1. A method of data processing in a cache coherent data processing system including at least first and second coherency domains, said method comprising: in a first cache memory within said first coherency domain of said data processing system, holding a memory block in a storage location associated with an address tag and a coherency state field; setting said coherency state field to 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.