In computing, the term 3 GB barrier refers to a limitation of some 32-bit operating systems running on x86 microprocessors.
Whatever the actual position of the "barrier", there is no code in operating system software nor any hardware architectural limit that directly imposes it.
[8][9][10] There are other factors that may limit this ability to use up to 64 GiB of memory, and lead to the "3 GB barrier" under certain circumstances, even on processors that implement PAE.
[11] This, however, is not sufficient to explain the "3 GB barrier" that appears even when running some x86 versions of Microsoft Windows on platforms that can access more than 4 GiB of RAM.
Modern personal computers are built around a set of standards that depend on, among other things, the characteristics of the original PCI bus.
On machines with large amounts of video memory, MMIO locations have been found to occupy as much as 1.8 GB of the 32-bit address space.
The "non-server", or "client", x86 SKUs of Windows XP and later operate x86 processors in PAE mode by default when the CPU present implements the NX bit.
Either the BIOS simply disables the conflicting RAM; or, the BIOS remaps the conflicting RAM to physical addresses above the 4 GiB point,[citation needed] but x86 Windows client editions refuse to use physical addresses higher than that, even though they are running with PAE enabled.