GATE | Gate IT 2007 | Question 6

A processor takes 12 cycles to complete an instruction I. The corresponding pipelined processor uses 6 stages with the execution times of 3, 2, 5, 4, 6 and 2 cycles respectively. What is the asymptotic speedup assuming that a very large number of instructions are to be executed?
(A) 1.83
(B) 2
(C) 3
(D) 6


Answer: (B)

Explanation: For non pipeline processor,
It takes, 12 cycles to complete 1 instruction
So, for n instructions it will take 12n cycle
For pipelined processor,
Each stage time = max{each stage cycles} = max{3, 2, 5, 4, 6 and 2} = 6 cycles
So, for n instructions it will take = 6*6+ (n-1)*6 {6*6 for 1st instruction and for rest of n-1 it will take 6}
For a large number of instructions:
Limn->∞ 12n/36 + (n-1)*6 = 12/6 =2

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