Instruction, interrupts & io processing

Unit 5 -INSTRUCTION, INTERRUPTS and IO
PROCESSING

Abhineet Anand

Computer Science and Engg. Department
University of Petroleum and Energy Studies, Dehradun

December 9, 2012

Abhineet Anand (UPES, Dehradun) INST n , INTERRUPTS, IO PROCESSING December 9, 2012 1 / 23

Outline
1 Instruction Codes
Introduction
Operation Code
Operands

2 Instruction Formats
Single accumulator organization
General register organization
Stack organization
Evaluation of Arithmetic Statement

3 Addressing Modes
Type of Addressing Modes

4 Program Interrupt
Types of Interrupts


Instruction Codes

A Computer instruction is binary code that speciﬁes a sequence of
micro operation for the Computer.
The Computer reads each instruction from memory and places it in a
control register.
The control then interprets the binary code of the instruction and
proceeds to execute it by issuing a sequence of micro operations.


Instruction Codes

An instruction code is a group of bits that instruct the computer to
perform a speciﬁc task.
It is usually divided into parts, each having its own particular
interpretation.
They are:
Operation Code, and
Operands.
The most basic part of an instruction code is its operation part.


Operation Code

The Operation Code (OpCode) of an instruction is a group of bits that
deﬁne each operation such add, subtract, multiply, shift, and
complement.
It must consist of at least n bits for a given 2n distinct operations.
Suppose we are having 64 (26 ) operation then the length of OpCode
will be 6.
The control unit decode the OpCode and do the required operation.


Operands

The Operation must be performed on some data stored in processor
register or in memory.
Every Computer has its own particular instruction code format.
The Simplest way to organize a computer is to have an instruction
code format with two parts.
The first part specifies the operation to be performed and the second
specifies an address.


Instruction Formats

The bits of the instruction are divided into groups called fields. The
most common fields found in instruction formats are:
An Operation Code field that specifies the operation to be performed.
An Address field that designates a memory address or a processor
register.
A mode field that specifies the way the operand or the effective
address is determined.


Instruction Formats

Computer may have instruction of several different lengths containing
varying numbers of addresses.
The number of address ﬁelds in the instruction format of a computer
depends on the internal organization of its registers.
Most computers fall into one of three types of CPU organization:
1 Single accumulator organization.
2 General register organization.
3 Stack organization.


Single accumulator organization

All Operation are performed with an implied accumulator register. The
instruction format in this type of computers uses one address ﬁeld.
Example:

ADD X
where, X is the address of the operand.
The ADD instruction in this case results in the operation
AC < − AC + M[X].


General register organization

The instruction format in this type of computer needs three register
address ﬁelds.
Thus the instruction for an arithmetic addition may be written in an
assembly language as

ADD R1, R2, R3
to denote the operation R1 < − R2 + R3.
The number of address ﬁelds in the instruction can be reduced from
three to two if the destination register is the same as one of the
source registers.

ADD R1, R2
would denote the operation R1 < − R1 + R2.


Stack organization

The stack-organized CPU would have PUSH and POP instruction
which requires an address ﬁled.
Example:

PUSH X
will push the word at address X to the top of the stack. The stack
pointer is updated automatically.
The operation is performed on the two items that are on top of the
stack.



X=(A+B)∗(C+D)
Three - Address Instruction
ADD R1, A, B R1 < − M[A] + M[B]
ADD R2, C, D R2 < − M[C] + M[D]
MUL X, R1, R2 M[X] < − R1 ∗ R2.
Two - Address Instruction
MOV R1, A R1 < − M[A]
ADD R1, B R1 < − R1 + M[B]
MOV R2, C R2 < − M[C]
ADD R2, D R2 < − R2 + M[D]
MUL R1, R2 R1 < − R1 ∗ R2
MOV X, R1 M[X] < − R1


One - Address Instruction
LOAD A AC< − M[A]
ADD B AC < − AC + M[B]
STORE T M[T] < − AC
LOAD C AC < − M[C]
ADD D AC< − AC + M[D]
MUL T AC < − AC ∗ M[T]
STORE X M[X] < − AC
Zero - Address Instruction
PUSH A TOS < − A
PUSH B TOS < − B
ADD TOS < − (A + B)
PUSH C TOS < − C
PUSH D TOS < − D
ADD TOS < −(C + D)
MUL TOS < − (C + D) * (A+ B)
POP X M[X] < − TOS

Addressing Modes

The way the operands are chosen during program execution is
dependent on the addressing mode of the instruction.
The addressing mode specifies a rule for interpreting or modifying the
address field of the instruction before the operand is actually
referenced.
An Operation Code field that specifies the operation to be performed.
An Address field that designates a memory address or a processor
register.
A mode field that specifies the way the operand or the effective
address is determined.


Addressing Modes

Computer use addressing mode techniques for the purpose of
accommodating one or both of the following provisions:
To give programming versatility to the user by providing such facilities
as pointers to memory, counters for loop control, indexing of data, and
program relocation.
To reduce the number of bits in the addressing ﬁeld of the instruction.



Although most addressing mode modify the address field of the
instruction, there are two modes that need no address field at all.
These are:
Implied Mode: Operands are specified implicitly in the definition of the
instruction. Like ”Complement Accumulator”.
Zero-Address instruction in a stack-organized computer are
implied-mode instruction since the operands are implied to be on top of
the stack.
Immediate Mode: In this mode the operand is specified in the
instruction itself.
Immediate-mode instruction are useful for initializing registers to a
constant value.



Register Mode: In this mode the operands are in register that reside
within the CPU.
The particular register is selected from a register filed in the
instruction.
Register Indirect Mode: In this mode the instruction specifies a
register in the CPU whose content give the address of the operand in
memory.
The advantage is that the address field of the instruction uses a fewer
bits to select a register than would have been required to specify a
memory address directly.



Auto-increment Mode or Auto-decrement Mode
Direct Address Mode
Indirect Address Mode
Relative Address Mode
Indexed Addressing Mode
Base Register Addressing Mode


Program Interrupt

The concept of program interrupt is used to handle a variety of
problems that arise out of normal program sequence.
Program interrupt refers to the transfer of program control from a
currently running program to another service program as a result of
an external or internal generated request.
Control returns to the original program after the service program is
executed.


Program Interrupt

The interrupt procedure is, in principal, quite similar, to subroutine call
except for three variation:
1 The interrupt is usually initiated by an internal or external signal rather
than from the execution of an instruction;
2 The address of the interrupt service program is determined by the
hardware rather than from the address ﬁeld of instruction; and
3 an interrupt procedure usually stores all the information necessary to
deﬁne the state of the CPU rather than storing only the program
counter.


Program Interrupt

These three procedure concept are clariﬁed further as:
1 The content of the program counter
2 The content of all processor register
3 The content of certain status conditions
The collection of all status bit conditions in the CPU is sometimes
called a Program Status Word or PSW.


Types of Interrupts

There are three major types of interrupts that cause a break in the
normal execution of a program. They are:
1 External Interrupts - come from input-output (I/O) devices, from a
timing devices, from a circuit monitoring the power supply, or from any
other external source.
Example: I/O devices requesting for transfer of data, I/O devices
finished transfer of data, elapsed time of an event, or power failure.
2 Internal Interrupts - arise from illegal or erroneous use of an instruction
or date, also known as trap.
Example: register overflow, divide by zero, invalid operation code, stack
overflow, protection violation.
3 Software Interrupts - initiated by executing an instruction. It is a special
call instruction that behaves like an interrupt rather than a subroutine
call.


THANK YOU


Instruction, interrupts & io processing

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