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

Question

A computer has a cache, main memory and disk used for virtual memory. If a reference word is in the cache 5 ns are required to access it. If it is in the main memory but not in the cache, 50ns are required to load it into the cache and then reference is started again. If the word is not in the main memory, 11ms are required to fetch from disk, followed by 50ns to copy it into the cache and then the reference is started again. The cache hit ratio is 0.8 and the main memory his ratio is 0.6.

What is the average time in nanoseconds required to access a referenced word on this system?

Please provide an explanation if possible.

Explanation / Answer

Dear,         Given data:                  In the cache,reference word takes time to access = 5ns                                                                      cache hit ratio = 0.8                  In the main-memory,the reference word takes time to access =50ns                                                                  memory hit ration =0.6                  In the disk,the reference word takes time to access =11ms     Average memory access time(AMAT)                                      =Hit-time+Miss-rate*Miss-penality     Hit-time(L1) =5ns     Miss-rate(L1)=1-0.8                           =0.2     Hit-time(L2)=50ns     Miss-rate(L2)=1-0.6                           =0.4     Hit-time(L3)=11ms                         =11*10^6 ns     Miss-rate(L3) = 0     Therefore,AMAT = 5+0.2*(50+0.4*(11*10^6 +0))                                  =5+0.2*(50+0.4*(11*10^6))                                  =(5.2)*(50.4)*(11*10^6)                                  =2.882*10^9                                  =2.882ns                                                                           

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