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21555

Non-Transparent PCI-to-PCI Bridge

Intel 

Non-Transparent PPB

It was a primary goal of the PCI-to-PCI bridge architecture that a PPB be transparent to devices

and device drivers. For example, no changes are needed to a device driver when a PCI peripheral is

located behind a PPB. Once configured during system initialization, a PPB operates without the aid

of a device driver. A PPB does not require a device driver of its own since it does not have any

resources that must be managed by software during run-time. This requirement for transparency

forced the usage of a flat addressing model across PCI-to-PCI bridges. This means that a given

physical address exists at only one location in the PCI bus hierarchy and that this location may be

accessed by any device attached at any point in the PCI bus hierarchy. As a consequence, it is not

possible for a PPB to isolate devices or address ranges from access by devices on the opposite

interface of a PPB. The PPB architecture assumes that the resources of any device in a PCI system

are configured and managed by the host processor.

However, there are applications where the transparency of a PCI-to-PCI bridge is not desired. For

example, Figure 1 shows a hypothetical PCI add-in card used for an intelligent subsystem

application.

Figure 1. 21555 Intelligent Controller Application

Intelligent Subsystem

DRAM/

ROM

PCI

Device

PCI

Device

PCI

Device

Memory

Local

CPU

CPU-

PCI

Bridge

PCI Bus

Intel®

21555

Device

PCI Bus

Host

Core

Logic

Host

CPU

A8826-01

Assume that the local processor on the add-in card is used to manage the resources of the devices

attached to the add-in card’s local PCI bus. Assume also that it is desirable to restrict access to

these same resources from other PCI bus masters in the system and from the host processor. In

addition, there is a need to resolve address conflicts that may exist between the host system and the

local processor. The non transparency of the 21555 is perfectly suited to this kind of configuration,

where a transparent PCI-to-PCI bridge is problematic.

Because the 21555 is not transparent, the device driver for the add-in card must be aware of the

presence of the 21555 and manage its resources appropriately. The 21555 allows the entire

subsystem to appear as a single virtual device to the host. This enables configuration software to

identify the appropriate driver for the subsystem.

With a transparent PCI-to-PCI bridge, a driver does not need to know about the presence of the

bridge and manage its resources. The subsystem appears to the host system as individual PCI

devices on a secondary PCI bus, not as a single virtual device.

Table 1 shows a comparison between a 21555 and a standard transparent PCI-to-PCI bridge.

6

Datasheet

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