The CPU story is not over, however. The need for processors capable of handling far more than 4GB of memory has led to development of two competing 64-bit architectures. Intel developed and promoted a 64-bit architecture called IA-64 or Itanium, intended primarily for database and network server computers. Advanced Micro Devices also produced a 64-bit architecture, called x64 or AMD-64. Intel is now manufacturing chips that use the x64 instruction set. x64 processors typically yield better performance than Itanium running 32-bit Windows applications, and thus are better performers in desktop computers. Both new architectures are supported by Microsoft.

Thus, at the time this was written, Windows is available for three architectures:

1) IA-32 (x86) for all versions of Windows
2) IA-64 (Itanium) for Windows Server 2003
3) x64 for Windows XP and Windows Server 2003

Usage of the x64 version of Windows XP should be very similar to the 32-bit versions with the following exceptions:

1) 64-bit versions of Windows will run 32-bit Windows applications, but will not run 16-bit (Windows 3.1) applications, nor can they use 32-bit drivers that is, drivers written for the standard version of Windows XP. This means that hardware support may be limited until vendors produce 64-bit driver versions.
2 MS-DOS applications cannot be run.
3) DirectX graphics support does not take advantage of hardware graphics acceleration, so gaming performance will likely be less than satisfactory.
4) DirectX support may be improved in a future Windows version or in a future service pack.