GATE | GATE CS 2011 | Question 65

Consider an instruction pipeline with four stages (S1, S2, S3 and S4) each with combinational circuit only. The pipeline registers are required between each stage and at the end of the last stage. Delays for the stages and for the pipeline registers are as given in the figure:

GATECS2011Q41

What is the approximate speed up of the pipeline in steady state under ideal conditions when compared to the corresponding non-pipeline implementation?
(A) 4.0
(B) 2.5
(C) 1.1
(D) 3.0


Answer: (B)

Explanation:



Pipeline registers overhead is not counted in normal 
time execution

So the total count will be

5+6+11+8= 30 [without pipeline]

Now, for pipeline, each stage will be of 11 n-sec (+ 1 n-sec for overhead).
and, in steady state output is produced after every pipeline cycle. Here,
in this case 11 n-sec. After adding 1n-sec overhead, We will get 12 n-sec
of constant output producing cycle.

dividing 30/12 we get 2.5 


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