Average access time in two level cache system
First Let's define all given things


L1 Cache hit: 90% &amp; Time: 1ns


L2 and RAM has hit rate 10% out of which 95% which is 9.5% in total


L2 Cache hit: 9.5% &amp; Time 20ns


RAM has hit rate 5% of 10% = 0.5%


RAM hit: 0.5% &amp; Time 220ns


Formula Avg Memory Access Time

AMAT = L1_hit * L1_T + L2_hit * L2_T + RAM_hit * RAM_T

AMAT = 0.9*1 + 9.5*20 + 0.5*220

AMAT = 300.9ns

What is 2 level cache system:

First cache called L1 is reside on CPU is too fast. When CPU needs data, it checks in L1 cache but if it is not there it will go to L2. L2 cache is sometimes on CPU or outside CPU it depends on architecture of CPU. It does similarly and here also it missed then it fetch data from RAM which is expensive process.