A method and apparatus is disclosed for maintaining coherency between a
primary cache and a secondary cache in a directory-based cache system.
Upon identifying a parity error in the primary cache, a tag parity packet
and a load instruction are sent from the primary cache to the secondary
cache. In response to the tag parity packet, each tag entry in the
secondary cache that is associated with the parity error is invalidated.
Upon receiving an acknowledgment of receipt of the tag parity packet, the
primary cache functions to invalidate each tag entry in the primary cache
that is associated with the parity error. Then, the secondary cache
communicates data requested in the load instruction to the primary cache.