Paging

The percentage of time that the desired information is found in the TLB is termed the hit ratio. The miss ratio is equal to (1 - hit ratio).

We can compute the average memory access time by knowing the (1) hit ratio and (2) the speed of accessing a physical memory address. This is possible because we know that we access memory only once if the entry is in the TLB and twice if it is not in the TLB.

Examples:

• If the hit ratio is standing at 95%, and the speed of accessing a physical memory is 100 nanoseconds (ns), the average memory access time is: 0.95 * 100 + (1 - 0.95) * 2 * 100 = 0.95 * 100 + 0.05 * 200 = 105 ns.
• If the hit ratio is standing at 80%, and the speed of accessing a physical memory is 85 nanoseconds (ns), the average memory access time is: 0.80 * 85 + (1 - 0.80) * 2 * 85 = 0.80 * 85 + 0.20 * 170 = 102 ns.