A computer system has three levels of memory: Cache, Main Memory, and…

1. Memory hierarchy concept

   • Computer memory is organized in multiple levels: cache, main memory, and secondary memory.

   • Faster memory is smaller and expensive, while slower memory is larger and cheaper.

     Introduction to Memory Hierarchy

2. Basic hierarchy structure

CPU
 │
Cache Memory
 │
Main Memory (RAM)
 │
Secondary Memory (Disk)

1. Need for locality

   • CPU frequently accesses only a small portion of memory at a time.

   • If frequently used data stays in cache, access time becomes very small.

2. Types of locality

   Temporal locality

   • Recently used data is likely to be used again soon.

   • Example: loop variables.

   Spatial locality

   • Nearby memory locations are likely to be accessed.

   • Example: accessing elements of an array.

3. Example

for(i=0;i<100;i++)
{
   sum = sum + A[i];
}

Explanation

• A[0], A[1], A[2] are stored close to each other.

• Once A[0] is fetched, nearby values may already be in cache.

• This demonstrates spatial locality.

1. Impact on performance

   • If data is found in cache → cache hit → very fast access.

   • If data not found → cache miss → data fetched from slower memory.

2. If locality did not exist

   • CPU would constantly access main memory or disk.

   • Cache would become useless.

   • Average memory access time would increase drastically.

3. Conclusion

   • Locality of reference makes cache memory effective and improves system performance significantly.