Channels

PicoLink uses exactly two channels:

Channel	Direction	Purpose
A	Core → CM	Requests, writebacks, and invalidation acknowledgments
B	CM → Core	Grants, invalidations, acknowledgments, and denials

A Channel Messages (Core → CM)

Message Type	Meaning
`ReadShared`	Request Shared (S) permission for a cache line.
`ReadExclusive`	Request Exclusive (E) permission for a cache line.
`Upgrade`	Request S → M transition (core already holds line in S).
`WriteBack`	Evict a Modified (M) line: carries dirty data payload.
`InvAck`	Acknowledge a received `Invalidate` from the CM.

Note

When a core in M state receives an Invalidate, its WriteBack response implicitly serves as the InvAck. The CM shall not expect a separate InvAck in this case.

B Channel Messages (CM → Core)

Message Type	Meaning
`GrantS`	Data + Shared permission.
`GrantE`	Data + Exclusive permission.
`GrantM`	Write permission only (no data): `Upgrade` response only.
`Invalidate`	Demand the core write back (if dirty) then go to `I` state.
`WriteBackAck`	Acknowledge the core’s `WriteBack` (memory has committed data).
`Nack`	Request denied: core must retry as `ReadExclusive`.

Grant Message Usage

Message	Carries Data?	Used For
`GrantS`	Yes	`ReadShared` response when other sharers exist.
`GrantE`	Yes	`ReadShared` response (no other sharers) or `ReadExclusive` response.
`GrantM`	No	`Upgrade` response only.