Introduction

The main task performed by the CPU is the execution of instructions. In the previous unit, we have discussed about the instruction set of computer system. But, one thing, which remained unanswered is: how these instructions will be executed by the CPU?

The above question can be broken down into two simpler questions. These are:

What are the steps required for the execution of an instruction? How are these steps performed by the CPU?

The answer to the first question lies in the fact that each instruction execution consists of several steps. Together they constitute an instruction cycle. A micro-operation is the smallest operation performed by the CPU. These operations put together execute an instruction.

For answering the second question, we must have an understanding of the basic structure of a computer. As discussed earlier, the CPU consists of an Arithmetic Logic Unit, the control unit and operational registers. We will be discussing the register organisation in this unit, whereas the arithmetic-logic unit and control unit organisation are discussed in subsequent units. 

In this unit we will first discuss the basic CPU structure and the register organisation in general. This is followed by a discussion on micro-operations and their implementation. The discussion on micro-operations will gradually lead us towards the discussion of a very simple ALU structure. The detail of ALU structure is the topic of the next unit. 

Objectives

After going through this unit, you will be able to:

• describe the register organisation of the CPU;
• define what is a micro-operation;
• differentiate among various micro-operations;
• discuss an instruction execution using the micro-operations; and
• define the concepts of instruction pipelining