This basic definition includes two separate aspects of the actions of the memory system, both of which are important for writing accurate shared memory programs. the first aspect, called coherence, defines which values a reading can return. the second aspect, called consistency, determines when a read returns a written value. supporting migration and replication in accessing shared data is critical to performance; so many multiprocessors are introducing a hardware protocol to maintain coherent caches. such protocols are called cache coherence protocols to ensure compatibility for multiple processors.