845GL datasheet(141/193 Pages) INTEL

141 / 193 page

Intel® 82845G/82845GL/82845GV GMCH Datasheet

141

System Address

5.1.1

Compatibility Area

This area is divided into the following address regions:

• 0 – 640 KB DOS Area

• 640 – 768 KB Video Buffer Area

• 768 – 896 KB in 16-KB sections (total of 8 sections) - Expansion Area

• 896 – 960 KB in 16-KB sections (total of 4 sections) - Extended System BIOS Area

• 960 KB – 1 MB Memory (BIOS Area) - System BIOS Area

There are fifteen memory segments in the compatibility area. Thirteen of the memory ranges can

be enabled or disabled independently for both read and write cycles.

DOS Area (00000h–9FFFFh)

The DOS area is 640 KB in size and is always mapped to the main memory controlled by the

GMCH.

Legacy VGA Ranges (A0000h–BFFFFh)

The legacy 128-KB VGA memory range A0000h–BFFFFh (Frame Buffer) can be mapped to IGD

(Device 2), to AGP/PCI_B (Device 1), and/or to the Hub Interface depending on the programming

of the VGA steering bits. Priority for VGA mapping is constant in that the GMCH always decodes

internally mapped devices first. Internal to the GMCH, decode precedence is given to IGD. The

GMCH always positively decodes internally mapped devices (IGD and AGP/PCI_B). Subsequent

decoding of regions mapped to AGP/PCI_B or the hub interface depends on the Legacy VGA

configurations bits (VGA Enable and MDAP). This region is also the default for SMM space.

Table 5-1. Memory Segments and Their Attributes

Memory Segments

Attributes

Comments

000000h–09FFFFh

fixed - always mapped to main

SDRAM

0 to 640K – DOS Region

0A0000h–0BFFFFh

mapped to Hub Interface, AGP, or

IGD - configurable as SMM space

Video Buffer (physical SDRAM

configurable as SMM space)

0C0000h–0C3FFFh

WE, RE

Add-on BIOS

0C4000h–0C7FFFh

WE, RE

Add-on BIOS

0C8000h–0CBFFFh

WE, RE

Add-on BIOS

0CC000h–0CFFFFh

WE, RE

Add-on BIOS

0D0000h–0D3FFFh

WE, RE

Add-on BIOS

0D4000h–0D7FFFh

WE, RE

Add-on BIOS

0D8000h–0DBFFFh

WE, RE

Add-on BIOS

0DC000h–0DFFFFh

WE, RE

Add-on BIOS

0E0000h–0E3FFFh

WE, RE

BIOS Extension

0E4000h–0E7FFFh

WE, RE

BIOS Extension

0E8000h–0EBFFFh

WE, RE

BIOS Extension

0EC000h–0EFFFFh

WE, RE

BIOS Extension

0F0000h–0FFFFFh

WE, RE

BIOS Area

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