Consider a processor executing a program consisting of 3 different instructions

Question

Consider a processor executing a program consisting of 3 different instructions classes (A, B, C), each with the same CPI, and with distributions A=40%, B=40%, C=20%. As currently implemented, the program executis in 500 milliseconds. How long would it take for the program to fully execute if we modify the processor such that the speed of class B instructions is doubled (i.e., class B instructions take half as long after the improvement)? In this scenario, how much faster is the improved processor relative to the original?

Explanation / Answer

Solution:

if the speed of the class B instruction is doubled then CPI will be affected and now the effective CPI will be 0.4 + 0.2 + 0.2 = 0.8

So the original CPI was 1 and the new is 0.8.

so 1/0.8 = 1.25

which means the modified processor is 1.25 times faster compared to the old one.

and overall CPU time will be = 500/1.25 = 400 ms.

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