Computer Organization

Central Processing Unit (CPU) (continued)

A brief introduction to machine instructions and their representation.

Machine instructions are the building blocks of all computer programs.
Each instruction does only one very simple step in a computation.
Typical processors have from a few dozen to a few hundred different machine instructions.
Each instruction is representated by a unique binary string made up of fields:
- opcode field: a few bits representing what type of instructions it is; e.g., ADD, SUBTRACT, COMPARE, JUMP, ...
- operand fields: bits representing one or more register numbers or a memory address where operands are to be found and/or results are to be stored.
As in a simple expression, an opcode is the designator of the operation to be executed, and the operands are the designators of the values to be used. Some machine architectures only permit one specific operand and assume that the other is contained in a special register (usually called the accumulator). Other machines permit multiple operands, including the possibility of one operand designating the next instruction to be executed.