Basic Computer Organization and Design

Instruction Formats and their Execution

• The instruction length of basic computer is 16-bit.
• 16-bit instruction of basic computer has three fields:

Mode

Op-code

Operand (Address)

i) Mode field

• MSB (bit 15) of the 16-bit instruction.
• Value 0 = direct addressing, value 1 = indirect addressing mode.

ii) Op-code field

• Defines what operation to be performed.
• Contains 3 bits (bits 14-12)

iii) Operand (Address) field

• Contains 12 bits (bits 11-0)

The basic computer has 3 instruction code formats. Type of the instruction is recognized by the computer control from 4-bit positions 12 through 15 of the instruction.

i) Memory-Reference Instructions

ii) Register-Reference Instructions

iii) Input-Output Instructions

<< Basic Computer Registers and Common Bus System

Details on Basic Computer instructions >>