The NoC request packet from the NMU carries the triple {Source ID, AXI-ID, Tag} for read requests and {Source ID, AXI-ID} for write requests. Non-DDR memory controller NSUs send a compressed AXI-ID to the downstream AXI slave. There are two programmable compression modes:
- A 2-bit compressed AXI-ID is created by choosing any two bits of the {Source ID, AXI-ID}. The bit selection is programmed as part of the NSU configuration.
- A 2-bit fixed value determined by a configuration register. This mode effectively forces read and write requests to remain in order.
By default the NSU will select the least significant two bits of the request AXI-ID as the compressed ID. DDR memory controller NSUs do not compress the AXI-ID.