MadeEasy Subject Test: CO & Architecture - Cache Memory

Question

ASAIK for Tavg(read) =  T(read) = HR(read) * Tc + (1-HR) * (Tc+Tm)

but while calculating they have neglected Tc in (Tc+Tm)...

Please verify ??

Comments

please can u say where im wrong 

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Your read calculation is correct.

But for write operation in write-through protocol, you should remember that every time you have to access the main memory for each write operation.

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why cache access time is not considered in write operation ?

what i think is

cache data is updated and on update the memory access is done

but to perform operation on data cache is also to be accesed

why we are not doing that

Answer 1

given

Cache access time = 45ns

Main memory access time =750ns

hit ratio for read = 0.9

hit ratio for write = 1 { which means they are using simultaneous access}

avg time for read = hit in cache * access time of cache + miss in cache * MM access time

                               =0.9*45+0.1*750 =115.5ns

avg time for write =  1*750ns {directly accessing last level for write} = 750ns

avg time for read and write = 0.75*115.5 +0.25*750 =274.125