Discussion
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 ?
Answer: A
The average access time is 157.5 nano sec.
Reads: a hit takes the cache time, 50 ns; on a miss the cache is checked and then main memory is read, 50 + 300 = 350 ns. Writes: with write-through, every write goes to main memory, 300 ns.
\[ \begin{aligned} t_{read} &= 0.8(50) + 0.2(350) = 110 \\ t &= 0.75(110) + 0.25(300) \\ &= 82.5 + 75 = 157.5\ \text{ns} \end{aligned} \]
110 ns is the read time alone and 82.5 ns is the read share only, both ignoring the writes; 75 ns is just the write share.
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