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




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