for class on Thursday Nov. 9, 2000
Please read Sections 6.4 and 6.5 of the Patterson & Hennessy text, and be prepared to discuss the following issues:
1. One advantage of pictures like Figures 6.11 and 6.36 (sometimes called "waterfall charts") is that they can show exactly how one instruction's results are communicated to future instructions. In a waterfall chart, it is possible to insist that all signals travel left-to-right or top-to-bottom. This is like insisting that information can only go forward in time: to a later pipeline stage or to a later instruction. How would you add explicit forwarding paths to a waterfall chart?
2. Can you reconcile Figure 6.45 with its prose description on page 490? The "air bubble" business seems not well depicted in the figure; for one thing, the air bubble never "exits." Also, what's the ALU supposed to be doing in cycle CC4? Please come up with an alternative version of Figure 6.45, draw a picture, and be ready to explain (or defend!) your idea.