The Pentium architecture supports 4GB of memory space. Chipsets limit physical RAM.
Intel supports 4GB in the 450 server chipsets, and starting with the 865 desktop chipset.
If you have 4GB RAM, a portion has to be disabled for PCI and system purposes.
If you are interested in how this is done, you must download
"Intel(R) Chipset 4 GB System Memory Support White Paper"
from
http://www.polywell.com/us/support/faq/4GB_Rev1.pdf .
You can fire up Device Manager and view resources by type - Memory.
The entry for PCI Bus shows the RAM-PCI boundry (TOUD/TOLD in chipset).
This is mostly set up by the BIOS, and retrieved with INT15 E820h and ACPI.
I gather most systems will reserve 256MB, 512MB with PCIe, or 1GB with large video.
All versions of 32-bit Windows should make this RAM available,
but XP SP2 seems to be limited to 3.12GB or less.
See
http://support.microsoft.com/kb/888137, The amount of RAM reported by the System Properties
dialog box and the System Information tool is less than you expect after you install Windows XP
Service Pack 2. I don't really understand this restriction.
If you are interested in PAE, see
http://www.microsoft.com/whdc/system...e/default.mspx
Hope this clears up an issue that is not really documented anywhere.
