82845PE Datasheet, PDF (104/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

System Address
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.
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 (82845GE only), 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 (G)MCH must decode cycles in the MDA range and
forward them either to IGD (82845GE only), 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 (G)MCH decodes I/O cycles at 3B4h, 3B5h, 3B8h, 3B9h,
3BAh and 3BFh and forwards them to the either the IGD (82845GE only), 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 (G)MCH 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 (G)MCH can “shadow” BIOS into the main SDRAM.
When disabled, this segment is not remapped.
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Intel^® 82845GE/82845PE Datasheet