A microprocessor including a control unit and a cache connected with the control unit for storing data to be used by the control, wherein the cache is selectively configurable as either a single cache or as a partitioned cache having a locked cache portion and a normal cache portion. The normal cache portion is controlled by a hardware implemented automatic replacement process. The locked cache portion is locked so that the automatic replacement process cannot modify the contents of the locked cache. An instruction is provided in the instruction set that enables software to selectively allocate lines in the locked cache portion to correspond to locations in an external memory, thereby enabling the locked cache portion to be completely managed by software.