![]() And that address space is private, it is not shared with other processes. This larger address space is an enormous advantage for some applications. Most applications have no idea how much RAM is in the system. But it is important to understand that this address space is in no way limited or even influenced by how much RAM you have. Through the "magic" of the CPU and memory management functions of the OS each 64 bit process sees an address space of 8 TB. Both articles linked to by sygnus21 mention this but never really explain what it means. The reason why 64 bit systems were developed never was about the ability to access more RAM. Generally, the only advantage of 64 bit operating systems is that they will address more RAM and speed itself is not very much a factor, unless doing heavy video encoding or the like, where it takes advantage of a large RAM space. ![]() The conflicting RAM is therefore unavailable to the operating system whether it is remapped or not." Either the BIOS simply disables the conflicting RAM or, the BIOS remaps the conflicting RAM to physical addresses above the 4 GB point, but x86 Windows client editions refuse to use physical addresses higher than that, even though they are running with PAE enabled. In both, RAM near the 4 GB point conflicts with memory-mapped I/O space. ![]() Thus, the "3 GB barrier" under x86 Windows "client" operating systems can therefore arise in two slightly different scenarios. This is not an architectural limit it is a limit imposed by Microsoft as a workaround for device driver compatibility issues that were discovered during testing. Nevertheless, these operating systems do not permit addressing of physical memory above the 4 GB address boundary. ![]() ![]() "In Microsoft's "non-server", or "client", x86 editions of Microsoft Windows: Windows XP, Windows Vista, Windows 7, Windows 8, and Windows 8.1, the 32-bit (x86) versions of these are able to operate x86 processors in PAE mode, and do so by default as long as the CPU present supports the NX bit. It's artificial restriction imposed by Microsoft. ![]()
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