Consider a classical 5-stage pipeline (Fetch, Decode, Execute, Memory, Writeback

Question

Consider a classical 5-stage pipeline (Fetch, Decode, Execute, Memory, Writeback), Consider an equal execution time across all stages. Also assume that 15% of instructions are conditional branches. Answer the following questions: What is the ideal achievable speedup over non-pipelined data path, assuming no branch prediction You are assigned to design a branch prediction for this pipelined data path. What would be the minimum acceptable prediction confidence for your designed branch prediction, if miss-prediction cost is 20 cycles.

Explanation / Answer

a) The ideal achievable speedup over non pipelined data path when no branch prediction can be given as,

= 5x speedup considering CPI 1.

as speed up can be given as,

Speedup = old CT / new CT.

or

speedup = (Avg execution time on non pipeline system) / (Avg execution time on pipeline system)

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