
PRELIMINARY
11
A
PCIset Overview
.
Figure 5. Expanded View of Extended Memory (EISA).
System BIOS
The top 2 Mbytes of the EISA Extended Memory region is used for System BIOS (High BIOS). This is where
the Pentium Pro processor begins execution after reset. If the PCI bus is bridged to an ISA bus, this region is
aliased to the top 128 Kbyte of the ISA Extended Memory range.
The actual address space required for system BIOS is less than 2 Mbytes. However, the minimum Pentium Pro
processor MTTR range for this region is 2 Mbytes. This establishes the minimum size for this gap. The MC
supports enabling or disabling this region for access to the MC memory via the HBIOSR Register.
I/O APIC Configuration Space
The FEC00000h (4GB minus 20 MB) to FEC0FFFFh range is reserved for APIC configuration space which
includes the default I/O APIC configuration space. Note that there is no I/O APIC unit in either the MC or PB.
The default Local APIC configuration space is FEE00000h–FEE00FFFh.
Pentium Pro processor accesses to the Local APIC configuration space do not result in external bus activity
since the Local APIC configuration space is internal to the Pentium Pro processor. However, an MTRR must be
programmed to make the Local APIC range uncacheable (UC). The Local APIC base address in each Pentium
Pro processor should be relocated to the FEC00000h (4GB minus 20 MB) to FEC0FFFFh range so that one
MTRR can be programmed to 64 Kbyte for the Local and I/O APICs.
System BIOS
I/O APIC
128KB
100_0000
FEC0_0000
FECO_FFFF
FFFE_0000
FFFF_FFFF
System Memory
(Memory/PCI)
Configuration
Space
Compatibility
Area
Extended
Memory
(ISA)
Extended
Memory
(EISA)
FFFF_FFFF
100_0000
0
F_FFFF
FF_FFFF
10_0000
Extended
Memory
(above 4GB)
F_FFFF_FFFF
System Memory
(Memory/PCI)
64KB
PB Only
MC Only
Note:
If the PCI Frame Buffer Range is enabled in the PB (accesses forwarded to PCI), the Low Memory
Gap in the MC can be used to create a gap in main memory for the corresponding memory
address range (MC ignores these accesses).
High Memory
Gap Range
Memory
Gap Range
PCI Frame
Gap Range
Low Memory
Gap Range