IDT PCI Express 89HPES32NT24xG2, User Manual, Page: 82 / 744

IDT Switch Core
PES32NT24xG2 User Manual 4 - 4 January 30, 2013
Notes
In addition to providing HOLB relief, the EFB is used as a dynamically-sized replay buffer. This allows for
efficient use of the egress buffer space: when transmitted packets are not being acknowledged by the link
partner the replay buffer grows to allow further transmission; when transmitted packets are successfully
acknowledged by the link partner the replay buffer shrinks and this space is used as egress buffer space to
provide more HOLB relief to the IFBs. Assuming a link partner issues ACK DLLPs at the rates recom-
mended in the PCI Express Specification 2.1, the replay buffer naturally grows to the optimal size for the
port’s link width and speed. Table 4.3 shows the maximum number of TLPs that may be stored in the EFB’s
replay buffer.
Crossbar Interconnect
The crossbar is a matrix of pathways, capable of concurrently transferring data between all the memory
modules associated with the port IFBs and EFBs, as well as the two DMA modules. As mentioned before,
the port IFBs and EFBs are implemented using shared memory modules. A memory module is capable of
Stack
Mode
EFB
Queue
Total Size and Limitations
(per-port)
x8
Merged
Posted 8192 Bytes and up to 128 TLPs
Non Posted 2048 Bytes and up to 128 TLPs
Completion 8192 Bytes and up to 128 TLPs
x4
Bifurcated
Posted 4096 Bytes and up to 64 TLPs
Non Posted 1024 Bytes and up to 64 TLPs
Completion 4096 Bytes and up to 64 TLPs
x2
Bifurcated
Posted 2048 Bytes and up to 32 TLPs
Non Posted 512 Bytes and up to 32 TLPs
Completion 2048 Bytes and up to 32 TLPs
x1
Bifurcated
Posted 1024 Bytes and up to 16 TLPs
Non Posted 256 Bytes and up to 16 TLPs
Completion 1024 Bytes and up to 16 TLPs
Table 4.2 EFB Buffer Sizes
Stack
Mode
Replay Buffer Storage
Limit
x8
Merged
128 TLPs
x4
Bifurcated
64 TLPs
x2
Bifurcated
32 TLPs
x1
Bifurcated
16 TLPs
Table 4.3 Replay Buffer Storage Limit