Windows XP Professional x64 Edition
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 random-access memory (RAM). 32-bit editions of Windows XP are limited to a total of 4 gigabytes. Although the theoretical memory limit of a 64-bit computer is about 16 exabytes (17.1 billion gigabytes), Windows XP x64 is limited  to 128GB of physical memory and 16 terabytes of virtual memory. Windows XP Professional x64 Edition uses the same kernel and code tree as Windows Server 2003 and is serviced by the same service packs. However, it includes client features of Windows XP such as System Restore, Windows Messenger, Fast User Switching, Welcome Screen, Security Center and games, 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 Itanium architecture. During the initial development phases, Windows XP Professional x64 Edition  was named "Windows XP 64-Bit Edition for 64-Bit Extended Systems". Software compatibility. Windows XP Professional x64 Edition uses a technology named Windows-on-Windows 64-bit (WoW64), which permits the execution of 32-bit software. It was first used in Windows XP 64-bit Edition (for Itanium architecture). Later, it was adopted for 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 program cannot use a 32-bit dynamic-link library (DLL) and  similarly a 32-bit program cannot use a 64-bit DLL. This may lead to the need for library developers to provide both 32-bit and 64-bit binary versions of their libraries. Specifically, 32-bit shell extensions for Windows Explorer fail to work with 64-bit Windows Explorer. Windows XP x64 Edition ships with both 32-bit and 64-bit versions of Windows Explorer. The 32-bit version can become the default Windows Shell. Windows XP x64 Edition also includes both 32-bit and 64-bit versions of Internet Explorer 6, so that user can still use browser  extensions or ActiveX controls that are not available in 64-bit versions. Only 64-bit drivers are supported in Windows XP x64 Edition, but 32-bit codecs are supported as long as the media player that uses them is 32-bit. Installation of programs. By default, 64-bit (x86-64) Windows programs are installed onto their own folders under folder location "C:\Program Files", while 32-bit (x86-32) Windows programs are installed onto their own folders under folder location "C:\Program Files (x86)". Known limitations. There are some common changes that applied with Windows XP Professional x64 Edition. Service packs. The  RTM version of Windows XP Professional x64 Edition was built from the Windows Server 2003 Service Pack 1 codebase. 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 March 13, 2007, is the same as Service Pack 3 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 x64 counterpart. Service Pack 2 is the last released service pack for Windows XP Professional x64 Edition. Upgrade. A machine running Windows XP Professional x64 Edition can be upgraded to Windows Vista or Windows 7. Unlike other versions of Windows XP, Windows XP Professional x64 Edition is not upgradable to Windows Vista as the 64 bit DVD recognizes the OS as a 32-bit system. Vista can still be installed, but it requires a clean install. The last version of Microsoft Office to be compatible with Windows  XP Professional x64 Edition is Office 2007, and the last version of Internet Explorer compatible with the operating system is Internet Explorer 8 (Service Pack 2 is required). However it is possible to install Office 2010 setting the compatibility of the installer to Windows XP (32 bit), although the 32 bit is not stated.