<P> Distributed shared memory systems mimic these mechanisms in an attempt to maintain consistency between blocks of memory in loosely coupled systems . </P> <P> Coherence protocols apply cache coherence in multiprocessor systems . The intention is that two clients must never see different values for the same shared data . </P> <P> The protocol must implement the basic requirements for coherence . It can be tailor - made for the target system or application . </P> <P> Protocols can also be classified as snoopy or directory - based . Typically, early systems used directory - based protocols where a directory would keep a track of the data being shared and the sharers . In snoopy protocols, the transaction requests (to read, write, or upgrade) are sent out to all processors . All processors snoop the request and respond appropriately . </P>

Difference between write invalidate and write update protocol