METHOD AND APPARATUS FOR DETERMINING AN EXACT MATCH IN A CONTENT ADDRESSABLE MEMORY DEVICE

A method and apparatus for determining an exact match in a ternary CAM device. Each ternary CAM cell includes CAM cells for storing CAM data, local mask cells for storing prefix mask data for the corresponding CAM cells, and a mask override circuit. Each local mask cell includes a masking circuit that masks the prefix mask data or CAM data provided to the comparison circuit, or masks the comparison result from the match line of a CAM cell. The mask override circuit effectively overrides the prefix mask data stored in the local mask cell. The mask override circuit performs the override function by negating the operation of the mask circuit such that no masking operation occurs when an exact match compare or invalidate function is performed by the ternary CAM device. For example, during an exact match operation, the CAM cells compare comparand data with unmasked CAM data and provide the compare results to CAM match lines. The local mask cells also compare mask data with the stored prefix mask data and provide the results to mask match lines. If both compares result in a match, then an exact match entry is located in the ternary CAM device. The locations or indexes of the exact match entries may then be output from the CAM device. One or more of the exact match locations may also be invalidated or deleted.