1. A computer apparatus for use in a multiprocessor computer system having a main memory for storing a plurality of data items, wherein the computer apparatus is coupled to the main memory by a bus and wherein a plurality of other devices are also coupled to said bus for communication with said main memory, the bus being operated according to a SNOOPY protocol, the computer apparatus comprising: a processor; a cache memory coupled to the processor and containing a subset of the data items stored in the main memory, for access by the processor; the cache memory including a TAG store comprising a plurality of VALID indicators, one VALID indicator for each of the data items currently stored in the cache memory; a bus interface coupled to the cache memory and adapted for coupling to the bus, the bus interface operating according to the SNOOPY protocol to monitor transactions on the bus for write transactions corresponding to data items in the cache memory having set VALID indicators, said bus interface further comprising: means for identifying which one of said plurality of other devices initiated a write transaction on the bus, said means for identifying further comprising means for determining a type of said identified device; means, responsive to said determined type of said identified device, for either invalidating or updating a data item in the cache memory corresponding to the write transaction on the bus.