450NX Datasheet, PDF (83/248 Pages) Intel Corporation – Intel 450NX PCIset

4.1 Memory Address Map
Top Of
Memory
˜
High
Gap
PCI space
—
Low
Gap
ISA space
–
˜
wasted
—
–
Host Bus Address
Physical Memory
Figure 4-2: Gaps, Spaces and Reclaiming Physical Memory
• The 64 KB System BIOS Region is treated as a single block and is normally mapped to the
compatibility PCI bus. Selecting appropriate read/write attributes for this region allows
the BIOS to be “shadowed” into RAM. After power-on reset, the Intel^® 450NX PCIset has
this area configured to direct accesses to PCI memory, allowing fetches from the boot
ROM during system initialization.
High Compatibility Region
The High Compatibility Region spans 20 MB immediately below the 4 GB address boundary
(address range FEC0_0000h to FFFF_FFFFh). This region supports four fixed spaces with
predefined functions for compatibility with PC-based systems.
• The 2 MB High BIOS Space is where the processor begins execution after reset. Following
power-on, the Intel^® 450NX PCIset has this space enabled; accesses will be directed to the
compatibility PCI bus. If an ISA bridge is also used, this area is then aliased by the ISA
bridge to the top of the ISA address range (14-16 MB). If this space is disabled, accesses
will be directed to memory (unless superceded by an expansion gap.)
• The 1 MB Local APIC Space is reserved for use by the processor. In Pentium^® II Xeon™
processors, this contains the default local APIC space (which can be remapped to the I/O
APIC space, below). Accesses to this region will not be claimed by the Intel 450NX PCIset.
No resources should be mapped to this region.
• The 1 MB Reserved Space is defined for future use. No resources should be mapped to this
region.
• The 1 MB I/O APIC Configuration Space provides an area where I/O APIC units in the
system can be mapped, and the I/O APICs within the processors can be remapped for
consistency of access. At least one I/O APIC must be included in an Intel 450NX PCIset-
based system. The I/O APIC space may be relocated anywhere in the 4 GB boundary.
Intel® 450NX PCIset
4-3