Static information storage and retrieval – Associative memories – Ferroelectric cell
Reexamination Certificate
2000-06-08
2001-11-27
Nelms, David (Department: 2818)
Static information storage and retrieval
Associative memories
Ferroelectric cell
C365S189070, C365S230030
Reexamination Certificate
active
06324087
ABSTRACT:
BACKGROUND
1. Field of Invention
This invention relates generally to semiconductor memories and specifically to content addressable memories.
2. Description of Related Art
Content addressable memories (CAMs) are frequently used for address look-up functions in Internet data routing. For example, routers used by local Internet Service Providers (ISPs) typically include one or more CAMs for storing a plurality of Internet addresses and associated data such as, for instance, corresponding address routing information. When data is routed to a destination address, the destination address is compared with all CAM words, e.g., Internet addresses, stored in the CAM array. If there is a match, routing information corresponding to the matching CAM word is output and thereafter used to route the data.
A CAM device includes a CAM array having a plurality of memory cells arranged in an array of rows and columns. Each memory cell stores a single bit of digital information, i.e., either logic zero or logic one. The bits stored within a row of memory cells constitute a CAM word. During compare operations, a comparand word is received at appropriate input terminals of a CAM device and driven into the CAM array using comparand lines to be compared with all the CAM words in the device. For each CAM word that matches the comparand word, a corresponding match line signal is asserted to indicate a match condition. If the comparand word matches more than one of the CAM words, the match line corresponding to each of the matching CAM words is asserted, and a “multiple match” flag is also asserted to indicate the multiple match condition. The match line signals from each CAM block are combined in a priority encoder to determine the index or address of the highest-priority matching CAM word. Associative information corresponding to the highest-priority matching CAM word stored in, for instance, an associated RAM, may also be provided.
A single CAM device can be used to store multiple tables each storing and maintaining different classes of data. All entries, however, typically participate in a compare operation. This can cause an undesirable amount of power to be drawn during the compare operation. It would be desirable to limit a search to only those entries associated with a particular class of data to reduce power consumption during the operation.
SUMMARY
A method and apparatus are disclosed that may be used to partition a CAM device having a plurality of CAM blocks into a number of individually searchable partitions, where each partition may include one or more CAM blocks of the CAM device. In accordance with one embodiment of the present invention, each CAM block is connected to a block select circuit that stores a class code indicating what class or type of data is stored in the block. The same class code may be stored in any number of the block select circuits to define a partition as including the corresponding number of CAM blocks. During compare operations between a comparand word and data stored in the CAM device, a search code is provided to the block select circuits. Each block select circuit compares the search code with its class code and, in response thereto, selectively enables or disables the corresponding CAM block for the compare operation. In some embodiments, the block select circuit enables the corresponding CAM block if the search class matches the class code and, conversely, disables the corresponding CAM block if the search code does not match the class code.
In one embodiment, the block select circuit disables a corresponding CAM block by driving the comparand lines of the CAM block to a predetermined state to preclude the comparand word from being driven onto the comparand lines during the compare operation. By driving the comparand word only on the comparand lines of the selected (i.e., enabled) CAM blocks during the compare operation, present embodiments not only allow for selective searching across CAM blocks according to class codes, but also reduce power consumption in un-selected (i.e., disabled) CAM blocks during such selective compare operations.
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Le Thong
Nelms David
NetLogic Microsystems, Inc.
Paradice William L.
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