ITPEC FE Morning April 2018 Question 10
ITPEC FE Morning April 2018 — Question 10 of 80
Cache effective access time — calculate average memory access time with hit ratio.
Given: cache = 0.01 µs, main memory = 0.1 µs, hit ratio = 90%. On a cache miss, data is transferred from main memory to cache, then read from cache — miss access time = 0.11 µs.
Formula:effective_time = hit_ratio × cache_time + (1 - hit_ratio) × miss_time
Calculation:
- Hit: 0.9 × 0.01 = 0.009
- Miss: 0.1 × 0.11 = 0.011
- Total: 0.009 + 0.011 = 0.020 µs
Answer: c) 0.020 µs
Why not others:
- (a) 0.011 — would require ~99% hit ratio
- (b) 0.019 — incorrectly uses 0.1 as miss time, ignoring the explicit miss penalty of 0.11
- (d) 0.091 — inverts the hit ratio (treats 90% as miss rate)
Key rule: When the problem explicitly states the miss access time (here 0.11 µs), use that value directly — do not substitute the raw main memory time.
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