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Added comment explaining the difference between global history and local history basic implementations.

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Ross Thompson 2023-05-02 11:01:46 -05:00
parent 0904a9b97f
commit 08b237b878
1 changed files with 6 additions and 0 deletions
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src/ifu/bpred/localHistoryPredictor.sv
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@ -74,6 +74,12 @@ module localHistoryPredictor #(parameter m = 6, // 2^m = number of local history
assign BPDirPredWrongE = PCSrcE != BPDirPredE[1] & BranchE;
// this is the main difference between global and local history basic implementations rather than
// having multiple history registers. In global, the ghr wraps back into itself directly without
// being pipelined. IE. GHR is not read in F and then pipelined to M where it is updated. Instead
// GHR is just read in M and updated. GHR is still pipelined so that the PHT is updated with the correct
// GHR. Local history in contrast must pipeline the specific history register read during F and then update
// that same one in M. This implementation does not forward if a branch matches in the D, E, or M stages.
assign LHRNextW = BranchM ? {PCSrcM, LHRM[k-1:1]} : LHRM;
// this is local history