The Dragon cache coherence protocol is the schema used in the Xerox Dragon multiprocessor workstation, developed by Xerox PARC. This protocol uses a write-back policy.
In this protocol, the following states can be assigned to each block:
Invalid: No data in cache block.
Clean: Clean, only copy.
Shared-Clean: Clean, might be shared.
Dirty: Modified, only copy.
Shared-Dirty: Modified, might be shared. This implies that there might be up-to-date other copies of the data (in Shared-Clean state) but that the memory copy is not up-to-date.
These five states correspond to the five states of the MOESI protocol, although they are listed above in IESMO order.
For any given pair of caches, the permitted states of a given cache line are as follows (the states abbreviated in the order above):
In order to identify which transitions must be made, the protocol detects sharing using a special bus line named Shared. All accesses on the main memory bus are snooped by all caches, which assert the Shared line when a snoop hit occurs. The following rules are then applied to the transitions:
Read hit: The data is supplied by the local cache. No state change.
Read miss: If there is any cache with a copy of the cache line, it indicates this with the Shared line and supplies the data to the requesting cache, which keeps a copy in the Shared-Clean state. The supplying cache leaves its copy of the line in the Shared-Dirty or Shared-Clean state, as appropriate. Otherwise fetch the data from main memory and mark the cache Clean.
Write hit: If the data in the cache is in Dirty or Clean state, updated the cached data and mark Dirty. If the state is Shared-Clean or Shared-Dirty then update other caches. If the Shared line is asserted change the local cache to Shared-Dirty and all others to Shared-Clean, otherwise the local cache changes to Dirty.
Write miss: If there is any cache with a copy, that cache supplies the data. The writer generates a write broadcast, the local cache changes to Shared-Dirty and all others to Shared-Clean. Otherwise main memory supplies the data, the local cache state changes to Dirty.
