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A hypothetical 5 stage processor is designed in which branch is predicted at 3 stage and each stage takes 1 cycle to compute its task. If f is the probability of an instruction being a branch instruction then what is the value of F such that speed up is atleast 3?

1 vote

As the branch is predicted at third stage, there will be 2 stall cycles.

Speedup > 3

5 / (1+f*2) > 3

5 > 3+6f

6f < 2

f < 0.33

The probability that instruction being branch, so that the speed up is at least 3 is 0.33

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Thanks...
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@balchandar reddy san what exactly formula you used?

5/1+2p can you explain/

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Speedup is given as:

Number of stages / (1 + dependency/branch instruction frequency *  no.of stalls )

dependency/branch instructions in terms of probability ranging from 0-1.

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