WOW64

In computing on Microsoft platforms, WoW64 (Windows 32-bit on Windows 64-bit) is a subsystem of the Windows operating system capable of running 32-bit applications that is included in all 64-bit versions of Windows—including Windows XP Professional x64 Edition, IA-64 and x64 versions of Windows Server 2003, as well as 64-bit versions of Windows Vista, Windows Server 2008, Windows 7, Windows 8, Windows Server 2012, Windows 8.1 and Windows 10. In Windows Server 2008 R2 Server Core, it is an optional component. In Nano Server it is not present. WoW64 aims to take care of many of the differences between 32-bit Windows and 64-bit Windows, particularly involving structural changes to Windows itself.

The WoW64 subsystem comprises a lightweight compatibility layer that has similar interfaces on all 64-bit versions of Windows. It aims to create a 32-bit environment that provides the interfaces required to run unmodified 32-bit Windows applications on a 64-bit system. Technically, WoW64 is implemented using three dynamic-link libraries (DLLs):

Despite its outwardly similar appearance on all versions of 64-bit Windows, WoW64's implementation varies depending on the target processor architecture. For example, the version of 64-bit Windows developed for the Intel Itanium 2 processor (known as the IA-64 architecture) uses Wow64win.dll to set up the emulation of x86 instructions within the Itanium 2's unique instruction set. This emulation is a much more computationally expensive task than the Wow64win.dll's functions on the x86-64 architecture (alias "Intel 64" in the Intel terminology, or "AMD64" in the original AMD implementation of this 64-bit mode, also used in the architecture type name of Windows installers), which switches the processor hardware from its 64-bit mode to compatibility mode when it becomes necessary to execute a 32-bit thread, and then handles the switch back to 64-bit mode.

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