Set Associative Mapping

Set Associative Mapping:- In this post, we discuss Set Associative Mapping in Cache Memory. It was mentioned previously that the disadvantage of direct mapping is that two words with the same index in their address but with different tag values cannot reside in cache memory at the same time.

 

Set Associative Mapping

 

The third type of cache organization, called set-associative mapping, is an improvement over the direct mapping organization in that each word of cache can store two or more words of memory under the same index address. Each data word is stored together with its tag and the number of tag-data items in one word of cache is said to form a set. An example of a set-associative cache organization for a set size of two is shown in Fig. Each index address refers to two data words and their associated tags. Each tag requires six bits and each data word has 12 bits, so the word length is 2 (6+ 12) = 36 bits. An index address of nine bits can accommodate 512 words. Thus the size of cache memory is 512 x 36. It can accommodate 1024 words of main memory since each word of cache contains two data words. In general, a set-associative cache of set size k will accommodate k words of main memory in each word of cache.

 

Set Associative Mapping

Set Associative Mapping in Cache Memory Figure

 

When the CPU generates a memory request, the index value of the address is used to access the cache. The tag field of the CPU address is then compared with both tags in the cache to determine if a match occurs. The comparison logic is done by an associative search of the tags in the set similar to an associative memory search: thus the name “set-associative.” The hit ratio will improve as the set size increases because more words with the same index but different tags can reside in cache. However, an increase in the set size increases the number of bits in words of cache and requires more complex comparison logic.

When a miss occurs in a set-associative cache and the set is full, it is necessary to replace one of the tag-data items with a new value. The most common replacement algorithms used are random replacement, first-in, first-out (FIFO), and least recently used (LRU). With the random replacement policy, the control chooses one tag-data item for replacement at random. The FIFO procedure selects for replacement of the item that has been in the set the longest. The LRU algorithm selects for replacement the item that has been least recently used by the CPU. Both FIFO and LRU can be implemented by adding a few extra bits in each word of cache.

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