"Tim Anderson" wrote in message news
"Rick" wrote in message
t...

Because *applications* have to be written to be PAE-aware,
and have to be written in 64-bit address space to make use of
anything over 4GB. This was (and still is) very much a niche
market -- very few desktop/workstation users will come close
to using 2GB, let alone 4, which is why MS originally split the
4GB address range between 2GB user space and 2GB
system space.

Rick,

Thanks for the response. I understand about that; what I'm talking about is
the hardware issue. When you boot with 4GB installed on one of these
motherboards, typically 1GB is not made available to the OS at all; it is
"consumed" before the OS boots (because of address shadowing). Even with an
AMD64 (EM64T) CPU installed.

The Intel D915PBL (the board referenced in your link) is a desktop
board:
ftp://download.intel.com/design/moth...l/C6859901.pdf

The board utilizes 4 GB of addressable system memory. Typically
the address space that is allocated for PCI Conventional bus add-in
cards, PCI Express configuration space, BIOS (firmware hub), and
chipset overhead resides above the top of DRAM (total system
memory). On a system that has 4 GB of system memory installed, it
is not possible to use all of the installed memory due to system
address space being allocated for other system critical functions.

These functions include the following:

BIOS/firmware hub (2 MB)
Local APIC (19 MB)
Digital Media Interface (40 MB)
Front side bus interrupts (17 MB)
PCI Express configuration space (256 MB)
MCH base address registers, internal graphics ranges, PCI Express ports (up to 512 MB)
Memory-mapped I/O that is dynamically allocated for PCI Conventional and
PCI Express add-in cards

The amount of installed memory that can be used will vary based
on add-in cards and BIOS settings. Figure 14 shows a schematic
of the system memory map. All installed system memory can be
used when there is no overlap of system addresses.