GATE | GATE-CS-2006 | Question 42

A CPU has a five-stage pipeline and runs at 1 GHz frequency. Instruction fetch happens in the first stage of the pipeline. A conditional branch instruction
computes the target address and evaluates the condition in the third stage of the pipeline. The processor stops fetching new instructions following a conditional branch until the branch outcome is known. A program executes 109 instructions out of which 20% are conditional branches. If each instruction takes one cycle to complete on average, the total execution time of the program is:
(A) 1.0 second
(B) 1.2 seconds
(C) 1.4 seconds
(D) 1.6 seconds


Answer: (C)

Explanation: In the 3rd stage of pipeline, there will be 2 stall cycles i.e. 2 delay slots.

Total number of instructions = 109

20% out of 109 are conditional branches.

Therefore, Cycle penalty = 0.2 * 2 * 109 = 4 * 109

Clock speed is 1 GHz and each instruction on average takes 1 cycle.

Total execution time = (109 / 109) + 4 * (108 / 109)
= 1.4 seconds

 
Thus, total execution time of the program is 1.4 seconds.

 
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