82845G Datasheet, PDF (141/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

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.
Table 5-1. Memory Segments and Their Attributes
Memory Segments
000000h–09FFFFh
0A0000h–0BFFFFh
0C0000h–0C3FFFh
0C4000h–0C7FFFh
0C8000h–0CBFFFh
0CC000h–0CFFFFh
0D0000h–0D3FFFh
0D4000h–0D7FFFh
0D8000h–0DBFFFh
0DC000h–0DFFFFh
0E0000h–0E3FFFh
0E4000h–0E7FFFh
0E8000h–0EBFFFh
0EC000h–0EFFFFh
0F0000h–0FFFFFh
Attributes
fixed - always mapped to main
SDRAM
mapped to Hub Interface, AGP, or
IGD - configurable as SMM space
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
WE, RE
Comments
0 to 640K – DOS Region
Video Buffer (physical SDRAM
configurable as SMM space)
Add-on BIOS
Add-on BIOS
Add-on BIOS
Add-on BIOS
Add-on BIOS
Add-on BIOS
Add-on BIOS
Add-on BIOS
BIOS Extension
BIOS Extension
BIOS Extension
BIOS Extension
BIOS Area
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.
Intel^® 82845G/82845GL/82845GV GMCH Datasheet
141