16. Instruction Processing

• Central idea of von Neumann model is that both program and data stored in computer memory:

  • Program is a sequence of instructions

  • Instruction is a binary encoding of operations and operands:

  • For example, an arithmetic expression

    • -a + b * c

    could be computed by a program with three machine instructions

        neg  ax       ; negate (multiply by -1)
        imul bx, cx   ; multiply and store result in bx
        add  ax, bx   ; add and store result in ax
    

    where NEG, IMUL, ADD are arithemtic instructions, AX, BX, and CX are operands.

  • Instructions are most basic units of processing.

  • Instructions are executed under control of the control unit.