A processor has a branch?target buffer. If a branch is in the buffer and it is correctly predicted, there is no branch penalty. The prediction rate is 85% correct. If it is incorrectly predicted, the penalty is 4 cycles. If the branch is not in the buffer, and not taken, the penalty is 2 cycles. Seventy percent of branches are taken. If the branch is not in the buffer and is taken the penalty is 3 cycles. The probability that a branch is in the buffer is 90%. What is the average branch penalty?
What will be an ideal response?
Branch penalty = mispredict penalty (in buffer) + taken penalty (not in buffer) + not taken penalty (not in buffer) =
90% × 15% × 4 + 10% × 70% × 3 + 10% × 30% × 2
= 0.54 + 0.21 + 0.06 = 0.81 cycles per branch.
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