(n.) Also referred to as a bus-snooping protocol, a protocol for maintaining cache coherency in symmetric multiprocessing environments. In a snooping system, all caches on the bus monitor (or snoop) the bus to determine if they have a copy of the block of data that is requested on the bus. Every cache has a copy of the sharing status of every block of physical memory it has. Multiple copies of a document in a multiprocessing environment typically can be read without any coherence problems; however, a processor must have exclusive access to the bus in order to write.
Snooping Protocol Types
There are two main types of snooping protocol:
Write-invalidate
The processor that is writing data causes copies in the caches of all other processors in the system to be rendered invalid before it changes its local copy. The local machine does this by sending an invalidation signal over the bus, which causes all of the other caches to check for a copy of the invalidated file. Once the cache copies have been invalidated, the data on the local machine can be updated until another processor requests it.
Write-update
The processor that is writing the data broadcasts the new data over the bus (without issuing the invalidation signal). All caches that contain copies of the data are then updated. This scheme differs from write-invalidate in that it does not create only one local copy for writes.
