Cache synonym detection and handling means

Abstract

The disclosure detects and handles synonyms for a store-in-cache (SIC). A processor cache directory (PD) is searched in a principle class addressed by a subset of bits taken from a processor request's logical address. The class address has both translatable and non-translatable bits. If any of the set-associative line entries in the principle class contains the request's translated address, the data is accessed in a corresponding line location in the cache. If the principle class does not have any entry with the request's translated address, a cache miss signal occurs which causes a line fetch command to be generated for main storage to fetch the required line. The line fetch command also causes synonym search circuits to generate the address of every potential synonym class by permutating the translatable bits in the principle class address provided in the line fetch command. Then each potential synonym class is accessed in a copy directory (CD) (which is a copy of essential information in all entries in PD) and compared to the translated request address in order to detect for any existing synonym. Each line entry in the PD and CD also has an exclusive (EX) shareability control bit which controls the handling of a request after detection of a synonym in the CD. If the EX bit is off representing a read only (RO) state, the line cannot be locked during any checkpoint interval. Then the data is not accessed in the detected synonym location, but instead the RO line is copied into an entry in the principle class to improve system performance due to subsequent requests expected to the same line. The synonym line is not invalidated unless the processor is requesting the data exclusively (EX). If the EX bit is on representing an exclusive state in a found synonym entry, the line may be locked in the cache during a checkpoint interval, and therefore the data is accessed in the detected synonym location in the SIC because it cannot then be moved.