Physical Address Extension - Operating System Support

Operating System Support

The x86 versions of the following releases of Microsoft Windows support PAE: Itanium versions of these operating systems (if they exist) do not use PAE because the Itanium does not need nor implement PAE. x86-64 editions of Windows always implement PAE, because it is a mandatory feature of long mode.

Values for Windows Server 2008 64-bit also apply to Windows Server 2008 R2 (which dropped 32-bit support). Values for Windows Server 2003 64-bit depend on:

1. service pack level
2. the release being R2 or not

The highest possible values appear here.

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 no-execute (NX) or execute-disable (XD) feature, runs in PAE mode in order to allow NX. The no execute (NX, or XD for execution disable) 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 and licensing reasons, even though these versions do run in PAE mode if NX support is enabled.

The Linux kernel includes full PAE mode support starting with version 2.3.23, 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. As of 2009, some common Linux distributions have started to use a PAE-enabled kernel as the distribution-specific default because it adds the NX bit.

Versions 10.4.4 through 10.5.8 of Mac OS X will run on both x86 and PowerPC processors. Version 10.6 and 10.7 version of OS X only run on an x86 processors. So far, all x86 Macs have used Intel (not AMD) CPUs. OS X versions that are compatible with x86 fully support PAE and the NX bit on all Intel Macs. Mac Pro and Xserve systems can use up to 64 GB of RAM. The Mac OS X 10.5 (Leopard) kernel remains 32-bit. Mac OS X 10.6 (Snow Leopard) can be booted into a 64-bit version of the kernel on certain systems; Mac OS X 10.7 (Lion) and Mac OS X 10.8 (Mountain Lion) ship with a 64-bit enabled kernel by default.

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 PAE configuration-option. Loadable kernel modules 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.

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.

Initial support for PAE was added to the Haiku operating system sometime after the R1 Alpha 2 release. With the release of R1 Alpha 3 PAE is now officially supported.