Intel 21555 user manual Dram PCI ROM, Cpu Pci

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/	PCI	PCI
	ROM	Device	Device
Local	CPU-	PCI Bus		Intel®
CPU	PCI			21555
	Bridge			Device

PCI

Device

PCI Bus

Memory

Host	Host
Core
	CPU
Logic

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.

•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 21555 Non-Transparent PCI-to-PCI Bridge User Manual