Physical Address Extension
A workaround first developed in the Pentium Pro, known as Physical Address Extension, allows certain 32-bit operating systems to access up to 36-bit memory addresses, even though individual programs are still limited to operating within 32 bits of address space. Each program can have its own 4 gigabyte addressing space, together utilizing up to 64 gigabytes of memory across all programs. This was fully supported in Windows XP up to the Service Pack 1 (SP1) release, but then withdrawn for SP2; the only 32-bit versions of Microsoft Windows to fully support this are certain high-end server versions of Windows Server 2003 and earlier; it remained in use by some 32-bit Linux distributions. Microsoft disabled the support in Windows XP SP2 and later operating systems because there were many compatibility problems with graphics card and other devices, which needed PAE-aware drivers, distinct from both standard 32-bit and later 64-bit drivers. Many versions of MS Windows can activate what is still called Physical Address Extensions for the purpose of using the NX bit, but this no longer extends the address space.
Read more about this topic: PCI Hole
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