Editing Physical Address Extension (section)

{{short description|Memory management feature}}
{{more citations needed|date=March 2014}}

In [[computing]], '''Physical Address Extension''' ('''PAE'''), sometimes referred to as '''Page Address Extension''',<ref>{{cite book 
|author=<!--Staff writer(s); no by-line.--> 
|title=Dual-Core Intel® Xeon® Processor 2.80 GHz Specification Update
|url= https://www.intel.com/content/dam/support/us/en/documents/processors/xeon/sb/309159.pdf
|publisher= Intel Corporation
|page= 18
|date=October 2006
}}</ref>
is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the [[Pentium Pro]], and later by AMD in the [[Athlon]] processor.<ref name="Athlon PAE">{{cite book|url=http://pdf.datasheetcatalog.com/datasheet/AdvancedMicroDevices/mXvyvs.pdf |access-date=2017-04-13|publisher=AMD, Inc.|title=AMD Athlon™ Processor x86 Code Optimization Guide|chapter=Appendix E|page=250 |date=February 2002|edition=Revision K|quote=A 2-bit index consisting of PCD and PWT bits of the page table entry is used to select one of four PAT register fields when PAE (page address extensions) is enabled, or when the PDE doesn’t describe a large page.}}</ref> 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&nbsp;[[gigabyte]]s (2<sup>32</sup> bytes).

The page table structure used by [[x86-64]] CPUs when operating in [[long mode]] further extends the page table hierarchy to four or more levels, extending the virtual address space, and uses additional physical address bits at all levels of the page table, extending the physical address space. It also uses the topmost bit of the 64-bit page table entry as a no-execute or [[NX bit|"NX" bit]], indicating that code cannot be executed from the associated page. The NX feature is also available in [[X86-64#Operating modes|protected mode]] when these CPUs are running a 32-bit operating system, provided that the operating system enables PAE.