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In [[computing]], '''Windows on Windows''' (commonly referred to as '''WOW'''),<ref>{{cite web|url=http://support.microsoft.com/kb/181333|title=WOW Environment Remains in Memory After Quitting 16-Bit Program|website=Support|publisher=[[Microsoft]]|access-date=February 7, 2017|date=February 22, 2007|url-status=dead|archive-url=https://web.archive.org/web/20071023060218/http://support.microsoft.com/kb/181333|archive-date=October 23, 2007}}</ref><ref>{{cite web|url=http://support.microsoft.com/kb/153544|title=Starting 16-Bit WOW Subsystem on Windows NT Server|date=November 1, 2016|access-date=February 7, 2017|website=Support|publisher=[[Microsoft]]|url-status=dead|archive-url=https://web.archive.org/web/20070509051612/http://support.microsoft.com/kb/153544|archive-date=May 9, 2007}}</ref><ref>{{cite web|url=https://support.microsoft.com/kb/220159|title=Disabling the MSDOS and WOWEXEC Subsystems on Terminal Server|date=November 1, 2006|website=Support|publisher=[[Microsoft]]|access-date=February 7, 2017|url-status=live|archive-url=https://web.archive.org/web/20080113000651/http://support.microsoft.com/kb/220159|archive-date=January 13, 2008}}</ref> is a [[compatibility layer]] of [[32-bit]] versions of the Microsoft [[Windows NT]] family of [[operating system]]s from 1993 with the release of [[Windows NT 3.1]] up until [[Windows 10]], which extends [[Virtual DOS machine#Windows NTVDM|NTVDM]] to provide limited support for running [[legacy code|legacy]] [[16-bit]] programs written for [[Windows 3.x]] or earlier. There is a similar subsystem, known as [[WoW64]], on 64-bit Windows versions that runs 32-bit programs.
This subsystem is available in 32-bit editions of Windows NT only. The 64-bit editions prior to [[Windows 11]] (including [[Windows Server 2008 R2]] and later which only have 64-bit editions) cannot run 16-bit software without third-party emulation software (e.g. [[DOSBox]]).▼
With Windows 11 dropping support for [[32-bit computing|32-bit]] [[IA-32]] processors, this subsystem has been discontinued.▼
==Background==
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All 16-bit programs run by default in a single [[virtual DOS machine]] with shared memory space. However, they can be configured to run in their own separate memory space, in which case each 16-bit process has its own dedicated virtual machine. The separate memory space increases system stability by preventing buggy 16-bit programs from interfering with one another, at the expense of reduced 16-bit [[inter-process communication]] and increased memory utilization.
▲This subsystem is available in 32-bit editions of Windows NT only. The 64-bit editions prior to [[Windows 11]] (including [[Windows Server 2008 R2]] and later which only have 64-bit editions) cannot run 16-bit software without third-party emulation software (e.g. [[DOSBox]]).
The WOWEXEC.EXE process on a [[Windows NT]] system facilitates Windows-on-Windows.<ref>{{cite web|url=http://support.microsoft.com/kb/105992|title=Windows NT Subsystems and Associated Files|date=October 31, 2006|access-date=February 7, 2017|website=Support|publisher=[[Microsoft]]|url-status=dead|archive-url=https://web.archive.org/web/20070316022744/http://support.microsoft.com/kb/105992|archive-date=March 16, 2007}}</ref><ref>{{cite web|url=http://support.microsoft.com/kb/199671|title=PRB: Relocation of Ntvdm.exe Fails on Multiprocessor Computers|website=Support|publisher=[[Microsoft]]|access-date=February 7, 2017|date=November 21, 2006|url-status=dead|archive-url=https://web.archive.org/web/20090222173939/http://support.microsoft.com/kb/199671|archive-date=February 22, 2009}}</ref> In addition to Windows-on-Windows emulating the [[Windows 95]] and [[Windows 98]] kernels, the WIN.COM file emulates a [[Windows 3.x]] kernel for [[Virtual DOS machine#Windows NTVDM|NTVDM]], which runs the 16-bit DOS-based Windows applications on Windows NT.
▲With Windows 11 dropping support for [[32-bit computing|32-bit]] [[IA-32]] processors, this subsystem has been discontinued.
===Shimming===
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