A computer has the following parameters.





If the average performance of the computer (in terms of its CPI) is to be increased by 20% while executing the

same instruction mix, what target must be achieved for the cycles per conditional branch instruction?


This question assumes that all other factors are the same. The current performance is 0.65 × 1 + 0.10 × 5 + 0.05
× 2 + 0.20 × 8 = 0.65 + 0.50 + 0.10 + 1.60 = 2.85 CPI.

If the performance is improved by 20%, the new CPI will be 2.85 × 0.80 = 2.28. We can now plug this in using
the previous figures

0.65 × 1 + 0.10 × 5 + 0.05 × 2 + 0.20 × B = 2.28 (where B is the cycles per branch)
0.65 + 0.50 + 0.10 + 0.20B = 2.28
0.20B = 2.28 ? 1.25 = 1.03
B = 1.03/0.20 = 5.15.

Therefore conditional branch instructions must take, on average, 5 cycles.

Computer Science & Information Technology

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