ARM Exception and interrupts

ARM Exceptions
&
ARM Interrupt Controller
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Mapping exceptions to modes
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Exception
Condition that needs to halt the normal sequential execution of
instructions

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Figure: ARM Vector table
Exception Priority:

• B<address> provides a branch relative from the pc.
• LDR pc, [pc, #offset]—This load register instruction
loads the handler address from memory to the pc.
• LDR pc, [pc, #-0xff0]—This load register instruction
loads a speciﬁc interrupt service routine address from
address 0xfffff030 to the pc.
• MOV pc, #immediate—This move instruction copies
an immediate value into the pc.
MOV pc, #immediate

RESET:
Happens when the processor powers up.
Initializes the system, sets up stacks for different processor modes.
Highest priority exception
Upon entry into the reset handler the CPSR is in SVC mode and
both IRQ and FIQ bits are set to 1, masking any interrupts
DATAABORT:
2nd highest priority.
 Happens when we try to read/write into
an invalid address or access with the wrong
access permission.
 Upon entry into the Data Abort Handler,
IRQ’s will be disabled (I-bit set 1), and FIQ
will be enabled (F-bit set 0).
FIQ:
Highest priority interrupt
 IRQ & FIQs are disabled till FIQ is handled. 5

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IRQ:
2nd Highest priority interrupt
IRQ handler is entered only if there
is no FIQ & Data Abort on-going.
Undefined Instruction:
Undefined Instruction exception occurs
when an instruction not in the ARM or Thumb
instruction set reaches the execute stage of the
pipeline and none of the other exceptions have
been flagged
 Same priority as SWI as one can happen at a
time. Meaning as the instruction being
executed cannot both be an SWI instruction
and an undefined instruction at the same time.
Pre-Fetch Abort:
Similar to data abort, but happens on
address fetch failure.
 On entry to the handler, IRQs are
disabled, but FIQs remain enabled and
can happen during a Pre-Fetch abort.
SWI:
A Software Interrupt (SWI) exception
occurs when the SWI instruction is
executed and none of the other higher-
priority exceptions have been flagged.

• saves the cpsr to the spsr of the exception mode
• saves the pc to the lr of the exception mode
• sets the cpsr to the exception mode
• sets pc to the address of the exception handler
CPSR
LR.exp
SPSR.exp
PC PC
CPSR
User mode Exception mode
ARM Exception handling
Return from Exception:

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Locating the offending instruction:

This example shows that a typical method of
returning from an IRQ and FIQ handler is to
use a SUBS instruction:
handler
<handler code>
...
SUBS pc, r14, #4 ; pc=r14-4
Because there is an S at the end of the SUB
instruction and the pc is the destination register,
the cpsr is automatically restored from the spsr
register.
This example shows another method that
subtracts the offset from the link register r14 at
the beginning of the handler.
handler
SUB r14, r14, #4 ; r14-=4
...
<handler code>
...
MOVS pc, r14 ; return
After servicing is complete, return to normal
execution occurs by moving the link register r14
into the pc and restoring cpsr from the spsr.

The ﬁnal example uses the interrupt stack to store the link register. This method ﬁrst
subtracts an offset from the link register and then stores it onto the interrupt stack.
handler
SUB r14, r14, #4 ; r14-=4
STMFD r13!,{r0-r3, r14} ; store context
...
<handler code>
...
LDMFD r13!,{r0-r3, pc}ˆ ; return
To return to normal execution, the LDM instruction is used to load the pc. The ˆ
symbol in the instruction forces the cpsr to be restored from the spsr.

ARM Interrupt handling
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Interrupts
SWI instructionIRQ and FIQ
• Software Interrupts are normally reserved to call privileged operating system routines.
Eg: Change a program running in user mode to a privileged mode.
• Interrupt Requests are normally assigned for general-purpose interrupts.
IRQ exception has a lower priority and higher interrupt latency
• Fast Interrupt Requests are normally reserved for a single interrupt source that requires a
fast response time

The interval of time from an external interrupt request signal being raised to the ﬁrst
fetch of an instruction of a speciﬁc interrupt service routine (ISR).
Two main methods to minimize interrupt
latency
• use a nested interrupt handler -
achieved by re-enabling the interrupts as
soon as the interrupt source has been
serviced but before the interrupt handling
is complete.
• Prioritization - ignore interrupts of the
same or lower priority than the interrupt
you are handling, so only a higher-priority
task can interrupt your handler.
Interrupt latency

• When an IRQ occurs, the processor moves into state 2.
• Sets the IRQ =1, disabling any further IRQ exceptions.
• FIQ has a higher priority and therefore doesn't get disabled
when a low-priority IRQ exception is raised.
• The cpsr processor mode changes to IRQ mode.
• spsr_irq = cpsr.
• PC = r14_irq.
• pc = IRQ entry +0x18 in the vector table.
• In state 3 the software handler takes over
• Upon completion, the processor mode reverts back to the
original user mode code in state 1.
Interrupt request IRQ
Figure: State-Machine for an IRQ

Interrupt request FIQ
• In state 3 the software handler takes over
• Upon completion, the processor mode reverts back to the original user mode code in state 1.
• FIQ is ideal for servicing a single-source, high-priority, low-latency interrupt.
• When an FIQ occurs, the processor moves into state 2.
• Sets the IRQ =1 and FIQ =1, disabling any further IRQ and
FIQ exceptions.
• The cpsr processor mode changes to FIQ mode.
• No need to save registers r8 to r12
• spsr_fiq = cpsr.
• r14_fiq = pc.
• pc = FIQ entry +0x1C in the vector table.

The immediate value on the data
processing BIC or ORR instruction
has to be changed to 0xc0 to enable
or disable both interrupts.

Stack memory layout
The main advantage of layout B over A is that B does not corrupt
the vector table when a stack overflow occurs, and so the system
has a chance to correct itself when an overflow has been identified.

Supervisor mode stack
LDR r13, SVC_NewStack ; r13_svc
...
SVC_NewStack
DCD SVC_Stack
IRQ mode stack
MOV r2, #NoInt|IRQ32md
MSR cpsr_c, r2
LDR r13, IRQ_NewStack ; r13_irq
...
IRQ_NewStack
DCD IRQ_Stack
NoInt EQU 0xc0 ; Disable interrupt
User mode stack
MOV r2, #Sys32md
MSR cpsr_c, r2
LDR r13, USR_NewStack ; r13_usr
...
USR_NewStack
DCD USR_Stack

Non-nested Interrupt Handler
• The interrupts are disabled until control is returned back to
the interrupted task or process.
• Can service only a single interrupt at a time.
1. Disable interrupt/s—When the IRQ exception is raised, the ARM
processor will disable further IRQ exceptions from occurring.
2. Save context—On entry the handler code saves a subset of the
current processor mode non banked registers.
3. Interrupt handler—The handler then identiﬁes the external interrupt
source and executes the appropriate interrupt service routine (ISR).
4. Interrupt service routine—The ISR services the external interrupt
source and resets the interrupt.
5. Restore context—The ISR returns back to the interrupt handler,
which restores the context.
6. Enable interrupts— pc = lr−4 cpsr = spsr_{mode}

What happens when an exception happens?
1. Store the CPSR to the SPSR of the exception mode.
2. PC is stored in the LR of the exception mode.
3. Link register is set to a specific address based on the current instruction..
For e.g. for ISR, LR = last executed instruction + 8
4. Update the CPSR about the exception
5. Set the PC to the address of the exception handler.
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ARM Exception and interrupts

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