A computer has three kinds of instructions. The CPI for type A instructions is 5

Question

A computer has three kinds of instructions. The CPI for type A instructions is 5. For type B, the CPI is 6. And for type C, it is 12. For a particular application, 60% of the executed instructions are type A. 30% are type B. and the remaining 10% are type C. 1) What is the avernge CPI for this instruction mix? Compute the overall speed up (compared to the original system) for improvements that 2) Reduce the CPI for type A instructions from 5 to 3 3) Reduce the CPI for type B instructions from 6 to 4. 4) Reduce the CPI for type C instructions form 12 to 6

Explanation / Answer

1)Calculating the wighetd avrage for the instructions we get:

5 * 0.6 + 6 * 0.3 + 12 * 0.1 = 6

So avrage CPI for the mix instruction is 6

2) CPI of type A reduced from 5 to 3
   So we have
   0.6 * 3 + 6*0.3 + 12 * 0.1 = 4.8

   So avrage CPI for the mix instruction is 4.8 (20% speed up)

3)CPI of type B reduced from 6 to 4
   So we have
   0.6 * 5 + 4*0.3 + 12 * 0.1 = 5.4

   So avrage CPI for the mix instruction is 5.4 (10% speed up)

4)CPI of type B reduced from 6 to 4
   So we have
   0.6 * 5 + 6*0.3 + 6 * 0.1 = 5.4

   So avrage CPI for the mix instruction is 5.4 (10% speed up)

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