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Host Bridge/DRAM Controller Registers (D0:F0)

Datasheet

4.1.14

PCIEXBAR—PCI Express* Register Range Base Address

(D0:F0) (Intel

® 82915G/82915P/82915PL Only)

PCI Device:

Address Offset:

48h

Default Value:

E0000000h

Access:

R/W

Size:

32 bits

This is the base address for the PCI Express configuration space. This window of addresses

contains the 4 KB of configuration space for each PCI Express device that can potentially be part

of the PCI Express hierarchy associated with the (G)MCH. There is not actual physical memory

within this 256-MB window that can be addressed. Each PCI Express hierarchies require a PCI

Express BASE register. The (G)MCH supports one PCI Express hierarchy.

The 256 MB reserved by this register does not alias to any PCI 2.3 compliant memory-mapped

space. For example, MCHBAR reserves a 16-KB space and reserves a 4-KB space both outside of

PCIEXBAR space. They cannot be overlaid on the space reserved by PCIEXBAR for devices 0.

On reset, this register is disabled and must be enabled by writing a 1 to PCIEXBAREN [Dev 0,

offset 54h, bit 31]

If the PCI Express Base Address [bits 31:28] were set to Fh, an overlap with the High BIOS area,

APIC ranges would result. Software must guarantee that these ranges do not overlap. The PCI

Express Base Address cannot be less than the maximum address written to the Top of physical

memory register (TOLUD). If a system is populated with more than 3.5 GB, either the PCI

Express Enhanced Access mechanism must be disabled or the value in TOLUD must be reduced

to report that only 3.5 GB are present in the system to allow a value of Eh for the PCI Express

Base Address (assuming that all PCI 2.3 compatible configuration space fits above 3.75 GB).

Bit

Access &

Default

Description

31:28

R/W

PCI Express* Base Address: This field corresponds to bits 31 to 28 of the

base address for PCI Express enhanced configuration space.

BIOS will program this register resulting in a base address for a 256-MB block

of contiguous memory address space. Having control of those particular 4 bits

insures that this base address will be on a 256-MB boundary, above the lowest

256 MB and still within total addressable memory space, currently 4 GB.

The address used to access the PCI Express configuration space for a specific

device can be determined as follows:

PCI Express Base Address + Bus Number * 1 MB + Device Number * 32 KB +

Function Number * 4 KB

The address used to access the PCI Express configuration space for Device 1

in this component would be PCI Express Base Address + 0 * 1 MB + 1 * 32 KB

+ 0 * 4 KB = PCI Express Base Address + 32 KB. Remember that this

address is the beginning of the 4-KB space that contains both the PCI

compatible configuration space and the PCI Express extended configuration

space.

27:0

Reserved

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