Windows XP Professional x64 Edition
|Company / developer||Microsoft|
|OS family||Windows NT|
|Source model||Closed source, shared source|
|Initial release||April 25, 2005|
|Latest stable release||SP2 (5.2.3790.3959) / March 13, 2007|
|Kernel type||Hybrid kernel|
|Default user interface||Graphical User Interface|
|License||Proprietary commercial software|
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Microsoft Windows XP Professional x64 Edition released on April 25, 2005 is an edition of Windows XP for x86-64 personal computers. It is designed to use the expanded 64-bit memory address space provided by the x86-64 architecture.
The primary benefit of moving to 64-bit is the increase in the maximum allocatable system memory (RAM). Windows XP 32-bit is limited to a total of 4 gigabytes. Windows XP Professional x64 Edition can support much more memory; although the theoretical memory limit of a 64-bit computer is about 16 exabytes (16 billion Gigabytes), Windows XP x64 is limited to 128 GB of physical memory and 16 terabytes of virtual memory.
Windows XP Professional x64 Edition is in fact an edition of Windows Server 2003. Both Windows Server 2003 x64 and Windows XP Professional x64 Edition use identical kernels and are built based on the same code bases. Although based on the Windows Server 2003 code base, Windows XP Professional x64 Edition does include client features of 32-bit Windows XP such as System Restore, Windows Messenger, Fast User Switching, Welcome Screen, Security Center, Games, etc. which Windows Server 2003 does not have.
Windows XP Professional x64 Edition is not to be confused with Windows XP 64-bit Edition, as the latter was designed for Intel Itanium processors. During the initial development phases, Windows XP Professional x64 Edition was named Windows XP 64-Bit Edition for 64-Bit Extended Systems.
- Can address 128 GB memory (many consumer motherboards supported 8 GB in 2006, 16 GB and greater is common on high-end motherboards).
- Uses the Windows Server 2003 kernel which is newer than 32-bit Windows XP and has improvements intended to improve scalability. Windows XP Professional x64 Edition also introduces Kernel Patch Protection (also known as PatchGuard) which can help improve security by helping to eliminate rootkits.
- Supports GPT-partitioned disks for data (but not booting) after SP1,  which allows using disks greater than 2 TB to be used as a single GPT partition for storing data.
- Allows faster encoding of audio/video, higher performance video gaming and faster 3D rendering in software optimized for 64-bit hardware.
- Ships with Internet Information Services 6.0 (all other 32-bit versions of Windows XP have IIS 5.1).
- Ships with Windows Media Player 10 (the 32-bit Windows XP Professional has WMP 9 as of SP2 and shipped with WMP 8).
- Benefits from IPsec new features and improvements made in Windows Server 2003.
- Benefits from Shadow Copy new features in Windows Server 2003  and all the storage improvements.
- Remote Desktop server supports Unicode keyboard input, client-side time-zone redirection, GDI+ rendering primitives for improved performance, FIPS encryption, fallback printer driver, auto-reconnect and new Group Policy settings.
- The Files and Settings Transfer Wizard supports migrating settings from 32-bit Windows XP and 64-bit Windows XP PCs.
Compatibility with 32-bit applications
Windows XP Professional x64 Edition uses a technology named Windows-on-Windows 64-bit (WoW64), which permits the execution of 32-bit x86 applications. It was first employed in Windows XP 64-bit Edition (for the Itanium), but then reused for the “x64 Editions” of Windows XP and Windows Server 2003.
Since the x86-64 architecture includes hardware-level support for 32-bit instructions, WoW64 simply switches the process between 32- and 64-bit modes. As a result, x86-64 architecture microprocessors suffer no performance loss when executing 32-bit Windows applications. On the Itanium architecture, WoW64 was required to translate 32-bit x86 instructions into their 64-bit Itanium equivalents—which in some cases were implemented in quite different ways—so that the processor could execute them. All 32-bit processes are shown with *32 in the task manager, while 64-bit processes have no extra text present.
Although 32-bit applications can be run transparently, the mixing of the two types of code within the same process is not allowed. A 64-bit application cannot use a 32-bit Dynamic-Link Library (DLL) and similarly a 32-bit application cannot use a 64-bit DLL. This may lead to the need for library developers to provide both 32- and 64-bit binary versions of their libraries. Windows XP x64 Edition includes both 32- and 64-bit versions of Internet Explorer 6, in order to allow for the possibility that some third-party browser extensions or ActiveX controls may not yet be available in 64-bit versions.
32-bit drivers and services are not supported by 64-bit Windows, but video and audio codecs are supported as long as the media player that uses them is 32-bit as well.
Most applications may run without any problems owing to the nature of WoW64, but may be unsupported and/or untested. Some Microsoft applications, such as Microsoft Office 2010 and the Zune Software, are not available for Windows XP Professional x64 Edition.
16-bit applications cannot run on Windows XP Professional x64 Edition.
There are some common issues that arise with Windows XP Professional x64 Edition.
- 64-bit Windows versions do not include NTVDM or Windows on Windows, so MS-DOS or 16-bit Windows applications will not run. Some 32-bit (typically older) programs have 16-bit installers that will not run on the x64 Edition with the exception of certain 16-bit installers such as ACME Setup versions 2.6, 3.0, 3.01, 3.1 and InstallShield 5.x hardcoded into WoW64.
- Only 64-bit drivers are supported.
- Any 32-bit Windows Explorer shell extensions fail to work with 64-bit Windows Explorer. However, Windows XP x64 Edition also ships with a 32-bit Windows Explorer. It is possible to make it the default Windows Shell. 
- Command prompts will not load in full-screen.
- No native support for Type 1 fonts.
- Does not contain a Web Extender Client component for Web Folders (WebDAV).
- Spell checking is not available in Outlook Express.
- IEEE1394 (FireWire) audio is not supported.
The RTM version of Windows XP Professional x64 Edition is based on Windows Server 2003 Service Pack 1 codebase, as seen in System Properties applet in Control Panel. Because Windows XP Professional x64 Edition comes from a different codebase than 32-bit Windows XP, its service packs are also developed separately. For the same reason, Service Pack 2 for Windows XP x64 Edition, released on the March 13, 2007, is not the same as Service Pack 2 for 32-bit versions of Windows XP. In fact, due to the earlier release date of the 32-bit version, many of the key features introduced by Service Pack 2 for 32-bit (x86) editions of Windows XP were already present in the RTM version of its 64-bit (x86-64) counterpart. Service Pack 2 is the last released service pack for Windows XP Professional x64 Edition.
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- "General FAQs About 64-bit Windows". Msdn.microsoft.com. Retrieved June 7, 2013.
- "Remote Desktop for Windows XP Professional x64 Edition". Technet.microsoft.com. Retrieved June 7, 2013.
- "Files and Settings Transfer Wizard for Windows XP Professional x64 Edition". Technet.microsoft.com. Retrieved June 7, 2013.
- "64-bit versions of Windows do not support 16-bit components, 16-bit processes, or 16-bit applications". Support. Microsoft. September 11, 2011. Retrieved April 19, 2013.
- "Release Notes for Windows XP Contained in the Relnotes.htm File". Support.microsoft.com. January 9, 2006. Retrieved June 7, 2013.
- "Some Windows Explorer extensions and some Control Panel items are not displayed on computers that are running an x64-based version of Windows". Support.microsoft.com. February 4, 2008. Retrieved June 7, 2013.
- "How to run the 32-bit Explorer shell on Windows x64". Extended64.com. Retrieved June 7, 2013.
- "You cannot connect to a Web folder from a Windows Server 2003 or Windows XP x64 computer". Support.microsoft.com. Retrieved June 7, 2013.
- "http://technet.microsoft.com/en-us/library/bb490863.aspx#EGAA". Technet.microsoft.com. Retrieved June 7, 2013.
- "Appendix B: Features Not Supported in Windows XP Professional x64 Edition". Technet.microsoft.com. Retrieved June 7, 2013.
- "Windows Server 2003 & Windows XP x64 Service Pack Technical Overview". Technet.microsoft.com. January 25, 2011. Retrieved June 7, 2013.
- Benefits of Microsoft Windows x64 Editions. Microsoft Corporation. February 8, 2006. Retrieved January 12, 2011.
- Da Costa, Andre (April 25, 2006). "Microsoft Windows XP x64 Edition: Year in Review". ActiveWin.com. Retrieved January 12, 2011.
- "List of updates in Windows Server 2003 Service Pack 2 (Revision 15.2)". Microsoft Support. Microsoft Corporation. February 27, 2009. Retrieved January 12, 2011.