A computer has a cache, main memory, and a disk used for virtual memory. If a re

Question

A computer has a cache, main memory, and a disk used for virtual memory. If a referenced word is in the cache, 20 ns are required to access it. If it is in main memory but not in the cache, 60 ns are needed to load it into cache (this includes the time to originally check the cache), the reference is started again. If the word is not in main memory, 12 ms are required to fetch the word from disk, followed by 60 ns to copy it to the cache, then the reference is started again. The cache hit ratio is 0.9 and the main memory hit ratio is 0.6, what is the average time in ns required to access a referenced word in the system.

Explanation / Answer

Tavg=hitcache × timecache + ( 1hitcache)( hitmemory) [timecache + timememory] + (1 hitcache) (1 hitmemory)[timecache + timememory+timedisk]

= .9 * 20 + .1( .6 * (60 +20) + .4 ( 60+20+12000000) )

= 480026 ns.

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