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Windows on Windows: Wikis


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Windows on Windows, commonly referred to by its acronym WOW or WoW, is a software component of 32-bit versions of the Microsoft Windows NT family of operating systems that provides limited support for running legacy Win16 applications, that is, applications written for Windows 3.x. Alternatively WOW may also refer to support for running 32-bit applications on 64-bit versions of Windows—something that is known as WOW64.


Many Win16 applications can run as-is on 32-bit editions of Windows, complete with the limitations of such applications compared with applications written for win32. 16-bit API's are thunked in the operating system to their underlying 32-bit equivalents in order to provide support for 16-bit pointers, memory models and address space. Long filenames are shortened into 8.3 names to allow their use by legacy applications. Application compatibility issues, notably around multiple users and the concept of least privilege, may prevent some applications that incorrectly assume write access to the whole file system from working on newer platforms. Furthermore, legacy applications that attempt to access hardware directly will find that they are unable to do so in user mode.

The Windows 9x series of operating systems were hybrid 16/32-bit systems in the sense that the underlying operating system was not truly 32-bit, was therefore still able to run Win16 applications natively without the use of a distinct WoW component.

64-bit versions of Windows do not include the WoW Win16-support subsystem and therefore cannot run Win16 applications, nor is the NTVDM DOS PC emulator provided. DOS applications therefore cannot be run on a 64-bit edition of Windows unless a third party emulator software is installed.

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