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In the case of Hit writing time would be ....WT of cache  + WT of MM(as write through) = 40+50

In case of miss writing time would be...Miss penalty+ WT in MM+ WT in cache(again we have to write in cache)

                                                                = 40+50+40

I don't know ...whether it is right or not.
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you can see all the answered questions in "cache-memory" here first. I'm sure if I take ME test for CO I'll score negative...

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L1 miss 65% means 1-H = 65 % and H = 35%

formula is :

TAvg writing Time  = H * T mem update time + ( 1 - H ) * ( T mem update time ) 

= 0.35 * 50  + 0.65 * 50 

=  17.5 +  32.5

= 50

So average writing time  for system is 50 ns .

In write through both cache and MMU update same time .

 Write Through  uses Write No-Allocate policy, in Write No-Allocate Missed main memory block is updated while residing in the Main memory itself and NOT brought to the cache , that's why only MMU update time is consider here . 

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In case of cache hit, we consider memory update time  in our formula , because it uses Write through technique, where cache and MMU update at same time.

In case of cache Miss , we consider mem update time again in formula , because Write Through  uses Write No-Allocate policy, in Write No-Allocate, Missed main memory block is updated while residing in the Main memory itself and NOT brought to the cache.

Answer:

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