Our favorite program runs in 10 seconds on computer A, which has a 4 GHz clock.

Question

Our favorite program runs in 10 seconds on computer A, which has a 4 GHz clock. We are trying to help a computer designer build a new machine B, that will run this program in 6 seconds. The designer can use new (or perhaps more expensive) technology to substantially increase the clock rate, but has informed us that this increase will affect the rest of the CPU design, causing machine B to require 1.2 times as many clock cycles as machine A for the same program. What clock rate should we tell the designer to target?

Explanation / Answer

Let C = number of cycles Execution time = C X clock cycle time = C/ clock rate

On computer A, C/ 4GHz = C/ 4X 10^9 = 10 seconds => C = 4 X 10^10 On computer B, number of cycles = 1.2XC

What should be B’s clock rate so that our favorite program has smaller execution time?

1.2 X C/ clock rate < 10 => 1.2 X 4X 10^10/ 10 < clock rate I.e. clock rate > 480 MHz

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