Instruction Format | Computer Architecture

Instruction Format in Computer Architecture – The instruction set of a computer is 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 mean of controlling CPU.

Computer Instruction Format

An user can interact with the processor through the instructions. These instructions are the command to the micro-processor 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 operand field that provides the data on which 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 is shown in below figure.

Instruction Design Issues

The design of an instruction format is a complex art and 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 system. The basic design issue is trade-off between powerful instruction set and the memory space to save instruction set.

The programmer wants more opcodes and more operands to make small programs. For example if opcode for multiplication is directly available, then there will be no need to write a separate program for multiplication operation. But as the opcodes and operands are increased, 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 be kept up with increase in processor speed. This is important to note that 16-bit instructions can be fetched at twice the rate of 32-bit instructions.

Orthogonality is also an important issue to design an instruction set of a processor. Instructions are said 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.