A compromise between a ref="module.php?name=Lexikon&file=search&eid=1&query=direct mapped cache">direct mapped cache and a ref="module.php?name=Lexikon&file=search&eid=1&query=fully associative cache">fully associative cache where each address is mapped to a certain set of cache locations. The address space is divided into blocks of 2^m bytes (the ref="module.php?name=Lexikon&file=search&eid=1&query=cache line">cache line size), discarding the bottom m address bits. An "n-way set associative" cache with S sets has n cache locations in each set. Block b is mapped to set "b mod S" and may be stored in any of the n locations in that set with its upper address bits as a tag. To determine whether block b is in the cache, set "b mod S" is searched associatively for the tag. A direct mapped cache could be described as "one-way set associative", i.e. one location in each set whereas a fully associative cache is N-way associative (where N is the total number of blocks in the cache). Performance studies have shown that it is generally more effective to increase the number of entries rather than associativity and that 2- to 16-way set associative caches perform almost as well as fully associative caches at little extra cost over direct mapping. (2004-10-18)
