Computer architecture Question 5) You have the following characteristics on your

Question

Computer architecture

Question 5) You have the following characteristics on your company’s processor for a
certain benchmark, which runs at 300 MHz:

Arithmetic and logical - frequency 40% - cycles 1
Load and store - frequency 25% - cycles 2
Branches - frequency 20% - cycles 3
Floating Point(FP) - frequency 15% - cycles 6

You are asked to consider a cheaper, lower-performance version of this processor, by removing
some of the FP hardware to reduce the die size. FP instructions in the new chip will now take 13
cycles to execute. What are the old and new CPI (Cycles Per Instructions)?

Explanation / Answer

Let there be a total of N instructions.

Therefore,

Number of arithmetic and logical instructions = 0.4N

Number of load and store instructions = 0.25N

Number of branch instructions = 0.2N

Number of floating point instructions = 0.15N

Total cycles required to execute all arithmetic and logical instructions = 0.4N*1 = 0.4N

Total cycles required to execute all load and store instructions = 0.25N*2 = 0.5N

Total cycles required to execute all branch instructions = 0.2N*3 = 0.6N

Total cycles required to execute all floating point instructions = 0.15N*6 = 0.9N

Therefore, total time taken to execute all instructions = 0.4N + 0.5N + 0.6N + 0.9N = 2.4N

Therefore, CPI of old hardware = 2.4N/N = 2.4

For new hardware, total time taken to execute all floating point instructions = 0.15N*13 = 1.95N

Therefore, total time taken to execute all instructions on new hardware= 0.4N + 0.5N + 0.6N + 1.95N = 3.45N

Therefore, CPI of new hardware = 3.45N/N = 3.45

Thus,

Old CPI = 2.4

New CPI = 3.45

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