Windows on Windows, commonly referred to as WOW or WoW, is an abstraction subsystem of Microsoft Windows operating system. Wow is a critical component for backwards compatibility of legacy code as Windows progresses. The first Wow appeared in Windows NT to support applications using the Win16 API and was relying on NTVDM.
Supporting Win32 applications on Windows XP Professional x64 Edition, Windows Server 2003 x64 and other x64 versions of Windows requires a new Wow called WoW64. The x64 versions of Windows no longer include the 16 bit WoW subsystem (as well as the NTVDM) and can therefore no longer run 16-bit Windows nor DOS applications, because the AMD64 architecture does not allow 16-bit code to execute while the processor is in "long" (64-bit) mode.