A program uses four major instruction types. The instruction mix and the CPI…
2021
A program uses four major instruction types. The instruction mix and the CPI for each type are given in the table. If the processor clock frequency is 400 MHz, what is the processor's average CPI?
Answer: B. 2.24 — ConceptAverage CPI is a weighted mean: each instruction type's CPI is multiplied by its fraction of the instruction mix. If the mix fractions sum to 1, then…
- A.
3.75
- B.
2.24
- C.
1.87
- D.
1.54
- E.
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Correct answer: B
Concept
Average CPI is a weighted mean: each instruction type's CPI is multiplied by its fraction of the instruction mix.
If the mix fractions sum to 1, then Average CPI = Σ(CPI × instruction-mix fraction). Clock frequency affects execution time, but it is not needed to compute CPI.
Application
Convert the percentages to fractions: 60% = 0.60, 18% = 0.18, 12% = 0.12, and 10% = 0.10.
Compute the weighted contribution of each type: 1 × 0.60 = 0.60; 2 × 0.18 = 0.36; 4 × 0.12 = 0.48; 8 × 0.10 = 0.80.
Add the contributions: 0.60 + 0.36 + 0.48 + 0.80 = 2.24 cycles per instruction.
Cross-check
The mix fractions sum to 0.60 + 0.18 + 0.12 + 0.10 = 1.00, so the weighted-average setup is complete.
For 100 instructions, the expected cycle count is 60 + 36 + 48 + 80 = 224 cycles; 224/100 = 2.24 CPI.
Result
The processor's average CPI is 2.24.