Physical Address Extension
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In computing, Physical Address Extension (PAE) refers to a feature of x86 processors that allows for up to 64 gigabytes of physical memory to be used in 32-bit systems, given appropriate operating system support. PAE is provided by Intel Pentium Pro and above CPUs (including all later Pentium-series processors except the 400MHz bus versions on the Pentium M), as well as by some compatible processors such as those from AMD. The CPUID flag PAE is assigned for the purpose of identifying CPUs with this capability.
The processor hardware is augmented with additional address lines used to select the additional memory, and 36 bit page tables, but regular application software continues to use instructions with 32 bit addresses and a flat memory model limited to 4 gigabytes. The operating system uses PAE to map this 32 bit address space onto the 64 gigabytes of total memory, and the map can be and usually is different for each process. In this way the extra memory is useful even though regular applications cannot access it all simultaneously.
For application software which needs access to more than 4 gigabytes of memory some special mechanism may be provided by the operating system in addition to the regular PAE support. On Microsoft Windows this mechanism is called Address Windowing Extensions (AWE), while on Unix systems a variety of tricks are used, such as using mmap() to map regions of a file into and out of the address space as needed, none having been blessed as a standard.