Discussion
If the TLB miss rate = 2% and memory access time = 60 ns, TLB lookup = 2 ns, page table walk = 100 ns, what is effective memory access time (EMAT) ?
Answer: D
The effective memory access time is 64 ns.
Every access first looks in the TLB (2 ns). On a hit (98%) the frame number is known and one memory access follows. On a miss (2%) the page table must be walked (100 ns) before the memory access.
\[ \begin{aligned} T_{hit} &= 2 + 60 = 62\ \text{ns} \\ T_{miss} &= 2 + 100 + 60 = 162\ \text{ns} \end{aligned} \]
\[ \begin{aligned} \text{EMAT} &= 0.98(62) + 0.02(162) \\ &= 60.76 + 3.24 = 64\ \text{ns} \end{aligned} \]
62 ns is the hit time alone and 162 ns the miss time alone; neither weights the two cases. 164 ns matches the miss path with the TLB lookup counted twice. Because misses are rare, the answer sits just above the hit time.
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