Scoreboarding

Scoreboarding is a centralized method, used in the CDC 6600 computer, for dynamically scheduling a pipeline so that the instructions can execute out of order when there are no conflicts and the hardware is available. In a scoreboard, the data dependencies of every instruction are logged. Instructions are released only when the scoreboard determines that there are no conflicts with previously issued and incomplete instructions. If an instruction is stalled because it is unsafe to continue, the scoreboard monitors the flow of executing instructions until all dependencies have been resolved before the stalled instruction is issued.

Instructions are decoded in order and go through the following four stages.

To control the execution of the instructions, the scoreboard maintains three status tables:

The detailed algorithm for the scoreboard control is described below:

The scoreboarding method must stall the issue stage when there is no functional unit available. In this case, future instructions that could potentially be executed will wait until the structural hazard is resolved. Some other techniques like Tomasulo algorithm can avoid the structural hazard and also resolve WAR and WAW dependencies with Register renaming.

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