AN APPARATUS AND METHOD FOR PROVIDING COHERENCE DATA FOR USE WHEN IMPLEMENTING A CACHE COHERENCY PROTOCOL

摘要

An apparatus and method are provided for receiving a request from a plurality of processing units, where multiple of those processing units have associated cache storage. A snoop unit is used to implement a cache coherency protocol when a request is received that identifies a cacheable memory address. The snoop unit has snoop filter storage comprising a plurality of snoop filter tables organised in a hierarchical arrangement. The snoop filter tables comprise a primary snoop filter table at a highest level in the hierarchy, and each snoop filter table at a lower level in the hierarchy forms a backup snoop filter table for an adjacent snoop filter table at a higher level in the hierarchy. Each snoop filter table is arranged as a multi-way set associative storage structure, and each backup snoop filter table has a different number of sets than are provided in the adjacent snoop filter table at the higher level in the hierarchy. Each entry in the snoop filter tables is used to store, for an associated address, coherence data used to determine which cache storages need to be subjected to a snoop operation in response to a request specifying that associated address. Snoop control circuitry controls allocation of coherence data into entries of the snoop filter tables, and is arranged, when new coherence data is to be allocated into the snoop filter storage, to apply an allocation policy to determine into which of the plurality of snoop filter tables the new coherence data is allocated.