Instruction Formats

The most common fields found in instruction format are:-

(1) An operation code field that specified the operation to be performed
(2) An address field that designates a memory address or a processor registers.
(3) A mode field that specifies the way the operand or the effective address is determined.

Computers may have instructions of several different lengths containing varying number of addresses. The number of address field 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 ADD X AC ® AC + M [×]
(2) General Register Organization ADD R1, R2, R3 R ® R2 + R3
(3) Stack Organization PUSH X

Three address Instruction

Computer with three addresses instruction format can use each address field to specify either processor register are memory operand.

ADD R1, A, B A1 ® M [A] + M [B]
ADD R2, C, D R2 ® M [C] + M [B] X = (A + B) * (C + A)
MUL X, R1, R2 M [X] R1 * R2

The advantage of the three address formats is that it results in short program when evaluating arithmetic expression. The disadvantage is that the binary-coded instructions require too many bits to specify three addresses.

Two Address Instruction

Most common in commercial computers. Each address field can specify either a processes register on a memory word.

MOV R1, A R1 ® M [A]
ADD R1, B R1 ® R1 + M [B]
MOV R2, C R2 ® M [C] X = (A + B) * ( C + D)
ADD R2, D R2 ® R2 + M [D]
MUL R1, R2 R1 ® R1 * R2
MOV X1 R1 M [X] ® R1

One Address instruction

It used an implied accumulator (AC) register for all data manipulation. For multiplication/division, there is a need for a second register.

LOAD A AC ® M [A]
ADD B AC ® AC + M [B]
STORE T M [T] ® AC X = (A +B) × (C + A)

All operations are done between the AC register and a memory operand. It’s the address of a temporary memory location required for storing the intermediate result.

Zero – Address Instruction

A stack organized computer does not use an address field for the instruction ADD and MUL. The PUSH & POP instruction, however, need an address field to specify the operand that communicates with the stack (TOS ® top of the stack)

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

CISC Characteristics

A computer with large number of instructions is called complex instruction set computer or CISC. Complex instruction set computer is mostly used in scientific computing applications requiring lots of floating point arithmetic.

1. A large number of instructions - typically from 100 to 250 instructions.
2. Some instructions that perform specialized tasks and are used infrequently.
3. A large variety of addressing modes - typically 5 to 20 different modes.
4. Variable-length instruction formats
5. Instructions that manipulate operands in memory.

RISC Characteristics

A computer with few instructions and simple construction is called reduced instruction set computer or RISC. RISC architecture is simple and efficient. The major characteristics of RISC architecture are,

1. Relatively few instructions
2. Relatively few addressing modes
3. Memory access limited to load and store instructions
4. All operations are done within the registers of the CPU
5. Fixed-length and easily-decoded instruction format.
6. Single cycle instruction execution
7. Hardwired and micro programmed control
   

Important Questions

1. What is the Instruction formats ?
2. Discuss in detail Instruction formats with its various types ?
3. What is the Three address Instruction formats ?
4. What is the Two address Instruction formats ?
5. What is the one address Instruction formats ?
6. What is the zero address Instruction formats ?