Computer instructions are a group of machine language instructions and are executed by a specific computer processor.
Whatever instruction we give to the computer will work on its basis.
There are fields in an instruction, these fields are the following: –
• operation code (Opcode) field –
This specifies the operation to be performed.
• Address field –
It stores the location of the operand.
• mode field –
This field specifies how operands will be detected.
A basic computer has three instruction code formats which are given below: –
1. memory – reference instruction
2. register – reference instruction
3. input-output instruction
memory- reference instruction
In this instruction, a memory of 12 bits is used to specify the address and a bit memory is used to specify the addressing mode.
register -reference instruction
These instructions are presented in opcode 111 (with 0 in the leftmost bit).
The register reference instruction is specified in the AC (accumulator) register operation.
It is presented in operation code 111 (with 1 in the leftmost bit). And the remaining 12 bits are used to specify the type of input-output operation.