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LINKED LIST OF MULTIPLE IDENTICAL CONTENT ADDRESSABLE MEMORY TAG FIELDS

IP.com Disclosure Number: IPCOM000006157D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2001-Dec-10

Publishing Venue

Motorola

Related People

Authors:
William D. Atwell, Jr. Grady L. Giles Jesse R. Wilson

Abstract

WHAT ARE THE PROBLEMS SOLVED BY THIS STRUCTURE? There is a general problem of CAM design con- cerning how to handle multiple identical CAM tag fields. The solutions have generally been either to disallow multiple identical tags or to provide some hardware means to resolve which one of the multiple matches to recognize. AI1 of such hardware means known to the authors are effective at the time of the associative operation that results in a multiple match condition. The standard state of the art solution is the use of some kind of priority encoder to resolve one of the multiple matching entries. These priority encoder circuits are problematical in that their performance degrades rapidly as the depth (number of entries) of the CAM is increased due to the use of gate-pass chains and look-ahead logic. That is, for a given desired performance level, there is a practical limit on the size of the CAM that uses any of these priority encoder circuits. This performance penalty grows linearly with CAM depth whereas the penalty associated with a simple encoder grows as log2 (CAM depth). A benefit of the structure being discussed is that it provides for the use of simple match encoders while accommodating multiple identical CAM tag fields.