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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 ? 

  1. $157.5$ n.sec. 
  2. $110$ n.sec.
  3. $75$ n.sec.
  4. $82.5$ n.sec. 
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2 Answers

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Option A). 

TRead = 0.8 * 50 + 0.2 * (300 + 50) = 110 ns

TWrite = 1 * max(300,50) = 1* 300 = 300 ns

Hence, Average access time for both read and write requests = 0.75 * 110 + 0.25 * 300

= 157.5 ns

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The average access time of the system for memory read cycle is
0.8x50+0.2x(300+50)=110ns
The average access time of the system for both read and writes request is
0.75x110+0.25x300=82.5+75=157.5
 
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