Windows XP Professional X64 Edition - Compatibility With 32-bit Applications

Compatibility With 32-bit Applications

Windows XP Professional x64 Edition uses a technology named Windows-on-Windows 64-bit (WoW64), which permits the execution of 32-bit x86 applications. It was first employed in Windows XP 64-bit Edition (for the Itanium), but then reused for the “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 application cannot use a 32-bit Dynamic-Link Library (DLL) and similarly a 32-bit application cannot use a 64-bit DLL. 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 6, in order to allow for the possibility that some third-party browser extensions or ActiveX controls may not yet be available in 64-bit versions.

32-bit drivers and services are not supported by 64-bit Windows, but video and audio codecs are supported as long as the media player that uses them is 32-bit as well.