Pipelining

Tom Kelliher, CS 240

Apr. 21, 1999

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Announcements

Assignment

Due in 1 week. 6.2, 6.3, 6.4, 10 pts. each. 6.32 or 6.33, 20 pts.

From Last Time

Reviewed midterm

Outline

1. Pipelining.

2. Superscalar pipelines.

Coming Up

Caches.

Pipelining

A pipelined datapath:

Observations:

1. Not a true pipeline: feedback.

2. How do we re-design control?

Hazards

1. Structural hazards.

   Example: unified L1 cache/memory.

2. Control hazards. Consider the following example:

   

   Solutions:

   1. Stall.

   2. Predict.

      Static prediction. Truly static. Compile-time determined.

      Dynamic prediction. Branch history tables. One-, two-bit counters.

   3. Delayed branch.

      Assumes you know branch outcome early.

      Code scheduling:

      

   Consideration: deeper pipelines.

3. Data hazards.

   Data not available when needed.

   ALU example:

   

   Fixed by forwarding.

   Memory example:

   

   How can this be fixed?

Superscalar Pipelines

Dynamic scheduling:

Logical vs. physical registers.

Generic structure of Pentium Pro, PowerPC 604: