# Query related to cache memory

Discussion in 'Engineering Concepts' started by aryan123, Jan 9, 2011.

1. ### aryan123New Member

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I request to all the ppl who hv knwledge abt this topic.. atleast leave a hint please.

The access time of a cache memory is 100 ns and that of main memory 1000 ns. It is estimated that 80% of the memory request are for read and 20% for write. The hit ratio for READ ACCESS only is 0.9 , A write through procedure is used.

1. What is the average access time of the system considering only memory read cycle?

2. What is the average access time of the system for both read and write requests?

3. What is the hit ratio taking into consideration the write cycle?

2. ### xpi0t0sMentor

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Tricky to leave hints without answering the questions for you. These are not difficult questions and you have all the information you need.

1. I find it can help to use a specific number when working out averages. So let's say you have 100 read requests. How many will be from cache (HINT: look at the hit ratio)? How long will that take (HINT: look at the access time, then call this value T1)? How many will be from main memory? How long will that take (call this T2)? How long will all 100 requests take (HINT: a simple operation containing T1 and T2). Now, how do you calculate averages?

2. OK, let's do the same as above, but add in write requests. Let's have 100 requests total, that's 80 reads and 20 writes. Recalculate T1 and T2 for 80 reads. Now calculate T3 which is the total time of the writes (HINT: what is the access time for main memory). Now calculate the average.

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