Editing Bus snooping (section)

== Types of snooping protocols ==
There are two kinds of snooping protocols depending on the way to manage a local copy of a write operation:

=== Write-invalidate ===
{{Unreferenced section|date=June 2023}}
When a processor writes on a shared cache block, all the shared copies in the other caches are [[Cache invalidation|invalidated]] through bus snooping.<ref>{{Cite book |last1=Eggers |first1=S. J. |last2=Katz |first2=R. H. |chapter=Evaluating the performance of four snooping cache coherency protocols |date=1989 |title=Proceedings of the 16th annual international symposium on Computer architecture - ISCA '89 |chapter-url=http://portal.acm.org/citation.cfm?doid=74925.74927 |language=en |publisher=ACM Press |pages=2–15 |doi=10.1145/74925.74927 |isbn=978-0-89791-319-5}}</ref> This method ensures that only one copy of a datum can be exclusively read and written by a processor. All the other copies in other caches are invalidated. This is the most commonly used snooping protocol. [[MSI protocol|MSI]], [[MESI protocol|MESI]], [[MOSI protocol|MOSI]], [[MOESI protocol|MOESI]], and [[MESIF protocol|MESIF]] protocols belong to this category.

=== Write-update ===
When a processor writes on a shared cache block, all the shared copies of the other caches are updated through bus snooping. This method broadcasts a write data to all caches throughout a bus. It incurs larger bus traffic than write-invalidate protocol. That is why this method is uncommon. [[Dragon protocol|Dragon]] and [[Firefly (cache coherence protocol)|firefly]] protocols belong to this category.<ref>{{Cite book |last1=Hennessy |first1=John L |title=Computer Architecture: A Quantitative Approach |last2=Patterson |first2=David A. |publisher=Elsevier |year=2011 |isbn=978-0123838728}}</ref><ref>{{cite book |title=Computer Architecture A Quantitative Approach |last1=Patterson |first1=David A. |last2=Hennessy |first2=John L. |isbn=1-55860-069-8 |date=1990 |pages=469–471 |publisher=Morgan Kaufmann Publishers}}</ref>