PIPELINING

Pipelining is an implementation technique where multiple instructions are overlapped in execution. The computer pipeline is divided in stages. Each stage completes a part of an instruction in parallel. The stages are connected one to the next to form a pipe - instructions enter at one end, progress through the stages, and exit at the other end.

Pipelining does not decrease the time for individual instruction execution. Instead, it increases instruction throughput. The throughput of the instruction pipeline is determined by how often an instruction exits the pipeline.

Because the pipe stages are hooked together, all the stages must be ready to proceed at the same time. We call the time required to move an instruction one step further in the pipeline a machine cycle . The length of the machine cycle is determined by the time required for the slowest pipe stage.

The pipeline designer's goal is to balance the length of each pipeline stage . If the stages are perfectly balanced, then the time per instruction on the pipelined machine is equal to

Time per instruction on nonpipelined machine
Number of pipe stages

Under these conditions, the speedup from pipelining equals the number of pipe stages. Usually, however, the stages will not be perfectly balanced; besides, the pipelining itself involves some overhead.

We will describe the principles of pipelining using DLX and a simple version of its pipeline. Those principles apply to more complex instruction sets than DLX , although the resulting pipelines are more complex.