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What is the difference between Shared-modified state and modified state? What does it mean multiple caches may have line? Looks like Shared-modified state is a mixture of Shared-clean state and Modified state?


I think shared modified state means there may be other up to date copies of the data in shared-clean state but the memory copy is not up to date.


It seems like shared-modified state is the case when multiple caches have a line and only one of them (the one in the SM state owns the data) is the owner, whereas in the modified state only the owner has the data and noone else. I think "to have" a line is saying that the line is loaded in the cache and up-to-date, so when multiple caches have a line then that means multiple caches have the up-to-date value of that line. I think it's reasonable to think of shared-modified state as a mixture of SC and M where only one cache is the owner and the other caches with the line are SC. However, one thing to note is that in SM and M memory is not up-to-date, but in SC it can depend.

This slide from Berkeley seems to put the states in slightly simpler terms.