A
data processing apparatus and method are provided for managing cache coherency. The
data processing apparatus comprises a plurality of
processing units, each having a cache associated therewith, and each cache having indication circuitry containing segment filtering data. The indication circuitry is responsive to an address portion of an address specified by an access request from an associated
processing unit to reference the segment filtering data in order to provide, for each of at least a subset of the segments of the associated cache, an indication as to whether the data is either definitely not stored in that segment or is potentially stored in that segment. Further, in accordance with the present invention, cache coherency circuitry is provided which employs a cache coherency protocol to ensure data accessed by each
processing unit is up-to-date. The cache coherency circuitry has snoop indication circuitry associated therewith whose content is derived from the segment filtering data of each indication circuitry. For certain access requests, the cache coherency circuitry initiates a coherency operation during which the snoop indication circuitry is referenced to determine whether any of the caches require subjecting to a snoop operation. For each cache for which it is determined a snoop operation should be performed, the cache coherency circuitry is arranged to issue a notification to that cache identifying the snoop operation to be performed. By taking
advantage of information already provided in association with each cache in order to form the content of the snoop indication circuitry, significant hardware
cost savings are achieved when compared with prior art techniques. Further, through use of such an approach, it is possible in embodiments of the present invention to identify the snoop operation not only on a cache-by-cache basis, but also for a particular cache to identify which segments of that cache should be subjected to the snoop operation.