64 Bit Windows is Windows modified to use 64 bit addressing on 64 bit CPUs. As of the Spring of 2004, Microsoft has several 64 bit versions of Windows available or under discussion. One is Windows Advanced Server, Limited Edition released in 2001. It runs on the Intel Itanium only. Another is a Windows XP-64. This runs on the AMD Athlon-64 or Opteron only. It is in Beta release and is available for testing. Another 64 bit windows under discussion is a major Windows release codenamed "Longhorn" that is to be released in the 2005-2006 timeframe. It will apparently be available for a 64 bit Desktop CPU from Intel that has not yet been released.

64 Bit Windows continues to support 32 bit Windows programs, running them via an emulation called Windows on Windows (WOW64). The following comparison of maximum capacities applies to Windows XP-64, and may or may not be indicative of limits of other 64 bit Windows releases.

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