Firefly is an update-based snoopy cache coherence protocol from DEC's Systems Research Center that uses a dedicated “shared wire” on the bus to signal whether a loaded line is exclusively held or shared. This lets caches determine exclusivity at load time, avoiding ambiguity on the first write similar to MESI's Exclusive state.
Firefly and Dragon are the canonical update-based protocols, differing mainly in how they detect exclusivity at load time.
Shared-wire detection eliminates exclusive-to-modified ambiguity.
read request -> any sharer asserts shared-signal wire requester knows exclusivity immediately -> can optimize first write