Locality of Reference:

Principle that memory accesses are clustered, not random — basis for caches and virtual memory.

• Temporal Locality: Recently accessed locations tend to be accessed again soon (e.g., loops, stack variables). → Keep them in cache.
• Spatial Locality: If a location is accessed, nearby locations will be accessed soon (e.g., sequential instructions, arrays). → Fetch blocks/lines.

Due to locality, a small fast cache can achieve hit rate ~90-95% and make average access time close to cache speed despite large, slow main memory.