The NMU contains a 64 entry write tracker buffer. This buffer tracks the set of responses received for each chopped write transaction and returns a single write response to the master when all of the chopped responses have been received.
The write tracker is also responsible for ensuring that only transactions to a single slave per AXI-ID is allowed to be outstanding at any given time (SSID check). Back pressure will be applied on a write request if there is an outstanding write transaction with the same AXI-ID but to a different destination.
The maximum number of outstanding write requests is programmable from 2 to 64, with a default value of 64.