Second level cache controller unit and system

Abstract

A second level cache memory controller, implemented as an integrated circuit unit, operates in conjunction with a secondary random access cache memory and a main memory (system) bus controller to form a second level cache memory subsystem. The subsystem is interfaced to the local processor (CPU) bus and to the main memory bus providing independent access by both buses, thereby reducing traffic of the main memory bus when the data required by the CPU is located in secondary cache. Similarly, CPU bus traffic is minimized when secondary cache access by the main memory bus for snoops and write-backs to main memory. Snoop latches interfaced with the main memory bus provide snoop access to the cache memory via the cache directory in the secondary cache controller unit. The controller also supports parallel look-up in the controller tag array and the secondary cache using most-recently-used (MRU) main memory write-through and pipelining of memory bus cycle requests.