Define HIT and MISS ratio in memory with an example.

The hit ratio is the fraction of CPU memory references that are found in the cache. The miss ratio is the fraction that are not found, so

$$\text{hit ratio} + \text{miss ratio} = 1$$

For example, suppose the CPU makes 200 references and 180 are found in the cache. Then the hit ratio is $180/200 = 0.9$ and the miss ratio is $20/200 = 0.1$. On a miss, the block must be fetched from the slower main memory.