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3.2 Pipelines     continued...

Figure 5 shows a type of diagram, known as a Gantt chart, that is commonly used to illustrate the operation of a pipeline. The horizontal axis represents time. There is one row for each stage of the pipeline. A line segment during cycle means a stage is active in that cycle; a blank means the stage is inactive. The figure illustrates the pipelined floating point adder of the previous example in the case when two successive instructions can be overlapped and in the case where the second instruction must wait for the first to complete. In the case of overlapped instructions, note that in cycle the first stage is busy with the second instruction while the second stage is busy with the first instruction. In each successive cycle the two instructions are passed down the ``assembly line'' to the next stage. In the first case, the second sum is done after 6 cycles, but in the second case it is not finished until after the 10th cycle. The ``bubble'' in the pipeline is the 4-cycle dead period in each stage caused by delaying the second instruction.

Figure 5: Gantt Charts for Pipelined Floating Point Adder.