9.cs instrset

INSTRUCTION
SETS
DEEPAK SHARMA
12KSSB6031
BCA 5th SEM

What is an Instruction Set?
• The complete collection of instructions that are
understood by a CPU
• Machine language: binary representation of
operations and (addresses of) arguments
• Assembly language: mnemonic
representation for humans, e.g.,
OP A,B,C (meaning A <- OP(B,C))

Elements of an Instruction
 Operation code (opcode)
 Do this: ADD, SUB, MPY, DIV, LOAD, STOR
 Source operand reference
 To this: (address of) argument of op, e.g.
register, memory location
 Result operand reference
 Put the result here (as above)
 Next instruction reference (often implicit)
 When you have done that, do this: BR

Simple Instruction Format
(using two addresses)

Instruction Types
 Data transfer: registers, main memory, stack or
I/O
 Data processing: arithmetic, logical
 Control: systems control, transfer of control

Data Transfer
• Store, load, exchange, move, clear, set, push, pop
• Specifies: source and destination (memory,
register, stack), amount of data
• May be different instructions for different (size,
location) movements.

Input/Output
 May be specific instructions, e.g. INPUT, OUTPUT
 May be done using data movement instructions
(memory mapped I/O)
 May be done by a separate controller (DMA): Start
I/O, Test I/O

Instruction format
 An instruction is normally made up of a combination of an operation code
and some way of specifying an operand, most commonly by its location or
address in memory. A computer has a variety of instruction code formats.
The control unit in CPU interprets each instruction code and provides the
necessary control functions needed to process the instruction. There are 5
types of computer instructions. They are:
 Four address instruction
 Three address instruction
 Two address instruction
 One address instruction
 Zero address instruction

Complex Instruction Set
Computer [CISC]
 A computer with a large number of instructions is called Complex
Instruction Set Computer. In CSIC architecture it is very essential that a
single machine instruction for each statement that is written in a high
level language must be provided.
 FEATURES:
 Variable length instruction formats are used.
 The instructions provide direct manipulation of operands residing in
memory.
 A large number of instructions about 100 to 200 are provided.
 A large variety of addressing modes about 5 to 20 are provided
 Instructions performing specialized tasks are used infrequently

Reduce Instruction Set
Computer
 Computer that uses fewer instructions with simple constructs so that
they can be executed much faster within the CPU without having to
use memory often is called RISC.
 FEATURES:
 It has relatively fewer instructions.
 There are relatively fewer and simple addressing modes.
 Memory access is limited only to load and store instructions.
 All instructions are carried out using the registers of the CPU.
 Instructions are easily decodable.
 Instruction format is of fixed length.

Logical
 Bitwise operations: AND, OR, NOT, XOR, TEST,
CMP, SET
 Shifting and rotating functions, e.g.
 logical right shift for unpacking: send 8-bit
character from 16-bit word
 arithmetic right shift: division and truncation for
odd numbers
 arithmetic left shift: multiplication without
overflow

Types of Operand
 Addresses: immediate, direct, indirect, stack
 Numbers: integer or fixed point (binary, twos
complement), floating point (sign, significand,
exponent), (packed) decimal (246 = 0000 0010
0100 0110)
 Characters: ASCII (128 printable and control
characters + bit for error detection)
 Logical Data: bits or flags, e.g., Boolean 0 and 1

9.cs instrset

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