Snooping Protocol

What is Snooping Protocol?
Snooping protocol ensures memory cache coherence in SMP (Symmetric Multiprocessing) systems. Each processor cache on a bus monitors or sniffs the bus to verify that it has a copy of a requested block of data.
Before any processor writes data, other processor cache copies must be invalidated or updated. The snooping protocol is also known as the bus snooping protocol.

The two types of snooping protocols are:

Write invalidates: Before changing the copy, a data write processor invalidates copies of data in all other system processor caches. This action is communicated by an invalid signal sent over the bus.

Write update: A data write processor announces new data over the bus. All affected caches are updated with the new data.

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Further explanations for the first letter S.