Dragon protocol

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.

States

In this protocol, the following states can be assigned to each block:

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):

 I   C   SC   D   SD 
 I  Green tick Green tick Green tick Green tick Green tick
 C  Green tick Red X Red X Red X Red X
 SC  Green tick Red X Green tick Red X Green tick
 D  Green tick Red X Red X Red X Red X
 SD  Green tick Red X Green tick Red X Red X

Transitions

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:

References

See also

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