Unformatted text preview: word in the cache
memory and helps in further improving the processor speed. Many computer systems are
also designed to have multiple levels of caches (such as level one and level two caches,
often referred to as LI and L2 caches). LI cache is smaller than L2 cache and is used to
store more frequently accessed instructions/data as compared to those in the L2 cache.
The use of cache memory requires several design issues to be addressed. A detailed
discussion of cache design is beyond the scope of this book. Key design issues are briefly
1. Cache size. Cache memory is very expensive as compared to the main memory and
hence its size is normally kept very small. It has been found through statistical studies
that reasonably small caches can have a significant impact on processor performance.
Hence cache memory size is usually decided based on statistical studies of program
behavior and is chosen so that the probability of the needed instruction/data being in the
cache is more than 90%. As a typical example of cache size, a system having 1 GB of
main memory may have about 1 MB of cache memory. Many of today's personal
computers have 64 KB, 128 KB, 256 KB, 512 KB, or 1 MB of cache memory.
Block size. Block size refers to the unit of data (few memory words) exchanged
between cache and main memory. As the block size increases from very small to larger
sizes, the hit ratio (fraction of time that referenced instruction/data is found in cache) will
at first increase because of the principle of locality since more and more useful words are
brought into the cache. However, the hit ratio will begin to decrease as the block size
further increases because the probability of using the newly fetched words becomes less
than the probability of reusing the words that must be moved out of the cache to make
room for the new block. Based on this fact, the block size is suitably chosen to
maximize the hit ratio.
3. Replacement policy. When a new block is to be fetched into the cache, another may
have to be repl...
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This document was uploaded on 04/07/2014.
- Spring '14