1.7 Cache Memory

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Cache Memory

Cache Memory is a special very high-speed memory. It is used to speed up and synchronizing with high-speed CPU. Cache memory is costlier than main memory or disk memory but economical than CPU registers. Cache memory is an extremely fast memory type that acts as a buffer between RAM and the CPU. It holds frequently requested data and instructions so that they are immediately available to the CPU when needed.

Cache memory is used to reduce the average time to access data from the Main memory. The cache is a smaller and faster memory which stores copies of the data from frequently used main memory locations. There are various different independent caches in a CPU, which store instructions and data.

Figure \(\PageIndex{1}\): Cahce Memory. ("Cache Memory" by VaibhavRai3, Geeks for Geeks is licensed under CC BY-SA 4.0)

Levels of memory:

Level 1 or Register
It is a type of memory in which data is stored and accepted that are immediately stored in CPU. Most commonly used register is accumulator, Program counter, address register etc.
Level 2 or Cache memory
It is the fastest memory which has faster access time where data is temporarily stored for faster access.
Level 3 or Main Memory (Primary Memory in the image above)
It is memory on which computer works currently. It is small in size and once power is off data no longer stays in this memory.
Level 4 or Secondary Memory
It is external memory which is not as fast as main memory but data stays permanently in this memory.

Cache Performance:
When the processor needs to read or write a location in main memory, it first checks for a corresponding entry in the cache.

If the processor finds that the memory location is in the cache, a cache hit has occurred and data is read from cache
If the processor does not find the memory location in the cache, a cache miss has occurred. For a cache miss, the cache allocates a new entry and copies in data from main memory, then the request is fulfilled from the contents of the cache.

The performance of cache memory is frequently measured in terms of a quantity called Hit ratio.

Hit ratio = hit / (hit + miss) =  no. of hits/total accesses

We can improve Cache performance using higher cache block size, higher associativity, reduce miss rate, reduce miss penalty, and reduce the time to hit in the cache.

Application of Cache Memory

Usually, the cache memory can store a reasonable number of blocks at any given time, but this number is small compared to the total number of blocks in the main memory.
The correspondence between the main memory blocks and those in the cache is specified by a mapping function.

Types of Cache

Primary Cache
A primary cache is always located on the processor chip. This cache is small and its access time is comparable to that of processor registers.
Secondary Cache
Secondary cache is placed between the primary cache and the rest of the memory. It is referred to as the level 2 (L2) cache. Often, the Level 2 cache is also housed on the processor chip.

Locality of reference
Since size of cache memory is less as compared to main memory. So to check which part of main memory should be given priority and loaded in cache is decided based on locality of reference. Locality will be discussed in greater detail later on.

Adapted from:
"Cache Memory in Computer Organization" by VaibhavRai3, Geeks for Geeks is licensed under CC BY-SA 4.0