Apparatus for maintaining multilevel cache hierarchy coherency in a

Abstract

A computer system comprising a plurality of caching agents with a cache hierarchy, the caching agents sharing memory across a system bus and issuing memory access requests in accordance with a protocol wherein a line of a cache has a present state comprising one of a plurality of line states. The plurality of line states includes a modified (M) state, wherein a line of a first caching agent in M state has data which is more recent than any other copy in the system; an exclusive (E) state, wherein a line in E state in a first caching agent is the only one of the agents in the system which has a copy of the data in a line of the cache, the first caching agent modifying the data in the cache line independent of other said agents coupled to the system bus; a shared (S) state, wherein a line in S state indicates that more than one of the agents has a copy of the data in the line; and an invalid (I) state indicating that the line does not exist in the cache. A read or a write to a line in I state results in a cache miss. The present invention associates states with lines and defines rules governing state transitions. State transitions depend on both processor generated activities and activities by other bus agents, including other processors. Data consistency is guaranteed in systems having multiple levels of cache and shared memory and/or multiple active agents, such that no agent ever reads stale data and actions are serialized as needed.