Consider a system with memory mapping done on a page basis and using a single le

Question

Consider a system with memory mapping done on a page basis and using a single level page table. Assume that the necessary page table is always in memory. a. If a memory reference takes 200 ns, how long does a paged memory reference take? b. Now we add a TLB that imposes zero time overhead on a hit or a miss. If we assume that 85% of all memory references hit in the TLB, i.e., a hit ratio of 85%, what is the Effective Access Time (EAT)? c. In practice, TLB does impose a small overhead. Suppose the overhead due to TLB access is 5ns for a hit or a miss, what is the EAT with a hit ratio of 90%?

Explanation / Answer

a) In memroy referencing, a paged memory reference need 2 memory accesses, one to get the frame number and other for actual accessing of the data.

So, if memory reference takes 200 ns so page memory reference will take 200*2 = 400 nanoseconds.

b) If the hit ratio is 85%, then effective memory access time will be calculates as below:

cache access time + (1-hit rate)* main memory access time

= (85/100) * 200 + (0.15) * 400

= 170+60 = 230 nanoseconds.

c) EAT = hit ratio(TLB + Main Memory access time) + (1-hit ratio) ( TLB + 2* MMA Time)

= (90/100) ( 5 + 200) + (1-0.90)* ( 5 + 2*200)

= 184.50 + 40.50 = 225ns

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