IDT PCI Express 89HPES32NT24xG2, User Manual, Page: 44 / 744

IDT PES32NT24xG2 Device Overview
PES32NT24xG2 User Manual 1 - 8 January 30, 2013
Notes
Non-Transparent Operation
The PCI architecture defines a hierarchy of buses interconnected by PCI-to-PCI bridges. This hierarchy
forms a tree and is referred to as a PCI domain.
– A PCI domain consists of a single memory address space, I/O address space, and ID address
space.
– The PCI ID consists of a bus, device and function number that uniquely defines an element in the
domain.
Although PCI Express switches support direct transfers between ports, the logical view seen by soft-
ware remains that of a hierarchy of buses as defined by the PCI architecture and illustrated in Figure 1.3.
The portion of a PCI domain emanating from a PCI Express root complex is referred to as the PCI Express
domain.
In many applications, a need exists to interconnect two independent PCI domains. A Non-Transparent
Bridge (NTB) enables this inter-domain communication. The architecture of an NTB is illustrated in Figure
1.5.
Figure 1.5 Non-Transparent Bridge
An NTB consists of two PCI functions each defined by a Type 0 PCI header that are interconnected by a
bridging function. The two Type 0 PCI functions are referred two as Non-Transparent (NT) endpoints (a.k.a.
NT functions). Each function advertises one or more memory windows using PCI Base Address Registers
(BARs). Software executing on each hierarchy allocates PCI memory space to the BAR. Memory opera-
tions that target a memory window defined by an NT endpoint are routed within the PCI domain to that
endpoint. When the non-transparent bridge receives a memory operation that targets a BAR used for
mapping through the bridge, it translates the address of the transaction to a new address in the opposite
domain and forwards the transaction to the other domain. Completions are handled in a similar manner.
The first non-transparent bridge was developed in 1997 Digital Semiconductor and called Drawbridge
(a.k.a. 21554). Drawbridge has been widely used to construct PCI based multi-processors and intelligent I/
O adapters. In 2004 PLX extended the Drawbridge NTB architecture to PCI Express by introducing
Requester ID translation. The PLX approach limited the number of masters to 8 on one side of the bridge
and 32 on the other side.
While maintaining the architectural concepts of the original Digital architecture, the switch extends non-
transparent bridging to allow direct non-transparent switching between two or more domains, and between
up to 64 masters. As shown in Figure 1.6, the switch allows two or more non-transparent endpoints to
directly communicate over a non-transparent (NT) interconnect. This extension of non-transparent bridging
from two ports to multiple ports parallels the evolution of two-port PCI bridges to multi-port PCI Express
switches.
PCIe Domain 0
Non-
Transparent
Bridge
PCIe Domain 1
NT
Endpoint
NT
Endpoint