
142
Intel
82865G/82865GV GMCH Datasheet
System Address Map
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 always given to IGD.
The GMCH always positively decodes internally mapped devices, namely the 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.
Compatible SMRAM Address Range (A0000h–BFFFFh)
When compatible SMM space is enabled, SMM-mode processor accesses to this range are routed
to physical system SDRAM at this address. Non-SMM-mode processor accesses to this range are
considered to be to the video buffer area as described above. AGP and HI originated cycles to
enabled SMM space are not allowed and are considered to be to the video buffer area.
Monochrome Adapter (MDA) Range (B0000h–B7FFFh)
Legacy support requires the ability to have a second graphics controller (monochrome) in the
system. Accesses in the standard VGA range are forwarded to IGD, AGP/PCI_B, and the hub
interface (depending on configuration bits). Since the monochrome adapter may be mapped to
anyone of these devices, the GMCH must decode cycles in the MDA range and forward them
either to IGD, AGP/PCI_B, or to the hub interface. This capability is controlled by VGA steering
bits and the legacy configuration bit (MDAP bit). In addition to the memory range B0000h to
B7FFFh, the GMCH decodes I/O cycles at 3B4h, 3B5h, 3B8h, 3B9h, 3BAh, and 3BFh and
forwards them to the either the IGD, AGP/PCI_B, and/or the hub interface.
Expansion Area (C0000h–DFFFFh)
This 128-KB ISA Expansion region is divided into eight, 16-KB segments. Each segment can be
assigned one of four read/write states: read-only, write-only, read/write, or disabled. Typically,
these blocks are mapped through GMCH and are subtractively decoded to ISA space. Memory that
is disabled is not remapped.
Extended System BIOS Area (E0000h–EFFFFh)
This 64-KB area is divided into four, 16-KB segments. Each segment can be assigned independent
read and write attributes so it can be mapped either to main system memory or to hub interface.
Typically, this area is used for RAM or ROM. Memory segments that are disabled are not
remapped elsewhere.
System BIOS Area (F0000h–FFFFFh)
This area is a single, 64-KB segment. This segment can be assigned read and write attributes. It is
by default (after reset) read/write disabled and cycles are forwarded to the hub interface. By
manipulating the read/write attributes, the GMCH can “shadow” BIOS into the main system
memory. When disabled, this segment is not remapped.