82845G Datasheet, PDF (66/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

Register Description
Microsoft MS-DOS* Application Area (00000h–9FFFh)
The MS-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 GMCH, 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 GMCH 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 GMCH.
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^® 82845G/82845GL/82845GV GMCH Datasheet