A TLB lookup takes 10 ns, a memory access takes 100 ns and the TLB hit ratio is 90 percent. What is the effective memory access time with a single-level page table?
Correct answer A. 120 ns
Explanation
A hit costs 10 + 100 = 110 ns. A miss costs 10 + 100 + 100 = 210 ns, because the page table in memory must be read before the data. EAT = 0.9 x 110 + 0.1 x 210 = 99 + 21 = 120 ns. The answer 110 ns ignores misses entirely, and 200 ns is the cost with no TLB. Higher hit ratios bring EAT closer to the raw memory time.
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