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Introduction
A primary goal of the PPB architecture is that PPB are 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 PPBs. 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.
Figure 1
shows a hypothetical PCI add-in card used for an intelligent controller application. In some applications
the transparency of a PPB is not desired. For example,
Figure 1. 21555 Intelligent Controller Application
Intelligent Subsystem
DRAM/
ROM
CPU-
Local
PCI
CPU
Bridge
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.
•
That it is desirable to restrict access to these same resources from other PCI bus masters in the system and from
the host processor.
•
That 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 PPB would
be problematic.
Since the 21555 is non 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 PPB, 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.
16
PCI
PCI
Device
Device
Intel
®
PCI Bus
21555
Device
21555 Non-Transparent PCI-to-PCI Bridge User Manual
PCI
Memory
Device
Host
PCI Bus
Core
Logic
Host
CPU
A8826-01
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