-Branch Prediction and Speculative Execution

1.5 Program Order and Dependencies

Branch Prediction and Speculative Execution

Although one or more instructions may begin execution during each cycle, each instruction takes several (or many) cycles to complete. Thus, when a branch instruction is decoded, its branch condition may not yet be known. However, the R10000 processor can predict whether the branch is taken, and then continue decoding and executing subsequent instructions along the predicted path.

When a branch prediction is wrong, the processor must back up to the original branch and take the other path. This technique is called speculative execution. Whenever the processor discovers a mispredicted branch, it aborts all speculatively-executed instructions and restores to the processor to the state it held before the branch.

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