Pipelining a Datapath

Tom Kelliher, CS 240

Apr. 15, 2002

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Assignment

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Overview of pipelining.

Outline

1. Pipelining: a pipelined datapath. Hazards.

2. Simple example: a single lw.

Coming Up

Pipelined control.

Pipelining

A pipelined datapath:

Consider four instructions: R-mode, a branch, LW, SW.

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?

Simple Example

Let's follow a lw. What's going on during each clock cycle?