Given the following information, calculate the speedup. Assume that the lw instr

Question

Given the following information, calculate the speedup. Assume that the lw instruction has a CPI of 5 but all other instructions in our MIPS subset (including beq and j) have a CPI of 4. A program consisting of 2,800,460 instructions (all taken from our MIPS subset) is executed on the multi-cycle implementation of our MIPS data-path running at a fixed 1 GHz clock rate. The program uses the following mix of instructions: 40% R-type, 20% lw, 10% sw,   20% beq and 10% j. What speedup would result if the j (jump) and beq instructions could be improved so that they each require 3 cycles rather than 4 cycles and this is the only improvement?

Explanation / Answer

Solution:

Number of instructions= 2,800,460 instructions?

The mix instruction 40% R-type, 20% lw, 10% sw,   20% beq and 10% j

Average CPI without modification is= 0.4*4 + 0.2*5 + 0.1 * 4 + 0.2 * 4 + 0.1 * 4= 4.2

Now that beq is improved and required 3 cycles only

the new average CPI will be= 0.4*4 + 0.2*5 + 0.1 * 4 + 0.2 * 3 + 0.1 * 4= 4.0

The speedup= 4.2/4= 1.05

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