Coherency Manager (CM) Behavior

Directory State

The CM maintains a directory entry for each cache line that is present in any core’s L1 cache. Each entry tracks:

• Sharer bitmask: which cores hold a copy of the line.

• State: the coherence state from the CM’s perspective (I, S, E, or M).

• Owner: the core ID of the exclusive/modified owner (if applicable).

CM Response Logic

Incoming Request	Directory State	CM Action
ReadShared	No sharers (I)	Fetch from memory, respond GrantE.
ReadShared	Other sharers exist (S)	Fetch from memory, respond GrantS, add requester to sharers.
ReadShared	Another core has E	Send Invalidate to E holder, wait for InvAck, fetch from memory, respond GrantS (or GrantE if now the sole copy).
ReadShared	Another core has M	Send Invalidate to M holder, wait for WriteBack, commit to memory, respond GrantS to requester.
ReadExclusive	No sharers (I)	Fetch from memory, respond GrantE.
ReadExclusive	Other sharers exist (S)	Send Invalidate to all sharers, wait for all InvAcks, fetch from memory, respond GrantE.
ReadExclusive	Another core has E	Send Invalidate to E holder, wait for InvAck, fetch from memory, respond GrantE.
ReadExclusive	Another core has M	Send Invalidate to M holder, wait for WriteBack, commit to memory, respond GrantE.
Upgrade	Requester is sole sharer	Respond GrantM immediately.
Upgrade	Other sharers exist	Send Invalidate to other sharers, wait for all InvAcks, respond GrantM.
WriteBack	Core is M owner	Commit data to memory, respond WriteBackAck, remove core from directory.