authors: Jason Lowe-Power
last edited: 2023-01-11 16:05:55 +0000

Cache Coherence Protocols

Common Notations and Data Structures

Coherence Messages

These are described in the <protocol-name>-msg.sm file for each protocol.

Message Description
ACK/NACK positive/negative acknowledgement for requests that wait for the direction of resolution before deciding on the next action. Examples are writeback requests, exclusive requests.
GETS request for shared permissions to satisfy a CPU’s load or IFetch.
GETX request for exclusive access.
INV invalidation request. This can be triggered by the coherence protocol itself, or by the next cache level/directory to enforce inclusion or to trigger a writeback for a DMA access so that the latest copy of data is obtained.
PUTX request for writeback of cache block. Some protocols (e.g. MOESI_CMP_directory) may use this only for writeback requests of exclusive data.
PUTS request for writeback of cache block in shared state.
PUTO request for writeback of cache block in owned state.
PUTO_Sharers request for writeback of cache block in owned state but other sharers of the block exist.
UNBLOCK message to unblock next cache level/directory for blocking protocols.

AccessPermissions

These are associated with each cache block and determine what operations are permitted on that block. It is closely correlated with coherence protocol states.

Permissions Description
Invalid The cache block is invalid. The block must first be obtained (from elsewhere in the memory hierarchy) before loads/stores can be performed. No action on invalidates (except maybe sending an ACK). No action on replacements. The associated coherence protocol states are I or NP and are stable states in every protocol.
Busy TODO
Read_Only Only operations permitted are loads, writebacks, invalidates. Stores cannot be performed before transitioning to some other state.
Read_Write Loads, stores, writebacks, invalidations are allowed. Usually indicates that the block is dirty.

Data Structures

Coherence controller FSM Diagrams