Content-addressable memory

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. (Hannum et al., 2004) Several custom computers, like the Goodyear STARAN, were built to implement CAM, and were referred to as associative computers.


Hardware associative array

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 data word, 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.[1]

Standards for content-addressable memories

A major interface definition for CAMs and other Network Search Elements (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, Netlogic Micro Systems and others to the LA interface agreement. On December 11, 2007, the OIF published the serial lookaside (SLA) interface agreement.

Semiconductor implementations

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.

Alternative implementations

To achieve a different balance between speed, memory size and cost, some implementations emulate the function of CAM by using standard tree search or hashing designs in hardware, using hardware tricks like replication or pipelining to speed up effective performance. These designs are often used in routers.

Ternary CAMs

Binary CAM is the simplest type of CAM which uses data search words consisting entirely of 1s and 0s. Ternary CAM (TCAM) allows a third matching state of "X" or "Don't Care" for one or more bits in the stored dataword, thus adding flexibility to the search. For example, a ternary CAM might have a stored word of "10XX0" which will match any of the four search words "10000", "10010", "10100", or "10110". The added search flexibility comes at an additional cost over binary CAM as the internal memory cell must now encode three possible states instead of the two of binary CAM. This additional state is typically implemented by adding a mask bit ("care" or "don't care" bit) to every memory cell.

Holographic associative memory provides a mathematical model for "Don't Care" integrated associative recollection using complex valued representation.

Example applications

Content-addressable memory is often used in computer networking devices. For example, when a network switch receives a data frame from one of its ports, it updates an internal table with the frame's source MAC address and the port it was received on. It then looks up the destination MAC address in the table to determine what port the frame needs to be forwarded to, and sends it out on that port. The MAC address table is usually implemented with a binary CAM so the destination port can be found very quickly, reducing the switch's latency.

Ternary CAMs are often used in network routers, where each address has two parts: the network address, which can vary in size depending on the subnet configuration, and the host address, which occupies the remaining bits. Each subnet has a network mask that specifies which bits of the address are the network address and which bits are the host address. Routing is done by consulting a routing table maintained by the router which contains each known destination network address, the associated network mask, and the information needed to route packets to that destination. Without CAM, the router compares the destination address of the packet to be routed with each entry in the routing table, performing a logical AND with the network mask and comparing it with the network address. If they are equal, the corresponding routing information is used to forward the packet. Using a ternary CAM for the routing table makes the lookup process very efficient. The addresses are stored using "don't care" for the host part of the address, so looking up the destination address in the CAM immediately retrieves the correct routing entry; both the masking and comparison are done by the CAM hardware.

Other CAM applications include:

See also


  1. ^ TRW Computer Division, 1963, p. 17.


External links

Wikimedia Foundation. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • Content addressable memory — Mémoire adressable par contenu Pour les articles homonymes, voir CAM. La mémoire adressable par contenu (CAM, en anglais Content Addressable Memory) est un type de mémoire informatique spécial utilisé dans certaines applications de recherche à… …   Wikipédia en Français

  • content addressable memory — asociatyvioji atmintis statusas T sritis informatika apibrėžtis ↑Atmintis, kurioje duomenys randami pagal jų turinį, bet ne pagal adresus. Turinys atspindimas ryšiais tarp duomenų. Iš kiekvieno duomens srities eina saitai į kitų su juo susijusių… …   Enciklopedinis kompiuterijos žodynas

  • Content Addressable Memory — Ein Assoziativspeicher oder auch inhaltsadressierbarer Speicher (engl. Content Addressable Memory, CAM) ist eine Speicherform, bei der mit der Assoziation von Inhalten gearbeitet wird, um auf einzelne Speicherinhalte zuzugreifen. Bereits 1943… …   Deutsch Wikipedia

  • content addressable memory —    (CAM)    A memory device that stores and retrieves data based on a key value as opposed to an address or name …   IT glossary of terms, acronyms and abbreviations

  • Content-addressable storage — Content addressable storage, also referred to as associative storage or abbreviated CAS, is a mechanism for storing information that can be retrieved based on its content, not its storage location. It is typically used for high speed storage and… …   Wikipedia

  • Content Addressable Parallel Processor — A Content Addressable Parallel Processor (CAPP) is a type of parallel processor which uses content addressing memory (CAM) principles. CAPPs are intended for bulk computation. The syntactic structure of their computing algorithm are simple,… …   Wikipedia

  • Content Addressable File Store — The Content Addressable File Store (CAFS) was a hardware device developed by International Computers Limited (ICL) that provided a disk storage with built in search capability. The motivation for the device was the discrepancy between the high… …   Wikipedia

  • Memory disambiguation — is a set of techniques employed by high performance out of order execution microprocessors that execute memory access instructions (loads and stores) out of program order. The mechanisms for performing memory disambiguation, implemented using… …   Wikipedia

  • Content processor — Content processors are sometimes confused with network processors that inspect the packet payload of an IP packet travelling through a computer network. These components allow for the design and deployment of next generation networking systems… …   Wikipedia

  • Volatile memory — Memory typesVolatile memory, also known as volatile storage or primary storage device, is computer memory that requires power to maintain the stored information, unlike non volatile memory which does not require a maintained power supply.Most… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”