Cache Coherence: System Config for C++ implementation

Now let’s talk about the system config which will be used in C++ implementation of MOESI cache coherence.

There are three main components:

1. CPU: Containing eight cores
2. Bus: Point of coherence and serialization
3. Memory

Let’s see what these queues are used for.

1. core2bus_req: This queue contains requests sent by the core to bus.

These requests can be:

→ CoreRead request: Core sends read request to bus which will either send to Memory or get data from cache.

→ CoreMemWriteBack: Core sends write-back request to bus which will forward to memory.

→ CoreCacheInvalidateReq: Whenever there is a write request, Core sends an invalidate bus request which will send to other cores having copy of that line.

2. core2bus_rsp: This queue contains responses sent by core to bus.

The responses can be:

→ CoreInvalidateAck: When the core invalidates its line copy, it sends Ack to bus.

→ CoreDataResponse: Whenever there is read request from bus to core having the line, core responds with the data.

3. bus2core: This queue contains requests and responses from bus to core.

Request includes:

→ BusReadReq: Bus sends read request to core which has the copy of line.

→ BusCacheInvalidate: Bus receives invalidate request from core (source) and then bus sends an invalidate req to cores having that line.

Possible responses:

→ BusDataResponse: Bus forwards the data it received from the source core.

→ BusInvalidateAck: Once all cores having line invalidate their copy and send invalidateAck to bus, bus sends source core the acknowledgement meaning that it can perform write operation on the line.

→ BusWriteAck: Bus forwards the acknowledgement from memory to Core meaning that memory is now updated and the core can remove the line.

4. mem2bus: This queue contains MemWriteAck and MemDataResponse provided by bus. The bus will then forward it to the corresponding core in CPU.

5. bus2mem: This queue has MemRead and MemWrite request

I have added some documentation to the codebase and one can find it in doc directory in the GitHub repository:

https://github.com/himanshu5-prog/coherent_protocols/tree/main