14
for class on
Monday
Nov. 15, 2010
Please read Section 4.7 of the text, and look at pages 4.12-16 through 4.12-30 (on the CD) for more examples of forwarding and stalls.
Please turn in a written response to these questions:
1. The Elaboration on page 371 of your text proposes forwarding to allow a load/store pair to avoid stalling. Draw a sketch of the datapath for just the MEM and WB stages that includes this forwarding.
2. What control would be needed for your datapath addition in question 1? Write the statement for your new control signal using the style of Section 4.7 (e.g., the ForwardA and ForwardB formulations at the bottom of page 366).
3. Extra credit: If you make the above changes, then the stall formula on page 372 will need some adjustment. Why? Re-write it please.
Then, be prepared to discuss the following in class:
4. One advantage of pictures like Figures 4.43 and 4.52 (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 (not counting obstacle-avoidance, obviously). 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?