CPUs and architecture

If the cache access time is 10 ns, the main memory access time is 100 ns, and the cache hit rate is 90%, what is the average access time?

  1. A19 ns
  2. B55 ns
  3. C91 ns
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Answer: A. 19 ns

Average access time = hit rate × cache access time + (1 − hit rate) × main memory access time = 0.9 × 10 + 0.1 × 100 = 9 + 10 = 19 ns. A small drop in hit rate greatly lengthens the average access time, so cache design directly affects performance.

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