Intel B940 Datasheet Page 19

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Datasheet, Volume 2 19
Processor Configuration Registers
2.2.1 Legacy Address Range
This area is divided into the following address regions:
0 – 640 KB — DOS Area
640 – 768 KB — Legacy Video Buffer Area
768 – 896 KB in 16 KB sections (total of 8 sections) — Expansion Area
896 – 960 KB in 16 KB sections (total of 4 sections) — Extended System BIOS Area
960 KB – 1 MB Memory — System BIOS Area
2.2.1.1 DOS Range (0000_0000h – 0009_FFFFh)
The DOS area is 640 KB (0000_0000h – 0009_FFFFh) in size and is always mapped to
the main memory controlled by the processor.
2.2.1.2 Legacy Video Area (000A_0000h – 000B_FFFFh)
The legacy 128 KB VGA memory range, frame buffer, (000A_0000h – 000B_FFFFh) can
be mapped to IGD (Device 2), to PCI Express (Device 1), and/or to the DMI Interface.
The appropriate mapping depends on which devices are enabled and the programming
of the VGA steering bits. Based on the VGA steering bits, priority for VGA mapping is
constant. The processor always decodes internally mapped devices first. Internal to the
processor, decode priority is:
1. IGD
2. PCI Express
3. DMI Interface (subtractive)
Figure 2-2. DOS Legacy Address Range
Expansion Area
128 KB (16 KB x 8)
000C_0000h
000D_FFFFh
896 KB
Extended System BIOS (Lower)
64 KB (16 KB x 4)
000E_0000h
000E_FFFFh
960 KB
Legacy Video Area
(SMM Memory)
128 KB
000A_0000h
000B_FFFFh
768 KB
DOS Area
0000_0000h
0009_FFFFh
640 KB
System BIOS (Upper)
64 KB
000F_0000h
000F_FFFFh
1 MB
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