Consider a paging system where translation lookaside buffer $($TLB$)$, a special type of associative memory, is used with a hit ratio of $80%$.

Assume that memory reference takes $80$ nanoseconds and reference time to TLB is $20$ nanoseconds. What will be the effective memory access time given an $80%$ hit ratio?

TLB access time $=20$ ns

Memory access time $=80$ ns

Hit ratio $=80%=0.8$

Miss ratio $=20%=0.2$

For TLB hit:

Effective time $=20+80=100$ ns

For TLB miss:

First TLB is checked, then page table is accessed from memory, then actual memory is accessed.

Effective time $=20+80+80=180$ ns

Now,

Effective memory access time:

$EAT=0.8(100)+0.2(180)$

$EAT=80+36$

$EAT=116$ ns