- x86 hardware reserves the physical address space between 3GB to 4GB for PCI memory mapped I/O (i.e. talking to devices the same way we access memory), so this part of the RAM is hidden by design of hardware architecture on the motherboard.
- Some motherboard hardware supports 36-bit addressing on the bus, and the RAM between the 3GB to 4GB range is remapped to a physical address higher than what 32-bit physical address can access.
- Intel CPU after Pentium Pro comes with Physical Address Extension that allows a 32-bit virtual address to be mapped to a 36-bit physical address.
This boils down to:
- If the motherboard supports 36-bit address and remaps the shadowed RAM between 3GB--4GB range, then an operating system (such as Linux, Mac OS X, Solaris) could use that RAM using Physical Address Extension, as well as any RAM beyond 4GB.
However, the licensing restriction on Windows works like this:
- Prior to Windows XP SP2, there is no licensing restriction, so on a capable hardware, 32-bit Windows XP could use as much physical memory as supported by the hardware.
- Due to licensing restriction, the kernel in and after Windows XP SP2 ignores RAM located at physical address space beyond 32-bit, effectively capping usable RAM to 3GB even if the hardware has 36-bit physical address and remaps the 3GB--4GB RAM.
- This licensing restriction is lifted on 32-bit versions of Windows Server and Windows Datacenter, as well as 64-bit Windows. These versions of Windows can access all available RAM supported by the hardware.
- Windows Vista SP2 now reports the amount of RAM installed on the system, not the amount of RAM used by the operating system.