Systems that support hot-plug, hot-swap, and hot-add PCI capabilities, with 4GB of memory installed may not report or have access to all physical memory. An indication of this is the operating system reporting less available memory than the amount of memory physically present in the system.
Hot-plug, hot-swap and hot-add PCI capabilities require that a certain amount of memory in the upper range of the 4GB address space be reserved. PCI devices installed in a system require all of their resources to be defined and mapped in the 32-bit address (4GB) space. The Flash Memory and APIC controllers are located at the top of memory at address 0xFEC00000 and above, other PCI devices are defined below this address range. Physical memory, which exists in this high address space, may be unusable by the operating system.
Windows operating systems that support hot-plug, hot-swap, and hot-add PCI capabilities have this limitation by design, with the exception of Microsoft® Windows® 2000 which uses a 36-bit address space that allows you to use the /PAE (physical address extension) switch in the boot.ini file to reclaim this memory.
For more information refer to Microsoft Knowledge Base Article ID: Q283037 Title "Large Memory Support Is Available in Windows 2000" at:
http://support.microsoft.com/directory/
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Computers with 4GB of memory installed may not report or have access to all physical memory.
