- For 64-bit Windows for Itanium systems, see Windows XP 64-bit Edition.
Windows XP Professional x64 Edition is based upon Windows Server 2003 SP1 (build 5.2.3790.1830) as that was the latest version of Microsoft Windows during the operating system's development, but takes Windows XP as its name. It is designed to use the expanded 64-bit memory address space provided by the AMD64 architecture; Intel refers to its implementation of the technology as EM64T.
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 GB, which is equally divided between Kernel and application usage. Windows XP x64 can support much more memory; although the theoretical memory limit a 64-bit computer can address is about 18 billion GB (18 exabytes), Windows XP x64 is currently limited to 128 GB ( bytes) of physical memory and 16 TB ( bytes) of virtual memory. Microsoft claims this limit will be increased as hardware capabilities improve. In practice, most motherboards compatible with 64-bit processors do not support nearly the maximum, and often retain the 4 GB limit.
The “x64 Edition” is not to be confused with the “64-bit Edition”, as the latter was designed for IA-64 (Intel Itanium) processors.  They both are commonly referred to as '64-bit Windows' by Microsoft due to their similarities from the developers' point of view.
Compatibility with 32-bit applications
Windows XP x64 Edition uses a technology named WOW64, which permits the execution of 32-bit x86 applications. It was first employed in Windows XP 64-bit Edition, but then reused for the “x64 Editions” of Windows XP and Windows Server 2003.
Since the AMD64 architecture includes hardware-level support for 32-bit instructions, WOW64 simply switches the process between 32- and 64-bit modes. However, on the IA-64 architecture, WOW64 was required to translate 32-bit x86 instructions into their 64-bit IA-64 equivalents—which in some cases were implemented in quite different ways—so that the processor could execute them. As a result, AMD64 microprocessors suffer no performance loss when executing 32-bit Windows applications, whereas IA-64 implementations experience noticeable performance loss.
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 link against a 32-bit library (DLL) and similarly a 32-bit application cannot link against a 64-bit library. 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, in order to allow for the possibility that some third-party browser plugins or ActiveX controls may not yet be available in 64-bit versions.
Older 32-bit drivers and services are not supported by 64-bit Windows, but video and audio codecs such as XviD or OggDS, which are in fact 32-bit DLLs, are supported as long as the media player that uses them is 32-bit as well. This includes popular video players such as Media Player Classic.
Compatibility with other applications
Unlike prior versions of the Windows NT line, 64-bit Windows versions do not include NTVDM so there is no support for the execution of MS-DOS, POSIX, OS/2 1.x and 16-bit Windows applications, although there are 3rd-party software emulators such as DOSBox that can be used to run DOS programs.
Another solution is to use virtualization software like VMware or VirtualPC to run other versions of Windows or MS-DOS.
During the initial development phases (2003–2004), Windows XP x64 Edition was named Windows XP 64-bit Edition for x86 Extended systems and later, Windows XP 64-bit Edition for Extended systems, as opposed to '64-bit Edition for Itanium systems'.
Because of their lineage to Windows 2003, Windows XP x64 Edition (along with Windows XP 64-bit Edition Version 2003) are the only versions of Windows XP to ship with IIS 6.0 (all other versions of XP have IIS 5.1).
- Microsoft.com - Windows XP Professional x64 Edition Home Page
- PlanetAMD64.com - Forums, 64bit Software Database, 64bit Driver Database, 64bit News
- 64-bit Support, Forum, 64-bit News and Database
- Extended64.com - 64bit forums, help, news and database of drivers/software
- Windows 64-bit Compatibility list - lists compatible/incompatible software and native x64 software