
8
PRELIMINARY
PCIset Overview
A
4.1
Compatibility Area
The first region of memory is called the Compatibility Area because it was defined for early PCs. This region is
divided into 5 subregions, as shown in Figure 3.
Figure 3. Expanded View of Compatibility Area.
DOS Region
The DOS Region is 640 Kbytes in the address range 00000h–9FFFFh. DOS applications execute here. This
region is further divided into two parts. The 512 Kbyte area at 00000h–7FFFFh is always mapped to memory
on the Pentium Pro processor bus (enabled in the MC), while the 128 Kbyte area from 80000h–9FFFFh can be
mapped to memory on the Pentium Pro processor bus or PCI memory (enabled in the PB). This region can be
programmed as disabled, read/write, write only, or read only.
Graphics Adapter Memory
The 128 Kbyte Graphics Adapter Memory region at A0000h–BFFFFh is normally mapped to a video device on
the PCI bus. Typically, this is a VGA controller. If there are no graphics compatible devices, this region can be
used as system memory. The range A0000h–AFFFFh (64 Kbytes) is also the default region for SMM space.
The SMM region can be re-mapped by programming the SMM Range Register in the PB and MC.
ISA Expansion
The 128 Kbyte ISA expansion region is divided into eight 16 Kbyte blocks that can be independently
programmed as disabled, read/write, write only, or read only providing the capability to “shadow” these regions
in main memory. Typically, these blocks are mapped through the PB to ISA space.
System BIOS
Extended
System BIOS
ISA
Video BIOS
Graphics
Adapter
ISA Window /
Memory
DOS Area
64KB
128KB
640KB
0
A_0000
C_0000
E_0000
F_0000
F_FFFF
ISA
Expansion/
DOS
Region
Channel I/O
Memory
8_0000
C_8000
Memory
Compatibility
Area
Extended
Memory
(ISA)
Extended
Memory
(EISA)
1MB
FFFF_FFFF
100_0000
0
F_FFFF
FF_FFFF
10_0000
Extended
Memory
(above 4GB)
F_FFFF_FFFF
1_0000_0000