450NX Datasheet, PDF (81/248 Pages) Intel Corporation – Intel 450NX PCIset

System Address Maps ₄
4.1
Memory Address Map
A Pentium^® II Xeon™ processor system based on the Intel^® 450NX PCIset supports up to
64 GBytes of addressable memory space. Within this memory address range the Intel 450NX
PCIset has two structured compatibility regions, two expansion gaps, and two general
purpose memory-mapped I/O spaces, as illustrated in Figure 4-1. The two compatibility
regions are the 1 MB Low Compatibility Region at the bottom of the address space, and the
20 MB High Compatibility Region just below the 4 GB boundary. The two expansion gaps
allow holes to be opened in the address space, where accesses can be directed to the PCI buses
or to a third-party agent, instead of to memory. The two I/O spaces allow control over which
addresses are forwarded to each of the four PCI buses supported by the Intel 450NX PCIset.
Spaces and Gaps
The Intel 450NX PCIset memory address map is based on spaces and gaps.
A space is an address range where the access is directed to a specific destination, usually (but
not always) a PCI bus. Any DRAM behind the space is not reclaimed, unless it is also covered
by a gap (described below). The Intel 450NX PCIset supports a variety of spaces with fixed or
configurable address ranges and individual enables.
A gap is a memory-mapped address range where the access is specifically not directed to
DRAM. The DRAM behind the gap is reclaimed; that is, the effective address presented to the
memory has the gaps subtracted from it, presenting a contiguous address space to the
memory. The gap does not control where the access is directed. Accesses may be directed
through an overlapping space, or left unclaimed on the system bus for a third-party agent to
claim. In typical maps, large spaces will be contained within gaps, to reclaim the DRAM that
would otherwise be wasted. The Intel 450NX PCIset supports two configurable gaps.
Low Compatibility Region
The Low Compatibility Region spans the first 1 MB address range (0h to F_FFFFh). This
region is divided into five subregions, some of which are further subdivided.
• The 640 KB DOS Region is split into a 512 KB DOS area (memory only) and a 128 KB ISA
Window, which can be mapped to either main memory or the PCI memory.
• The 128 KB Graphics Adapter Memory is normally mapped to a video device on the PCI
bus, typically a VGA controller. This region is also the default location of the
configuration SMM RAM space.
Intel® 450NX PCIset
4-1