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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 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, 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. And content addressable memory is using comparison logic with each bit of storage. A data value is broadcast to all words of storage and compared with the values there. Words which match are flagged in some way. Subsequent operations can then work on flagged words, e.g. read them out one at a time or write to certain bit positions in all of them. A CAM can thus operate as a data parallel (SIMD) processor , Writing to a CAM is exactly like writing to a conventional RAM. However, the read operation is actually a search of the CAM for a match to an input "tag." In addition to storage cells, the CAM requires one or more comparators

For more please visit

http://en.wikipediawiki/Content-addressable_memory
http://citeseerx.ist.psu.edu/viewdoc/dow...1&type=pdf
http://acteldocuments/CAM_AN.pdf

(08-02-2010, 09:14 PM)muthu kumar Wrote: i need a seminar report with ppt

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thanku and can i get sm more info abt CAM topics

This article is presented by:
Vahid Tabatabaee

Content Addressable Memories

References
Title: Network Processors Architectures, Protocols, and Platforms
Author: Panos C. Lekkas
Publisher: McGraw-Hill
Kostas Pagiamtzis, Ali Sheikholeslami, “Content-Addressable Memory (CAM) Circuits and Architectures: A Tutorial and Survey,” IEEE J of Solid-State Circuits vol. 41, No.3, March 2006.
NetLogic MicroSystems Application Note, “Intradevice Configuration of Network Search Engines”.
NetLogic MicroSystems Application Note, “High Performance Layer 3 Forwarding”.
IDT White Paper, “Taking Packet Processing to the Next Level”.

Classification and Search Engines

Classification engine receives streams of packets as its input.
It applies a set of application-specific sorting rules and policies continuously on the packets.
It ends up compiling a series of new parallel packet streams in queues of packets.ored.

For classification the NP should consult a memory bank, a lookup table or even a data base where the rules are stored.
Search engines are used for consultation of a lookup table or a database based on rules and policies for the correct classification.

What is CAM?
Content Addressable Memory is a special kind of memory!
Read operation in traditional memory:
Input is address location of the content that we are interested in it.
Output is the content of that address.
In CAM it is the reverse:
Input is associated with something stored in the memory.
Output is location where the associated content is stored.
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This article is presented by:
AJIN R
OBJECTIVES

To familiarise with structure,features and working of CAM
Types of CAM and CAM cells.
Applications of CAM.
Working of CAM as search engines in Networking.
Future scopes of CAM.

INTRODUCTION

Memory devices store and retrieve data by addressing specific memory location.
The time required to find an item stored in memory an be reduced if the item is identified by its content rather than by its address.
Content-addressable memories (CAMs) are hardware search engines that are much faster than algorithmic approaches for search-intensive applications.

What is CAM?

Content Addressable Memory is a special kind of memory!
Read operation in traditional memory:
Input is address location of the content that we are interested in it.
Output is the content of that address.
In CAM it is the reverse:
Input is associated with something stored in the memory.
Output is location where the associated content is stored

CAM Features
CAM Cascading:
We can cascade up to 8 pieces without incurring performance penalty in search time (72 bits x 512K).
We can cascade up to 32 pieces with performance degradation (72 bits x 2M).
Terminology:
Initializing the CAM: writing the table into the memory.
Learning: updating specific table entries.
Writing search key to the CAM: search operation
Handling wider keys:
Most CAM support 72 bit keys.
They can support wider keys in native hardware.
Shorter keys: can be handled at the system level more efficiently.

This type of memory has become a very good option for almost all the people across the world. It has made the system of storing quite easy.
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[attachment=10652]
Single Port CAM
Content Addressable Memory is a memory which its locations are accessed through comparing Tags rather than providing their addresses. Each stored data is associated with a unique tag. When we like to retrieve the data, we apply its tag with a read signal to all locations simultaneously. The applied tag is compared with all stored tags simultaneously. If any of the stored tags matches the applied tag, the equality signal of the location comparator enables the output of the location, and the data is placed on the data bus in order to be read by the processor. Figure 1 depicts a single port CAM.
Dual Port CAM
In a multiprocessor interconnection network architecture proposed by Dr. Ayyad, the need arises for implementing CAM cache memory modules at the cross-point of the crossbar switch, through which the processors exchange the variables. The source processor can write the variable to a selected set of cross-point CAM modules simultaneously, and can search for a variable in the set of CAMs connected to its input bus simultaneously. So, we need a dual port CAM like the one shown in figure 2. Through the left (Write port), one processor can write a variable to the first available memory location. The written variable includes the data (the value of the variable) and an associated tag stored in the tag field of the memory location. Another processor can search in the CAM for the variable by applying the tag to the tag field and a read signal through the right port (the read port). The tag will be compared to the stored tags in all location simultaneously, and, if a match occurs, the equality signal of the comparator will enable the output of the data field to output the data on the output data bus [O31-O0].
The control circuit of the CAM includes a pointer circuit which points to the first available memory location for writing. When the system is reset, this pointer points to the first memory location. After writing to the current location, the pointer points to the next location, and so on. In the case of simultaneous write and read process, the circuit gives the priority for writing and gives the reading processor a WAIT signal. The WAIT signal can be obtained by ORing the active low RD and WR signal coming from the writing and the reading processors. The WAIT signal also, separates the tag bus of the reading processor by disabling the tri-state buffer Tr

[attachment=15121]
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.
A content-addressable memory (CAM) is a critical device for applications involving asynchronous transfer mode (ATM), communication networks, LAN bridges/switches, databases, lookup tables, and tag directories, due to its high-speed data search capability. A CAM is a functional memory with a large amount of stored data that simultaneously compares the input search data with the stored data. Once matching data are found, their addresses are returned as output as shown in Fig. 1. The vast number of comparison operations required by CAMs consumes a large amount of power.
1.1 Introduction
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.
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 comprised entirely of 1s and 0s. Ternary CAM allows a third matching state of "X" or "Don't Care" for one or more bits in the stored data word, 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. 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 addresses. 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.