A processor has a branch-target buffer. If a branch is in the buffer and it is c

Question

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 two 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 if 90%. What is the average branch penalty? 1.575 1.23 0.81 0.5

Explanation / Answer

p(B) = 90%

PR(Branch present) = 85%

Percentage of Branch taken (Branch not present) =70%

Average Branch Penalty = p(B)*[(1- PR(Branch present)) * 4] + [1-p(B)] * [70% *3 +30%*2]

= .9 * .15 * 4 + .1 * [.21 + .6] = .54 + .27 = 0.81

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