In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension,[1] is a memory management feature for the x86 architecture. Intel introduced PAE in the Pentium Pro and later by AMD in the Athlon processor.[2] It defines a page table hierarchy of three levels (instead of two), with table entries of 64 bits each instead of 32, allowing these CPUs to directly access a physical address space larger than 4 gigabytes (232 bytes).
When operating in extended mode, the page table structure used by x86-64 CPUs further extends the page table hierarchy to four levels, boosts the virtual address space, and uses additional physical address bits at all page table levels, opening the physical address space. It also uses the topmost bit of the 64-bit page table entry as a no-execute or “NX” bit, indicating that code cannot be executed from the associated page. The NX feature is also available in protected mode when these CPUs run a 32-bit operating system, provided that the operating system enables PAE.
