Processor A has a CPI 1.2 and a clock rate of 1 GHz. Processor B has a CPI of 2.

Question

Processor A has a CPI 1.2 and a clock rate of 1 GHz. Processor B has a CPI of 2.0 and a clock rate of 2 GHz. Processor C has a clock rate of 2.5 GHz (400 ps/cycle) and a CPI of 2.6. All three processors implement the same instruction set. Determine the time per instruction for each processor and write it in the correct spot. Which processor is faster? How many times faster is it than the slowest processor? Show your work! Processor A time per instruction ______ Processor B time per instruction ______ Processor C time per instruction ______ Which is fastest? How many times faster than the slowest?

Explanation / Answer

Processor A:

CPI is cycles per instruction. So, Processor A takes 1.2 Clock cycles to execute 1 instruction. The clock for processor A is 1Ghz, so each clock cycle takes t = 1 ns (nano second).

So for processor A, time per instruction = CPI * t = 1.2 * 1ns = 1.2 ns

Processor B:

CPI is cycles per instruction. So, Processor B takes 2 Clock cycles to execute 1 instruction. The clock for processor B is 2 Ghz, so each clock cycle takes t = 1 / 2 Ghz = 0.5 ns (nano second)

So for processor B, time per instruction = CPI * t = 2 * 0.5ns = 1 ns

Processor C:

Processor C takes 2.6 Clock cycles to execute 1 instruction. The clock for processor B is 2.5 Ghz, so each clock cycle takes t = 1 / 2.5 Ghz = 400 ps (pico seconds) = 0.4 ns (nano second)

So for processor B, time per instruction = CPI * t = 2.6 * 0.4 ns = 1.04 ns

From the above calculations, it can be observed that processor B takes least amount of time to execute one instruction, and so it is the fastest. The slowest is Processor A. It is 1.2 times faster.

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