82845G Datasheet, PDF (142/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

System Address
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 hub interface 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 any
one of these devices, the GMCH must decode cycles in the MDA range and forward them either to
IGD, AGP/PCI_B, or the hub interface. This capability is controlled by a 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 SDRAM or to the 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 SDRAM.
When disabled, this segment is not remapped.
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Intel^® 82845G/82845GL/82845GV GMCH Datasheet