# Pipeline question

I have this multiple choice question from a computer architecture class, a mips processor executes a program at 10 sec, without pipelining and clock rate 100 MHz. When running the program on a processor with 500 MHz clock rate and 5 stages of pipelining which of the following execution times isn't possible. a:2,5 sec, b:3 sec, c:1,5 sec, d:3,3 sec.

My approach to it was to find the cycles of the clock for executing the program on the first processor which would be execution time * clock rate = 1000 * 10^6. From that the number of instructions would be 1000 * 10^6/ first processor's CPI. Since the second processor uses 5 stages of pipelining, I assume it's CPI to be 1 and thus the execution time for it should be 1000 * 10^6/(500 * 10^6 * first processor's CPI) = 2/first processor's CPI and since the CPI of the first processor would have to be 1 or higher the execution time for the second processor would have to be lower than 2 which would mean all choices are correct but c, yet we have been told that the correct answer is c. So c is in reality the only execution time that isn't possible for the second processor but I simply can't figure out why.

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– Raphael
Sep 9, 2018 at 22:08