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 PIPELINE Definition
We define PIPELINE |
PIPELINE - The technique of processing multiple parts of an instruction at the same time. Many processors have two or more instruction pipelines--think of them as automobile assembly lines. As one instruction is executed, the next instruction is being decoded, and the one after that is being fetched from memory (i.e., Ellen welds the frame of one car, while Frank paints another one, and Jimmy puts the tires onto the finished frame of another car). The pipeline is only as fast as its slowest member (i.e., if it takes Jimmy an hour to put on some tires then it doesn't matter that Ellen can weld the frame in 20 minutes; either the line will slow down, or you need three Jimmys for optimal performance). The speed-up you get from having a pipeline is about equal to the amount of pieces of pipe, though it's slightly slower due to the fact that the first instruction executed begins to fill the empty pipeline (i.e., Jimmy can't put on any tires if Ellen is still welding the frame of the first car through the line!). If something happens that empties the pipeline then performance suffers--the penalty is greater with larger pipelines. For some practical examples, the original PowerPC G4 has a 7-stage pipeline, while the Pentium 4 has a 20-stage pipeline. Processors with smaller pipelines are typically seen as more efficient, but at higher speeds the short pipeline can be limiting. |
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