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== Operating system support == === Microsoft Windows === 32-bit versions of [[Microsoft Windows]] support PAE if booted with the appropriate option. According to Microsoft Technical Fellow [[Mark Russinovich]], some drivers were found to be unstable when encountering physical addresses above 4GB.<ref name="markr200807">{{cite web |url=http://blogs.technet.com/markrussinovich/archive/2008/07/21/3092070.aspx |title=Pushing the Limits of Windows: Physical Memory |date=2008-07-21 |access-date=2010-07-11 |author=Mark Russinovich |archive-url=https://web.archive.org/web/20080725002837/http://blogs.technet.com/markrussinovich/archive/2008/07/21/3092070.aspx |archive-date=2008-07-25 |url-status=dead }}</ref> The following table shows the memory limits for 32-bit versions of Microsoft Windows: {| class="wikitable sortable" |+ Memory limits on 32-bit editions of Microsoft Windows,<br/>with PAE support<ref>{{cite web|title = Memory Limits for Windows releases|website = [[MSDN]]|publisher = [[Microsoft]]|date = December 5, 2007|url = http://msdn2.microsoft.com/en-us/library/aa366778.aspx|access-date = 2015-11-16|archive-url = https://web.archive.org/web/20071217093949/http://msdn2.microsoft.com/en-us/library/aa366778.aspx|archive-date = December 17, 2007|url-status = dead}}</ref><ref>{{cite web |title = Intel Physical Addressing Extensions (PAE) in Windows 2000 |website=Support |publisher = [[Microsoft]] |date = October 26, 2007 |url = http://support.microsoft.com/kb/268363/ |access-date = 2007-12-29 |archive-url=https://web.archive.org/web/20080101144938/http://support.microsoft.com/kb/268363/ |archive-date=2008-01-01 |url-status=dead}}</ref><ref>{{cite web |title = Overview of Windows Server 2003 R2 Datacenter Edition |website=[[Microsoft TechNet|TechNet]] |publisher = [[Microsoft]] |url = https://technet.microsoft.com/en-au/windowsserver/bb429508.aspx |access-date = 2009-05-15 |archive-url=https://web.archive.org/web/20111128004758/https://technet.microsoft.com/en-au/windowsserver/bb429508.aspx |archive-date=2011-11-28 |url-status=dead}}</ref> ! Windows version ! data-sort-type=number | Memory limit |- | [[Windows 2000]] Professional, Server | style="text-align:right;" | 4 GB |- | [[Windows 2000]] Advanced Server | style="text-align:right;" | 8 GB |- | [[Windows 2000]] Datacenter | style="text-align:right;" | 32 GB |- | [[Windows XP]] Starter | style="text-align:right;" | 512 MB |- | [[Windows XP]] (other editions) | style="text-align:right;" | 4 GB |- | [[Windows Server 2003]] Web SP2 | style="text-align:right;" | 2 GB |- | [[Windows Server 2003]] Standard SP2 | style="text-align:right;" | 4 GB |- | [[Windows Server 2003]] Enterprise/Datacenter SP2 | style="text-align:right;" | 64 GB |- | [[Windows Storage Server|Windows Storage Server 2003]] Enterprise | style="text-align:right;" | 8 GB |- | [[Windows Storage Server|Windows Storage Server 2003]] (other editions) | style="text-align:right;" | 4 GB |- | [[Windows Home Server]] | style="text-align:right;" | 4 GB |- | [[Windows Vista]] Starter | style="text-align:right;" | 1 GB |- | [[Windows Vista]] (other editions) | style="text-align:right;" | 4 GB |- | [[Windows Server 2008]] Standard, Web | style="text-align:right;" | 4 GB |- | [[Windows Server 2008]] Enterprise, Datacenter | style="text-align:right;" | 64 GB |- | [[Windows 7]] Starter | style="text-align:right;" | 2 GB |- | [[Windows 7]] (other editions) | style="text-align:right;" | 4 GB |- | [[Windows 8]] (all editions) | style="text-align:right;" | 4 GB |- |[[Windows 10]] (all editions) | style="text-align:right;" | 4 GB |} The original releases of Windows XP and Windows XP SP1 used PAE mode to allow RAM to extend beyond the 4 GB address limit. However, it led to compatibility problems with 3rd party drivers which led Microsoft to remove this capability in Windows XP Service Pack 2. Windows XP SP2 and later, by default, on processors with the [[NX bit|no-execute (NX)]] or [[NX bit|execute-disable (XD)]] feature, runs in PAE mode in order to allow NX.<ref>{{cite web |url=http://support.microsoft.com/kb/888137 |title=The RAM reported by the System Properties dialog box and the System Information tool is less than you expect in Windows Vista or in Windows XP Service Pack 2 or later version (MSKB 888137) |access-date=2009-01-30 |work=Knowledge Base |publisher=Microsoft |archive-url=https://web.archive.org/web/20090204015716/http://support.microsoft.com/kb/888137 |archive-date=2009-02-04 |url-status=dead}}</ref> The NX bit resides in bit 63 of the page table entry and, without PAE, page table entries on 32-bit systems have only 32 bits; therefore PAE mode is required in order to exploit the NX feature. However, "client" versions of 32-bit Windows (Windows XP SP2 and later, Windows Vista, Windows 7) limit physical address space to the first 4 GB for driver compatibility <ref name="markr200807"/> even though these versions do run in PAE mode if NX support is enabled. [[Windows 8]] and later releases will only run on processors which support PAE, in addition to [[NX bit|NX]] and [[SSE2]].<ref>{{cite web|last=Khurshid |first=Usman |url=http://www.technize.net/how-to-check-if-your-processor-supports-pae-nx-and-sse2-for-windows-8-installation/?ModPagespeed=noscript |title=How To Check If Your Processor Supports PAE, NX And SSE2 For Windows 8 Installation |work=technize.net |publisher=Technize |date=2 November 2012 |access-date=20 April 2014}}</ref><ref>{{cite web|url=https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-8.1-and-8/dn482072(v=win.10)|title=PAE/NX/SSE2 Support Requirement Guide for Windows 8|publisher=Microsoft Docs|date=10 February 2014|access-date=11 July 2023}}</ref> === macOS === [[Mac OS X Tiger]] through [[Mac OS X Snow Leopard]] support PAE and the [[NX bit]] on IA-32 processors; Snow Leopard was the last version to support IA-32 processors. On x86-64 processors, all versions of [[macOS]] use 4-level paging (IA-32e paging rather than PAE) to address memory above 4GB. [[Mac Pro]] and [[Xserve]] systems can use up to 64 GB of RAM.<ref>{{cite web | title = Road to Mac OS X 10.6 Snow Leopard: 64-Bits | date = 2008-09-26 | url = http://www.appleinsider.com/articles/08/08/26/road_to_mac_os_x_10_6_snow_leopard_64_bits.html | access-date = 2008-09-26}}</ref> === Linux === {{see also|Executable space protection#Linux}} The [[Linux kernel]] includes full PAE-mode support starting with version 2.3.23,<ref>{{cite mailing list | title = 2.3.23-pre4 x86 64 GB RAM changes [HIGHMEM patch] explained a bit | url = http://lkml.indiana.edu/hypermail/linux/kernel/9910.2/0542.html |first=Ingo |last=Molnar |date=20 October 1999 |mailing-list=linux-kernel}}</ref> in 1999 enabling access of up to 64 GB of memory on 32-bit machines. A PAE-enabled Linux kernel requires that the CPU also support PAE. The Linux kernel supports PAE as a build option and major distributions provide a PAE kernel either as the default or as an option. The NX bit feature requires a kernel built with PAE support.<ref>{{cite book |url=https://books.google.com/books?id=-6zvRFEfQ24C |title=Professional Linux Kernel Architecture |isbn=978-1-118-07991-1 |at=Figure 3.16 Code flow for paging_init |quote=Execute Disable Protection is also enabled if supported by processor and if the kernel was compiled with PAE support; unfortunately, the feature is otherwise not available. |last1=Mauerer |first1=Wolfgang |date=11 March 2010 }}</ref> [[Linux distributions]] now commonly use a PAE-enabled kernel as the default, a trend that began in 2009.<ref name=fedora11>{{cite web | title = x86 Specifics for Fedora 11 | url = http://docs.fedoraproject.org/en-US/Fedora/11/html/Release_Notes/sect-Release_Notes-Architecture_Specific_Notes.html#sect-Release_Notes-x86_Specifics_for_Fedora |archive-url=https://web.archive.org/web/20100704051357/http://docs.fedoraproject.org/en-US/Fedora/11/html/Release_Notes/sect-Release_Notes-Architecture_Specific_Notes.html |archive-date=2010-07-04 |url-status=dead}}</ref> {{as of|2012}} many, including [[Ubuntu]] (and derivatives like [[Xubuntu]] and [[Linux Mint]]),<ref name=xubuntu-non-pae-end>{{cite web|title=Xubuntu 12.04 released|url=http://xubuntu.org/news/12-04-release/|website=Xubuntu.org|access-date=24 October 2015|date=April 26, 2012|quote= The non-PAE kernel will not be available in future Xubuntu releases.}}</ref><ref>{{cite web | title = PAE | url = https://help.ubuntu.com/community/PAE | website = Ubuntu Community Help Wiki | access-date = 2023-07-11}}</ref> [[Red Hat Enterprise Linux]] 6.0,<ref>{{cite web|url=https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/6.0_Release_Notes/kernel.html|title=RHEL 6 Release Notes, 12.6. General Kernel Updates 12.6.1. Physical Address Extension (PAE)|publisher=RedHat|access-date=27 November 2013}}</ref> and [[CentOS]], have stopped distributing non-PAE kernels, thus making PAE-supporting hardware mandatory. Linux distributions that require PAE may refuse to boot on [[Pentium M]] family processors because they do not show the PAE support flag in their CPUID information (even though it is supported internally).<ref name="Pentium-M-PAE"/> However, this can be easily bypassed with the <code>forcepae</code> option.<ref>{{cite web |url=https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html |title=The kernel's command-line parameters |work=The Linux Kernel documentation}}</ref> Distributions that still provide a non-PAE option, including [[Debian]] (and derivatives like [[Linux Mint#Debian-based edition|LMDE 2 (Linux Mint Debian Edition)]]<ref>{{cite web|title=Known problems in Linux Mint Debian|url=http://linuxmint.com/rel_debian.php|quote=To guarantee compatibility with non-PAE processors, the 32-bit versions of Linux Mint Debian come with a 486 kernel by default.|access-date=2015-10-24|archive-date=2015-10-16|archive-url=https://web.archive.org/web/20151016052124/http://www.linuxmint.com/rel_debian.php|url-status=dead}}</ref>), [[Slackware]], and [[LXLE Linux|LXLE]], typically do so with "i386", "i486", or "retro" labels.<ref>{{cite web|url=http://puppylinux.org/wikka/PuppyPrecise |title=Precise Puppy |work=puppylinux.org |publisher=PuppyLinux |access-date=20 April 2014 |archive-url=https://web.archive.org/web/20140813071203/http://puppylinux.org/wikka/PuppyPrecise |archive-date=2014-08-13 |url-status=dead}}</ref><ref>{{cite web|url=https://www.debian.org/releases/stable/i386/ch02s01.html.en |title=2.1. Supported Hardware |work=Debian GNU/Linux Installation Guide |publisher=SPI |access-date=20 April 2014 |archive-url=https://web.archive.org/web/20140513011042/https://www.debian.org/releases/stable/i386/ch02s01.html.en |archive-date=2014-05-13 |url-status=dead}}</ref> The article [[Light-weight Linux distribution]] does list some others, allowing to install Linux onto old computers. === Others === [[FreeBSD]] and [[NetBSD]] also support PAE as a kernel build option. [[FreeBSD]] supports PAE in the 4.x series starting with 4.9, in the 5.x series starting with 5.1, and in all 6.x and later releases. Support requires the kernel <code>PAE</code> configuration-option. [[Loadable kernel module]]s can only be loaded into a kernel with PAE enabled if the modules were built with PAE enabled; the binary modules in FreeBSD distributions are not built with PAE enabled, and thus cannot be loaded into PAE kernels. Not all drivers support more than 4 GB of physical memory; those drivers won't work correctly on a system with PAE.<ref>{{cite web | title = FreeBSD i386 5.5-RELEASE PAE(4) man page | date = 2003-04-08 | url = https://man.freebsd.org/cgi/man.cgi?query=pae&apropos=0&sektion=4&manpath=FreeBSD+5.5-RELEASE&arch=i386&format=html | access-date = 2023-07-11}}</ref> [[OpenBSD]] has had support for PAE since 2006 with the standard GENERIC i386 kernel. GeNUA mbH supported the initial implementation.<ref>{{cite web | title = PAE for OpenBSD/i386 by Michael Shalayeff, NYC | date = 2006 | url = https://www.openbsd.org/papers/nyc2006/index.html | access-date = 2018-02-03}}</ref> Since release 5.0 PAE has had a series of changes, in particular changes to i386 MMU processing for PMAP, see pmap(9).<ref>{{man|9|pmap|OpenBSD}}</ref>{{failed verification|date=July 2023}} [[Solaris (operating system)|Solaris]] supports PAE beginning with Solaris version 7. However, third-party drivers used with version 7 which do not specifically include PAE support may operate erratically or fail outright on a system with PAE.<ref>{{cite web | title=Added Support for Physical Address Extension (PAE) Mode | work=Solaris 7 5/99 Release Notes (Intel Platform Edition), Appendix B: Hardware Compatibility List and Device Configuration Guide (Intel Platform Edition) 5/99 | year=1999 | url=https://docs.oracle.com/cd/E19620-01/806-0225/app-hcl-21/index.html | access-date=2018-03-23 }}</ref> [[Haiku (operating system)|Haiku]] added initial support for PAE sometime after the R1 Alpha 2 release. With the release of R1 Alpha 3 PAE is now officially supported. [[ArcaOS]] has limited support for PAE for the purpose of creating RAM disks above the 4 GB boundary.<ref>{{cite web | title=ArcaOS 5.0 from Arca Noae is the new release of OS/2 for the 21st Century | year=2017 | url=https://www.arcanoae.com/arcaos/ | access-date=2019-12-16 }}</ref>
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