CHI-B Spec Page number 2-93 states that writes must not be merged in the device memory type description.
I am unable to understand:
a) What is this merging?
b) How to merge different writes?
Also later it talks about combining different requests to the same location into one request:
a) What is this request combining?
Write merging is the action of the hardware to combine multiple individual stores into a single write, in order to make these more efficient.
For example, in software you might execute a 4 separate stores to a contiguous region of memory. The hardware could execute this as either 4 separate CHI writes requests to memory, or it could merge these into a single large CHI write. This is typically controlled by the page table attributes within the RN.
Request combining is the same concept, but extended to other requests. For example, you write and then a read could be issued as two separate write and then read requests, or the read could have the data directly returned from the write (e.g. from a store buffer) and so the read is never issued externally.