Content-addressable memory

Content-addressable memory (CAM) is a special type of computer memory used in certain very-high-speed searching applications. It is also known as associative memory, associative storage, or associative array, although the last term is more often used for a programming data structure. It compares input search data (tag) against a table of stored data, and returns the address of matching data (or in the case of associative memory, the matching data). Several custom computers, like the Goodyear STARAN, were built to implement CAM, and were designated associative computers.

Unlike standard computer memory (random access memory or RAM) in which the user supplies a memory address and the RAM returns the data word stored at that address, a CAM is designed such that the user supplies a data word and the CAM searches its entire memory to see if that data word is stored anywhere in it. If the data word is found, the CAM returns a list of one or more storage addresses where the word was found (and in some architectures, it also returns the contents of that storage address, or other associated pieces of data). Thus, a CAM is the hardware embodiment of what in software terms would be called an associative array. The data word recognition unit was proposed by Dudley Allen Buck in 1955.

A major interface definition for CAMs and other network search engines (NSEs) was specified in an interoperability agreement called the Look-Aside Interface (LA-1 and LA-1B) developed by the Network Processing Forum, which later merged with the Optical Internetworking Forum (OIF). Numerous devices have been produced by Integrated Device Technology, Cypress Semiconductor, IBM, Broadcom and others to the LA interface agreement. On December 11, 2007, the OIF published the serial lookaside (SLA) interface agreement.

Because a CAM is designed to search its entire memory in a single operation, it is much faster than RAM in virtually all search applications. There are cost disadvantages to CAM however. Unlike a RAM chip, which has simple storage cells, each individual memory bit in a fully parallel CAM must have its own associated comparison circuit to detect a match between the stored bit and the input bit. Additionally, match outputs from each cell in the data word must be combined to yield a complete data word match signal. The additional circuitry increases the physical size of the CAM chip which increases manufacturing cost. The extra circuitry also increases power dissipation since every comparison circuit is active on every clock cycle. Consequently, CAM is only used in specialized applications where searching speed cannot be accomplished using a less costly method. One successful early implementation was a General Purpose Associative Processor IC and System.

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