INTERVIEW QUESTIONS COMPUTER HARDWARE COMPUTER ARCHITECTURE AND DESIGN DETAILS

Question: What is a Snooping cache?

Answer:

Source: CoolInterview.com

Snooping is the process where the individual caches monitor address lines for accesses to memory locations that they have cached. When a write operation is observed to a location that a cache has a copy of, the cache controller invalidates its own copy of the snooped memory location.

Snarfing is where a cache controller watches both address and data in an attempt to update its own copy of a memory location when a second master modifies a location in main memory

Source: CoolInterview.com

Snooping protocol is 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.
There are two 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.

Source: CoolInterview.com

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