Windows on Windows: Difference between revisions

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{{Short description |SubsystemDiscontinued subsystem for 32-bit Windows for running 16-bit Windows programs}}

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{{primary sources|date=October 2018}}

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| license = [[Proprietary software|Proprietary]] [[commercial software]]

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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 discontinued [[compatibility layer]] of [[32-bit computing|32-bit]] versions of the [[Windows NT]] family of [[operating system]]s. sinceSince 1993, with the release of [[Windows NT 3.1]], whichWoW extends [[Virtual DOS machine#Windows NTVDM|NTVDM]] to provide limited support for running [[legacy codesystem|legacy]] [[16-bit computing|16-bit]] programs written for [[Windows 3.x]] or earlier. There is a similar subsystem, known as [[WoW64]], on [[64-bit computing|64-bit]] Windows versions that runs 32-bit programs.

This subsystem haswill sincebe beenretired discontinuedwith asthe [[Product lifecycle#Phase 4: Service|end of 2021support]] of [[Windows 10]] in October 14, 2025. The last version of Windows to include this subsystem is [[Windows 10]], as [[Windows 11]] (and [[Windows Server 2008 R2]] and later) droppedonly supportrun forthe 32[[x86-bit64]] processorsprocessor in [[long mode]] and therefore cannot run 16-bit software without third-party emulation software (e.g. [[DOSBox]]).

==Background==

Many 16-bit Windows legacy programs can run without changes on newer [[32-bit]] editions of Windows. The reason designers made this possible was to allow software developers time to remedy their software during the industry transition from [[Windows 3.1x1]] to [[Windows 95]] and later, without restricting the ability for the operating system to be upgraded to a current version before ''all'' programs used by a customer had been taken care of.

The [[Windows 9x]] series of operating systems, reflecting their roots in [[DOS]], functioned as hybrid 16- and 32-bit systems in the sense that the underlying operating system was not truly 32-bit,{{citation needed |reason=What is the source of this assertion? The fact that you could boot to DOS? |date=January 2017}} and therefore could run 16-bit software natively without requiring any special emulation; [[Windows NT]] operating systems differ significantly from Windows 9x in their architecture, and therefore require a more complex solution. Two separate strategies are used in order to let 16-bit programs run on 32-bit versions of Windows (with some [[Execution_(computing)#Runtime|runtime]] limitations). They are called [[thunk]]ing and [[Shim (computing)|shimming]].