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Suppose the cache access time is 15ns, main memory access time is 220ns, and the cache hit rate is 95%. Assuming parallel (overlapped) access (CPU starts the data request in parallel to both cache and to main memory at the same, so if a cache missing occurs, we don't have to add this cache search time to the memory access), what is the average access time for the processor to access an item

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  1. 17 June, 15:09
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    Check the explanation

    Explanation:

    Generally, average access time is:

    hit rate * (cache access time) / /since data found in cache

    +

    miss rate * (cache access time + time to hit memory to get data from memory) / / miss occurred

    In this case, parallel access is happening, so we don't need to add cache access time when

    a miss occurs. Therefore, in this case the formula becomes:

    avg (time) = hit rate * (cache access time)

    +

    miss rate * (time to hit memory to get data from memory)

    Given,

    hit rate = 0.95

    miss rate = 1 - hit rate = 1-0.95 = 0.05

    cache access time = 15 ns

    memory access time = 220 ns

    Put values in formula:

    avg (time) = 0.95 * 15 + 0.05 * 220

    =25.25 ns
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