82845PE Datasheet, PDF (64/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

Register Description
For details on overall system address mapping scheme, refer to Chapter 4.
DOS Application Area (00000h–9FFFh)
The DOS area is 640 KB in size and it is further divided into two parts. The 512-KB area at 0 to
7FFFFh is always mapped to the main memory controlled by the (G)MCH, while the 128-KB
address range from 080000h to 09FFFFh can be mapped to PCI_A or to main DRAM. By default
this range is mapped to main memory and can be declared as a main memory hole (accesses
forwarded to PCI_A) via the (G)MCH FDHC configuration register.
Video Buffer Area (A0000h–BFFFFh)
Attribute bits do not control this 128-KB area. The host -initiated cycles in this region are always
forwarded to either PCI_A or AGP unless this range is accessed in SMM mode. Routing of
accesses is controlled by the Legacy VGA control mechanism of the “virtual” PCI-to-PCI
bridge device embedded within the (G)MCH.
This area can be programmed as SMM area via the SMRAM register. When used as a SMM space,
this range cannot be accessed from the hub interface or AGP.
Expansion Area (C0000h–DFFFFh)
This 128-KB area is divided into eight, 16-KB segments that can be assigned with different
attributes via the PAM control registers as defined by Table 3-2.
Extended System BIOS Area (E0000h–EFFFFh)
This 64-KB area is divided into four, 16-KB segments that can be assigned with different attributes
via the PAM control registers as defined by Table 3-2.
System BIOS Area (F0000h–FFFFFh)
This area is a single, 64-KB segment, which can be assigned with different attributes via the PAM
control registers as defined by Table 3-2.
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Intel^® 82845GE/82845PE Datasheet