Multi-processor computer system using partition group directories to maintain cache coherence

In a multi-processor computer system, the processors are divided into broadcast groups and partition groups, with each processor belonging to exactly one broadcast group and exactly one partition group. Each processor holds a partition group directory indicative of contents of the caches associated with each other processor of the partition group. For read or write operations a processor broadcasts a message to other members of its broadcast group. The members of the broadcast group consult their respective partition directories to provide a complete response to a request for a needed item of data.