Cache coherent computer system that minimizes invalidation and copyback operations

A multi-processor computer system having a plurality of processor subsystems, comprising:

• shared memory having a plurality of read-only datablocks;
• a first processor subsystem having an external cache memory including external tags for datablocks stored in the cache memory and that maintains the external tags according to a first coherency protocol;
• and
• memory interconnect that enables access to the shared memory and that includes duplicate tags that correspond to the external tags and that maintains the duplicate tags according to a second coherency protocol;
• wherein the first processor subsystem generates a read to share always request that specifies one of said plurality of read-only datablocks if a cache miss on the specified read-only datablock to the external cache memory occurs, and wherein the memory interconnect in response to the read to share always request causes the external tags for the specified read-only datablock to indicate a shared state;
• and
• wherein the first processor subsystem generates a read to share request that specifies a requested datablock if a cache miss on the requested datablock to the external cache memory occurs and wherein the memory interconnect causes the external tags for the requested datablock to indicate an exclusive state in response to the read to share request only if no other processor subsystem has a copy of the requested datablock.

A cache coherency method in a multi-processor computer system, comprising the steps of:

• maintaining external tags for datablocks stored in an external cache memory of a processor according to a first coherency protocol;
• maintaining duplicate tags that correspond to the external tags according to a second coherency protocol;
• and one of
• generating a read to share always request that specifies a datablock if a cache miss on the datablock to the external cache memory occurs and the datablock lies in a read-only location in a main memory, and causing the external tags for the datablock to indicate a shared state in response to the read to share always request, and
• generating a read to share request that specifies the datablock if a cache miss on the datablock to the external cache memory occurs, and causing the external tags for the datablock to indicate an exclusive state in response to the read to share request after determining that no other processor owns the datablock.