Altera IP Compiler for PCI Express - Manual (Page 202)

11–6 Chapter 11: Flow Control
Throughput of Non-Posted Reads
IP Compiler for PCI Express User Guide August 2014 Altera Corporation
You can also control the maximum amount of outstanding read request data. This
amount is limited by the number of header tag values that can be issued by the
application and by the maximum read request size that can be issued. The number of
header tag values that can be in use is also limited by the IP Compiler for PCI Express.
For the ×8 function, you can specify 32 tags. For the ×1 and ×4 functions, you can
specify up to 256 tags, though configuration software can restrict the application to
use only 32 tags. In commercial PC systems, 32 tags are typically sufficient to maintain
optimal read throughput.