Cache memory | Definition and concept

Cache memory | Definition and concept

Cache memory is a special memory that acts between main memory and CPU (Central Processing Unit). Cache memory is a fast memory, managed entirely by hardware. Cache memory is not visible to CPU. It contains most recently data used by the CPU. It uses SRAM technology, which is faster but costly.

If CPU request an element, then this element is first search in Cache memory but if element is not present in cache memory, then this element is search in main memory. If this element is again not present in main memory, this condition is called page fault.

The efficiency of cache memory is measured by cache hit ratio.

Cache hit : If CPU request an data and this data exists in cache memory, then it is called cache hit and no involvements of main memory is required to complete the operation.

Cache miss : If CPU request an data and this data does not exists in cache memory, then such situation  is called cache miss.

Cache memory operates on two major principles of locality:

Temporal locality: Data which have been used most recently by CPU is more likely to be used again.

Spatial locality: If the data is referenced, it is very likely that nearby data will be accessed soon.

Cache levels

In a system, more than one cache can be used. These caches are organised into levels from one to four.

Need of levelling

Larger cache memory is very costly. It has very hit rate but longer latency (delay). Multilevel cache generally operates by checking the smallest cache first. If it hits, processor processes at very speed and if misses, then next larger cache will be checked and so on for last higher level cache.

Level 1 ranges from 8 kb to 64 kb, level 2 ranges from 64 kb to 8 mb, level 3 ranges from 4 mb to 256 mb, level 3 is also known as victim cache and level 4 is greater than 512 mb and known as remote cache.

Cache

1.   Inclusive cache: Inclusive cache means that content of level 1 cache memory are also available in level 2. We can say that level 1 is subset of level 2.

2.   Exclusive cache: Exclusive cache means that the data is stored in one cache level only.