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Computer Science Question....

M

Moonark

Senior member
Can anyone figure this out...

The access time for a cache memory is 30ns and that for primary memory is 125ns. (1ns = 1nanosecond = 10^(-9) sec)
75% of memory accesses are for RD's and the hit ratio is 0.85. The cache uses write through on WRs.

a) What is the average access time for the RDs?
b) What is the average access time for the WRs?
c) What is the hit ratio for WRs?
d) What is the overall access time for all memory accesses(that is including all RDs and WRs)?
e) What is the overall hit ratio for all memory accesses?

I have absolutely no idea how to do this.....
 
I'll try to answer part (a) for you, once you can do part (a) the
rest is easy.

a) What is the average access time for the RDs ?

Hit ratio is 0.85, meaning that 85% going to be
accessing cache instead of accessing memory, i.e
if you have 100 RDs, 85 RDs is accessing cache and
15 RDs have to access the primary memory.

Thus, you have 0.85 * 30ns + 0.15 * 125 ns = 44.25 ns

ANSWER : average access time for the RDs = 44.25 ns

And part (b-e) can be answered once you understand this.
 
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