Definition:
$$\text{Hit Ratio } (h) = \frac{\text{Number of hits}}{\text{Total number of accesses}} = \frac{\text{Hits}}{\text{Hits + Misses}}$$

$$Miss\ Ratio = 1 - h$$

Where hit = data found in faster memory (Cache), miss = data not found and must be fetched from lower level (Main memory).

Hit ratio is usually expressed for cache: $h_{cache}$, also applicable to TLB, etc.

Significance - AMAT (Average Memory Access Time):

The effective access time depends critically on hit ratio:

$$AMAT = h \times T_{hit} + (1-h)\times T_{miss}$$

or for hierarchical memory:

$$AMAT = T_{cache} + (1-h) \times T_{memory}$$

• High $h$ ($\approx$ 0.9-0.99 due to locality of reference) → $AMAT \approx T_{cache}$ → system performs near cache speed, despite large cheap main memory.
• Low $h$ → AMAT → approaches main memory time, cache is ineffective and penalty dominates.
• Used to evaluate/design cache: organization, block size, associativity, replacement policy all aim to maximize $h$. Helps in deciding cache size vs. cost trade-off and in calculating effective CPU stall cycles due to memory.

Example: $T_{cache}=10ns$, $T_{memory}=100ns$, $h=0.95$ → $AMAT = 0.95\times10 + 0.05\times(10+100)=14.5ns$ vs $100ns$ without cache.