Windows on Windows

This article is about the Win16 subsystem in 32-bit Windows NT. For the 32-bit compatibility layer in 64-bit Windows, see WOW64.

In computing, 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 - applications written for Windows 3.x. Alternatively "WOW" may also refer to support for running 32-bit applications on 64-bit versions of Windows - known as WOW64.

Background

Many Win16 applications can run without changes on 32-bit editions of Windows, complete with the limitations of such applications compared with applications written for Win32. The operating system thunks 16-bit APIs to their underlying 32-bit equivalents in order to provide support for 16-bit pointers, memory models and address space. 32-bit Windows shortens long filenames into 8.3 filenames 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 cannot do so in user mode.

The Windows 9x series of operating systems functioned as hybrid 16/32-bit systems in the sense that the underlying operating system was not truly 32-bit, and therefore could 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 do they provide the NTVDM DOS PC emulator. DOS applications therefore cannot run on a 64-bit edition of Windows unless a system administrator has installed third-party emulator-software or configured a virtual DOS machine in a guest 32-bit virtual machine.

The WIN.COM file in a Windows NT System32 folder facilitates Windows-on-Windows. In addition to Windows-on-Windows emulating the Windows 95, Windows NT 4.0, and Windows 98 kernels, the WIN.COM file emulates a Windows 3.x kernel for NTVDM, which runs the 16-bit DOS-based Windows applications on Windows NT. In 64-bit Windows NT environments Microsoft has removed both NTVDM and the emulated Windows 3.x kernel; thus 64-bit Windows-on-Windows only emulates the kernels of Windows 95, Windows NT 4.0, Windows 98, and later (where the emulated kernel of Windows Vista and later were only 32-bit).

See also

• WoW64
• Shim (computing)
• Compatibility layer

External links

• Windows NT subsystems
• What are NTVDM and WOW?
• Monitoring 16-bit Windows applications