A hierarchical memory system that uses cache memory has cache access time of…
2014
A hierarchical memory system that uses cache memory has cache access time of 50 nano seconds, main memory access time of 300 nano seconds, 75% of memory requests are for read, hit ratio of 0.8 for read access and the write-through scheme is used. What will be the average access time of the system both for read and write requests ?
- A.
157.5 n.sec.
- B.
110 n.sec.
- C.
75 n.sec.
- D.
82.5 n.sec.
Attempted by 150 students.
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Correct answer: A
Given: cache access time = 50 ns, main memory access time = 300 ns, reads = 75% of requests, read hit ratio = 0.8, write-through policy.
Average read access time = cache access time + (miss fraction) × main memory time = 50 + (1 - 0.8) × 300 = 50 + 60 = 110 ns.
Average write access time (write-through): each write goes to main memory, so the dominant time is the main memory access. Using that assumption, write time ≈ 300 ns. (If cache access is also counted separately, one could use 50 + 300 = 350 ns; the standard simplification here uses 300 ns.)
Overall average across all memory requests = 0.75 × (read average) + 0.25 × (write average) = 0.75 × 110 + 0.25 × 300 = 82.5 + 75 = 157.5 ns.
Answer: Overall average access time = 157.5 ns. Per-type: read ≈ 110 ns, write ≈ 300 ns (under the write-through assumption used above).
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