Games - WOW64

	Games - WOW64

WOW64 (Windows-on-Windows 64-bit) is a Microsoft operating system subsystem capable of running 32-bit applications and is included on all 64-bit versions of Microsoft Windows—including Windows XP Professional x64 Edition, Windows Server 2003 x64 Edition and Windows XP 64-bit Edition. WOW64 takes care of all the differences between 32-bit Windows and 64-bit Windows, particularly involving structural changes to Windows itself.

Translation libraries

The WOW64 subsystem is a lightweight translation layer that has similar interfaces on all 64-bit versions of Windows. Its primary purpose is to create a 32-bit environment that provides the interfaces required to allow 32-bit Windows apps to run unmodified in the 64-bit system. Technically, WOW64 is implemented using three dynamic-link libraries (DLLs): Wow64.dll, which is the core interface to the NT kernel that translates between 32-bit and 64-bit calls, including pointer and stack manipulations; Wow64win.dll, which provides the appropriate entry points for 32-bit apps; and Wow64cpu.dll, which takes care of switching the processor from 32-bit to 64-bit mode.

Architectures

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 uses Wow64win.dll to set up the emulation of x86 instructions within the Itanium 2's unique instruction set. That's a more computationally expensive task than the Wow64win.dll's functions on the AMD64 architecture, which switches the processor hardware from its 64-bit mode to 32-bit mode when it's time to execute a 32-bit thread, and then handles the switch back to 64-bit mode. No emulation is required for WOW64 on AMD64.

The registry and the file system

The WOW64 subsystem also handles other key aspects of running 32-bit applications. For example, it's involved in managing the interaction of 32-bit apps with the Windows registry, which is somewhat different in 64-bit versions of the OS, and in providing an interface to the storage subsystem.

The operating system rather awkwardly uses the %SystemRoot%\system32 directory for its 64-bit library and executable files. When executing 32-bit applications, WOW64 redirects requests for DLLs from that directory to %SystemRoot%\sysWOW64, which contains legacy libraries and executables.

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