Thursday, September 25, 2008


My DSP have two caches with 32k each.

Cache is a block of memory for temporary storage of data duplicating original values stored elsewhere(such as main memory(RAM)), the data is likely to be used again. (CPU and Hard disk use a cache). The original data is expensive to fetch or compute, compared to the cost of reading the cache. So frequently accessed data can be stored in cache for rapid access.

Once the data is stored in the cache, future use can be made by accessing the cached copy rather than re-fetching or recomputing the original data, so that the average access time is shorter. A cache, therefore, helps expedite data access that the CPU would otherwise need to fetch from main memory.

A cache is made up of a pool of entries. Each entry has a datum (a nugget of data) which is a copy of the datum in some backing store. Each entry also has a tag, which specifies the identity of the datum in the backing store of which the entry is a copy.

When the CPU wishes to access a datum presumably in the backing store, it first checks the cache. If an entry can be found with a tag matching that of the desired datum, the datum in the entry is used instead. This situation is known as a cache hit. The percentage of accesses that result in cache hits is known as the hit rate or hit ratio of the cache.

The alternative situation, when the cache is consulted and found not to contain a datum with the desired tag, is known as a cache miss. [from wiki]

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