, some of them also generate Invalid Operation exceptions, while others generate an unordered condition code result. The condition code result is also unordered in all cases where an Invalid Operation exception is generated but the exception is untrapped. For more details, see Comparison instructions on page C3-6. All other VFP instructions that process NaNs have floating-point result values. Instructions with floating-point results If one or more of the operands to an operation with a floating-point result is a NaN, the IEEE 754 standard requires that: • If any of the NaN operands is a signaling NaN, an Invalid Operation exception must be generated. If this exception is untrapped, the result must be a quiet NaN. • If all of the NaN operands are quiet NaNs, the result must be a quiet NaN, and must be equal to one of the NaN operands. Note For this purpose, the standard permits some copy operations on floating-point numbers to be treated as non floating-point operations, so that they do not process NaNs in this fashion. The VFP architecture requires these copy operations to be treated as non floating-point operations. Instructions affected by this are described in Copy, negation and absolute value instructions on page C3-11, Load and Store instructions on page C3-13 and Register transfer instructions on page C3-17. Most floating-point instructions in the VFP architecture use the same format for their operands and results. For these, the VFP architecture specifies that the correct quiet NaN result in either of the above cases is determined as follows: C2-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 589. VFP Programmer’s Model 1. For instructions acting on vector operands, rules 2 through 4 below are applied independently to each individual operation on vector elements. 2. The FMAC, FMSC, FNMAC, and FNMSC instructions each specify two floating-point operations, each with two operands. If either of the operands to the first operation is a NaN, its result is determined according to rules 3 and 4 below. Then the third operand and result of the first operation (with its sign bit inverted for FNMAC and FNMSC) become the operands of the second operation. If either of them is a NaN, the final result is determined according to rules 3 and 4 below. 3. If an operand is a signaling NaN, the result is the quiet NaN constructed by taking a copy of that operand and changing its most significant fraction bit from 0 to 1. If both operands of a two-operand operation are signaling NaNs, the first operand is the one used to generate the result in this fashion. 4. If no operand is a signaling NaN, but an operand is a quiet NaN, the result is a copy of the quiet NaN operand. If both operands of a two-operand operation are quiet NaNs, the first operand is the one copied to generate the result. The IEEE 754 standard also specifies that an Invalid Operation exception must be generated for certain operations whose operands are not NaNs. The following operations yielding floating-point results can cause this to happen: • Additions, when the two operands are infinities with opposite signs. VFP instructions affected by this are FADD, FMAC, and FNMAC. • Subtractions, when the two operands are infinities with the same sign. VFP instructions affected by this are FMSC, FNMSC, and FSUB. • Multiplications, when one operand is a zero and the other is an infinity. VFP instructions affected by this are FMAC, FMSC, FMUL, FNMAC, FNMSC, and FNMUL. • Divisions, when both operands are zeros or both operands are infinities. The only VFP instruction affected by this is FDIV. • Square roots, whose operands are negative, including -¥ (minus infinity) but excluding -0. The only VFP instruction affected by this is FSQRT. In each case, if the exception is untrapped, the result must be a quiet NaN. The VFP architecture specifies that the quiet NaN produced in these cases must have sign bit equal to 0, most significant fraction bit equal to 1, and all remaining fraction bits equal to 0. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-7
- 590. VFP Programmer’s Model Special cases There are two instructions whose operands and results have different floating-point formats. These have special rules for handling NaNs, as follows: • The FCVTDS instruction converts a single-precision value to double precision. If its operand is a single-precision quiet NaN, the result is the double-precision quiet NaN with: S = S bit of operand fraction[51:29] = fraction[22:0] of operand fraction[28:0] = 0 If its operand is a single-precision signaling NaN, an Invalid Operation exception is generated. If the exception is untrapped, the result is the double-precision quiet NaN with: S = S bit of operand fraction[51] = 1 fraction[50:29] = fraction[21:0] of operand fraction[28:0] = 0 • The FCVTSD instruction converts a double-precision value to single precision. If its operand is a double-precision quiet NaN, the result is the single-precision quiet NaN with: S = S bit of operand fraction[22:0] = fraction[51:29] of operand If its operand is a double-precision signaling NaN, an Invalid Operation exception is generated. If the exception is untrapped, the result is the single-precision quiet NaN with: S = S bit of operand fraction[22] = 1 fraction[21:0] = fraction[50:29] of operand C2-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 591. VFP Programmer’s Model 2.2 Rounding Floating-point arithmetic inherently only has limited accuracy, because the exact mathematical result of an arithmetic operation often has more significant bits than can fit in its destination format. To deal with this, the result is rounded to fit in the destination format, by choosing a representable number in that format which is a close approximation to the exact result. The IEEE 754 standard specifies four rounding modes, each of which specifies how the exact result of an operation is rounded. In the following descriptions of the rounding modes, the rounding error is defined to be the value of: (rounded result) - (exact result) The rounding modes are as follows: Round to Nearest (RN) mode In this mode, the rounded result is the nearest representable number to the unrounded result, that is, the representable number that minimizes abs(rounding error). If the unrounded result lies precisely halfway between two representable numbers, the one whose least significant bit is 0 is used. This is the default rounding mode, and generally yields the most accurate results. The other rounding modes are mostly used for specialized purposes, such as interval arithmetic. Round towards Plus Infinity (RP) mode In this mode, the rounded result is the nearest representable number which is greater than or equal to the exact result, that is, the one that minimizes abs(rounding error) subject to the requirement (rounding error) ³ 0. If the exact result is greater than the largest positive normalized number of the destination format, the rounded result is +¥ (plus infinity). Round towards Minus Infinity (RM) mode In this mode, the rounded result is the nearest representable number which is less than or equal to the exact result, that is, the one that minimizes abs(rounding error) subject to the requirement (rounding error) £ 0. If the exact result is less than the largest negative normalized number of the destination format, the rounded result is -¥ (minus infinity). Round towards Zero (RZ) mode In this mode, results are rounded to the nearest representable number which is no greater in magnitude than the unrounded result, that is, the one that minimizes abs(rounding error) subject to the requirement abs(rounded result) £ abs(exact result). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-9
- 592. VFP Programmer’s Model 2.3 Floating-point exceptions The IEEE 754 standard specifies five classes of floating-point exception: Invalid Operation exceptions Occur in various cases where neither a numeric value nor an infinity is a sensible result of a floating-point operation, and also when an operand of a floating-point operation is a signaling NaN. For more details of Invalid Operation exceptions, see NaNs on page C2-5. Division by Zero exceptions Occur when a normalized or denormalized number is divided by a zero. Overflow exceptions Occur when result of a floating-point operation is too big in magnitude for it to be represented in the destination format without an unusually large rounding error for the rounding mode in use. More precisely, the ideal rounded result of a floating-point operation is defined to be the result that its rounding mode would produce if the destination format had no limits on the unbiased exponent range. If the ideal rounded result has unbiased exponent too big for the destination format (that is, >127 for single precision or >1023 for double precision), it differs from the actual rounded result, and an Overflow exception occurs. Underflow exceptions Occur when a floating-point operation’s result is too small in magnitude for it to be represented in the destination format without an unusually large rounding error for the rounding mode in use. The exact conditions for an Underflow exception to occur vary slightly depending on the value of the Underflow exception enable (UFE) bit (which is bit[11] of the FPSCR), that is, on whether the Underflow exception is to be trapped. In each case, one condition is that the ideal rounded result (as defined above) has an unbiased exponent too small for the destination format (that is, < -126 for single precision or < -1022 for double precision). If UFE == 1, this is the only condition and a trapped Underflow exception occurs when it is detected. If UFE == 0, an untrapped Underflow exception only occurs if the ideal rounded result is also different from the result produced by actual rounding (which might be a zero, a denormalized number or the minimum possible normalized number). Note The IEEE 754 standard leaves two choices open in its definition of the underflow exception. In the terminology of the standard, the above description means that the VFP architecture requires these choices to be: • the after rounding form of tininess • the denormalization loss form of loss of accuracy. C2-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 593. VFP Programmer’s Model Inexact exceptions Occur whenever the rounded result of a floating-point operation is different from its exact unrounded result, that is, whenever a non-zero rounding error occurs. They also occur: • whenever an untrapped Overflow exception occurs • whenever a denormalized operand is treated as +0 or a result is forced to +0 by flush-to-zero mode. Note Despite being called an exception, the Inexact exception occurs frequently in normal floating-point calculations. Other than in some specialized uses of floating-point arithmetic, the Inexact exception does not indicate that anything unusual has occurred. Each of these exceptions can be handled in one of two ways, selected by a trap enable bit associated with the exception: Trap enable bit is 0 Untrapped exception handling is selected. This causes the result of the operation to be a default value specified by the IEEE 754 standard, and a cumulative exception bit associated with the exception becomes 1. Table 2-1 on page C2-12 shows how the result value is determined for each exception. The cumulative exception bits can only become 0 as the result of an explicit write to the FPSCR using the FMXR instruction. Other floating-point instructions only leave them unchanged (if no untrapped exceptions occurred) or set one or more of them to 1 depending on which untrapped exceptions occurred. A program can therefore test whether untrapped exceptions occurred during a calculation, by setting these bits to zero before the calculation and testing them afterwards. Trap enable bit is 1 Trapped exception handling is selected. This causes a trap handler routine for the exception to be called. Details of how trap handlers are selected and of the interfaces via which they are called are IMPLEMENTATION DEFINED. The call to the trap handler routine is allowed to be imprecise, that is, it might occur at a later point during program execution than the floating-point instruction that caused the exception. However, it always occurs before execution of any subsequent instruction that depends on the results of that instruction, or of any serializing instruction (see FMRX on page C4-58 and FMXR on page C4-70). Trapped exception handling does not cause the cumulative exception bit to become set. If this behavior is desired, the trap handler routine can use an FMRX/ORR/FMXR sequence on the FPSCR to set the bit. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-11
- 594. VFP Programmer’s Model Table 2-1 Exception default results Exception type Default result for positive sign Default result for negative sign Invalid Operation Quiet NaN Quiet NaN Division by Zero +¥ (plus infinity) -¥ (minus infinity) Overflow RN,RP: RM,RZ: +¥ (plus infinity) +MaxNorm RN,RM: RP,RZ: -¥ (minus infinity) -MaxNorm Underflow Normal rounded result Normal rounded result Inexact Normal rounded result Normal rounded result The following notes apply to Table 2-1: • For Invalid Operation exceptions, see NaNs on page C2-5 for details of which quiet NaN is produced as the default result. • For Division by Zero exceptions, the default result depends on the sign bit as normally determined for a division - that is, on the exclusive OR of the two operand sign bits. • For Overflow exceptions, the default result depends on the sign bit as normally determined for the overflowing operation, and also on which rounding mode is being used. MaxNorm means the maximum normalized number of the destination precision. 2.3.1 Combinations of exceptions It is possible for more than one exception to occur on the same operation. The only combinations of exceptions that can occur are Overflow/Inexact and Underflow/Inexact. In these cases, the Inexact exception is treated as lower priority, as follows: • If the Overflow or Underflow exception is trapped, its trap handler is called. It is IMPLEMENTATION DEFINED whether the parameters to the trap handler include information about the Inexact exception. Apart from this, the Inexact exception is ignored in this case. • If the Overflow or Underflow exception is untrapped, its cumulative bit is set to 1 and its default result is evaluated. Then the Inexact exception is handled normally, with this default result being treated as the normal rounded result of the operation. C2-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 595. VFP Programmer’s Model 2.4 Flush-to-zero mode The performance of some VFP implementations is significantly lower than normal when performing calculations involving denormalized numbers and Underflow exceptions. Typically, this occurs for hardware implementations which only handle normalized numbers and zeros in hardware, and invoke their support code when they encounter other types of value. If a significant number of the operands and intermediate results in an algorithm are denormalized numbers, this can result in a considerable loss of performance. In some (but not all) of these algorithms, this performance can be recovered by replacing the denormalized operands and intermediate results with zeros, without significantly affecting the accuracy of their final results. To allow this optimization, VFP implementations have a special processing mode called flush-to-zero mode. The behavior in flush-to-zero mode differs from normal IEEE 754 arithmetic in the following ways: • All inputs to floating-point operations which are denormalized numbers are treated as though they were +0. • All results of single-precision floating-point operations which lie in the range -2-126 < x < +2-126 before rounding are forced to +0. • All results of double-precision floating-point operations which lie in the range -2-1022 < x < +2-1022 before rounding are forced to +0. • Whenever an operation has an operand treated as +0 or a result forced to +0 in any of the above ways, an Inexact exception occurs (this normally just causes the IXC bit in the FPSCR to become set). • Underflow exceptions never occur in flush-to-zero mode. Copy operations are not treated as floating-point operations for the purpose of flush-to-zero mode. The operations not affected by flush-to-zero mode are precisely the same as those that do not generate Invalid Operation exceptions when their operands are signaling NaNs. For more details, see Copy, negation and absolute value instructions on page C3-11, Load and Store instructions on page C3-13, and Register transfer instructions on page C3-17. Note Flush-to-zero mode is incompatible with the IEEE 754 standard, and must not be used when IEEE 754 compatibility is a requirement. Flush-to-zero mode must be treated with great care. As stated above, it can lead to a major performance increase on some algorithms, but there are a number of pitfalls when using it. These pitfalls include: • On many algorithms, it has no noticeable effect, because the algorithm does not normally use denormalized numbers. • On many other algorithms, it can cause exceptions to occur or seriously impact the accuracy of the results of the algorithm. It is strongly recommended that flush-to-zero mode is only used after ascertaining that performance is indeed being degraded by the occurrence of denormalized numbers, and that its use is not going to upset the accuracy and/or stability of the algorithm. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-13
- 596. VFP Programmer’s Model 2.5 Floating-point general-purpose registers A VFP implementation contains 32 general-purpose registers, each capable of holding a single-precision floating-point number or a 32-bit integer. These are named S0-S31. In D variants of the VFP architecture, these registers are also treated as 16 double-precision registers, with names D0-D15. Double-precision register D0 overlaps single-precision registers S0 and S1, double-precision register D1 overlaps single-precision registers S2 and S3, and so on, as illustrated in Figure 2-1. S0 S1 D0 S2 S3 D1 S4 S5 D2 S6 S7 D3 S8 S9 D4 S10 S11 D5 S12 S13 D6 S14 S15 D7 overlapped with S16 S17 D8 S18 S19 D9 S20 S21 D10 S22 S23 D11 S24 S25 D12 S26 S27 D13 S28 S29 D14 S30 S31 D15 Figure 2-1 VFP general-purpose registers The VFP architecture does not specify how a double-precision register is overlapped with its pair of single-precision registers. Different implementations make different decisions about which bits are overlapped and about how the overlapping bits match up. Also, implementations are free to convert numbers of either precision (or both) to an internal register format when they are loaded (or transferred from the ARM), and convert them back when stored (or transferred to the ARM), as long as the overall results are correct. As a result, software must not rely on any particular type of overlap between the registers. More formally, the rules for this are: C2-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 597. VFP Programmer’s Model • When a single-precision value or a 32-bit integer is written to single-precision register Si, the value in the overlapped double-precision register D(i >> 1) becomes UNPREDICTABLE. • When a double-precision value is written to double-precision register Di, the values in the overlapped single-precision registers S(2 ´ i) and S(2 ´ i + 1) become UNPREDICTABLE. • Any instruction which reads one or more registers with UNPREDICTABLE contents produces UNPREDICTABLE results. Accordingly, most VFP instructions must only be used when it is known that the values in their source registers have the precision expected by the instruction. No facility exists in the architecture to detect or correct an inconsistency between the precision expected by an instruction and the precision of the numbers last written to its source registers. Such inconsistencies produce UNPREDICTABLE behavior. 2.5.1 Storing and reloading values of unknown precision Programs sometimes need to store register values to memory and later reload them without determining whether they contain single-precision or double-precision values. Two typical cases in which this happens are: • Procedure-calling standards often specify that registers are callee-save registers (that is, that the called procedure must preserve them). If the called procedure needs to use a callee-save register, its entry sequence must store the register value to the stack. Later, the return sequence of the procedure must reload the value from the stack in order to restore the original contents of the register. However, the contents of the register(s) being stored on the stack depend on how they were being used by the caller, and different callers can use the registers differently. So the entry sequence of the called procedure must treat the callee-save registers as containing values of unknown precision. • Process swap code needs to store the contents of registers when a process is swapped out, and later reload them when the process is swapped back in. As different processes probably use the registers in different ways, process swap code needs to treat the VFP registers as containing values of unknown precision. Two VFP instructions (FLDMX and FSTMX) are used in such situations. These instructions are exceptions to the normal rule that the source precision must match the precision of the instruction. FSTMX Stores one or more double-precision registers, using an IMPLEMENTATION DEFINED memory format FLDMX Reloads registers that have been stored in that format. The only architectural constraints on the FSTMX/FLDMX format are that: • N double-precision registers are stored in at most 2N+1 words. • A matching FLDMX reloads the original contents of the registers correctly, regardless of whether they originally contained single-precision or double-precision values. For this purpose, a matching FLDMX means one that loads precisely the same set of registers as the FSTMX stored and generates the same memory addresses as the FSTMX. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-15
- 598. VFP Programmer’s Model The only operation which is normally performed on data stored with FSTMX is to reload it using a matching FLDMX. However, debug software might need to interpret and/or modify the contents of stack frames or process control blocks, and so might need to know the FSTMX/FLDMX memory format. There are two standard options for the format: • In standard format 1, FSTMX stores the register contents in exactly the same way as an FSTMD of the same registers would store them. The first double-precision register value is stored as two words, in the correct order for the configured endianness of the processor. Then the next double-precision register value is stored similarly, and so on, until the N registers have been stored in 2N memory words. The (2N+1)th memory word is unused. The matching FLDMX instruction reloads the data as double-precision values, precisely like an FLDMD of the same registers. This format is only used for implementations in which storing and reloading double-precision registers in this fashion correctly reload their contents even if they happen to contain single-precision values. • In standard format 2, the first word stored is a format word, and then each double-precision register specified in the instruction is stored as two words, in ascending order of register number. This option uses all 2N+1 available words of memory. The format word has the following format: 31 16 15 0 SBZ format bits For each double-precision register Di which is stored by the FSTMX instruction, bit[i] of the format word indicates how it is stored: — If bit[i] == 0, the register contained single-precision data, and the two words stored are the contents of S(2 ´ i) followed by the contents of S(2 ´ i + 1), both in single-precision format. — If bit[i] == 1, the register contained double-precision data, and the two words stored are the contents of Di, stored as two words in the correct order for the configured endianness of the processor. For each double-precision register Di which is not stored by the FSTMX instruction, bit[i] of the format word is UNPREDICTABLE. Example For example, Figure 2-2 on page C2-17 shows how the instruction: FSTMIAX Rn,{D4-D6} would store the registers using each standard option, on the assumption that D4 and D6 contain double-precision values and S10 and S11 (which overlap D5) contain single-precision values. C2-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 599. VFP Programmer’s Model Address Standard format 1 Standard format 2 D6, second word D6, first word S11 S10 D4, second word D4, first word Unused D6, second word D6, first word D5, second word D5, first word D4, second word D4, first word Format word Rn+24 Rn+20 Rn+16 Rn+12 Rn+8 Rn+4 Rn Figure 2-2 Standard FSTMX/FLDMX memory formats In the second column of Figure 2-2, the format word is: 31 16 15 7 6 4 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 UNP 1 0 1 UNP 2.5.2 Short vectors The single-precision registers can be used to hold short vectors of up to 8 single-precision values. Arithmetic operations on all the elements of such a vector can be specified by just one single-precision arithmetic instruction. For details of how this is done, see Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 and Addressing Mode 3 - Single-precision vectors (monadic) on page C5-14. Similarly, the double-precision registers can be used to hold short vectors of up to 4 double-precision values, and double-precision arithmetic instructions can specify operations on these vectors. For details, see Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 and Addressing Mode 4 - Double-precision vectors (monadic) on page C5-19. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-17
- 600. VFP Programmer’s Model 2.5.3 Holding integers in single-precision registers Each single-precision register can hold a 32-bit integer instead of a single-precision floating-point number. The register contents are identical for a 32-bit integer and for the single-precision value represented by the same word. This means that FMRS, FMSR and the single-precision load/store instructions can be used to transfer either integers or single-precision values. The single-precision floating-point number represented by the same word as a 32-bit integer does not normally have the same value as the integer. For example, the integers 2 and -1 are represented by the words 0x00000002 and 0xFFFFFFFF. As single-precision floating-point numbers, the same words represent the denormalized number 2-148 and a quiet NaN respectively. An integer held in a floating-point register can therefore not be used directly as a single-precision value, nor can a single-precision value be used directly as an integer. If conversions between integers and floating-point values are wanted, explicit conversion instructions must be used, as described in the following two subsections. Floating-point to integer Two instructions are used to convert a floating-point number to an integer: 1. The first instruction is FTOSID, FTOSIS, FTOUID, or FTOUIS, depending on whether the floating-point operand is double-precision or single-precision, and whether a signed or unsigned integer result is wanted. After this instruction, the required result is held as an integer in a single-precision register. The special forms FTOSIZD, FTOSIZS, FTOUIZD, and FTOUIZS of these instructions allow the conversion to be done using Round towards Zero (RZ) mode, without changing the rounding mode specified by the FPSCR. This is the form of floating-point to integer conversion required by the C, C++ and related languages. 2. The second instruction is typically an FMRS instruction, which transfers the integer result to an ARM register, but can also be various other instructions (see Conversion instructions on page C3-10). Integer to floating-point Similarly, two instructions are used to convert an integer to a floating-point number: 1. The first instruction is typically FMSR, to transfer the integer operand to a single-precision register, but can also be various other instructions (see Conversion instructions on page C3-10). 2. The second instruction is FSITOD, FSITOS, FUITOD or FUITOS, depending on whether the integer operand is to be treated as signed or unsigned and whether a double-precision or single-precision floating-point result is wanted. C2-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 601. VFP Programmer’s Model 2.6 System registers A VFP implementation contains three or more special-purpose system registers: • The Floating-point System ID register (FPSID) is a read-only register whose value indicates which VFP implementation is being used. See FPSID on page C2-20 for details. • The Floating-point Status and Control register (FPSCR) is a read/write register which provides all user-level status and control of the floating-point system. See FPSCR on page C2-21 for details of the FPSCR. • The Floating-point Exception register (FPEXC) is a read/write register, two bits of which provide system-level status and control. The remaining bits of this register can be used to communicate exception information between the hardware and software components of the implementation, in an IMPLEMENTATION DEFINED manner. See FPEXC on page C2-24 for details of the FPEXC. • Individual VFP implementations can define and use further system registers for the purpose of communicating between the hardware and software components of the implementation. All such registers are IMPLEMENTATION DEFINED. They may not be used outside the implementation itself, except as described in implementation-specific documentation. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-19
- 602. VFP Programmer’s Model 2.6.1 FPSID The FPSID has the following format: 31 24 23 22 21 20 19 16 15 8 7 4 3 0 implementor SW format SNG architecture part number variant revision Bits[31:24] Contain an implementor code. The following code is defined: 0x41 = A (ARM Ltd) All other values of the implementor code are reserved by ARM Ltd. Bit[23] Contains 0 if the implementation contains a hardware coprocessor, or 1 if it is a pure software implementation. Bits[22:21] Indicate which FSTMX/FLDMX format is used (see Storing and reloading values of unknown precision on page C2-15): 0b00 Indicates standard format 1. 0b01 Indicates standard format 2. 0b10 Is reserved. 0b11 Indicates a non-standard format. Bit[20] Contains 0 if the implementation supports both single precision and double precision (a D variant of the architecture), or 1 if it only supports single precision (a non-D variant). Bits[19:16] Contain the architecture version number, encoded as follows: 0b0000 indicates VFPv1. All other values of this architecture version code are reserved by ARM Ltd. Bits[15:8] Contain an IMPLEMENTATION DEFINED representation of the primary part number of the VFP implementation. Bits[7:4] Contain an IMPLEMENTATION DEFINED variant number. This is typically used to distinguish variants of the same primary part. For example, two variants of the same VFP implementation might have hardware coprocessor interfaces designed to work with different ARM processors. Bits[3:0] Contain the IMPLEMENTATION DEFINED revision number of the part. The FPSID register is read-only, and can be accessed in both privileged and unprivileged modes. Attempts to write the FPSID register are ignored. C2-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 603. VFP Programmer’s Model 2.6.2 FPSCR The FPSCR has the following format: 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 N Z C V DNM FZRMODESTRIDE D N M LEN DNM I X E U F E O F E D Z E I O E DNM I X C U F C O F C D Z C I O C All of these bits can be read and written, and can be accessed in both privileged and unprivileged modes. Note All bits described as DNM (Do Not Modify) in the diagram are reserved for future expansion. They are initialized to zeros. Non-initialization code must use read/modify/write techniques when handling the FPSCR, in order to ensure that these bits are not modified. Failure to observe this rule can result in code which has unexpected side effects on future systems. The FPSCR bits are described in the following subsections. Condition flags Bits[31:28] of the FPSCR contain the results of the most recent floating-point comparison: N Is 1 if the comparison produced a less than result Z Is 1 if the comparison produced an equal result C Is 1 if the comparison produced an equal, greater than or unordered result V Is 1 if the comparison produced an unordered result. These condition flags do not directly affect conditional execution, either of ARM instructions or of VFP instructions. A comparison instruction is normally followed by an FMSTAT instruction. This transfers the FPSCR condition flags to the ARM CPSR flags, after which they can affect conditional execution. For more details of how comparisons are performed, see Comparison instructions on page C3-6. Flush-to-zero mode control Bit[24] of the FPSCR is the FZ bit and controls flush-to-zero mode. See Flush-to-zero mode on page C2-13 for details of this processing mode. FZ == 0 Flush-to-zero mode is disabled and the behavior of the floating-point system is fully compliant with the IEEE 754 standard. FZ == 1 Flush-to-zero mode is enabled. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-21
- 604. VFP Programmer’s Model Rounding mode control Bits[23:22] of the FPSCR select the current rounding mode. This rounding mode is used for almost all floating-point instructions. The only floating-point instructions which do not use it are FTOSIZD, FTOSIZS, FTOUIZD and FTOUIZS, which always use RZ mode. The rounding modes are encoded as follows: 0b00 Indicates Round to Nearest (RN) mode 0b01 Indicates Round towards Plus Infinity (RP) mode 0b10 Indicates Round towards Minus Infinity (RM) mode 0b11 Indicates Round towards Zero (RZ) mode. See Rounding on page C2-9 for details of the rounding modes. Vector length/stride control The LEN field (bits[18:16]) of the FPSCR controls the vector length for VFP instructions that operate on short vectors, that is, how many registers are in a vector operand. Similarly, the STRIDE field (bits[21:20]) controls the vector stride, that is, how far apart the registers in a vector lie in the register bank. The allowed combinations of LEN and STRIDE are shown in Table 2-2 on page C2-23. All other combinations of LEN and STRIDE produce UNPREDICTABLE results. The combination LEN == 0b000, STRIDE == 0b00 is sometimes called scalar mode. When it is in effect, all arithmetic instructions specify simple scalar operations. Otherwise, most arithmetic instructions specify a scalar operation if their destination lies in the range S0-S7 (for single precision) or D0-D3 (for double precision). The full rules used to determine which operands are vectors and full details of how vector operands are specified can be found in Chapter C5 VFP Addressing Modes and in the individual instruction descriptions. The rules for vector operands do not allow the same register to appear twice or more in a vector. The allowed LEN/STRIDE combinations listed in Table 2-2 never cause this to happen for single-precision instructions, so single-precision scalar and vector instructions can be used with all of these LEN/STRIDE combinations. For double-precision vector instructions, some of the allowed LEN/STRIDE combinations would cause the same register to appear twice in a vector. If a double-precision vector instruction is executed with such a LEN/STRIDE combination in effect, the instruction is UNPREDICTABLE. The last column of Table 2-2 indicates which LEN/STRIDE combinations this applies to. Double-precision scalar instructions work normally with all of the allowed LEN/STRIDE combinations. C2-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 605. VFP Programmer’s Model Table 2-2 Vector length/stride combinations LEN STRIDE Vector length Vector stride Double-precision vector instructions 0b000 0b00 1 - All instructions are scalar 0b001 0b00 2 1 Work normally 0b001 0b11 2 2 Work normally 0b010 0b00 3 1 Work normally 0b010 0b11 3 2 UNPREDICTABLE 0b011 0b00 4 1 Work normally 0b011 0b11 4 2 UNPREDICTABLE 0b100 0b00 5 1 UNPREDICTABLE 0b101 0b00 6 1 UNPREDICTABLE 0b110 0b00 7 1 UNPREDICTABLE 0b111 0b00 8 1 UNPREDICTABLE Exception status and control Bits[12:8] and bits[4:0] of the FPSCR are the trap enable bits and cumulative exception bits respectively for the five types of exception. For details of what these do, see Floating-point exceptions on page C2-10. Table 2-3 shows which bits are associated with each exception. Table 2-3 Exception status and control bits Exception type Trap enable bit Cumulative exception bit Invalid Operation IOE (bit[8]) IOC (bit[0]) Division by Zero DZE (bit[9]) DZC (bit[1]) Overflow OFE (bit[10]) OFC (bit[2]) Underflow UFE (bit[11]) UFC (bit[3]) Inexact IXE (bit[12]) IXC (bit[4]) ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-23
- 606. VFP Programmer’s Model 2.6.3 FPEXC The FPEXC register has the following format: 31 30 29 0 EXEN IMPLEMENTATION DEFINED This register can only be accessed in privileged modes. The EX bit The EX bit (bit[31]) is a status bit which specifies how much information needs to be saved to record the state of the floating-point system. It can be read on all VFP implementations, and is mainly of interest to process swap code. EX == 0 In this case, the only significant state in the floating-point system is the contents of the architecturally defined writable registers, that is, of the general-purpose registers, FPSCR and FPEXC. If EX == 0 when a process is swapped out, only these registers need to be saved, or reloaded when the process is swapped back in. Also, no unexpected ARM exceptions (such as an undefined instruction exception to process a pending exception in the hardware) must occur during the saving and reloading of the registers. EX == 1 Here, there is additional IMPLEMENTATION DEFINED significant state in the floating-point system which process swap code needs to handle. This typically occurs when VFP hardware requires support code assistance to handle a potential exception, and one or more of the additional hardware system registers contains details of the potential exception. (Some implementations describe this by saying that the hardware is in an exceptional state.) The actions required to swap a process out when EX == 1 and to swap such a process back in are IMPLEMENTATION DEFINED. The behavior of the EX bit when FPEXC is written is IMPLEMENTATION DEFINED, subject to the constraint that writing a 0 to the EX bit must be a legitimate action. Otherwise, the process swap technique described above for the case EX == 0 cannot work. The EN bit The EN bit (bit[30]) is a global enable bit, and can be both read and written. EN == 1 In this case, the floating-point system is enabled and operates normally. EN == 0 Here, the floating-point system is disabled. In this state, all VFP instructions are treated as undefined instructions when executed in an unprivileged ARM processor mode, and all except the following are treated as undefined instructions when executed in a privileged ARM processor mode: • an FMXR to the FPEXC or FPSID register • an FMRX from the FPEXC or FPSID register. C2-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 607. VFP Programmer’s Model Note An FMXR to the FPSCR or an FMRX from the FPSCR is treated as an undefined instruction when EN == 0. If a VFP implementation contains additional system registers besides FPSID, FPSCR, and FPEXC, the behavior of FMXR instructions to them and FMRX instructions from them is IMPLEMENTATION DEFINED. Other bits All bits of the FPSCR other than the EX and EN bits are IMPLEMENTATION DEFINED, including whether they are readable, writable or both. They are typically used in hardware implementations for communicating exception information between the VFP hardware and its support code. A constraint on how these bits are defined is that when the EX bit is 0, it must be possible to save and reload all significant state in the floating-point system by saving and reloading only the VFP general-purpose registers, FPSCR and FPEXC. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C2-25
- 608. VFP Programmer’s Model 2.7 Reset behavior and initialization When a hardware VFP implementation is reset, the FPEXC EN bit is reset to 0. The behavior of all other VFP registers and of the remaining bits of FPEXC on hardware reset is IMPLEMENTATION DEFINED. When the software component of a VFP implementation has finished initializing, the following are true: • The FPEXC EN bit is set to 1 • The FPEXC EX bit is set to 0 • All bits of the FPSCR are set to 0, with the possible exception of the condition code flags in some cases. This selects the following settings: — normal IEEE 754 mode, not flush-to-zero mode — the Round to Nearest rounding mode — scalar mode (vector length 1) — all exceptions are untrapped, and their cumulative status bits indicate that no exceptions of that type have been detected yet. It is IMPLEMENTATION DEFINED whether the VFP general-purpose registers and the FPSCR condition flags are initialized, and if so, what values they are initialized to. C2-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 609. Chapter C3 VFP Instruction Set Overview This chapter gives an overview of the VFP instruction set. It contains the following sections: • Data-processing instructions on page C3-2 • Load and Store instructions on page C3-13 • Register transfer instructions on page C3-17. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-1
- 610. VFP Instruction Set Overview 3.1 Data-processing instructions All VFP data-processing instructions are CDP instructions for coprocessors 10 or 11, with the following format: 31 30 29 28 27 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 1 1 1 0 p D q r Fn Fd cp_num N s M 0 Fm p, q, r, s These bits collectively form the instruction’s primary opcode. See Table 3-1 for the assignment of these opcodes. When all of p, q, r and s are 1, the instruction is a two-operand extension instruction, with an extension opcode specified by the Fn and N bits. Fd and D These bits normally specify the destination register of the instruction: • For a single-precision instruction, Fd holds the top 4 bits of the register number and D holds the bottom bit. • For a double-precision instruction, Fd holds the register number and D must be 0. If D is 1 in a double-precision instruction, the instruction is UNDEFINED. For multiply-accumulate instructions, this register is also the accumulate operand register. For comparison instructions, it is the first operand register rather than a destination register. Fn and N These bits normally specify the first operand register of the instruction. • For a single-precision instruction, Fn holds the top 4 bits of the register number and N holds the bottom bit. • For a double-precision instruction, Fn holds the register number and N must be 0. However, if p, q, r and s are all 1, the instruction is an extension instruction, and the Fn and N fields form an extension opcode instead of specifying a register. See Table 3-2 for the assignment of these extension opcodes. If N is 1 in a double-precision non-extension instruction, the instruction is UNDEFINED. Fm and M These bits specify the second operand register of the instruction, or the only operand register for some extension instructions. • For a single-precision instruction, Fm holds the top 4 bits of the register number and M holds the bottom bit. • For a double-precision instruction, Fm holds the register number and M must be 0. If M is 1 in a double-precision instruction, the instruction is UNDEFINED. cp_num If cp_num is 0b1010 (coprocessor number 10), the instruction is a single-precision instruction. If cp_num is 0b1011 (coprocessor number 11), the instruction is a double-precision instruction. For the instructions that convert between single-precision and double-precision (FCVTDS and FCVTSD), cp_num matches the source precision. C3-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 611. VFP Instruction Set Overview Table 3-1 and Table 3-2 show the assignment of VFP data-processing opcodes. In these tables, Fd is used to mean a destination register of the appropriate precision, that is, Sd for single-precision instructions and Dd for double-precision instructions. Fn and Fm are used similarly. Table 3-1 VFP data-processing primary opcodes p q r s Instruction name cp_num=10 Instruction name cp_num=11 Instruction functionality 0 0 0 0 FMACS FMACD Fd = Fd + (Fn * Fm) 0 0 0 1 FNMACS FNMACD Fd = Fd - (Fn * Fm) 0 0 1 0 FMSCS FMSCD Fd = -Fd + (Fn * Fm) 0 0 1 1 FNMSCS FNMSCD Fd = -Fd - (Fn * Fm) 0 1 0 0 FMULS FMULD Fd = Fn * Fm 0 1 0 1 FNMULS FNMULD Fd = -(Fn * Fm) 0 1 1 0 FADDS FADDD Fd = Fn + Fm 0 1 1 1 FSUBS FSUBD Fd = Fn - Fm 1 0 0 0 FDIVS FDIVD Fd = Fn / Fm 1 0 0 1 - - UNDEFINED 1 0 1 0 - - UNDEFINED 1 0 1 1 - - UNDEFINED 1 1 0 0 - - UNDEFINED 1 1 0 1 - - UNDEFINED 1 1 1 0 - - UNDEFINED 1 1 1 1 See Table 3-2 on page C3-4 See Table 3-2 on page C3-4 Extension instructions ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-3
- 612. VFP Instruction Set Overview Table 3-2 VFP data-processing extension opcodes Extension opcode Instruction name Fn N cp_num=10 cp_num=11 Instruction functionality 0000 0 FCPYS FCPYD Fd = Fm 0000 1 FABSS FABSD Fd = abs(Fm) 0001 0 FNEGS FNEGD Fd = -Fm 0001 1 FSQRTS FSQRTD Fd = sqrt(Fm) 001x x - - UNDEFINED 0100 0 FCMPS FCMPD Compare Fd with Fm, no exceptions on quiet NaNs 0100 1 FCMPES FCMPED Compare Fd with Fm, with exceptions on quiet NaNs 0101 0 FCMPZS FCMPZD Compare Fd with 0, no exceptions on quiet NaNs 0101 1 FCMPEZS FCMPEZD Compare Fd with 0, with exceptions on quiet NaNs 0110 x - - UNDEFINED 0111 0 - - UNDEFINED 0111 1 FCVTDS FCVTSD Single « double precision conversions 1000 0 FUITOS FUITOD Unsigned integer ® floating-point conversions 1000 1 FSITOS FSITOD Signed integer ® floating-point conversions 1001 x - - UNDEFINED 101x x - - UNDEFINED 1100 0 FTOUIS FTOUID Floating-point ® unsigned integer conversions 1100 1 FTOUIZS FTOUIZD Floating-point ® unsigned integer conversions, RZ mode 1101 0 FTOSIS FTOSID Floating-point ® signed integer conversions 1101 1 FTOSIZS FTOSIZD Floating-point ® signed integer conversions, RZ mode 111x x - - UNDEFINED C3-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 613. VFP Instruction Set Overview 3.1.1 Basic arithmetic instructions and square root The FADDS, FSUBS, FMULS, FDIVS, and FSQRTS instructions provide the four basic arithmetic operations and square root on single-precision values. Similarly, the FADDD, FSUBD, FMULD, FDIVD, and FSQRTD instructions supply these operations on double-precision values. In addition, the FNMULS and FNMULD instructions supply negated multiplications in single and double precision respectively. Their results are precisely equivalent to those of performing an FMULS or FMULD instruction followed by an FNEGS or FNEGD instruction (which inverts the sign of the result). All of these instructions can be made to operate on short vectors by setting the FPSCR LEN and STRIDE fields appropriately (see Chapter C5 VFP Addressing Modes for details). The operations performed by all these instructions are always treated as floating-point operations, both for NaN handling and flush-to-zero mode. In particular, signaling NaN operands cause Invalid Operand exceptions, and in flush-to-zero mode, denormalized operands are treated as +0 and sufficiently small results are forced to +0. 3.1.2 Multiply-accumulate instructions FMACS, FMACD, FNMACS, FNMACD, FMSCS, FMSCD, FNMSCS, and FNMSCD are multiply-accumulate instructions. They multiply their two main operands, possibly invert the sign bit of the product, add or subtract the value in the destination register and write the result back to the destination register. They are in all respects equivalent to the following sequences of basic arithmetic and negation instructions: FMACS Sd,Sn,Sm: FMULS St,Sn,Sm FADDS Sd,St,Sd FMACD Dd,Dn,Dm: FMULD Dt,Dn,Dm FADDD Dd,Dt,Dd FNMACS Sd,Sn,Sm: FMULS St,Sn,Sm FNEGS St,St FADDS Sd,St,Sd FNMACD Dd,Dn,Dm: FMULD Dt,Dn,Dm FNEGD Dt,Dt FADDD Dd,Dt,Dd FMSCS Sd,Sn,Sm: FMULS St,Sn,Sm FSUBS Sd,St,Sd FMSCD Dd,Dn,Dm: FMULD Dt,Dn,Dm FSUBD Dd,Dt,Dd FNMSCS Sd,Sn,Sm: FMULS St,Sn,Sm FNEGS St,St FSUBS Sd,St,Sd FNMSCD Dd,Dn,Dm: FMULD Dt,Dn,Dm FNEGD St,St FSUBD Dd,Dt,Dd ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-5
- 614. VFP Instruction Set Overview where St or Dt describes a notional register used to hold intermediate results, treated as being a scalar if Sd or Dd is a scalar and a vector if Sd or Dd is a vector. Note This implies that each multiply-accumulate operation involves two roundings: • one on the multiplication result • one on the result of the final addition or subtraction. Both of these roundings are performed fully and as defined by the IEEE 754 standard. In particular, these instructions do not specify fused multiply-accumulates as used in a number of other architectures. All of these instructions can be made to operate on short vectors by setting the FPSCR LEN and STRIDE fields appropriately (see Chapter C5 VFP Addressing Modes for details). The operations performed by all these instructions are always treated as floating-point operations, both for NaN handling and flush-to-zero mode. In particular, signaling NaN operands cause Invalid Operand exceptions, and in flush-to-zero mode, denormalized operands are treated as +0 and sufficiently small results are forced to +0. 3.1.3 Comparison instructions The FCMPS, FCMPD, FCMPES, and FCMPED instructions perform comparisons between two register values. The FCMPZS, FCMPZD, FCMPEZS, and FCMPEZD instructions perform comparisons between a register value and the constant +0. The IEEE 754 standard specifies that precisely one of four relationships holds between any two values being compared. These are as follows: • Two values are considered equal if any of the following conditions holds: — They are both numeric and have the same numerical value. This usually means that they have precisely the same representation, but also includes the case that one is +0 and the other is -0. — They are both +¥ (plus infinity). — They are both -¥ (minus infinity). • The first value is considered less than the second value if any of the following conditions holds: — They are both numeric and the numeric value of the first is less than that of the second. — The first is -¥ (minus infinity) and the second is numeric. — The first is numeric and the second is +¥ (plus infinity). — The first is -¥ (minus infinity) and the second is +¥ (plus infinity). • The first value is considered greater than the second value if any of the following conditions holds: — They are both numeric and the numeric value of the first is greater than that of the second. — The first is +¥ (plus infinity) and the second is numeric. — The first is numeric and the second is -¥ (minus infinity). — The first is +¥ (plus infinity) and the second is -¥ (minus infinity). • Two values are unordered if either or both of them are NaNs. C3-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 615. VFP Instruction Set Overview Note If both values are the same NaN, the comparison result is unordered, not equal. If an exact bit-by-bit comparison is wanted, the ARM comparison instructions must be used rather than VFP comparison instructions, both for this reason and because +0 and -0 compare as equal. For all the comparison instructions, the result of the comparison is placed in the FPSCR flags, as shown in Table 3-3: Table 3-3 VFP comparison flag values Comparison result N Z C V Equal 0 1 1 0 Less than 1 0 0 0 Greater than 0 0 1 0 Unordered 0 0 1 1 These FPSCR flag values need to be copied to the ARM CPSR flags before ARM conditional execution can be based on them. For this purpose, a special form of the FMRX instruction (called FMSTAT) is used. This is described in System register transfer instructions on page C3-20. When the result of the comparison is unordered, it is possible that the comparison can also generate an Invalid Operation exception because of the NaN operand(s). These instructions supply two distinct forms of Invalid Operation exception generation: • The FCMPS, FCMPD, FCMPZS, and FCMPZD instructions have the normal behavior of generating an Invalid Operation exception when either or both of their operands are signaling NaNs. If neither operand is a signaling NaN, but one or both are quiet NaNs, they generate an unordered result without an accompanying Invalid Operation exception. • The FCMPES, FCMPED, FCMPEZS, and FCMPEZD instructions generate an Invalid Operation exception when either or both of their operands are NaNs, regardless of whether they are signaling or quiet NaNs. It is not possible to get an unordered result from these instructions without an accompanying Invalid Operation exception. The VFP comparison instructions always treat their operands as scalars, regardless of the settings of the FPSCR LEN and STRIDE fields. The operations performed by all these instructions are always treated as floating-point operations, both for NaN handling and flush-to-zero mode. In particular, signaling NaN operands cause Invalid Operand exceptions, and in flush-to-zero mode, denormalized operands are treated as +0. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-7
- 616. VFP Instruction Set Overview Testing the IEEE 754 predicates The IEEE 754 standard specifies two ways in which a floating-point comparison can deliver its results: • As a condition code result, identifying one of the four relations: — equal — less than — greater than — unordered. • As a true-or-false result to one of twenty-six predicates, each of which specifies a particular test on the values. Six of these are the standard ==, !=, <, <=, > and >= comparisons, used in common languages like C, C++ and related languages. The VFP architecture uses the first approach. However, its condition code results have been carefully chosen to allow ARM conditional execution to test as many of the predicates as possible after a sequence of a VFP comparison instruction and an FMSTAT instruction. This includes all six of the commonly-used predicates. Table 3-4 shows how each predicate must be tested to get the correct results according to the IEEE 754 standard: Table 3-4 VFP predicate testing Common language condition IEEE predicate Instruction type ARM condition == = FCMP EQ != ?<> FCMP NE > > FCMPE GT >= >= FCMPE GE < < FCMPE MI or CC <= <= FCMPE LS ? FCMP VS <> FCMPE Two conditions <=> FCMPE VC ?> FCMP HI ?>= FCMP PL or CS ?< FCMP LT C3-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 617. VFP Instruction Set Overview Table 3-4 VFP predicate testing (Continued) Common language condition IEEE predicate Instruction type ARM condition ?<= FCMP LE ?= FCMP Two conditions NOT(>) FCMPE LE NOT(>=) FCMPE LT NOT(<) FCMPE PL or CS NOT(<=) FCMPE HI NOT(?) FCMP VC NOT(<>) FCMPE Two conditions NOT(<=>) FCMPE VS NOT(?>) FCMP LS NOT(?>=) FCMP MI or CC NOT(?<) FCMP GE NOT(?<=) FCMP GT NOT(?=) FCMP Two conditions In each case, the two main choices to be made are: • Whether to use an FCMP-type instruction (that is, the appropriate one of FCMPS, FCMPD, FCMPZS or FCMPZD) or an FCMPE-type instruction (the appropriate one of FCMPES, FCMPED, FCMPEZS or FCMPEZD). This choice causes the predicate to have the correct behavior with regard to Invalid Operation exceptions. • Which ARM condition is to be used. This is not always obvious. For example, a standard < comparison on floating-point numbers must use the ARM MI or LO/CC condition, not LT, despite the fact that floating-point comparisons are always signed. If this column contains two conditions, no single ARM condition can be used to test the predicate. Each of these predicates can be tested using a suitable combination of two ARM conditions, in several different ways. For example, the <> predicate can be tested by checking that NE and VC are both true, or that either of GT and MI is true. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-9
- 618. VFP Instruction Set Overview 3.1.4 Conversion instructions All of the VFP conversion instructions always treat their operands as scalars, regardless of the settings of the FPSCR LEN and STRIDE fields. Conversions between single and double precision The FCVTDS and FCVTSD instructions perform conversions between single-precision and double-precision values. FCVTDS converts single precision to double precision and is a coprocessor 10 instruction, while FCVTSD converts double precision to single precision and is a coprocessor 11 instruction. The FCVTDS and FCVTSD conversions are always treated as floating-point operations, both for NaN handling and flush-to-zero mode. In particular, signaling NaN operands cause Invalid Operand exceptions, and in flush-to-zero mode, denormalized operands are treated as +0. The only exception possible for FCVTDS is an Invalid Operation exception caused by a signaling NaN operand, as single-precision numbers can always be represented exactly in double precision. FCVTSD can additionally generate Overflow, Underflow and/or Inexact exceptions. Conversions from floating-point to integers The FTOSIS and FTOSID instructions convert floating-point values to signed integers, and the FTOUIS and FTOUID instructions convert floating-point values to unsigned integers, using the rounding mode specified by the FPSCR. Variants of these instructions called FTOSIZS, FTOSIZD, FTOUIZS, and FTOUIZD perform similar conversions, but using Round towards Zero mode. These are useful because C and related languages specify that floating-point ® integer conversions use this mode, whereas almost all other operations normally use Round to Nearest mode. Using these instructions avoids the need to change the FPSCR rounding mode every time a floating-point ® integer conversion is wanted. All of the floating-point ® integer conversion instructions place their integer result in a single-precision register. This result can then be used in any of the following ways: • store it to memory using FSTS or FSTMS • transfer it to an ARM register using FMRS • convert it to a floating-point number using any of FSITOS, FSITOD, FUITOS or FUITOD. The operations performed by all these instructions are always treated as floating-point operations, both for NaN handling and flush-to-zero mode. In particular, signaling NaN operands cause Invalid Operand exceptions, and in flush-to-zero mode, denormalized operands are treated as +0. Most exceptional conditions that can occur during these instructions are signaled as Invalid Operation exceptions. These cannot produce the normal quiet NaN value as their result, as the destination is an integer. Instead, the following list of values that generate Invalid Operation exceptions also specifies the integer default result in each case: C3-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 619. VFP Instruction Set Overview • If the operand is numeric, but converting it to an integer using the appropriate rounding mode would produce an integer that is greater than the maximum possible destination integer, the default result is the maximum possible destination integer. • If the operand is numeric, but converting it to an integer using the appropriate rounding mode would produce an integer that is less than the minimum possible destination integer, the default result is the minimum possible destination integer. • If the operand is +¥ (plus infinity), the default result is the maximum possible destination integer. • If the operand is -¥ (minus infinity), the default result is the minimum possible destination integer. • If the operand is a NaN (either signaling or quiet), the default result is 0. Apart from these Invalid Operation exceptions, the only exceptions that can be produced by the floating-point ® integer conversions are Inexact exceptions. Conversions from integers to floating-point The FSITOS and FSITOD instructions convert signed integers to floating-point values, and the FUITOS and FUITOD instructions convert unsigned integers to floating-point values. All of them take their integer operand from a single-precision register. This operand can have been placed in the register earlier in any of the following ways: • loading it from memory using FLDS or FLDMS • transferring it from an ARM register using FMSR • converting a floating-point number to an integer using any of FTOSIS, FTOSID, FTOSIZS, FTOSIZD, FTOUIS, FTOUID, FTOUIZS, or FTOUIZD. When an integer 0 is converted to floating-point, the result is +0. For the FSITOS and FUITOS instructions, some integer operands that exceed 224 in magnitude cannot be converted exactly. Conversions of these operands are rounded according to the rounding mode specified in the FPSCR, with an Inexact exception being generated. Otherwise, no exceptions are possible with the integer ® floating-point conversions. 3.1.5 Copy, negation and absolute value instructions The FCPYS and FCPYD instructions perform an exact copy of a floating-point value from one register to another. The FNEGS and FNEGD instructions do the same as FCPYS and FCPYD, except that they invert the sign bit during the copy. This negates numerical values and infinities, in the way described in the Appendix to the IEEE 754 standard. The FABSS and FABSD instructions do the same as FCPYS and FCPYD, except that they change the sign bit to 0 during the copy. This takes the absolute value of numerical values and infinities, in the way described in the Appendix to the IEEE 754 standard. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-11
- 620. VFP Instruction Set Overview All of these instructions can be made to operate on short vectors by setting the FPSCR LEN and STRIDE fields appropriately (see Chapter C5 VFP Addressing Modes). The IEEE 754 standard and its Appendix allow all these operations to be treated as non floating-point operations with regard to NaN handling. The VFP architecture requires this to be done. In particular, this implies the following: • The VFP architecture requires these instructions not to generate Invalid Operation when their operands are signaling NaNs. • The results of these instructions are generated by copying their operands (with appropriate sign bit adjustments), even when their operands are NaNs. This overrides the normal rules for generating the results of instructions with one or more NaN operands (described in NaNs on page C2-5). In addition, the VFP architecture requires these instructions to be treated as non floating-point operations with regard to flush-to-zero mode. In flush-to-zero mode, they copy denormalized operands in the same way as they do in normal mode, and do not treat the operands as +0. Note Calculating the value of -x using FNEGS or FNEGD does not produce exactly the same results as calculating either (+0 - x) or (-0 - x) using FSUBS or FSUBD. The differences are: • FSUBS or FSUBD produces an Invalid Operation exception if x is a signaling NaN, whereas FNEGS or FNEGD produces x with its sign bit inverted, without an exception. • FSUBS or FSUBD produces an exact copy of x if x is a quiet NaN, whereas FNEGS or FNEGD produces x with its sign bit inverted. • FNEGS or FNEGD applied to a zero always produces an oppositely signed zero. Calculating the value of (+0 - x) using FSUBS or FSUBD does this in RM rounding mode, but always produces +0 in RN, RP or RZ rounding mode. Calculating (-0 - x) always produces -0 in RM rounding mode, and produces an oppositely signed zero in RN, RP or RZ rounding mode. • In flush-to-zero mode, the calculation using FSUBS or FSUBD treats denormalized operands as +0, and therefore produce a zero result if x is denormalized. FNEGS or FNEGD ignore flush-to-zero mode and produce a result of x with its sign bit inverted. C3-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 621. VFP Instruction Set Overview 3.2 Load and Store instructions All VFP Load and Store instructions are LDC and STC instructions respectively for coprocessors 10 and 11, with the following format: 31 30 29 28 27 25 24 23 22 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 1 1 0 P U D W L Rn Fd cp_num offset P, U, W These bits specify an addressing mode for the LDC or STC instruction, as described in ARM Addressing Mode 5 - Load and Store Coprocessor on page A5-56. In addition, a VFP implementation uses them to determine which load/store operation is required, as shown in Table 3-5 on page C3-14. Fd and D These bits specify the destination floating-point register of a load instruction, or the source floating-point register of a store instruction. • For a single-precision instruction, Fd holds the top 4 bits of the register number and D holds the bottom bit. • For a double-precision instruction, Fd holds the register number and D must be 0. If D is 1 in a double-precision instruction, the instruction is UNDEFINED. For Load Multiple and Store Multiple instructions, the register specified by these fields is the lowest-numbered register to be transferred. Subsequent registers are transferred in order of register number, up to the number of registers determined by the offset field. If this would result in a register after S31 or D15 being transferred, the results are UNPREDICTABLE. L bit This bit determines whether the instruction is a load (L == 1) or a store (L == 0). Rn This specifies the ARM register used as the base register for the address calculation, as described in ARM Addressing Mode 5 - Load and Store Coprocessor on page A5-56. cp_num If cp_num is 0b1010 (coprocessor number 10), the instruction is a single-precision instruction. If cp_num is 0b1011 (coprocessor number 11), the instruction is either a double-precision instruction or one of the instructions used to handle values of unknown precision (see Storing and reloading values of unknown precision on page C2-15). offset These bits specify the word offset which is applied to the base register value to obtain the starting memory address for the transfer, as described in ARM Addressing Mode 5 - Load and Store Coprocessor on page A5-56. The least significant bit of this offset also helps to determine which load/store operation is required, as shown in Table 3-5 on page C3-14. In addition, for Load Multiple and Store Multiple instructions, the offset determines how many registers are to be transferred. Table 3-5 shows how the name and other details of the instruction are determined from the P, U, W, and L bits and the cp_num and offset fields: ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-13
- 622. VFP Instruction Set Overview Table 3-5 VFP load and store instructions cp num offset [0] Instruction L==0 Instruction L==1 Addr mode Registers transferred 0 0 x x x UNDEFINED - - - 0 1 0 0b1010 x FSTMS FLDMS Unindexed (offset) single-precision registers 0 1 0 0b1011 0 FSTMD FLDMD Unindexed (offset)/2 double-precision registers 0 1 0 0b1011 1 FSTMX FLDMX Unindexed (offset-1)/2 double-precision registers 0 1 1 0b1010 x FSTMS FLDMS Increment (offset) single-precision registers 0 1 1 0b1011 0 FSTMD FLDMD Increment (offset)/2 double-precision registers 0 1 1 0b1011 1 FSTMX FLDMX Increment (offset-1)/2 double-precision registers 1 0 0 0b1010 x FSTS FLDS Negative offset One single-precision register 1 0 0 0b1011 x FSTD FLDD Negative offset One double-precision register 1 0 1 0b1010 x FSTMS FLDMS Decrement (offset) single-precision registers 1 0 1 0b1011 0 FSTMD FLDMD Decrement (offset)/2 double-precision registers 1 0 1 0b1011 1 FSTMX FLDMX Decrement (offset-1)/2 double-precision registers 1 1 0 0b1010 x FSTS FLDS Positive offset One single-precision register 1 1 0 0b1011 x FSTD FLDD Positive offset One double-precision register 1 1 1 x x UNDEFINED - - - All load instructions perform a copy of the loaded value(s) from memory, and all store instructions perform a copy of the stored value(s) to memory. No exceptions are ever raised and the value(s) transferred are not changed, except possibly for a reversible conversion to the internal register format of an implementation.The copy is treated as a non floating-point operation for the purposes of NaN handling and flush-to-zero mode. In particular, the VFP architecture requires: • a load or store of a signaling NaN not to raise an Invalid Operation exception, nor to change the signaling NaN into a quiet NaN • a load or store of a denormalized number in flush-to-zero mode not to change it into +0. PUW C3-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 623. VFP Instruction Set Overview 3.2.1 Load/store one value The FLDS and FSTS instructions allow single-precision values and 32-bit integers to be loaded and stored, and the FLDD and FSTD instructions allow double-precision values to be loaded and stored. Each of these instructions transfers just one register of the type concerned. Of the addressing modes described in ARM Addressing Mode 5 - Load and Store Coprocessor on page A5-56, only the Immediate offset mode (see Load and Store Coprocessor - Immediate offset on page A5-58) is allowed for these instructions. This addressing mode allows the address to be specified by the base register value Rn, plus or minus an immediate offset which lies in the range 0 to 1020 and is a multiple of 4. No base register writeback is available. 3.2.2 Load/store multiple values The FLDMS and FSTMS instructions allow multiple single-precision values and/or integers to be loaded and stored, and the FLDMD and FSTMD instructions allow multiple double-precision values to be loaded and stored. Each of these instructions transfers a number of registers determined by the offset field of the instruction. The offset field is equal to the total number of words transferred for all of these instructions, that is, it is the number of registers for FLDMS and FSTMS, and twice the number of registers for FLDMD and FSTMD. In addition, the FSTMX instruction can be used to store double-precision registers when it is not known whether they contain single-precision or double-precision values, in a format that allows a matching FLDMX instruction to reload them correctly (see Storing and reloading values of unknown precision on page C2-15). In these instructions, the offset field is twice the number of double-precision registers to be transferred, plus one. This is the maximum number of words these instructions can transfer. Some implementations transfer one fewer word than this maximum, leaving a memory word unused. The FSTMX and FLDMX instructions are encoded as coprocessor 11 instructions, like FSTMD and FLDMD. They are distinguished from the latter by the fact that the offset field is odd in FSTMX and FLDMX instructions, and even in FSTMD and FLDMD instructions. The FSTMX and FLDMX instructions are the only coprocessor 11 instructions which are present in single-precision-only variants (non-D variants) of the VFP architecture. To aid software portability, it is recommended that programs written for such variants must use them in the same situations as a program written for a D variant would, even though the registers are known to hold single-precision values in non-D variants. The main situations affected are when storing and reloading callee-save registers, and in process swap code. Three addressing modes are available for these instructions: • Unindexed mode is the same as the LDC/STC Unindexed addressing mode (see Load and Store Coprocessor - Unindexed on page A5-64). The base register Rn determines the starting address for the transfer and is left unchanged. The offset field determines the number of registers to transfer, but does not affect the address calculations. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-15
- 624. VFP Instruction Set Overview • Increment mode is the same as the LDC/STC Immediate post-indexed addressing mode with a positive offset (see Load and Store Coprocessor - Immediate post-indexed on page A5-62). The base register Rn determines the starting address for the transfer. The offset field determines the number of registers to transfer, and is also multiplied by 4, added to the value of Rn and written back to Rn. After the transfer, Rn therefore points to the memory word immediately after the last word to be transferred (or the last word that could have been transferred in the case of FSTMX and FLDMX). This means that it is suitable for pushing values on to an Empty Ascending stack or for popping them from a Full Descending stack. • Decrement mode is the same as the LDC/STC Immediate pre-indexed addressing mode with a negative offset (see Load and Store Coprocessor - Immediate pre-indexed on page A5-60). The offset is multiplied by 4 and added to the value of the base register Rn to determine the starting address for the transfer, and this starting address is written back to Rn. The offset field also determines the number of registers to transfer. Before the transfer, Rn therefore points to the memory word immediately after the last word to be transferred (or the last word that could have been transferred in the case of FSTMX and FLDMX). This means that it is suitable for pushing values on to a Full Descending stack or for popping them from an Empty Ascending stack. Note There are no short vector forms of the load and store instructions as such, but the FLDMS, FLDMD, FSTMS and FSTMD instructions can be used to load and store many of the possible short vectors. However, note that short vectors wrap around within banks as described in Chapter C5 VFP Addressing Modes, while the load multiple and store multiple instructions simply advance linearly through S0-S31 or D0-D15. If a short vector that wraps around is to be loaded or stored, two or more instructions are needed. C3-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 625. VFP Instruction Set Overview 3.3 Register transfer instructions All VFP register transfer instructions are MCR and MRC instructions for coprocessors 10 and 11, with the following format: 31 30 29 28 27 24 23 21 20 19 16 15 12 11 8 7 6 5 4 3 0 cond 1 1 1 0 opcode L Fn Rd cp_num N 0 0 1 SBZ opcode This determines which register transfer operation is required, as shown in Table 3-6. L bit This bit determines the direction of the transfer: • from an ARM register to a VFP register (an MCR instruction, with L == 0) • from a VFP register to an ARM register (an MRC instruction, with L == 1). Fn and N bit These bits specify the VFP register involved in the transfer: • For a single-precision register, Fn holds the top 4 bits of the register number, and N holds the bottom bit. • For a double-precision register, Fn holds the register number, and N must be 0. • For a system register, Fn and N specify the register as shown in Table 3-7 on page C3-18. If N is 1 in an instruction that transfers a double-precision register, the instruction is UNDEFINED. Rd This specifies the ARM register involved in the transfer. If Rd is R15, the behavior is as specified for the generic ARM instruction: • For an MCR instruction (L == 0), the instruction is UNPREDICTABLE. • For an MRC instruction (L == 1), the top 4 bits of the value transferred are placed in the ARM condition code flags, and the remaining 28 bits are discarded. The FMSTAT instruction uses this behavior to transfer comparison results to the ARM. cp_num If cp_num is 0b1010 (coprocessor number 10), the instruction is a single-precision or system register transfer. If cp_num is 0b1011 (coprocessor number 11), the instruction is a double-precision register transfer. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-17
- 626. VFP Instruction Set Overview Table 3-6 shows the assignment of register transfer opcodes and other details of the instructions: Table 3-6 VFP register transfer instructions opcode cp_num L Instruction name Instruction functionality 000 0b1010 0 FMSR Sn = Rd 000 0b1010 1 FMRS Rd = Sn 000 0b1011 0 FMDLR Dn[31:0] = Rd 000 0b1011 1 FMRDL Rd = Dn[31:0] 001 0b1010 x - UNDEFINED 001 0b1011 0 FMDHR Dn[63:32] = Rd 001 0b1011 1 FMRDH Rd = Dn[63:32] 01x 0b101x x - UNDEFINED 10x 0b101x x - UNDEFINED 110 0b101x x - UNDEFINED 111 0b1010 0 FMXR SystemReg(Fn,N) = Rd 111 0b1010 1 FMRX Rd = SystemReg(Fn,N) 111 0b1011 x - UNDEFINED Table 3-7 shows how system registers are encoded in FMXR and FMRX instructions: Table 3-7 VFP system register encodings Fn N System register 0b0000 0 FPSID 0b0001 0 FPSCR 0b1000 0 FPEXC Encodings that are not shown in this table are: • Reserved for future expansion if the top bit of Fn is 0. FMXR and FMRX instructions using these encodings are UNDEFINED. • Reserved for additional IMPLEMENTATION DEFINED system registers if the top bit of Fn is 1. FMXR and FMRX instructions using these encodings are IMPLEMENTATION DEFINED. C3-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 627. VFP Instruction Set Overview 3.3.1 General-purpose register transfer instructions The FMRS instruction allows a single-precision value or a 32-bit integer in a single-precision register to be transferred to an ARM register, and the FMSR instruction allows a similar transfer from an ARM register to a single-precision register. The FMRDH and FMRDL instructions allow a double-precision value in a double-precision register to be transferred to a pair of ARM registers. The FMRDH instruction transfers the most significant word of the double-precision value, which contains the sign, exponent and 20 most significant fraction bits. The FMRDL instruction transfers the least significant word, which contains the remaining fraction bits. Similarly, the FMDHR and FMDLR instructions allow a double-precision value in a pair of ARM registers to be transferred to a double-precision register. FMDHR transfers the most significant word and FMDLR transfers the least significant word. Note The FMDHR and FMDLR instructions must be used in pairs, writing to the same double-precision register. These need not be executed consecutively, but while one of a pair has been executed and the other has not, the only valid uses of the destination double-precision register are: • as the destination register of the second instruction of the pair • storing it with FSTMX and reloading it with FLDMX, and using it for other purposes between the store and the reload. All of these instructions always treat their floating-point operand as a scalar, regardless of the settings of the FPSCR LEN and STRIDE fields. The register transfer performed is always a simple copy. No exceptions are ever raised and the value transferred is not changed, except possibly for a reversible conversion to or from the internal register format of an implementation. The copy is treated as a non floating-point operation for the purposes of NaN handling and flush-to-zero mode. In particular, the VFP architecture requires: • a load or store of a signaling NaN not to raise an Invalid Operation exception, nor to change the signaling NaN into a quiet NaN • a load or store of a denormalized number in flush-to-zero mode not to change it into +0. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C3-19
- 628. VFP Instruction Set Overview 3.3.2 System register transfer instructions The FMRX instruction transfers a system register value to an ARM register, and the FMXR instruction transfers an ARM register value to a system register. Their exact effects depend on the definition of the system register concerned. For more details, see System registers on page C2-19 for the architecturally defined system registers, or implementation documentation for IMPLEMENTATION DEFINED system registers. These instructions are serializing instructions. When one of them is executed, the register transfer is delayed until: • all floating-point operations in progress have determined whether they are going to generate an exception • any trapped exception handling or other software processing of floating-point operations in progress has completed • all effects of floating-point operations in progress on system register contents (such as setting cumulative exception flags for untrapped exceptions) have occurred • all floating-point operations in progress are no longer affected by changes to system register contents (for example, by rounding mode or flush-to-zero mode changes). C3-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 629. Chapter C4 VFP Instructions This chapter describes the syntax and usage of each VFP instruction, in the section: • Alphabetical list of VFP instructions on page C4-2. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-1
- 630. VFP Instructions 4.1 Alphabetical list of VFP instructions Each VFP instruction is described in detail on the following pages. 4.1.1 FABSD 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 0 0 0 0 Dd 1 0 1 1 1 1 0 0 Dm The FABSD (Floating-point Absolute Value, Double-precision) instruction writes the absolute value of a double-precision register to another double-precision register. It can also perform a vector version of this operation. Syntax FABSD{<cond>} <Dd>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dm> Specifies the source register. Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = abs(Dm[i]) C4-2 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 631. VFP Instructions Notes Absolute value function The function abs(x) means a copy of x with its sign bit forced to zero, as defined in the Appendix to the IEEE 754-1985 standard. Flush-to-zero mode The FZ bit of the FPSCR does not affect the operand or result of this instruction. Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FABSD performs just one absolute value operation, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FABSD might perform more than one absolute value operation. Addressing Mode 4 - Double-precision vectors (monadic) on page C5-19 describes how FABSD encodes the registers it uses and how vec_len, Dd[i], and Dm[i] are determined. Signaling NaNs To comply with the VFP architecture, FABSD must not generate an exception even if the value in its source register is a signaling NaN. This is a more stringent requirement than the one in the Appendix to the IEEE 754-1985 standard. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-3
- 632. VFP Instructions 4.1.2 FABSS 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 0 0 0 0 Fd 1 0 1 0 1 1 M 0 Fm The FABSS (Floating-point Absolute Value, Single-precision) instruction writes the absolute value of a single-precision register to another single-precision register. It can also perform a vector version of this operation. Syntax FABSS{<cond>} <Sd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom bit). <Sm> Specifies the source register. Its number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions None Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = abs(Sm[i]) C4-4 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 633. VFP Instructions Notes Absolute value function The function abs(x) means a copy of x with its sign bit forced to zero, as defined in the Appendix to the IEEE 754-1985 standard. Flush-to-zero mode The FZ bit of the FPSCR does not affect the operand or result of this instruction. Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FABSS performs just one absolute value operation, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FABSS might perform more than one absolute value operation. Addressing Mode 3 - Single-precision vectors (monadic) on page C5-14 describes how FABSS encodes the registers it uses and how vec_len, Sd[i], and Sm[i] are determined. Signaling NaNs To comply with the VFP architecture, FABSS must not generate an exception even if the value in its source register is a signaling NaN. This is a more stringent requirement than the one in the Appendix to the IEEE 754-1985 standard. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-5
- 634. VFP Instructions 4.1.3 FADDD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 1 1 Dn Dd 1 0 1 1 0 0 0 0 Dm The FADDD (Floating-point Addition, Double-precision) instruction adds together two double-precision registers and writes the result to a third double-precision register. It can also perform a vector version of this operation. Syntax FADDD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dn> Specifies the register that contains the first operand for the addition. <Dm> Specifies the register that contains the second operand for the addition. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = Dn[i] + Dm[i] C4-6 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 635. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FADDD performs just one addition, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FADDD might perform more than one addition. Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 describes how FADDD encodes the registers it uses and how vec_len, Dd[i], Dn[i], and Dm[i] are determined. Rounding The operation is a fully-rounded addition. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-7
- 636. VFP Instructions 4.1.4 FADDS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 D 1 1 Fn Fd 1 0 1 0 N 0 M 0 Fm The FADDS (Floating-point Addition, Single-precision) instruction adds together two single-precision registers and writes the result to a third single-precision register. It can also perform a vector version of this operation. Syntax FADDS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the addition. Its number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the addition. Its number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Overflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = Sn[i] + Sm[i] C4-8 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 637. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FADDS performs just one addition, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FADDS might perform more than one addition. Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 describes how FADDS encodes the registers it uses and how vec_len, Sd[i], Sn[i], and Sm[i] are determined. Rounding The operation is a fully-rounded addition. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-9
- 638. VFP Instructions 4.1.5 FCMPD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 0 1 0 0 Dd 1 0 1 1 0 1 0 0 Dm The FCMPD (Floating-point Compare, Double-precision) instruction compares two double-precision registers, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPD{<cond>} <Dd>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the register which contains the first operand for the comparison. <Dm> Specifies the register which contains the second operand for the comparison. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Dd is a signaling NaN) or (Dm is a signaling NaN) then raise Invalid Operation exception FPSCR N flag = if (Dd < Dm) then 1 else 0 FPSCR Z flag = if (Dd == Dm) then 1 else 0 FPSCR C flag = if (Dd < Dm) then 0 else 1 FPSCR V flag = if (Dd and Dm compare as unordered) then 1 else 0 C4-10 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 639. VFP Instructions Notes Vectors FCMPD always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If either or both of Dd and Dm are NaNs, they are unordered, and all three of (Dd < Dm), (Dd == Dm) and (Dd > Dm) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. FCMPD only raises an Invalid Operation exception if one or both operands are signaling NaNs, and is suitable for testing for ==, !=, unorderedness, and other predicates which do not raise an exception when the operands are unordered. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-11
- 640. VFP Instructions 4.1.6 FCMPED 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 0 1 0 0 Dd 1 0 1 1 1 1 0 0 Dm The FCMPED (Floating-point Compare (NaN Exceptions), Double-precision) instruction compares two double-precision registers, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPED{<cond>} <Dd>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the register which contains the first operand for the comparison. <Dm> Specifies the register which contains the second operand for the comparison. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Dd is a NaN) or (Dm is a NaN) then raise Invalid Operation exception FPSCR N flag = if (Dd < Dm) then 1 else 0 FPSCR Z flag = if (Dd == Dm) then 1 else 0 FPSCR C flag = if (Dd < Dm) then 0 else 1 FPSCR V flag = if (Dd and Dm compare as unordered) then 1 else 0 C4-12 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 641. VFP Instructions Notes Vectors FCMPED always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If either or both of Dd and Dm are NaNs, they are unordered, and all three of (Dd < Dm), (Dd == Dm) and (Dd > Dm) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. FCMPED raises an Invalid Operation exception if one or both operands are any type of NaN, and is suitable for testing for <, <=, >, >=, and other predicates which raise an exception when the operands are unordered. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-13
- 642. VFP Instructions 4.1.7 FCMPES 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 0 1 0 0 Fd 1 0 1 0 1 1 M 0 Fm The FCMPES (Floating-point Compare (NaN Exceptions), Single-precision) instruction compares two single-precision registers, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPES{<cond>} <Sd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the register which contains the first operand for the comparison. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sm> Specifies the register which contains the second operand for the comparison. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Sd is a NaN) or (Sm is a NaN) then raise Invalid Operation exception FPSCR N flag = if (Sd < Sm) then 1 else 0 FPSCR Z flag = if (Sd == Sm) then 1 else 0 FPSCR C flag = if (Sd < Sm) then 0 else 1 FPSCR V flag = if (Sd and Sm compare as unordered) then 1 else 0 C4-14 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 643. VFP Instructions Notes Vectors FCMPES always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If either or both of Dd and Dm are NaNs, they are unordered, and all three of (Dd < Dm), (Dd == Dm) and (Dd > Dm) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. FCMPES raises an Invalid Operation exception if the operand is any type of NaN, and is suitable for testing for <, <=, >, >=, and other predicates which raise an exception when the operands are unordered. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-15
- 644. VFP Instructions 4.1.8 FCMPEZD 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 cond 1 1 1 0 1 0 1 1 0 1 0 1 Dd 1 0 1 1 1 1 0 0 SBZ The FCMPEZD (Floating-point Compare (NaN Exceptions) with Zero, Double-precision) instruction compares a double-precision register with zero, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPEZD{<cond>} <Dd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the register which contains the first operand for the comparison. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Dd is a NaN) then raise Invalid Operation exception FPSCR N flag = if (Dd < 0.0) then 1 else 0 FPSCR Z flag = if (Dd == 0.0) then 1 else 0 FPSCR C flag = if (Dd < 0.0) then 0 else 1 FPSCR V flag = if (Dd is a NaN) then 1 else 0 C4-16 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 645. VFP Instructions Notes Vectors FCMPEZD always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If Dd is a NaN, it compares as unordered with zero, and all three of (Dd < 0.0), (Dd == 0.0) and (Dd > 0.0) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. FCMPEZD raises an Invalid Operation exception if the operand is any type of NaN, and is suitable for testing for <, <=, >, >=, and other predicates which raise an exception when the operands are unordered. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-17
- 646. VFP Instructions 4.1.9 FCMPEZS 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 cond 1 1 1 0 1 D 1 1 0 1 0 1 Fd 1 0 1 0 1 1 0 0 SBZ The FCMPEZS (Floating-point Compare (NaN Exceptions) with Zero, Single-precision) instruction compares a single-precision register with zero, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPEZS{<cond>} <Sd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the register which contains the first operand for the comparison. The register number is encoded as Fd (top 4 bits) and D (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Sd is a NaN) then raise Invalid Operation exception FPSCR N flag = if (Sd < 0.0) then 1 else 0 FPSCR Z flag = if (Sd == 0.0) then 1 else 0 FPSCR C flag = if (Sd < 0.0) then 0 else 1 FPSCR V flag = if (Sd is a NaN) then 1 else 0 C4-18 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 647. VFP Instructions Notes Vectors FCMPEZS always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If Dd is a NaN, it compares as unordered with zero, and all three of (Dd < 0.0), (Dd == 0.0) and (Dd > 0.0) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. FCMPEZS raises an Invalid Operation exception if the operand is any type of NaN, and is suitable for testing for <, <=, >, >=, and other predicates which raise an exception when the operands are unordered. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-19
- 648. VFP Instructions 4.1.10 FCMPS 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 0 1 0 0 Fd 1 0 1 0 0 1 M 0 Fm The FCMPS (Floating-point Compare, Single-precision) instruction compares two single-precision registers, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPS{<cond>} <Sd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the register which contains the first operand for the comparison. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sm> Specifies the register which contains the second operand for the comparison. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Sd is a signaling NaN) or (Sm is a signaling NaN) then raise Invalid Operation exception FPSCR N flag = if (Sd < Sm) then 1 else 0 FPSCR Z flag = if (Sd == Sm) then 1 else 0 FPSCR C flag = if (Sd < Sm) then 0 else 1 FPSCR V flag = if (Sd and Sm compare as unordered) then 1 else 0 C4-20 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 649. VFP Instructions Notes Vectors FCMPS always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If either or both of Dd and Dm are NaNs, they are unordered, and all three of (Dd < Dm), (Dd == Dm) and (Dd > Dm) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-21
- 650. VFP Instructions 4.1.11 FCMPZD 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 cond 1 1 1 0 1 0 1 1 0 1 0 1 Dd 1 0 1 1 0 1 0 0 SBZ The FCMPZD (Floating-point Compare with Zero, Double-precision) instruction compares a double-precision register with zero, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPZD{<cond>} <Dd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the register which contains the first operand for the comparison. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Dd is a signaling NaN) then raise Invalid Operation exception FPSCR N flag = if (Dd < 0.0) then 1 else 0 FPSCR Z flag = if (Dd == 0.0) then 1 else 0 FPSCR C flag = if (Dd < 0.0) then 0 else 1 FPSCR V flag = if (Dd is a NaN) then 1 else 0 C4-22 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 651. VFP Instructions Notes Vectors FCMPZD always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If Dd is a NaN, it compares as unordered with zero, and all three of (Dd < 0.0), (Dd == 0.0) and (Dd > 0.0) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. FCMPZD only raises an Invalid Operation exception if the operand is a signaling NaN, and is suitable for testing for ==, !=, unorderedness, and other predicates which do not raise an exception when the operands are unordered. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-23
- 652. VFP Instructions 4.1.12 FCMPZS 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 2 1 0 cond 1 1 1 0 1 D 1 1 0 1 0 1 Fd 1 0 1 0 0 1 0 0 SBZ The FCMPZS (Floating-point Compare with Zero, Single-precision) instruction compares a single-precision register with zero, writing the result to the FPSCR flags (which is normally transferred to the ARM flags by a subsequent FMSTAT instruction). Syntax FCMPZS{<cond>} <Sd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the register which contains the first operand for the comparison. The register number is encoded as Fd (top 4 bits) and D (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation Operation if ConditionPassed(cond) then if (Sd is a signaling NaN) then raise Invalid Operation exception FPSCR N flag = if (Sd < 0.0) then 1 else 0 FPSCR Z flag = if (Sd == 0.0) then 1 else 0 FPSCR C flag = if (Sd < 0.0) then 0 else 1 FPSCR V flag = if (Sd is a NaN) then 1 else 0 C4-24 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 653. VFP Instructions Notes Vectors FCMPZS always specifies a scalar operation, regardless of the LEN field of the FPSCR. NaNs The IEEE 754 standard specifies that the result of a comparison is precisely one of <, ==, > or unordered. If Dd is a NaN, it compares as unordered with zero, and all three of (Dd < 0.0), (Dd == 0.0) and (Dd > 0.0) are false. This results in the FPSCR flags being set as N=0, Z=0, C=1 and V=1. FCMPZS only raises an Invalid Operation exception if the operand is a signaling NaNs, and is suitable for testing for ==, !=, unorderedness, and other predicates which do not raise an exception when the operands are unordered. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-25
- 654. VFP Instructions 4.1.13 FCPYD 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 0 0 0 0 Dd 1 0 1 1 0 1 0 0 Dm The FCPYD (Floating-point Copy, Double-precision) instruction copies one double-precision register to another double-precision register. It can also perform a vector version of this operation. Syntax FCPYD{<cond>} <Dd>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dm> Specifies the source register. Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = Dm[i] C4-26 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 655. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FCPYD performs just one copy, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FCPYD might perform more than one copy. Addressing Mode 4 - Double-precision vectors (monadic) on page C5-19 describes how FCYPD encodes the registers it uses and how vec_len, Dd[i], and Dm[i] are determined. Flush-to-zero mode The FZ bit of the FPSCR does not affect the operand or result of this instruction. Signaling NaNs To comply with the VFP architecture, FCPYD must not generate an exception even if the value in its source register is a signaling NaN. This is a more stringent requirement than the one in the IEEE 754-1985 standard. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-27
- 656. VFP Instructions 4.1.14 FCPYS 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 0 0 0 0 Fd 1 0 1 0 0 1 M 0 Fm The FCPYS (Floating-point Copy, Single-precision) instruction copies one single-precision register to another single-precision register. It can also perform a vector version of this operation. Syntax FCPYS{<cond>} <Sd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sm> Specifies the source register. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions None Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = Sm[i] C4-28 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 657. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FCPYS performs just one copy, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FCPYD might perform more than one copy. Addressing Mode 3 - Single-precision vectors (monadic) on page C5-14 describes how FCYPS encodes the registers it uses and how vec_len, Sd[i], and Sm[i] are determined. Flush-to-zero mode The FZ bit of the FPSCR does not affect the operand or result of this instruction. Signaling NaNs To comply with the VFP architecture, FCPYS must not generate an exception even if the value in its source register is a signaling NaN. This is a more stringent requirement than the one in the IEEE 754-1985 standard. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-29
- 658. VFP Instructions 4.1.15 FCVTDS 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 0 1 1 1 Dd 1 0 1 0 1 1 M 0 Fm The FCVTDS (Floating-point Convert to Double-precision from Single-precision) instruction converts the value in a single-precision register to double precision and writes the result to a double-precision register. Syntax FCVTDS{<cond>} <Dd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Sm> Specifies the source register. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version D variants only Exceptions Floating-point exception: Invalid Operation Operation if ConditionPassed(cond) then Dd = ConvertSingleToDouble(Sm) Notes Vectors FCVTDS always specifies a scalar operation, regardless of the LEN field of the FPSCR. C4-30 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 659. VFP Instructions 4.1.16 FCVTSD 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 0 1 1 1 Fd 1 0 1 1 1 1 0 0 Dm The FCVTSD (Floating-point Convert to Single-precision from Double-precision) instruction converts the value in a double-precision register to single precision and writes the result to a single-precision register. Syntax FCVTSD{<cond>} <Sd>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Dm> Specifies the source register. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then Sd = ConvertDoubleToSingle(Dm) Notes Vectors FCVTSD always specifies a scalar operation, regardless of the LEN field of the FPSCR. Rounding FCVTSD performs a fully-rounded conversion. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-31
- 660. VFP Instructions 4.1.17 FDIVD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 0 0 Dn Dd 1 0 1 1 0 0 0 0 Dm The FDIVD (Floating-point Divide, Double-precision) instruction divides one double-precision register by another double-precision register and writes the result to a third double-precision register. It can also perform a vector version of this operation. Syntax FDIVD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dn> Specifies the register that contains the first operand for the division. <Dm> Specifies the register that contains the second operand for the division. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Division by Zero, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = Dn[i] / Dm[i] Usage Divisions take a large number of cycles on most implementations, and vector divisions take proportionately longer. This can have a major effect on performance. If a lot of divisions by the same number are wanted, the performance can usually be improved by using one division to calculate the number’s reciprocal, followed by a lot of multiplications by that reciprocal. This slightly reduces the accuracy of the calculations, since they incur two rounding errors rather than one, but this is often an acceptable tradeoff. Also see Interrupts on page C1-5 for a description of some implications for interrupt latency. C4-32 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 661. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FDIVD performs just one division, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FDIVD might perform more than one division. Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 describes how FDIVD encodes the registers it uses and how vec_len, Dd[i], Dn[i], and Dm[i] are determined. Rounding The operation is a fully-rounded division. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-33
- 662. VFP Instructions 4.1.18 FDIVS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 0 0 Fn Fd 1 0 1 0 N 0 M 0 Fm The FDIVS (Floating-point Divide, Single-precision) instruction divides one single-precision register by another single-precision register and writes the result to a third single-precision register. It can also perform a vector version of this operation. Syntax FDIVS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the division. The register number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the division. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Division by Zero, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = Sn[i] / Sm[i] C4-34 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 663. VFP Instructions Usage Divisions take a large number of cycles on most implementations, and vector divisions take proportionately longer. This can have a major effect on performance. If a lot of divisions by the same number are wanted, the performance can usually be improved by using one division to calculate the number’s reciprocal, followed by a lot of multiplications by that reciprocal. This slightly reduces the accuracy of the calculations, since they incur two rounding errors rather than one, but this is often an acceptable tradeoff. Also see Interrupts on page C1-5 for a description of some implications for interrupt latency. Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FDIVS performs just one division, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FDIVS might perform more than one division. The way FDIVS encodes the registers it uses and how vec_len, Sd[i], Sn[i], and Sm[i] are determined is described on Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2. Rounding The operation is a fully-rounded division. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-35
- 664. VFP Instructions 4.1.19 FLDD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 cond 1 1 0 1 U 0 0 1 Rn Dd 1 0 1 1 offset The FLDD (Floating-point Load, Double-precision) instruction loads a double-precision register from memory. Syntax FLDD{<cond>} <Dd>, [<Rn>{, #+/-(<offset>*4)}] where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Rn> Specifies the register holding the base address for the transfer. <offset> Specifies an offset to be multiplied by 4, then added to the base address (if U == 1) or subtracted from it (if U == 0) in order to form the actual address of the transfer. If this offset is omitted, it defaults to +0. Architecture version D variants only Exceptions Data Abort Operation if ConditionPassed(cond) then if (U == 1) address = Rn + offset * 4 else address = Rn - offset * 4 if (big-endian) Dd = [Memory[address,4] << 32) OR Memory[address+4,4] else Dd = [Memory[address+4,4] << 32) OR Memory[address,4] C4-36 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 665. VFP Instructions Notes Addressing mode This is a special case of Addressing Mode 5 - VFP load/store multiple on page C5-24. Conversions In the programmer’s model, FLDD does not perform any conversion on the value transferred. Implementations are free to convert the value transferred to an internal format, provided they can recover the correct double-precision value as necessary. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-37
- 666. VFP Instructions 4.1.20 FLDMD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 cond 1 1 0 P U 0 W 1 Rn Dd 1 0 1 1 offset The FLDMD (Floating-point Load Multiple, Double-precision) instruction loads a sequence of consecutive double-precision registers from memory. Syntax FLDM<addressing_mode>D{<cond>} <Rn>{!}, <registers> where: <addressing_mode> Specifies the addressing mode, which determines the values of start_address and end_address used by the instruction. See Addressing Mode 5 - VFP load/store multiple on page C5-24. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the base register used by <addressing_mode>. ! Sets the W bit of the instruction to 1, specifying that the base register <Rn> is to be updated by the instruction. If it is omitted, the W bit of the instruction is set to 0 and the base register <Rn> is left unchanged. Some combinations of <addressing_mode> and the presence or absence of ! are not allowed. For details, see Addressing Mode 5 - VFP load/store multiple on page C5-24. <registers> Specifies which registers are to be loaded, as a list of consecutively numbered double-precision registers, separated by commas and surrounded by brackets. It is encoded in the instruction by setting Dd to the number of the first register in the list, and offset to twice the number of registers in the list. At least one register must be specified in the list. For example, if <registers> is {D2,D3,D4}, the Dd field of the instruction is 2 and the offset field is 6. Architecture version D variants only Exceptions Data Abort C4-38 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 667. VFP Instructions Operation if ConditionPassed(cond) then address = start_address for i = 0 to (offset-2)/2 /* d is the number of register Dd; */ /* D(n) is the double-precision register numbered n */ if (big-endian) D(d+i) = (Memory[address,] << 32) OR Memory[address+4,4] else D(d+i) = (Memory[address+4,4] << 32) OR Memory[address,4] address = address + 8 assert end_address = address - 4 Notes Encoding If P=1 and W=0, the instruction is an FLDD instruction instead. Otherwise, if offset is odd, the instruction is an FLDMX instruction instead. Vectors The FLDMD instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and does not wrap around at bank boundaries in the way that vector operands to data-processing instructions do. Registers are loaded in simple increasing order of register number. Invalid register lists If Dd and offset do not specify a valid register list, the instruction is UNPREDICTABLE. This happens in two cases: • if offset == 0, that is, if an attempt is made to transfer no registers • if d + offset/2 > 16, that is, if an attempt is made to transfer another register after D15. Conversions In the programmer’s model, FLDMD does not perform any conversion on the value transferred. Implementations are free to convert the value transferred to an internal format, provided they can recover the correct double-precision value as necessary. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-39
- 668. VFP Instructions 4.1.21 FLDMS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 cond 1 1 0 P U D W 1 Rn Fd 1 0 1 0 offset The FLDMS (Floating-point Load Multiple, Single-precision) instruction loads a sequence of consecutive single-precision registers from memory. Syntax FLDM<addressing_mode>S{<cond>} <Rn>{!}, <registers> where: <addressing_mode> Specifies the addressing mode, which determines the values of start_address and end_address used by the instruction. See Addressing Mode 5 - VFP load/store multiple on page C5-24 for details. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the base register used by <addressing_mode>. ! Sets the W bit of the instruction to 1, specifying that the base register <Rn> is to be updated by the instruction. If it is omitted, the W bit of the instruction is set to 0 and the base register <Rn> is left unchanged. Some combinations of <addressing_mode> and the presence or absence of ! are not allowed. For details, see Addressing Mode 5 - VFP load/store multiple on page C5-24. <registers> Specifies which registers are to be loaded, as a list of consecutively numbered single-precision registers, separated by commas and surrounded by brackets. If d is the number of the first register in the list, the list is encoded in the instruction by setting Fd and D to the top 4 bits and the bottom bit respectively of d, and offset to the number of registers in the list. At least one register must be specified in the list. For example, if <registers> is {S5,S6,S7}, the Fd field of the instruction is 0b0010, the D bit is 1 and the offset field is 3. Architecture version All Exceptions Data Abort C4-40 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 669. VFP Instructions Operation if ConditionPassed(cond) then address = start_address for i = 0 to offset-1 /* d is as defined for <registers> above; */ /* S(n) is the single-precision register numbered n */ S(d+i) = Memory[address,4] address = address + 4 assert end_address = address - 4 Notes Encoding If P=1 and W=0, the instruction is an FLDS instruction instead. Vectors The FLDMS instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and does not wrap around at bank boundaries in the way that vector operands to data-processing instructions do. Registers are loaded in simple increasing order of register number. Invalid register lists If Fd, D and offset do not specify a valid register list, the instruction is UNPREDICTABLE. This happens in two cases: • if offset == 0, that is, if an attempt is made to transfer no registers • if d + offset > 32, that is, if an attempt is made to transfer another register after S31. Conversions In the programmer’s model, FLDMS does not perform any conversion on the values transferred. The memory words can hold either integers or single-precision floating-point numbers. Most VFP arithmetic instructions treat the loaded values as single-precision floating-point numbers. If they are integers, they need to be converted using the integer-to-floating-point conversion instructions before such arithmetic instructions can yield sensible results.Implementations are free to convert the values transferred to an internal format, provided they can recover either the correct single-precision value or the correct integer value for each one (depending on how the registers are subsequently used). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-41
- 670. VFP Instructions 4.1.22 FLDMX 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 cond 1 1 0 P U 0 W 1 Rn Dd 1 0 1 1 offset The FLDMX (Floating-point Load Multiple, Unknown precision) instruction loads a sequence of consecutive double-precision registers from memory, using the same IMPLEMENTATION DEFINED format as the FSTMX instruction. This allows the registers to be reloaded correctly with integers, single-precision values or double-precision values. Syntax FLDM<addressing_mode>X{<cond>} <Rn>{!}, <registers> where: <addressing_mode> Specifies the addressing mode, which determines the values of start_address and end_address used by the instruction. See Addressing Mode 5 - VFP load/store multiple on page C5-24 for details. <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rn> Specifies the base register used by <addressing_mode>. ! Sets the W bit of the instruction to 1, specifying that the base register <Rn> is to be updated by the instruction. If it is omitted, the W bit of the instruction is set to 0 and the base register <Rn> is left unchanged. Some combinations of <addressing_mode> and the presence or absence of ! are not allowed. For details, see Addressing Mode 5 - VFP load/store multiple on page C5-24. <registers> Specifies which registers are to be loaded, as a list of consecutively numbered double-precision registers, separated by commas and surrounded by brackets. It is encoded in the instruction by setting Dd to the number of the first register in the list, and offset to twice the number of registers in the list, plus 1. At least one register must be specified in the list. For example, if <registers> is {D2,D3,D4}, the Dd field of the instruction is 2 and the offset field is 7. Architecture version All C4-42 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 671. VFP Instructions Exceptions Data Abort Operation if ConditionPassed(cond) then /* d is the number of register Dd; */ /* D(n) is the double-precision register numbered n */ Load registers D(d) to D(d+(offset-3)/2) from memory words Memory[start_address,4] through to Memory[end_address-4,4] Usage FLDMX is used to reload VFP register values from memory when FSTMX was previously used to store them. Typical cases in which it is used are: • in procedure exit sequences when a callee-save procedure-calling standard is being used • in process swap code. Notes Encoding If P=1 and W=0, the instruction is an FLDD instruction instead. Otherwise, if offset is even, the instruction is an FLDMD instruction instead. Vectors The FLDMX instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and does not wrap around at bank boundaries in the way that vector operands to data-processing instructions do. Registers are loaded in simple increasing order of register number. Invalid register lists If Dd and offset do not specify a valid register list, the instruction is UNPREDICTABLE. This happens in two cases: • if offset == 0, that is, if an attempt is made to transfer no registers • if d + (offset-1)/2 > 16, that is, if an attempt is made to transfer another register after D15. Memory format The memory format used is IMPLEMENTATION DEFINED. The only requirements on it are: • FLDMX must reload the register values stored by a matching FSTMX instruction correctly, regardless of whether they originally contained integers, single-precision values or double-precision values. For an FLDMX instruction and an FSTMX instruction to count as matching, they must specify identical register lists and must generate the same start_address and end_address when executed. • It must not use more than the allowed number of memory words, that is, 2N+1 memory words for storing N double-precision registers. Non implementation-specific code must not do anything with FLDMX other than reload the results of a matching FSTMX instruction. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-43
- 672. VFP Instructions 4.1.23 FLDS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 0 cond 1 1 0 1 U D 0 1 Rn Fd 1 0 1 0 offset The FLDS (Floating-point Load, Single-precision) instruction loads a single-precision register from memory. Syntax FLDS{<cond>} <Sd>, [<Rn>{, #+/-(<offset>*4)}] where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom bit). <Rn> Specifies the register holding the base address for the transfer. <offset> Specifies an offset to be multiplied by 4, then added to the base address (if U == 1) or subtracted from it (if U == 0) in order to form the actual address of the transfer. If this offset is omitted, it defaults to +0. Architecture version All Exceptions Data Abort Operation if ConditionPassed(cond) then if (U == 1) address = Rn + offset * 4 else address = Rn - offset * 4 Sd = Memory[address,4] C4-44 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 673. VFP Instructions Notes Addressing mode This is a special case of Addressing Mode 5 - VFP load/store multiple on page C5-24. Conversions In the programmer’s model, FLDS does not perform any conversion on the value transferred. The memory word can hold either an integer or a single-precision floating-point number. Most VFP arithmetic instructions treat the Sd value as a single-precision floating-point number. If it is an integer, one of the integer-to-floating-point conversion instructions needs to be executed before such arithmetic instructions can yield sensible results. Implementations are free to convert the value transferred to an internal format, provided they can recover either the correct single-precision value or the correct integer value (depending on how Sd is subsequently used). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-45
- 674. VFP Instructions 4.1.24 FMACD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 0 Dn Dd 1 0 1 1 0 0 0 0 Dm The FMACD (Floating-point Multiply and Accumulate, Double-precision) instruction multiplies together two double-precision registers, adds a third double-precision register to the product and writes the result to the third register. It can also perform a vector version of this operation. Syntax FMACD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register, which is also used as the second operand for the addition. <Dn> Specifies the register that contains the first operand for the multiplication. <Dm> Specifies the register that contains the second operand for the multiplication. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = Dn[i] * Dm[i] + Dd[i] C4-46 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 675. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMACD performs just one multiply-add operation, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FMACD might perform more than one multiply-add operation. Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 describes how FMACD encodes the registers it uses and how vec_len, Dd[i], Dn[i] and Dm[i] are determined. Rounding The operation is a fully-rounded multiplication followed by a fully-rounded addition. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-47
- 676. VFP Instructions 4.1.25 FMACS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 D 0 0 Fn Fd 1 0 1 0 N 0 M 0 Fm The FMACS (Floating-point Multiply and Accumulate, Single-precision) instruction multiplies together two single-precision registers, adds a third single-precision register to the product and writes the result to the third register. It can also perform a vector version of this operation. Syntax FMACS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register, which is also used as the second operand for the addition. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the multiplication. The register number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the multiplication. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = Sn[i] * Sm[i] + Sd[i] C4-48 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 677. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMACS performs just one multiply-add operation, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FMACS might perform more than one multiply-add operation. Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 shows how FMACS encodes registers and determines vec_len, Sd[i], Sn[i] and Sm[i]. Rounding The operation is a fully-rounded multiplication followed by a fully-rounded addition. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-49
- 678. VFP Instructions 4.1.26 FMDHR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 1 0 Dn Rd 1 0 1 1 0 SBZ 1 SBZ The FMDHR (Floating-point Move to Double-precision High from Register) instruction transfers the contents of the ARM register Rd to the upper half of the double-precision register Dn. It is used in conjunction with FMDLR to transfer double-precision values between ARM registers and floating-point registers. Syntax FMDHR{<cond>} <Dn>, <Rd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dn> Specifies the destination register. <Rd> Specifies the source ARM register. Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then Dn[63:32] = Rd C4-50 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 679. VFP Instructions Notes Use with FMDLR FMDHR must be used in conjunction with an FMDLR instruction specifying the same destination register. Between these two instructions, the value of Dn is UNPREDICTABLE for all purposes except: • the execution of the second instruction must result in Dn containing the double-precision number transferred by the two instructions • if Dn is saved to memory by an FSTMX instruction and subsequently reloaded by a correctly matching FLDMX instruction, the final value of Dn must be functionally equivalent to its original value. Conversions In the programmer's model, the combination of FMDHR and FMDLR does not perform any conversion. Implementations are free to convert the value transferred to an internal format, provided they can recover the correct double-precision value when both the FMDHR and the FMDLR instructions have been executed. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-51
- 680. VFP Instructions 4.1.27 FMDLR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 0 Dn Rd 1 0 1 1 0 SBZ 1 SBZ The FMDLR (Floating-point Move to Double-precision Low from Register) instruction transfers the contents of the ARM register Rd to the lower half of the double-precision register Dn. Used with FMDHR, it transfers double-precision values between ARM registers and floating-point registers. Syntax FMDLR{<cond>} <Dn>, <Rd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dn> Specifies the destination register. <Rd> Specifies the source ARM register. Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then Dn[31:0] = Rd C4-52 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 681. VFP Instructions Notes Use with FMDHR FMDLR must be used in conjunction with an FMDHR instruction specifying the same destination register. Between these two instructions, the value of Dn is UNPREDICTABLE for all purposes except: • the execution of the second instruction must result in Dn containing the double-precision number transferred by the two instructions • if Dn is saved to memory by an FSTMX instruction and subsequently reloaded by a correctly matching FLDMX instruction, the final value of Dn must be functionally equivalent to its original value. Conversions In the programmer’s model, the combination of FMDHR and FMDLR does not perform any conversion. Implementations are free to convert the value transferred to an internal format, provided they can recover the correct double-precision value when both the FMDHR and the FMDLR instructions have been executed. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-53
- 682. VFP Instructions 4.1.28 FMRDH 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 1 1 Dn Rd 1 0 1 1 0 SBZ 1 SBZ The FMRDH (Floating-point Move to Register from Double-precision High) instruction transfers the upper half of the contents of the double-precision register Dn to the ARM register Rd. It is used in conjunction with FMRDL to transfer double-precision values between ARM registers and floating-point registers. Syntax FMRDH{<cond>} <Rd>, <Dn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination ARM register. <Dn> Specifies the source register. Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then Rd = Dn[63:32] Notes Conversions If an implementation uses an internal format for double-precision values, it must convert that format back to the external double-precision format. Otherwise, no conversion is required. C4-54 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 683. VFP Instructions 4.1.29 FMRDL 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 1 Dn Rd 1 0 1 1 0 SBZ 1 SBZ The FMRDL (Floating-point Move to Register from Double-precision Low) instruction transfers the lower half of the contents of the double-precision register Dn to the ARM register Rd. It is used in conjunction with FMRDH to transfer double-precision values between ARM registers and floating-point registers. Syntax FMRDL{<cond>} <Rd>, <Dn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination ARM register. <Dn> Specifies the source register. Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then Rd = Dn[31:0] Notes Conversions If an implementation uses an internal format for double-precision values, it must convert that format back to the external double-precision format. Otherwise, no conversion is required. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-55
- 684. VFP Instructions 4.1.30 FMRS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 1 Fn Rd 1 0 1 0 N SBZ 1 SBZ The FMRS (Floating-point Move to Register from Single-precision) instruction transfers the contents of the single-precision register Fn to the ARM register Rd. The value transferred can be an integer (typically generated by a FTOSID, FTOSIS, FTOUID or FTOUIS instruction) or a single-precision floating-point number (typically generated by other arithmetic instructions). Syntax FMRS{<cond>} <Rd>, <Sn> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination ARM register. <Sn> Specifies the source register. Its number is encoded as Fn (top 4 bits) and N (bottom bit). Architecture version All Exceptions None Operation if ConditionPassed(cond) then Rd = Sn C4-56 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 685. VFP Instructions Notes Conversions In the programmer’s model, FMRS does not perform any conversion on the value transferred. Both the source register Sn and the destination register Rd can contain either an integer or a single-precision floating-point number. Arithmetic instructions on the ARM treat the Rd value as an integer, whereas most arithmetic instructions on the VFP coprocessor treat the Sn value as a single-precision floating-point number. One of the floating-point-to-integer conversion instructions needs to be executed before the FMRS instruction if they are to agree on the number being represented. Implementations are free to hold the value in Sn in an internal format, provided that FMRS converts it to external format and this conversion recovers the correct data, regardless of whether the register contains a single-precision floating-point number or an integer. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-57
- 686. VFP Instructions 4.1.31 FMRX 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 1 1 1 reg Rd 1 0 1 0 0 SBZ 1 SBZ The FMRX (Floating-point Move to Register from System Register) instruction transfers the contents of one of the VFP system registers to the ARM register Rd. Syntax FMRX{<cond>} <Rd>, <reg> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Rd> Specifies the destination ARM register. <reg> Specifies the source system register as follows: <reg> = 0b0000: FPSID <reg> = 0b0001: FPSCR <reg> = 0b1000: FPEXC Other values of <reg> can be used by individual VFP implementations for IMPLEMENTATION DEFINED purposes. Typically, they are used to transfer data from a hardware coprocessor to the support code for that coprocessor. All other code must treat such values of <reg> as UNPREDICTABLE. Architecture version All Exceptions None Operation if ConditionPassed(cond) then Rd = reg C4-58 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 687. VFP Instructions Notes Serialization FMRX is a serializing instruction. See System register transfer instructions on page C3-20 for details of what this means. Exception processing After serialization, if the VFP system contains a hardware coprocessor, that coprocessor might have a pending exception to process. Whether the FMRX instruction triggers the processing of such an instruction depends on which system register is being transferred, as described in the following notes. If exception processing is triggered, this causes the FMRX instruction to take the ARM’s undefined instruction trap. Reading FPSID An FMRX instruction with source FPSID can be executed in any ARM processor mode. After serialization, it writes the value of the FPSID to Rd, and does not trigger exception processing. Reading FPSCR An FMRX instruction with source FPSCR can be executed in any ARM processor mode. After serialization, exception processing is triggered if necessary. Otherwise, the value of the FPSCR is written to Rd. Reading FPEXC An FMRX instruction with source FPEXC can only be executed in privileged ARM processor modes. An attempt to execute it in User mode causes the ARM’s undefined instruction trap to be taken. After serialization, it writes the value of FPEXC to Rd, and does not trigger exception processing. Because all but bits[31:30] of FPEXC is IMPLEMENTATION DEFINED, non implementation-specific code must only rely on bits[31:0] of the value written to Rd. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-59
- 688. VFP Instructions 4.1.32 FMSCD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 1 Dn Dd 1 0 1 1 0 0 0 0 Dm The FMSCD (Floating-point Multiply and Subtract, Double-precision) instruction multiplies together two double-precision registers, subtracts a third double-precision register from the product and writes the result to the third register. It can also perform a vector version of this operation. Syntax FMSCD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register, which is also the second operand for the subtraction. <Dn> Specifies the register that contains the first operand for the multiplication. <Dm> Specifies the register that contains the second operand for the multiplication. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = Dn[i] * Dm[i] - Dd[i] C4-60 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 689. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMSCD performs just one multiply-subtract operation, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FMSCD might perform more than one multiply-subtract operation. Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 describes how FMSCD encodes the registers it uses and how vec_len, Dd[i], Dn[i], and Dm[i] are determined. Rounding The operation is a fully-rounded multiplication followed by a fully-rounded subtraction. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-61
- 690. VFP Instructions 4.1.33 FMSCS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 D 0 1 Fn Fd 1 0 1 0 N 0 M 0 Fm The FMSCS (Floating-point Multiply and Subtract, Single-precision) instruction multiplies together two single-precision registers, subtracts a third single-precision register from the product and writes the result to the third register. It can also perform a vector version of this operation. Syntax FMSCS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register, which is also used as the second operand for the subtraction. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the multiplication. The register number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the multiplication. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = Sn[i] * Sm[i] - Sd[i] C4-62 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 691. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMSCS performs just one multiply-subtract operation, where vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FMSCS might perform more than one multiply-subtract operation. Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 shows how FMSCS encodes registers and determines vec_len, Sd[i], Sn[i], and Sm[i]. Rounding The operation is a fully-rounded multiplication followed by a fully-rounded subtraction. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-63
- 692. VFP Instructions 4.1.34 FMSR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 0 Fn Rd 1 0 1 0 N SBZ 1 SBZ The FMSR (Floating-point Move to Single-precision from Register) instruction transfers the contents of the ARM register Rd to the single-precision register Fn. The value transferred can subsequently be treated either as an integer (if used as the source register of a FSITOD, FSITOS, FUITOD or FUITOS instruction) or as a single-precision floating-point number (if used by other arithmetic instructions). Syntax FMSR{<cond>} <Sn>, <Rd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sn> Is the destination register. Its number is encoded as Fn (top 4 bits) and N (bottom bit). <Rd> Is the source ARM register. Architecture version All Exceptions None Operation if ConditionPassed(cond) then Sn = Rd C4-64 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 693. VFP Instructions Notes Conversions In the programmer’s model, FMSR does not perform any conversion on the value transferred. Both the source register Rd and the destination register Sn can contain either an integer or a single-precision floating-point number. Arithmetic instructions on the ARM treat the Rd value as an integer, whereas most VFP arithmetic instructions treat the Fn value as a single-precision floating-point number. If an integer is transferred, one of the integer-to-floating-point conversion instructions need to be executed after the FMSR instruction if subsequent VFP instructions are to yield sensible results. Implementations are free to convert the value transferred to an internal format, provided they can recover either the correct single-precision value or the correct integer value (depending on how Sn is subsequently used). ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-65
- 694. VFP Instructions 4.1.35 FMSTAT 31 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 5 4 3 0 cond 1 1 1 0 1 1 1 1 0 0 0 1 1 1 1 1 1 0 1 0 SBZ 1 SBZ The FMSTAT (Floating-point Move Status) instruction transfers the N, Z, C, and V flags in the FPSCR to the corresponding flags in the ARM’s CPSR, and is normally used after one of the VFP comparison instructions has set the FPSCR flags. Syntax FMSTAT{<cond>} where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. Architecture version All Exceptions None Operation if ConditionPassed(cond) then CPSR N Flag = FPSCR N Flag CPSR Z Flag = FPSCR Z Flag CPSR C Flag = FPSCR C Flag CPSR V Flag = FPSCR V Flag Notes Encoding The instruction FMSTAT{<cond>} is encoded as: FMRX{<cond>} r15, FPSCR See also FMRX on page C4-58. C4-66 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 695. VFP Instructions 4.1.36 FMULD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 1 0 Dn Dd 1 0 1 1 0 0 0 0 Dm The FMULD (Floating-point Multiply, Double-precision) instruction multiplies together two double-precision registers and writes the result to a third double-precision register. It can also perform a vector version of this operation. Syntax FMULD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dn> Specifies the register that contains the first operand for the multiplication. <Dm> Specifies the register that contains the second operand for the multiplication. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = Dn[i] * Dm[i] Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMULD performs one multiplication, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FMULD might perform more than one multiplication. Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 describes how FMULD encodes the registers it uses and how vec_len, Dd[i], Dn[i], and Dm[i] are determined. Rounding This is a fully-rounded multiplication. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-67
- 696. VFP Instructions 4.1.37 FMULS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 D 1 0 Fn Fd 1 0 1 0 N 0 M 0 Fm The FMULS (Floating-point Multiply, Single-precision) instruction multiplies together two single-precision registers and writes the result to a third single-precision register. It can also perform a vector version of this operation. Syntax FMULS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the multiplication. Its number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the multiplication. Its number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = Sn[i] * Sm[i] C4-68 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 697. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FMULS performs just one multiplication, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FMULS might perform more than one multiplication. Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 shows how FMULS encodes the registers it uses and determines vec_len, Sd[i], Sn[i], and Sm[i]. Rounding The operation is a fully-rounded multiplication. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-69
- 698. VFP Instructions 4.1.38 FMXR 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 1 1 0 reg Rd 1 0 1 0 0 SBZ 1 SBZ The FMXR (Floating-point Move to System Register from Register) instruction transfers the contents of the ARM register Rd to one of the VFP system registers. Syntax FMXR{<cond>} <reg>, <Rd> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <reg> Specifies the destination system register as follows: <reg> = 0b0000: FPSID <reg> = 0b0001: FPSCR <reg> = 0b1000: FPEXC Other values of <reg> can be used by individual VFP implementations for IMPLEMENTATION DEFINED purposes. Typically, they are used to transfer data to a hardware coprocessor from the support code for that coprocessor. All other code must treat such values of <reg> as UNPREDICTABLE and not to be relied upon. <Rd> Specifies the source ARM register. Architecture version All Exceptions None Operation if ConditionPassed(cond) then reg = Rd C4-70 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 699. VFP Instructions Notes Serialization FMXR is a serializing instruction. See System register transfer instructions on page C3-20 for details of what this means. Exception processing After serialization, if the VFP system contains a hardware coprocessor, that coprocessor might have a pending exception to process. Whether the FMXR instruction triggers the processing of such an instruction depends on which system register is being transferred, as described in the following notes. If exception processing is triggered, this causes the FMXR instruction to take the ARM’s undefined instruction trap. Writing FPSID An FMXR instruction with destination FPSID can be executed in any ARM processor mode. It is a serializing no-op, because FPSID is a read-only register, and does not trigger exception processing. Writing FPSCR An FMXR instruction with destination FPSCR can be executed in any ARM processor mode. After serialization, exception processing is triggered if necessary. Otherwise, the value of Rd is written to the FPSCR. Writing FPEXC An FMXR instruction with destination FPEXC can only be executed in privileged ARM processor modes. An attempt to execute it in User mode causes the ARM’s undefined instruction trap to be taken. After serialization, it writes the value of Rd to FPEXC, and does not trigger exception processing. Because all but bits[31:30] of FPEXC is IMPLEMENTATION DEFINED, non implementation-specific code must only use such an instruction as part of a read/modify bits[31:0]/write sequence. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-71
- 700. VFP Instructions 4.1.39 FNEGD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 0 0 0 1 Dd 1 0 1 1 0 1 0 0 Dm The FNEGD (Floating-point Negate, Double-precision) instruction negates the value of a double-precision register and writes the result to another double-precision register. It can also perform a vector version of this operation. Syntax FNEGD{<cond>} <Dd>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dm> Specifies the source register. Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = -(Dm[i]) C4-72 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 701. VFP Instructions Notes Negation The function -(x) means a copy of x with its sign bit reversed, as defined in the Appendix to the IEEE 754-1985 standard. Flush-to-zero mode The FZ bit of the FPSCR does not affect the operand or result of this instruction. Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNEGD performs just one negation operation, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FNEGD might perform more than one negation operation. Addressing Mode 4 - Double-precision vectors (monadic) on page C5-19 shows how FNEGD encodes its registers and determines the values of vec_len, Dd[i], and Dm[i]. Signaling NaNs To comply with the VFP architecture, FNEGD must not generate an exception even if the value in its source register is a signaling NaN. This is a more stringent requirement than the one in the Appendix to the IEEE 754-1985 standard. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-73
- 702. VFP Instructions 4.1.40 FNEGS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 0 0 0 1 Fd 1 0 1 0 0 1 M 0 Fm The FNEGS (Floating-point Negate, Single-precision) instruction negates the value of a single-precision register and writes the result to another single-precision register. It can also perform a vector version of this operation. Syntax FNEGS{<cond>} <Sd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. Its number is encoded as Fd (top 4 bits) and D (bottom bit). <Sm> Specifies the source register. Its number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions None Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = -(Sm[i]) C4-74 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 703. VFP Instructions Notes Negation The function -(x) means a copy of x with its sign bit reversed, as defined in the Appendix to the IEEE 754-1985 standard. Flush-to-zero mode The FZ bit of the FPSCR does not affect the operand or result of this instruction. Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNEGS performs just one negation operation, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FNEGS might perform more than one negation operation. Addressing Mode 3 - Single-precision vectors (monadic) on page C5-14 shows how FNEDS encodes its registers and determines vec_len, Sd[i], and Sm[i]. Signaling NaNs To comply with the VFP architecture, FNEGS must not generate an exception even if the value in its source register is a signaling NaN. This is a more stringent requirement than the one in the Appendix to the IEEE 754-1985 standard. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-75
- 704. VFP Instructions 4.1.41 FNMACD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 0 Dn Dd 1 0 1 1 0 1 0 0 Dm The FNMACD (Floating-point Negated Multiply and Accumulate, Double-precision) instruction multiplies together two double-precision registers, adds a third double-precision register to the negation of the product and writes the result to the third register. It can also perform a vector version of this operation. Syntax FNMACD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register, which is also used as the second operand for the addition. <Dn> Specifies the register that contains the first operand for the multiplication. <Dm> Specifies the register that contains the second operand for the multiplication. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = -(Dn[i] * Dm[i]) + Dd[i] C4-76 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 705. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMACD performs just one multiply-negate-add operation, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FNMACD might perform more than one multiply-negate-add operation. Addressing Mode 4 - Double-precision vectors (monadic) on page C5-19 shows how FNMACD encodes its registers and determines vec_len, Dd[i], Dn[i], and Dm[i]. Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the FPSCR, followed by reversal of the sign bit and a fully-rounded addition, using the same rounding mode. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-77
- 706. VFP Instructions 4.1.42 FNMACS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 D 0 0 Fn Fd 1 0 1 0 N 1 M 0 Fm The FNMACS (Floating-point Negated Multiply and Accumulate, Single-precision) instruction multiplies together two single-precision registers, adds a third single-precision register to the negation of the product and writes the result to the third register. It can also perform a vector version of this. Syntax FNMACS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register, which is also used as the second operand for the addition. Its number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the multiplication. Its number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the multiplication. Its number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = -(Sn[i] * Sm[i]) + Sd[i] C4-78 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 707. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMACS performs just one multiply-negate-add operation, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FNMACS might perform more than one multiply-negate-add operation. Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 describes how FNMACS encodes the registers it uses and how vec_len, Sd[i], Sn[i], and Sm[i] are determined. Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the FPSCR, followed by reversal of the sign bit and a fully-rounded addition, using the same rounding mode. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-79
- 708. VFP Instructions 4.1.43 FNMSCD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 0 1 Dn Dd 1 0 1 1 0 1 0 0 Dm The FNMSCD (Floating-point Negated Multiply and Subtract, Double-precision) instruction multiplies together two double-precision registers, subtracts a third double-precision register from the negation of the product and writes the result to the third register. It can also perform a vector version of this operation. Syntax FNMSCD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register, and is also used as the second operand for the subtraction. <Dn> Specifies the register that contains the first operand for the multiplication. <Dm> Specifies the register that contains the second operand for the multiplication. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = -(Dn[i] * Dm[i]) - Dd[i] C4-80 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 709. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMSCD performs just one multiply-negate-subtract operation, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FNMSCD might perform more than one multiply-negate-subtract operation. Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 describes how FNMSCD encodes the registers it uses and how vec_len, Dd[i], Dn[i], and Dm[i] are determined. Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the FPSCR, followed by reversal of the sign bit and a fully-rounded subtraction, using the same rounding mode. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-81
- 710. VFP Instructions 4.1.44 FNMSCS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 D 0 1 Fn Fd 1 0 1 0 N 1 M 0 Fm The FNMSCS Floating-point Negated Multiply and Subtract, Single-precision() instruction multiplies together two single-precision registers, subtracts a third single-precision register from the negation of the product and writes the result to the third register. It can also perform a vector version of this operation. Syntax FNMSCS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register, which is also used as the second operand for the subtraction. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the multiplication. The register number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the multiplication. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = -(Sn[i] * Sm[i]) - Sd[i] C4-82 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 711. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMSCS performs just one multiply-negate-subtract operation, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FNMSCS might perform more than one multiply-negate-subtract operation. Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 describes how FNMSCS encodes the registers it uses and how vec_len, Sd[i], Sn[i], and Sm[i] are determined. Rounding The operation is a fully-rounded multiplication with the rounding mode determined by the FPSCR, followed by reversal of the sign bit and a fully-rounded subtraction, using the same rounding mode. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-83
- 712. VFP Instructions 4.1.45 FNMULD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 0 1 0 Dn Dd 1 0 1 1 0 1 0 0 Dm The FNMULD (Floating-point Negated Multiply, Double-precision) instruction multiplies together two double-precision registers, and writes the negation of the result to a third double-precision register. It can also perform a vector version of this operation. Syntax FNMULD{<cond>} <Dd>, <Dn>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dn> Specifies the register that contains the first operand for the multiplication. <Dm> Specifies the register that contains the second operand for the multiplication. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = -(Dn[i] * Dm[i]) C4-84 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 713. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMULD performs just one negated multiplication, and vec_len=1, Dd[0]=Dd, Dn[0]=Dn, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FNMULD might perform more than one negated multiplication. Addressing Mode 2 - Double-precision vectors (non-monadic) on page C5-8 describes how FNMULD encodes the registers it uses and how vec_len, Dd[i], Dn[i], and Dm[i] are determined. Rounding The operation is a fully-rounded multiplication. The rounding mode is determined by the FPSCR, followed by reversal of the sign bit of the result. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-85
- 714. VFP Instructions 4.1.46 FNMULS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 0 D 1 0 Fn Fd 1 0 1 0 N 1 M 0 Fm The FNMULS (Floating-point Negated Multiply, Single-precision) instruction multiplies together two single-precision registers, and writes the negation of the result to a third single-precision register. It can also perform a vector version of this operation. Syntax FNMULS{<cond>} <Sd>, <Sn>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sn> Specifies the register that contains the first operand for the multiplication. The register number is encoded as Fn (top 4 bits) and N (bottom bit). <Sm> Specifies the register that contains the second operand for the multiplication. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Overflow, Underflow, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = -(Sn[i] * Sm[i]) C4-86 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 715. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FNMULS performs just one negated multiplication, and vec_len=1, Sd[0]=Sd, Sn[0]=Sn, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FNMULS might perform more than one negated multiplication. Addressing Mode 1 - Single-precision vectors (non-monadic) on page C5-2 shows how FNMULS encodes its registers and determines vec_len, Sd[i], Sn[i], and Sm[i]. Rounding The operation is a fully-rounded multiplication. The rounding mode is determined by the FPSCR, followed by reversal of the sign bit of the result. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-87
- 716. VFP Instructions 4.1.47 FSITOD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 1 0 0 0 Dd 1 0 1 1 1 1 M 0 Fm The FSITOD (Floating-point Convert Signed Integer to Double-precision) instruction converts a signed integer value held in a single-precision register to double precision and writes the result to a double-precision register. The integer value will normally have been transferred from memory by a single-precision load instruction or from an ARM register by an FMSR instruction. Syntax FSITOD{<cond>} <Dd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Sm> Specifies the source register. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version D variants only Exceptions None Operation if ConditionPassed(cond) then Dd = ConvertSignedIntegerToDouble(Sm) Notes Vectors FSITOD always specifies a scalar operation, regardless of the LEN field of the FPSCR. Zero If Sm contains an integer zero, the result is a double-precision +0.0, not a double-precision -0.0. C4-88 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 717. VFP Instructions 4.1.48 FSITOS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 1 0 0 0 Fd 1 0 1 0 1 1 M 0 Fm The FSITOS (Floating-point Convert Signed Integer to Single-precision) instruction converts a signed integer value held in a single-precision register to single precision and writes the result to a second single-precision register. The integer value will normally have been transferred from memory by a single-precision load instruction or from an ARM register by an FMSR instruction. Syntax FSITOS{<cond>} <Sd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sm> Specifies the source register. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exception: Inexact Operation if ConditionPassed(cond) then Sd = ConvertSignedIntegerToSingle(Sm) Notes Vectors FSITOS always specifies a scalar operation, regardless of the LEN field of the FPSCR. Zero If Sm contains an integer zero, the result is a single-precision +0.0, not a single-precision -0.0. Rounding Rounding is needed for some large operand values. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-89
- 718. VFP Instructions 4.1.49 FSQRTD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 0 1 1 0 0 0 1 Dd 1 0 1 1 1 1 0 0 Dm The FSQRTD (Floating-point Square Root, Double-precision) instruction calculates the square root of the value in a double-precision register and writes the result to another double-precision register. It can also perform a vector version of this operation. Syntax FSQRTD{<cond>} <Dd>, <Dm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the destination register. <Dm> Specifies the source register. Architecture version D variants only Exceptions Floating-point exceptions: Invalid Operation, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Dd[i] = sqrt(Dm[i]) Usage Square roots take a large number of cycles on most implementations, and vector square roots take proportionately longer. This can have a major effect on performance, and so the use of large numbers of square roots should be avoided where possible. Also see Interrupts on page C1-5 for a description of some implications for interrupt latency. C4-90 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 719. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FSQRTD performs just one square root operation, and vec_len=1, Dd[0]=Dd, and Dm[0]=Dm. When the LEN field indicates a vector mode (vector length > 1), FSQRTD might perform more than one square root operation. Addressing Mode 4 - Double-precision vectors (monadic) on page C5-19 describes how FSQRTD encodes the registers it uses and how vec_len, Dd[i], and Dm[i] are determined. Rounding The operation is a fully-rounded square root operation. The rounding mode is determined by the FPSCR. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-91
- 720. VFP Instructions 4.1.50 FSQRTS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 1 0 1 D 1 1 0 0 0 1 Fd 1 0 1 0 1 1 M 0 Fm The FSQRTS (Floating-point Square Root, Single-precision) instruction calculates the square root of the value in a single-precision register and writes the result to another single-precision register. It can also perform a vector version of this operation. Syntax FSQRTS{<cond>} <Sd>, <Sm> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Sd> Specifies the destination register. The register number is encoded as Fd (top 4 bits) and D (bottom bit). <Sm> Specifies the source register. The register number is encoded as Fm (top 4 bits) and M (bottom bit). Architecture version All Exceptions Floating-point exceptions: Invalid Operation, Inexact Operation if ConditionPassed(cond) then for i = 0 to vec_len-1 Sd[i] = sqrt(Sm[i]) Usage Square roots take a large number of cycles on most implementations, and vector square roots take proportionately longer. This can have a major effect on performance, and so the use of large numbers of square roots should be avoided where possible. Also see Interrupts on page C1-5 for a description of some implications for interrupt latency. C4-92 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 721. VFP Instructions Notes Vectors When the LEN field of the FPSCR indicates scalar mode (vector length 1), FSQRTS performs just one square root operation, and vec_len=1, Sd[0]=Sd, and Sm[0]=Sm. When the LEN field indicates a vector mode (vector length > 1), FSQRTS might perform more than one square root operation. Addressing Mode 3 - Single-precision vectors (monadic) on page C5-14 describes how FSQRTS encodes the registers it uses and how vec_len, Sd[i], and Sm[i] are determined. Rounding This is a fully-rounded square root operation. The FPSCR determines the rounding mode. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-93
- 722. VFP Instructions 4.1.51 FSTD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 0 1 U 0 0 0 Rn Dd 1 0 1 1 offset The FSTD (Floating-point Store, Double-precision) instruction stores a double-precision register to memory. Syntax FSTD{<cond>} <Dd>, [<Rn>{, #+/-(<offset>*4)}] where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <Dd> Specifies the source register. <Rn> Specifies the register holding the base address for the transfer. <offset> Specifies an offset to be multiplied by 4, then added to the base address (if U == 1) or subtracted from it (if U == 0) to form the actual address of the transfer. If offset is omitted, it defaults to +0. Architecture version D variants only Exceptions Data Abort Operation if ConditionPassed(cond) then if (U == 1) address = Rn + offset * 4 else address = Rn - offset * 4 if (big-endian) Memory[address,4] = Dd[63:32] Memory[address+4,4] = Dd[31:0] else Memory[address,4] = Dd[31:0] Memory[address+4,4] = Dd[63:32] C4-94 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 723. VFP Instructions Notes Addressing mode This is a special case of Addressing Mode 5 - VFP load/store multiple on page C5-24. Conversions An implementation using an internal format for double-precision values must convert that format back to the external double-precision format. Otherwise, no conversion is required. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-95
- 724. VFP Instructions 4.1.52 FSTMD 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 0 P U 0 W 0 Rn Dd 1 0 1 1 offset The FSTMD (Floating-point Store Multiple, Double-precision) instruction stores a sequence of consecutive double-precision registers to memory. Syntax FSTM<addressing_mode>D{<cond>} <Rn>{!}, <registers> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <addressing_mode> Specifies the addressing mode, which determines the values of start_address and end_address used by the instruction. See Addressing Mode 5 - VFP load/store multiple on page C5-24 for details. <Rn> Specifies the base register used by <addressing_mode>. ! Sets the W bit of the instruction to 1, specifying that the base register <Rn> is to be updated by the instruction. If it is omitted, the W bit of the instruction is set to 0 and the base register <Rn> is left unchanged. Some combinations of <addressing_mode> and the presence or absence of ! are not allowed. For details, see Addressing Mode 5 - VFP load/store multiple on page C5-24. <registers> Specifies which registers are to be stored, as a list of consecutively numbered double-precision registers, separated by commas and surrounded by brackets. It is encoded in the instruction by setting Dd to the number of the first register in the list, and offset to twice the number of registers in the list. At least one register must be specified in the list. For example, if <registers> is {D2,D3,D4}, the Dd field of the instruction is 2 and the offset field is 6. Architecture version D variants only Exceptions Data Abort C4-96 Copyright © 1996-2000 ARM Limited. All rights reserved. ARM DDI 0100E
- 725. VFP Instructions Operation if ConditionPassed(cond) then address = start_address for i = 0 to (offset-2)/2 /* d is the number of register Dd; */ /* D(n) is the double-precision register numbered n */ if (big-endian) Memory[address,4] = D(d+i)[63:32] Memory[address+4,4] = D(d+i)[31:0] else Memory[address,4] = D(d+i)[31:0] Memory[address+4,4] = D(d+i)[63:32] address = address + 8 assert end_address = address - 4 Notes Encoding If P=1 and W=0, the instruction is instead an FSTD instruction. Otherwise, if offset is odd, the instruction is instead an FSTMX instruction. Vectors The FSTMD instruction is unaffected by the LEN and STRIDE fields of the FPSCR, and does not wrap around at bank boundaries in the way that vector operands to data-processing instructions do. Registers are stored in simple increasing order of register number. Invalid register lists If Dd and offset do not specify a valid register list, the instruction is UNPREDICTABLE. This happens in two cases: • if offset == 0, that is, if an attempt is made to transfer no registers • if d + offset/2 > 16, that is, if an attempt is made to transfer another register after D15. Conversions If an implementation uses an internal format for double-precision values, it must convert that format back to the external double-precision format. Otherwise, no conversion is required. ARM DDI 0100E Copyright © 1996-2000 ARM Limited. All rights reserved. C4-97
- 726. VFP Instructions 4.1.53 FSTMS 31 28 27 26 25 24 23 22 21 20 19 16 15 12 11 10 9 8 7 6 5 4 3 0 cond 1 1 0 P U D W 0 Rn Fd 1 0 1 0 offset The FSTMS (Floating-point Store Multiple, Single-precision) instruction stores a sequence of consecutive single-precision registers to memory. Syntax FSTM<addressing_mode>S{<cond>} <Rn>{!}, <registers> where: <cond> Is the condition under which the instruction is executed. The conditions are defined in The condition field on page A3-5. If <cond> is omitted, the AL (always) condition is used. <addressing_mode> Specifies the addressing mode, which determines the values of start_address and end_address used by the instruction. See Addressing Mode 5 - VFP load/store multiple on page C5-24. <Rn> Specif