Below is an excerpt from a Windows XP product feature page:
Scalable memory and processor support up to 4 gigabytes (GB) of RAM and up to two symmetric multiprocessors.
Windows XP core only uses 32-bit addressing to allocate memory and as the result it' up to 4GB maximum. Re-map technology logically truncates a memory hole above 4GB and Intel processor's PAE (an official term) feature is capable of 36-bit addressing.
The technology is truncating memory area, which is overlapped by PCI MMIO to 4GB behind. In such case, 32bit addressing accesses up-to 4GB and the truncated memory area can not be used until /PAE is enabled to make it 36-bit addressing.
Overlapped PCI MMIO can apply to any type of add-on cards as long as they consumes huge PCI MMIO
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