Datasheet

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