A transaction layer packet (TLP) is transferred on each of the AXI4-Stream interfaces as a descriptor followed by payload data (when the TLP has a payload). The descriptor has a fixed size of 16 bytes on the request interfaces and 12 bytes on the completion interfaces. On its transmit side (towards the link), the integrated block assembles the TLP header from the parameters supplied by the user application in the descriptor. On its receive side (towards the user interface), the integrated block extracts parameters from the headers of received TLP and constructs the descriptors for delivering to the user application. Each TLP is transferred as a packet, as defined in the AXI4-Stream Interface protocol.
When a payload is present, there are two options for aligning the first byte of the payload with respect to the datapath.
1. Dword-aligned mode: In this mode, the descriptor bytes are followed immediately by the payload bytes in the next Dword position, whenever a payload is present.
2. Address-Aligned Mode: In this mode, the payload can begin at any byte position on the datapath. For data transferred from the integrated block to the user application, the position of the first byte is determined as:
n = A mod w
where A is the memory or I/O address specified in the descriptor (for message and configuration requests, the address is taken as 0), and w is the configured width of the data bus in bytes. Any gap between the end of the descriptor and the start of the first byte of the payload is filled with null bytes.
For data transferred from the integrated block to the user application, the data alignment is determined based on the starting address where the data block is destined to in user memory. For data transferred from the user application to the integrated block, the user application must explicitly communicate the position of the first byte to the integrated block using the tuser sideband signals when the address-aligned mode is in use.
In the address-aligned mode, the payload and descriptor are not allowed to overlap. That is, the transmitter begins a new beat to start the transfer of the payload after it has transmitted the descriptor. The transmitter fills any gaps between the last byte of the descriptor and the first byte of the payload with null bytes.
The Vivado IP catalog applies the data alignment option globally to all four interfaces. However, advanced users can select the alignment mode independently for each of the four AXI4-Stream interfaces. This is done by setting the corresponding alignment mode parameter. See the Completer Interface for more details on address alignment and example diagrams.