Core concepts

ACE channel extensions

ACE adds three snoop channels to AXI: AC (snoop address from interconnect to caches), CR (snoop response), and CD (snoop data). A coherent transaction can require snoop and/or memory actions according to its type, shareability domain and the interconnect's tracked state; this need not mean broadcasting every transaction to every cache.

AxDOMAIN identifies the transaction's shareability domain, while other attributes and transaction type complete the contract. A Non-shareable transaction is outside shared-cache coherence for that access; it is not inherently faster. A Shareable attribute alone does not prove that every required agent, region mapping and software ownership rule is configured correctly.

ACE SNOOP FLOW

RN (master) --AR/Aw--> ICN (interconnect)
                          |
                     AC (snoop addr) --> peer caches
                     CR (snoop resp) <-- peer caches
                     CD (snoop data) <-- peer caches
                          |
                     AR/R or AW/W completes with coherent data

Coherent read and write types

ReadShared: fetch line, other caches may keep shared copy. ReadUnique: fetch line, invalidate other copies. WriteUnique: write with exclusive ownership. WriteBack: evict dirty line to memory.

Each type triggers different snoop responses. A ReadUnique that gets a dirty copy from another cache receives data via CD channel instead of going to memory.

• ACE barrier transactions/fields have defined domain and completion semantics; map them to the selected architecture/software ordering contract rather than calling them a universal fence.

• DVM (Distributed Virtual Memory) ops for TLB/cache maintenance across cores.

• Incorrect shareability can change coherence participation or traffic; demonstrate the resulting outcome with the complete region and software contract.

Key takeaways

• ACE barrier transactions/fields have defined domain and completion semantics; map them to the selected architecture/software ordering contract rather than calling them a universal fence.

• DVM (Distributed Virtual Memory) ops for TLB/cache maintenance across cores.

• Incorrect shareability can change coherence participation or traffic; demonstrate the resulting outcome with the complete region and software contract.