Define instruction format.

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

An instruction format defines the layout of the bits of an instruction, in terms of its constituent parts. The bits of an instruction are divided into groups called fields. The most common fields found in instruction formats are:

·    An operation code held that specifies the operation to be performed.

·    An address field that designates a memory address or a processor registers.

·    A mode field that specifies the way the operand or the effective address is determined.

Op-code

Mode

Address

Figure: Instruction fields

Other special fields are sometimes employed under certain circumstances. The operation code field of an instruction is a group of bits that define various processor operations, such as add, subtract, complement and shift Address fields contain either a memory address field or a register address. Mode fields offer a variety of ways in which an operand is chosen.