Editing Protected mode (section)

=== Paging ===
{{details|Paging}}
[[File:Virtual address space and physical address space relationship.svg|thumb|Common method of using paging to create a virtual address space]]
[[File:080810-protected-386-paging.svg|thumb|Paging (on Intel 80386) with page size of 4K]]

In addition to adding virtual 8086 mode, the 386 also added paging to protected mode.<ref name="paging introduced">{{ cite web | url = http://www.deinmeister.de/x86modes.htm#c1 | title = ProtectedMode overview [deinmeister.de] | access-date = 2007-07-29 | format = Website }}</ref> Through paging, system software can restrict and control a task's access to pages, which are sections of memory. In many operating systems, paging is used to create an independent virtual address space for each task, preventing one task from manipulating the memory of another. Paging also allows for pages to be moved out of [[primary storage]] and onto a slower and larger [[secondary storage]], such as a [[hard disk drive]].<ref name="Paging usage">{{cite web | url = http://technet2.microsoft.com/windowsserver/en/library/efc41320-713f-4004-bc81-ddddfc8552651033.mspx?mfr=true | title = What Is PAE X86? | access-date = 2007-07-29 | date = May 28, 2003 | format = Article | publisher = Microsoft TechNet | quote = The paging process allows the operating system to overcome the real physical memory limits. However, it also has a direct impact on performance because of the time necessary to write or retrieve data from disk. | archive-date = 2008-04-22 | archive-url = https://web.archive.org/web/20080422184656/http://technet2.microsoft.com/windowsserver/en/library/efc41320-713f-4004-bc81-ddddfc8552651033.mspx?mfr=true | url-status = dead }}</ref> This allows for more memory to be used than physically available in primary storage.<ref name="Paging usage" />

The x86 architecture allows control of pages through two [[array data structure|array]]s: page directories and [[page table]]s. Originally, a page directory was the size of one page, four kilobytes, and contained 1,024 page directory entries (PDE), although subsequent enhancements to the x86 architecture have added the ability to use larger page sizes. Each PDE contained a [[pointer (computer programming)|pointer]] to a page table. A page table was also originally four kilobytes in size and contained 1,024 page table entries (PTE). Each PTE contained a pointer to the actual page's physical address and are only used when the four-kilobyte pages are used. At any given time, only one page directory may be in active use.<ref name="Only one page directory">{{cite web | url = http://www.embedded.com/98/9806fe2.htm | title = Advanced Embedded x86 Programming: Paging | access-date = 2007-07-29 | last = Gareau | first = Jean | format = Guide | publisher = Embedded.com | quote = Only one page directory may be active at a time, indicated by the CR3 register. | archive-date = 2008-05-16 | archive-url = https://web.archive.org/web/20080516202434/http://www.embedded.com/98/9806fe2.htm | url-status = dead }}</ref>