Data processing system and method for efficient communication utilizing an Ig coherency state

A cache memory for a cache coherent data processing system including at least first and second coherency domains each including at least one processing unit, wherein said first coherency domain includes the cache memory and said second coherency domain includes another coherent cache memory, said cache memory comprising:

• a cache controller;
• a data array including a data storage location for caching a memory block;
• a cache directory including:
• a tag field for storing an address tag in association with said memory block;
• a coherency state field associated with said tag field and said data storage location, wherein said coherency state field has a plurality of possible states including a state that indicates that said address tag is valid, that said storage location does not contain valid data, and that said memory block is possibly cached outside of said first coherency domain.