Physical Address Extension
Encyclopedia
In computing
, Physical Address Extension (PAE) is a feature to allow (32-bit) x86 processors to access a physical address space (including random access memory and memory mapped devices) larger than 4 gigabyte
s.
First implemented in the Intel Pentium Pro in 1995, it was extended by AMD
to add a level to the page table hierarchy, to allow it to handle up to 52-bit physical addresses, add NX bit
functionality, and make it the mandatory memory paging model in long mode
. PAE is provided by Intel Pentium Pro
and above CPUs
, including all later Pentium-series processors (except the 400 MHz-bus versions of the Pentium M
). It is also available on other processors with similar or more advanced versions of the same architecture, such as the AMD
Athlon
and later AMD processor models.
x86 processor hardware-architecture is augmented with additional address lines used to select the additional memory, so physical address size increases from 32 bits to 36 bits. This, theoretically, increases maximum physical memory size from 4 GB to 64 GB. The 32-bit size of the virtual address is not changed, so regular application software continues to use instructions with 32-bit addresses and (in a flat memory model
) is limited to 4 gigabytes of virtual address space. The operating system uses page table
s to map this 4-GB address space into the 64 GB of physical memory. The mapping is typically applied differently for each process
. In this way, the extra memory is useful even though no single regular application can access it all simultaneously.
To use PAE, operating system
support is required. Intel versions of Mac OS X
support PAE. The Linux kernel
supports PAE as a build option and most major distributions provide a PAE kernel either as the default or as an option (Red Hat Enterprise Linux
6+ kernels expect PAE). FreeBSD
and NetBSD
also support PAE as a kernel build option.
Microsoft Windows
implements PAE if booted with the appropriate option, but current 32-bit desktop editions enforce the physical address space within 4GB even in PAE mode. According to Geoff Chappell, Microsoft limits 32-bit versions of Windows to 4GB as a matter of its licensing policy, and Microsoft Technical Fellow Mark Russinovich says that some drivers were found to be unstable when encountering physical addresses above 4GB. Unofficial kernel patches for Windows Vista
and Windows 7 32-bit are available that break this enforced limitation, though the stability is not guaranteed.
For application software which needs access to more than 4 GB of RAM, operating systems may provide some special mechanisms in addition to the regular PAE support. On Windows
this mechanism is called Address Windowing Extensions
, while on Unix-like
systems a variety of techniques are used, such as using mmap
to map regions of a file into and out of the address space as needed.
, x86 processors use a two-level page translation scheme, where the control register
s, similarly consisting of 1024 × 4 byte entries pointing to 4 KB long pages
.
Enabling PAE (by setting bit 5, PAE, of the system register
The entries in the page directory have an additional flag in bit 7, named
). The NX bit
is another flag in the page directory, in bit 63, to mark pages as no execute. Because the 12 least significant bits of the page table entry's 64 bits are either similar flags or are available for OS-specific data, a maximum of 52 bits are available to address 252 bytes, or 4 petabytes, of physical memory.
Software can identify via the CPUID
flag PAE whether a CPU supports PAE mode or not.
On x86-64
processors in native long mode
, the address translation scheme uses PAE but adds a fourth table, the 512-entry page-map level 4 table, and extends the page directory pointer table to 512 entries instead of the original 4 entries it has in protected mode. Currently 48 bits of virtual page number are translated, giving a virtual address space of up to 256 TB. In the page table entries, in the original specification, 40 bits of physical page number are implemented.
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 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.
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. , some common Linux distributions have started to use a PAE-enabled kernel as the distribution-specific default because it adds the NX bit
.
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) ships with a 64-bit enabled kernel by default.
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.
Annotations: 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:
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 4GB 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.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, Physical Address Extension (PAE) is a feature to allow (32-bit) x86 processors to access a physical address space (including random access memory and memory mapped devices) larger than 4 gigabyte
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
s.
First implemented in the Intel Pentium Pro in 1995, it was extended by AMD
Advanced Micro Devices
Advanced Micro Devices, Inc. or AMD is an American multinational semiconductor company based in Sunnyvale, California, that develops computer processors and related technologies for commercial and consumer markets...
to add a level to the page table hierarchy, to allow it to handle up to 52-bit physical addresses, add NX bit
NX bit
The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data, a feature normally only found in Harvard architecture processors...
functionality, and make it the mandatory memory paging model in long mode
Long mode
In the x86-64 computer architecture, long mode is the mode where a 64-bit application can access 64-bit instructions and registers...
. PAE is provided by Intel Pentium Pro
Pentium Pro
The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel introduced in November 1, 1995 . It introduced the P6 microarchitecture and was originally intended to replace the original Pentium in a full range of applications...
and above CPUs
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
, including all later Pentium-series processors (except the 400 MHz-bus versions of the Pentium M
Pentium M
The Pentium M brand refers to a family of mobile single-core x86 microprocessors introduced in March 2003 , and forming a part of the Intel Carmel notebook platform under the then new Centrino brand...
). It is also available on other processors with similar or more advanced versions of the same architecture, such as the AMD
Advanced Micro Devices
Advanced Micro Devices, Inc. or AMD is an American multinational semiconductor company based in Sunnyvale, California, that develops computer processors and related technologies for commercial and consumer markets...
Athlon
Athlon
Athlon is the brand name applied to a series of x86-compatible microprocessors designed and manufactured by Advanced Micro Devices . The original Athlon was the first seventh-generation x86 processor and, in a first, retained the initial performance lead it had over Intel's competing processors...
and later AMD processor models.
x86 processor hardware-architecture is augmented with additional address lines used to select the additional memory, so physical address size increases from 32 bits to 36 bits. This, theoretically, increases maximum physical memory size from 4 GB to 64 GB. The 32-bit size of the virtual address is not changed, so regular application software continues to use instructions with 32-bit addresses and (in a flat memory model
Flat memory model
Flat memory model or linear memory model refers to a memory addressing paradigm in low-level software design such that the CPU can directly address all of the available memory locations without having to resort to any sort of memory segmentation or paging schemes.Memory management and...
) is limited to 4 gigabytes of virtual address space. The operating system uses page table
Page table
A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are those unique to the accessing process...
s to map this 4-GB address space into the 64 GB of physical memory. The mapping is typically applied differently for each process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
. In this way, the extra memory is useful even though no single regular application can access it all simultaneously.
To use PAE, operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
support is required. Intel versions of Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
support PAE. The Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
supports PAE as a build option and most major distributions provide a PAE kernel either as the default or as an option (Red Hat Enterprise Linux
Red Hat Enterprise Linux
Red Hat Enterprise Linux is a Linux-based operating system developed by Red Hat and targeted toward the commercial market. Red Hat Enterprise Linux is released in server versions for x86, x86-64, Itanium, PowerPC and IBM System z, and desktop versions for x86 and x86-64...
6+ kernels expect PAE). FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
and NetBSD
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
also support PAE as a kernel build option.
Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
implements PAE if booted with the appropriate option, but current 32-bit desktop editions enforce the physical address space within 4GB even in PAE mode. According to Geoff Chappell, Microsoft limits 32-bit versions of Windows to 4GB as a matter of its licensing policy, and Microsoft Technical Fellow Mark Russinovich says that some drivers were found to be unstable when encountering physical addresses above 4GB. Unofficial kernel patches for Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
and Windows 7 32-bit are available that break this enforced limitation, though the stability is not guaranteed.
For application software which needs access to more than 4 GB of RAM, operating systems may provide some special mechanisms in addition to the regular PAE support. On Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
this mechanism is called Address Windowing Extensions
Address Windowing Extensions
Address Windowing Extensions is a Microsoft Windows application programming interface that allows a 32-bit software application to access more physical memory than it has virtual address space. The process of mapping an application's virtual address space to physical memory under AWE is known as...
, while on Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems a variety of techniques are used, such as using mmap
Mmap
In computing, mmap is a POSIX-compliant Unix system call that maps files or devices into memory. It is a method of memory-mapped file I/O. It naturally implements demand paging, because initially file contents are not entirely read from disk and do not use physical RAM at all...
to map regions of a file into and out of the address space as needed.
Page table structures
In traditional 32-bit protected modeProtected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
, x86 processors use a two-level page translation scheme, where the control register
Control register
A control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and coprocessor control.-CR0:The CR0 register is 32 bits...
CR3 points to a single 4 kB long page directory divided into 1024 × 4 byte entries that point to 4 kB long page tablePage table
A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are those unique to the accessing process...
s, similarly consisting of 1024 × 4 byte entries pointing to 4 KB long pages
Page (computing)
A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory that is the smallest unit of data for the following:* memory allocation performed by the operating system for a program; and...
.
Enabling PAE (by setting bit 5, PAE, of the system register
CR4) causes major changes to this scheme. By default, the size of each page remains as 4 kB. Each entry in the page table and page directory grows to 64 bits (8 bytes) rather than 32 bits – to allow for additional address bits. However, the size of tables does not change, so both table and directory now have only 512 entries. Because this allows only one quarter of the entries of the original scheme, an extra level of hierarchy has been added, so CR3 now points to the Page Directory Pointer Table, a short table which contains pointers to 4 page directories.The entries in the page directory have an additional flag in bit 7, named
PS (for page size). If the system has set this bit to 1, the page directory entry does not point to a page table, but to a single large 2 MB page (Page Size ExtensionPage Size Extension
In computing, Page Size Extension refers to a feature of x86 processors that allows for pages larger than the traditional 4 KiB size. It was introduced in the original Pentium processor, but it was only publicly documented by Intel with the release of the Pentium Pro. The CPUID instruction can be...
). The NX bit
NX bit
The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data, a feature normally only found in Harvard architecture processors...
is another flag in the page directory, in bit 63, to mark pages as no execute. Because the 12 least significant bits of the page table entry's 64 bits are either similar flags or are available for OS-specific data, a maximum of 52 bits are available to address 252 bytes, or 4 petabytes, of physical memory.
Software can identify via the CPUID
CPUID
The CPUID opcode is a processor supplementary instruction for the x86 architecture. It was introduced by Intel in 1993 when it introduced the Pentium and SL-Enhanced 486 processors....
flag PAE whether a CPU supports PAE mode or not.
On x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
processors in native long mode
Long mode
In the x86-64 computer architecture, long mode is the mode where a 64-bit application can access 64-bit instructions and registers...
, the address translation scheme uses PAE but adds a fourth table, the 512-entry page-map level 4 table, and extends the page directory pointer table to 512 entries instead of the original 4 entries it has in protected mode. Currently 48 bits of virtual page number are translated, giving a virtual address space of up to 256 TB. In the page table entries, in the original specification, 40 bits of physical page number are implemented.
FreeBSD
FreeBSDFreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
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 module
Loadable Kernel Module
In computing, a loadable kernel module is an object file that contains code to extend the running kernel, or so-called base kernel, of an operating system...
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.
Haiku
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.Linux
The Linux kernelLinux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
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. , some common Linux distributions have started to use a PAE-enabled kernel as the distribution-specific default because it adds the NX bit
NX bit
The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data, a feature normally only found in Harvard architecture processors...
.
Mac OS X
Versions 10.4.4 through 10.5.8 of Mac OS XMac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
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
NX bit
The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data, a feature normally only found in Harvard architecture processors...
on all Intel Macs. Mac Pro
Mac Pro
The Mac Pro is a workstation computer manufactured by Apple Inc. The machines are based on Xeon microprocessors, but are similar to the Power Mac G5 they replaced in terms of outward appearance and expansion capabilities...
and Xserve
Xserve
Xserve was a line of rack unit computers designed by Apple Inc. for use as servers. When the Xserve was introduced in 2002, it was Apple's first designated server hardware design since the Apple Network Server in 1996...
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) ships with a 64-bit enabled kernel by default.
Microsoft Windows
The x86 versions of the following releases of Microsoft Windows support PAE:Itanium
Itanium
Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems...
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.
| Windows Version | 32-bit Editions | 64-bit Editions |
|---|---|---|
| Windows 2000 Windows 2000 Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the... Professional, Server |
4 GB | |
| Windows 2000 Windows 2000 Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the... Advanced Server |
8 GB | |
| Windows 2000 Windows 2000 Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the... Datacenter |
32 GB | |
| Windows XP Windows XP Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base... Starter |
512 MB | |
| Windows XP Windows XP Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base... Home & Media Center |
4 GB | |
| Windows XP Windows XP Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base... Professional |
4 GB | 128 GB |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... Web |
2 GB | |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... Small Business, Home, Storage |
4 GB | |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... Storage Server |
4 GB | |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... R2 Standard Edition |
4 GB | 32 GB |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... Standard Edition (SP1) |
4 GB | 32 GB |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... Standard Edition (SP2) |
4 GB | 32 GB |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... Enterprise Edition (SP1) |
16 GB with 4GT | |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... R2 Enterprise Edition |
64 GB | 1 TB |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... Datacenter (SP1) |
128 gb | |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... R2 Datacenter (SP1) |
128 GB | 1 TB |
| Windows Server 2003 Windows Server 2003 Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005... R2 Datacenter (SP2) |
128 GB | 2 TB |
| Windows Vista Windows Vista Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs... Starter |
1 GB | |
| Windows Vista Windows Vista Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs... Home Basic |
4 GB | 8 GB |
| Windows Vista Windows Vista Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs... Home Premium |
4 GB | 16 GB |
| Windows Vista Windows Vista Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs... Business, Enterprise, Ultimate |
4 GB | 128 GB |
| Windows Server 2008 Standard, Web | 4 GB | 32 GB |
| Windows Server 2008 Enterprise, Datacenter | 64 GB | 2 TB |
| Windows 7 Starter | 2 GB | |
| Windows 7 Home Basic | 4 GB | 8 GB |
| Windows 7 Home Premium | 4 GB | 16 GB |
| Windows 7 Professional, Enterprise, Ultimate | 4 GB | 192 GB |
| Windows Server 2008 R2 Windows Server 2008 R2 Windows Server 2008 R2 is a server operating system produced by Microsoft. It was released to manufacturing on July 22, 2009 and launched on October 22, 2009. According to the Windows Server Team blog, the retail availability was September 14, 2009. It is built on Windows NT 6.1, the same core... Foundation |
8 GB | |
| Windows Server 2008 R2 Windows Server 2008 R2 Windows Server 2008 R2 is a server operating system produced by Microsoft. It was released to manufacturing on July 22, 2009 and launched on October 22, 2009. According to the Windows Server Team blog, the retail availability was September 14, 2009. It is built on Windows NT 6.1, the same core... Standard |
32 GB | |
| Windows Server 2008 R2 Windows Server 2008 R2 Windows Server 2008 R2 is a server operating system produced by Microsoft. It was released to manufacturing on July 22, 2009 and launched on October 22, 2009. According to the Windows Server Team blog, the retail availability was September 14, 2009. It is built on Windows NT 6.1, the same core... Enterprise, Datacenter, or Itanium |
2 TB |
Annotations: 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:
- service pack level
- 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 4GB 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)
NX bit
The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data, a feature normally only found in Harvard architecture processors...
or execute-disable (XD)
NX bit
The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data, a feature normally only found in Harvard architecture processors...
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.
Solaris
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.See also
- PSE-36PSE-36In computing, PSE-36 refers to a feature of x86 processors that extends the physical memory addressing capabilities from 32 bits to 36 bits. This mechanism is a simpler alternative to the Physical Address Extension method. It uses the Page Size Extension mode and a modified page directory table...
: an alternative to Physical Address Extension - Page Size ExtensionPage Size ExtensionIn computing, Page Size Extension refers to a feature of x86 processors that allows for pages larger than the traditional 4 KiB size. It was introduced in the original Pentium processor, but it was only publicly documented by Intel with the release of the Pentium Pro. The CPUID instruction can be...
- Architecture of Windows NT
- 3 GB barrier3 GB barrierIn computing, the 3 GB barrier is a limitation of some 32-bit operating systems running on x86 microprocessors. It prevents the operating systems from using more than about 3 GB of main memory...
: why Windows often sees less than 4 GB RAM, even with PAE enabled