SOURCE AND RESULT OPERANDS AND INSTRUCTION FORMAT

Source and Result operands can be in any of the areas.

(i) Main or Virtual Memory : The source operand may be present in the memory and the result operand may need to be stored in memory or in virtual memory.

(ii) CPU Register: A CPU can contain one or more registers, and these can be referenced by machine instructions. If only one register exists, its reference may be implicit, if more than one register exists, then each register is given a unique number, and in the instruction the  number of the desired register must be specified.

(iii) I/O Device : The source operand may be present at an Input Device and the result may need to be send out to an output device. Hence the instruction must specify the I/O module and the device for the operation.

2.3 INSTRUCTION FORMAT

Each instruction is represented by a sequence of bit within the computer. These bits are divided into groups called fields. Each field consists of an element of the instruction. This layout of the instruction is known as the instruction format.

The format of an instruction is usually depicted in a rectangular box symbolizing the bits of the instructions as they appear in memory words or in a control register

The most common fields found in instruction formats are:

(i) An operation code field that specifies the operation to be performed

(ii) An address field that designates a memory address or a processor register.

(iii) A mode field that specifies the addressing mode, which specifies the way the operand or the effective address is determined.

With most instruction sets, more than one format is used.