Associative Memory

Many data-processing applications require the search of items in a table stored in memory.

The number of accesses to memory depends on the location of the item and the efficiency of the search algorithm. Many search algorithms have been developed to minimize the number of accesses while searching for an item in a random or sequential access memory. 

The time required to find an item stored in memory can be reduced considerably if stored data can be identified for access by the content of the data itself rather than by an address. A memory unit accessed by content is called content addressable an associative memory or content addressable memory (CAM).

When a word is written in an associative memory, no address is given. The memory is capable of finding an empty unused location to store the word. When a word is to be read from an associative memory, the content of the word, or part of the word, is specified. The memory locates all words which match the specified content and marks them for reading. 

The associative memory is uniquely suited to do parallel searches by data association. An associative memory is more expensive than a random access memory because each cell must have storage capability as well as logic circuits for matching its content with an external argument.

Hardware Organization:

The argument register A and key register K each of size n-bits, each bit for each bit of a word. The match register M has m-bits, each bit  for each memory word. The words that match the bits of the argument register set a corresponding bit in the match register. After the matching process, those bits in the match register that have been set indicate the fact that their corresponding words is matched. Reading is done by a sequential access to memory for those words whose corresponding bits in the match register is set. 

for example:

agumented register consist of word to be search.key register consist of 1's and 0's the bits which are filled with 1 are to be matched 

A               111 111 100  

K               111 000000 

Word 1     100 111 100     no match

Word 2     111 000001      match

Structure of associative memory with m words and n bits :

argumented register bits are individually denoted with A1,A2,A3.......An. key register and match registeris also represented as same as argumented register. The cells in the array are marked by the letter C with two subscripts. The first subscript gives the word number and the second specifies the bit position in the word. Thus cell Cij is the cell for bit j in word i.