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Content Addressable Memory for FPGA

상품코드 DimoCore-CAM-FPGA
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디모소프트에서는 FPGA 기반의 네트워크 변환장비에 적용할 수 있는 Content Addressable Memory IP 을 개발하였습니다. 과거의 FPGA는 하드웨어적으로 지원할 수 있도록 되어 있으나, 최신의 FPGA들은 이를 지원하지 않아서 어려움이 많았습니다.

디모소프트에서 개발한 CAM IP는 Cyclone 4e (EP4CE75F23C7)에서 135Mhz이상의 속도를 구현한 제품으로 64비트 단위의 메모리를 최대 512개까지 기억하며, 소자의 상태에 따라서 뱅크단위의 검색 기능도 수행이 가능합니다.

 

위와 같은 구조로 되어 있으며, 검색을 하는 클럭과 입력을 하는 클럭이 서로 달라도 문제가 없는 것이 특징입니다. 또한, 다음 데이터를 처리할 수 있도록 Busy 신호를 내주기 때문에 연속적인 입력에서도 뛰어난 성능의 발휘가 가능합니다. 검증(확인)을 할 때에는 두개의 서로 다른 64비트를 동시에 검증이 가능하며, 최대 10 Clock이내에 512개의 메모리를 검사하여 같은 값에 대한 결과를 나타내 줍니다.

현재는 Altera 제품군으로만 있으나, Xilinx, QuickLogic, ACTEL 등으로도 컨버젼을 하여, 사용자의 편의를 줄 예정입니다.

제공 형태는 Netlist, Source (Verilog HDL)등의 형태이며, Altera 버젼의 경우 암호화된 코드(Netlist, VerilogHDL, Altera의 심사 통과시 )로도 제공이 가능합니다. 또한, 사용되는 시스템에 맞도록 변경이 가능하며, 변경을 원할 때에는 별도의 협의가 필요합니다.

FPGA 형태로 Ethernet Network 엔진을 개발하는 업체들에게 많은 도움을 될 수 있는 코어입니다.

관련하여, WPKIPEDIA의 내용입니다.

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 memoryassociative storage, or associative array, although the last term is more often used for a programming data structure.[1] 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).[2] Several custom computers, like the Goodyear STARAN, were built to implement CAM, and were designated 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.[3]

Standards for content-addressable memories

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 TechnologyCypress SemiconductorIBMBroadcom 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. One successful early implementation was a General Purpose Associative Processor IC and System.[4]

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.

An alternative approach to implementation is based on Superimposed Code Words or Field Encoded Words which are used for more efficient database operations, information retrieval and logic programming, with hardware implementations based on both RAM and head-monitoring disk technology.[5][6]

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.

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