5.8. Programming Model for Avalon-MM Root Port
The Application Layer writes the Root Port TLP TX Data registers with TLP formatted data for Configuration Read and Write Requests, Message TLPs, Message TLPs with data payload, I/O Read and Write Requests, or single dword Memory Read and Write Requests. Software should check the Root Port Link Status register (offset 0x92) to ensure the Data Link Layer Link Active bit is set to 1'b1 before issuing a Configuration request to downstream ports.
The Application Layer data must be in the appropriate TLP format with the data payload aligned to the TLP address. Aligning the payload data to the TLP address may result in the payload data being either aligned or unaligned to the qword. The following figure illustrates three dword TLPs with data that is aligned and unaligned to the qword.
The following figure illustrates four dword TLPs with data that are aligned and unaligned to the qword.
The TX TLP programming model scales with the data width. The Application Layer performs the same writes for both the 64- and 128-bit interfaces. The Application Layer can only have one outstanding non-posted request at a time. The Application Layer must use tags 16–31 to identify non-posted requests.
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