H ENGINES 224 NETWORK PROCESSORS 97 96 95 94 43210 Match 10101 found 11010 01001 01110 11101 01010 Excluded from the search 97 96 95 94 43210 10100 Comparand register 97 96 95 94 43210 11111 Global register Partition to be searched Partitions to be excluded from the search FIGURE 12.3 Partitioning a CAM array into multiple tables and the method of accessing them individually. denote the maximum search capability of the CAM. This is because the actual search performance often depends on multiple additional factors, including the size of the key and the size of the lookup table, which may be SO large that it requires multiple CAM chips to be cascaded. The speed of a CAM denoted by millions of searches per second (Msps) or by millions of lookups per second (Mlps). The latency in lookup table operations based on a CAM is another important measure of per- formance as the systems designer must know how much time his or her design must wait every time it issues a search and until the CAM yields an answer. In the case of CAM, latency is therefore used to measure the time between the moment when the search key has been presented to the CAM's input and the moment when the result has been produced by the CAM's output. This does not include the time needed to access the associated data SRAM by using the CAM output as an address index to retrieve the necessary data. Therefore, be careful about how the numbers are interpreted. One of the great characteristics about CAMs (as opposed to other types of hashed-index memory with which one might be tempted to build a content-addressable memory) is that latency is deterministic. However, this depends on the actual CAM design and clock frequency. Typically, latency can be two or three clock cycles long, but it can also be twice as much or more-namely, for cascaded CAM configurations. In terms of lookup latency performance, it is also possible that vendor-published numbers may not necessarily be telling the truth. For example, a search engine's lookup latency numbers can be hidden in a