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: $0.95 \cdot 100 + (1 - 0.95) \cdot 2 \cdot 100 = 0.95 \cdot 100 + 0.05 \cdot 200 = 105$ 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: $0.80 \cdot 85 + (1 - 0.80) \cdot 2 \cdot 85 = 0.80 \cdot 85 + 0.20 \cdot 170 = 102$ ns.