ITPEC FE Subject A October 2024 Question 9
ITPEC FE Subject A October 2024 — Question 9 of 60
Cache Average Access Time — calculate effective memory access time given hit/miss ratio.
Given: miss ratio = 0.2, cache = 75 ns, main memory = 1500 ns (includes cache check time).
Formula: T = hit_ratio × cache_time + miss_ratio × main_memory_time
Calculation:
- hit ratio = 1 - 0.2 = 0.8
- T = 0.8 × 75 + 0.2 × 1500
- T = 60 + 300 = 360 ns
Why not others:
- (a) 315 — incorrectly adds cache time to main memory time on miss (0.2 × (75 + 1500))
- (c) 1,215 — swaps hit and miss ratios (0.2 × 75 + 0.8 × 1500)
- (d) 1,260 — similar swap with additional calculation error
Key rule: Read carefully whether main memory access time includes the cache check. If yes, do NOT add cache time again on a miss.
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