Image Number 6 for United States Patent #8015364.
An apparatus for implementing snooping cache coherence that locally reduces the number of snoop requests presented to each cache in a multiprocessor system. A snoop filter device associated with a single processor includes one or more "scoreboard" data structures that make snoop determinations, i.e., for each snoop request from another processor, to determine if a request is to be forwarded to the processor or, discarded. At least one scoreboard is active, and at least one scoreboard is determined to be historic at any point in time. A snoop determination of the queue indicates that an entry may be in the cache, but does not indicate its actual residence status. In addition, the snoop filter block implementing scoreboard data structures is operatively coupled with a cache wrap detection logic means whereby, upon detection of a cache wrap condition, the content of the active scoreboard is copied into a historic scoreboard and the content of at least one active scoreboard is reset.