Consider a 6-stage instruction pipeline, where all stages are perfectly…
2014
Consider a 6-stage instruction pipeline, where all stages are perfectly balanced.Assume that there is no cycle-time overhead of pipelining. When an application is executing on this 6-stage pipeline, the speedup achieved with respect to non-pipelined execution if 25% of the instructions incur 2 pipeline stall cycles is ______________________.
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Correct answer: 4
Key insight: a perfectly balanced 6-stage pipeline ideally issues one instruction per cycle, so without stalls the ideal speedup is 6.
Compute average cycles per instruction (CPI) on the pipelined machine: ideal CPI 1 plus stall penalty. 25% of instructions incur 2 stall cycles, so average pipelined CPI = 1 + 0.25 × 2 = 1.5.
Non-pipelined execution takes all 6 stages serially for each instruction, so non-pipelined CPI = 6.
Speedup = (non-pipelined CPI) / (pipelined average CPI) = 6 / 1.5 = 4.
Answer: 4
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