Channels

PicoLink uses exactly two channels per core, one in each direction. Each channel is an independent, unidirectional, flow-controlled message stream (see Signal Descriptions and Flow Control for the signal-level definition).

Channel	Direction	Purpose
A	Core → CM	Requests (ReadShared, ReadExclusive, Upgrade), voluntary and forced writebacks (WriteBack), and invalidation acknowledgments (InvAck).
B	CM → Core	Grants (GrantS, GrantE, GrantM), demand invalidations (Invalidate), writeback acknowledgments (WriteBackAck), and denials (Nack).

Channel usage rules

Rule	Requirement
PL-CHAN-01	A-channel opcodes shall only be transported on channel A; B-channel opcodes shall only be transported on channel B. The unified opcode space (see Message Field Definitions) makes the channel derivable from the opcode; a receiver may treat a mismatch as a protocol error.
PL-CHAN-02	A core shall have at most one outstanding request (ReadShared, ReadExclusive, Upgrade, or voluntary WriteBack) per cache line at any time. Multiple outstanding requests to different lines are permitted, bounded by the transaction-ID space (see Message Field Definitions).
PL-CHAN-03	The CM shall have at most one outstanding Invalidate per {core, cache line} pair at any time.
PL-CHAN-04	Nack shall only be issued in response to Upgrade. ReadShared, ReadExclusive, and WriteBack shall never be denied; the CM shall stall or queue such requests until it can serve them (see Coherency Manager (CM) Behavior). This restriction is what guarantees forward progress after a Nack retry (rule PL-PROG-01).

A channel messages (Core → CM)

Message	Class	Meaning
ReadShared	Request	Request at least Shared (S) permission for a cache line, with data. The CM may respond with GrantE when the requester would be the only holder (see Grant message usage).
ReadExclusive	Request	Request Exclusive (E) permission for a cache line, with data, typically ahead of a store.
Upgrade	Request	Request promotion S → M without data transfer. The requester shall hold the line in S when the request is issued. This is the only request that can be denied (Nack).
WriteBack	Writeback	Transfer a dirty (M) line back to memory and relinquish it. Issued voluntarily on eviction, or as the forced response of an M holder to Invalidate. Carries a full-line data payload.
InvAck	Response	Acknowledge a received Invalidate. Confirms the core no longer holds a valid copy of the line.

Rule	Requirement
PL-CHAN-05	A core that receives Invalidate shall respond with exactly one of WriteBack (if it holds the line in M) or InvAck (in all other cases, including I — see rule PL-MESI-06). When a core in M responds with WriteBack, that WriteBack is the implicit InvAck: the core shall not send a separate InvAck, and the CM shall not expect one.

B channel messages (CM → Core)

Message	Class	Meaning
GrantS	Response	Grants Shared permission. Carries a full clean line of data.
GrantE	Response	Grants Exclusive permission. Carries a full clean line of data.
GrantM	Response	Grants write (Modified) permission without data. Issued only in response to Upgrade: the requester already holds the current data in S.
Invalidate	Demand	Orders the core to relinquish the line: write back if dirty (WriteBack), acknowledge otherwise (InvAck), and transition to I. The only CM-initiated message in the protocol.
WriteBackAck	Response	Acknowledges a WriteBack; confirms the payload has been committed to memory. Completes the writeback transaction.
Nack	Response	Denies an Upgrade that lost a race. The requester shall retry using ReadExclusive (see Ordering, Races, and Atomicity).

Grant message usage

Message	Data?	Response to	Conditions
GrantS	Yes	ReadShared	Always legal in response to ReadShared.
GrantE	Yes	ReadShared, ReadExclusive	The only legal response to ReadExclusive. Legal in response to ReadShared only when the requester becomes the sole holder of the line.
GrantM	No	Upgrade	Only after all other sharers have been invalidated and every acknowledgment has been received (rule PL-CM-06).

Rule	Requirement
PL-CHAN-06	In response to ReadShared, the CM shall issue GrantS or GrantE. GrantE is permitted only when no other core holds the line at the time the grant is issued, and is the recommended response in that case (the Exclusive-grant optimization: it allows the requester to later write without an Upgrade). GrantS is always legal.
PL-CHAN-07	In response to ReadExclusive, the CM shall issue GrantE and nothing else. Note that a core intending to write receives E, not M: the subsequent store performs the silent E → M transition locally (see MESI State Transitions).

Request–response summary

Every A-channel message except InvAck completes with exactly one B-channel response:

A-channel message	Legal completions	Illegal responses
ReadShared	GrantS, GrantE	GrantM, Nack
ReadExclusive	GrantE	GrantS, GrantM, Nack
Upgrade	GrantM, Nack	GrantS, GrantE
WriteBack	WriteBackAck	any other
InvAck	(none — terminates the CM’s Invalidate)	any

Rule	Requirement
PL-CHAN-08	Every request and every WriteBack shall receive exactly one completing response. InvAck and Invalidate are themselves completions and shall not be responded to.