It is observed that organization enhancements to the CPU can improve performance. We have already seen that use of multiple registers rather than a single a accumulator, and use of cache memory improves the performance considerably. Another organizational approach, which is quite common, is instruction pipelining.

Pipelining is a particularly effective way of organizing parallel activity in a computer system. The basic idea is very simple. It is frequently encountered in manufacturing plants, where pipelining is commonly known as an assembly line operation.

By laying the production process out in an assembly line, product at various stages can be worked on simultaneously. This process is also referred to as pipelining, because, as in a pipeline, new inputs are accepted at one end before previously accepted inputs appear as outputs at the other end.

To apply the concept of instruction execution in pipeline, it is required to break the instruction in different task. Each task will be executed in different processing elements of the CPU.

As we know that there are two distinct phases of instruction execution: one is instruction fetch and the other one is instruction execution. Therefore, the processor executes a program by fetching and executing instructions, one after another.

Let and refer to the fetch and execute steps for instruction . Execution of a program consists of a sequence of fetch and execute steps is shown in the Figure 9.1.