1. A least recently used mechanism for a content addressable memory which holds data values having respective resource identifiers comprising: a control unit which stores a plurality of the resource identifiers in entries of the content addressable memory with respective count values indicating a time sequence in which the resource identifiers were stored, changes selected count values to higher count values in response to access requests for corresponding resource identifiers and, after each access request, searches the content addressable memory to find one of the entries having a lowest count value and writes the resource identifier of the entry having the lowest count value to a least recently used register wherein the content addressable memory is part of a data processing system having system software, said control unit further receives a request to store a new resource identifier in the content addressable memory from the system software, and determines that the new resource identifier is not currently stored in the content addressable memory and that the content addressable memory has no currently unused entries, and said system software reads the least recently used register to identify the resource identifier therein as a least recently used resource identifier, and allocates the entry corresponding to the least recently used resource identifier for storing the new resource identifier.