Combined content addressable memories

Details Combined content addressable memories Combined content addressable memories

2005-10-18

2005-10-18

Ellis, Kevin L. (Department: 2188)

Electrical computers and digital processing systems: memory

Storage accessing and control

Specific memory composition

Reexamination Certificate

active

06957299

ABSTRACT:
A method and circuit for a high speed interface that enables the integration of several Content Addressable memories into a larger, Combined Content Addressable Memory with only an insignificant delay in the original response time of the individual memories. The interface provides connections to the Bus system only and no connection between different CAM devices of the system is needed, whereby the combination of one CAM and one CAM interface in a single device, such as a chip is enabled. Such chips may be used as modules for increasing the CAM Memory by directly attaching them to the Bus System without any additional interface, as known for standard memory chips such as RAM. The inventive interface may also be used for creating a hierarchical structure within a CAM device by dividing the memory cells in groups, each group being interfaced to the larger combined CAM device via an interface according to the invention. In such a structure the priority function in each group operates on a smaller number of cells and due to the relatively small number of gates necessary for making the interface the implementation of a CAM device with a reduced number of gates is facilitated. In a combined CAM device several CAMs are connected to an Address Bus and to a Data Bus via their respective interfaces. In the Read and Write mode, the interface will connect the two sets of lines and thus be transparent for the Read or Write operation. In the search mode (Content Addressing mode), the Interface will select and set a number of lines of the Data Bus according to the CAM response so as to output the address of one matching cell that is the highest in a predefined direction. The interface circuit adds the logic functions and lines necessary to expand the Content addressable functions of the CAM module to a larger range of addresses, sets an additional number of bus lines in order to write the address of the cell within that larger range and checks that no other cell with a higher address responds throughout the system by masking the bits of the CAM address if they would change the address of a cell with a higher address. In this way the whole ensemble of CAMs is capable of functioning as a single CAM of larger size.

REFERENCES:
patent: 5010516 (1991-04-01), Oates
patent: 5568416 (1996-10-01), Kawana et al.
patent: 5805493 (1998-09-01), Hiroshi et al.
patent: 5973950 (1999-10-01), Shindo
patent: WO 01 65564 (2001-09-01), None

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