Assume that for a certain processor, a read request takes 50 nanoseconds on a…
2015
Assume that for a certain processor, a read request takes 50 nanoseconds on a cache miss and 5 nanoseconds on a cache hit. Suppose while running a program, it was observed that 80% of the processor's read requests result in a cache hit. The average read access time in nanoseconds is __________.
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Correct answer: 14
Key idea: compute the weighted average of hit time and miss time using the hit and miss rates.
Hit rate = 80% = 0.8, so miss rate = 1 - 0.8 = 0.2.
Hit time = 5 ns. Contribution from hits = 0.8 × 5 = 4 ns.
Miss time = 50 ns. Contribution from misses = 0.2 × 50 = 10 ns.
Average read access time = 4 ns + 10 ns = 14 ns.
Answer: 14 nanoseconds.
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