Update vs. invalidate policy for a snoopy bus protocol

Abstract

The present invention is directed to a computer apparatus for use in a multiprocessor computer system having a main memory storing a plurality of data items and being coupled to a bus. The bus is operated according to a SNOOPY protocol. The computer apparatus includes a processor and a cache memory coupled to the processor. The cache memory contains a subset of the data items stored in the main memory, for access by the processor and includes a TAG store comprising a plurality of VALID indicators, one VALID indicator for each of the data items currently contained in the cache memory. A bus interface is coupled to the cache memory and is adapted for coupling to the bus. The interface operates according to the SNOOPY protocol to monitor transactions on the bus for write transactions affecting data items of the subset having set VALID indicators and determines the identity of each initiator of a write transaction on the bus affecting a VALID data item of the subset. The interface operates further to invalidate or update each VALID data item of the subset when there is a write transaction affecting the data item. An invalidate operation includes a clearing of the corresponding VALID indicator for the affected data item by the interface and an update operation includes overwriting of the affected data item by the interface from the write transaction on the bus. The operation of the interface to invalidate or update is performed as a function of the identity of the initiator of the write transaction determined by the interface.