Instruction Format in Computer Architecture

Instruction Format in Computer Architecture – The instruction set of a computer is the most interesting aspect of computer architecture. The instruction set defines many of the functions performed by the processor and thus has a significant effect on the implementation of the CPU. In other words, the instruction set is the programmer’s means of controlling the CPU.

Computer Instruction Format

A user can interact with the CPU through the instructions. These instructions are written by the programmer to command the CPU to perform a specific task. The CPU executes the instructions in a sequential manner if there is no interrupt and no branching instruction.

The CPU can understand a variety of instruction code formats. It is the responsibility of the control unit to interpret the meaning of each instruction code format and provide the necessary control signal whenever required. If the computer system supports various types of addressing modes, the instruction format can be divided into three fields, as shown in the figure.

1. Address field
2. Operation code field
3. Mode field

The address field of the instruction gives the address of the operands. Operands residing in memory are specified by their memory address, and operands residing in the processor registers are specified with a register address.

The operation code field of the instruction is a group of bits that define various operations, like addition, subtraction, shift, etc., to be performed on the given operands. Each operation has its own separate code, and that code is provided in the operation code field of the instruction.

The third field is the mode field. It specifies how to find the address of the operands. Various addressing modes have been given in the previous chapter. This field indicates whether the instruction is using direct addressing or indirect addressing.

A user can interact with the processor through the instructions. These instructions are the command to the microprocessor to be executed in the given sequence to complete the specified task. Each instruction has two specific information fields.

One is the operation code (opcode), which specifies the operation to be performed. The operation is specified by binary code, hence the name is operation code. Second is the operand field that provides the data on which the operation is to be performed. The number of operands may be more than one in one instruction; however, there will be only one operation code. The layout and format of an instruction are shown below.

Note: Computer Instructions are a set of machine language instructions that a particular processor understands and executes. A computer performs tasks on the basis of the instructions provided.

A basic computer has three instruction code formats, which are:

1. Memory-reference instruction
2. Register-reference instruction
3. Input-Output instruction

Memory-reference instruction: In a memory-reference instruction, 12 bits of memory are used to specify an address and one bit to specify the addressing mode ‘I’. (Opcode = 000 through 110)

I

Opcode

Address

Register-reference instruction: The Register-reference instructions are represented by the Opcode 111 with a 0 in the leftmost bit (bit 15) of the instruction. A Register-reference instruction specifies an operation on or a test of the AC (Accumulator) register. (Opcode = 111, I=0)

0 1 1 1

Register Operation

Input-Output instruction: Just like the Register-reference instruction, an Input-Output instruction does not need a reference to memory and is recognized by the operation code 111 with a 1 in the leftmost bit of the instruction. The remaining 12 bits are used to specify the type of the input-output operation or test performed. (Opcode = 111, I=1)

1 1 1 1

I/O Operation

Instruction Design Issues

The design of an instruction format is a complex art, and a number of instruction designs may be implemented. The length of an instruction affects the memory size, memory organization, bus architecture, processor design, and speed of the system. The basic design issue is a trade-off between a powerful instruction set and the memory space to save instructions.

The programmer wants more opcodes and more operands to make small programs. For example, if the opcode for multiplication is directly available, then there will be no need to write a separate program for the multiplication operation. But as the opcodes and operands are increased, the required length of instruction will also increase. Longer instruction length will require more memory space, and it may be wasteful.

Another important consideration is that in the bus architecture, the data bus length should be equal to the instruction length, or one should be a multiple of the other. A related consideration is the memory transfer rate. This rate has to keep up with the increase in processor speed. It is important to note that 16-bit instructions can be fetched at twice the rate of 32-bit instructions.

Orthogonality is also an important consideration in designing an instruction set for a processor. Instructions are said to be orthogonal if they do not overlap or perform the same function. A good instruction set minimizes the overlap between instructions. This provides the programmer with the necessary functions in a minimum of instructions.

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