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{{Short description |Subsystem for 32-bit Windows for running 16-bit Windows programs}}
{{Short description |Discontinued subsystem for 32-bit Windows for running 16-bit Windows programs}}
{{multiple|
{{multiple|
{{primary sources|date=October 2018}}
{{primary sources|date=October 2018}}
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| license = [[Proprietary software|Proprietary]] [[commercial software]]
| license = [[Proprietary software|Proprietary]] [[commercial software]]
}}
}}
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> was a [[compatibility layer]] of [[32-bit]] versions of the [[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.
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]] versions of the [[Windows NT]] family of [[operating system]]s since 1993 with the release of [[Windows NT 3.1]], 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 not available in 64-bit editions prior to [[Windows 11]] (including [[Windows Server 2008 R2]] and later, which only have 64-bit editions) and therefore cannot run 16-bit software without third-party emulation software (e.g. [[DOSBox]]). With Windows 11 dropping support for [[32-bit computing|32-bit]] processors, this subsystem has been discontinued.
This subsystem has since been discontinued as of 2021. The last version of Windows to include this subsystem is [[Windows 10]], as [[Windows 11]] (and [[Windows Server 2008 R2]] and later) dropped support for 32-bit processors and therefore cannot run 16-bit software without third-party emulation software (e.g. [[DOSBox]]).


==Background==
==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 3.1x]] 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.
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 3.1x]] 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 runtime limitations). They are called [[thunk]]ing and [[Shim (computing)|shimming]].
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]].


===Thunking===
===Thunking===
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The WOW subsystem of the operating system {{Clarify|text=thunks legacy 16-bit APIs to their newer 32-bit equivalents|date=July 2020}} in order to provide support for 16-bit [[pointer (computer programming)|pointer]]s, memory models and [[address space]].
The WOW subsystem of the operating system {{Clarify|text=thunks legacy 16-bit APIs to their newer 32-bit equivalents|date=July 2020}} in order to provide support for 16-bit [[pointer (computer programming)|pointer]]s, memory models and [[address space]].


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.
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.


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.
The {{mono|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]] [[kernel (operating system)|kernels]], the {{mono|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.


===Shimming===
===Shimming===
{{Main article|Shim (computing)}}
{{Main article|Shim (computing)}}
Application compatibility issues, notably around [[long filename]]s, multiple users and the concept of [[Principle of least privilege|least privilege]], may prevent some applications from working. For example, they may incorrectly assume full write access to the whole file system whereas [[NTFS]] security is in place.
Application compatibility issues, notably around [[long filename]]s, multiple users and the concept of [[Principle of least privilege|least privilege]], may prevent some applications from working. For example, they may incorrectly assume full write access to the whole [[file system]] whereas [[NTFS]] security is in place.


When the Windows 95 line of operating systems was designed, a key requirement was for the file system to keep backward compatibility with [[8.3 filename]]s to allow legacy applications to continue to work on the platform. Windows 95 and later operating systems therefore support a compatibility mode whereby both a long filename and a short filename are stored in the [[File Allocation Table]].
When the Windows 95 line of operating systems was designed, a key requirement was for the file system to keep [[backward compatibility]] with [[8.3 filename]]s to allow legacy applications to continue to work on the platform. Windows 95 and later operating systems therefore support a compatibility mode whereby both a long filename and a short filename are stored in the [[Design_of_the_FAT_file_system#Directory_entry|directory entry]].


Furthermore, legacy applications that attempt to access hardware directly cannot do so in [[Protection ring|user mode]]. Legacy applications may also fail if system configuration files from the DOS and Windows 9x era are not present in Windows NT based kernels, hence the reason for zero-length versions of files like [[AUTOEXEC.BAT]] and [[CONFIG.SYS]] having to be carried forward on operating systems that do not use them.
Furthermore, legacy applications that attempt to access hardware directly cannot do so in [[Protection ring|user mode]]. Legacy applications may also fail if system configuration files from the DOS and Windows 9x era are not present in Windows NT based kernels, hence the reason for zero-length versions of files like {{mono|[[AUTOEXEC.BAT]]}} and {{mono|[[CONFIG.SYS]]}} having to be carried forward on operating systems that do not use them.


A considerable number of shims are present in the [[compatibility layer|application compatibility layer]] of later versions of Windows to intercept and modify [[Application programming interface|API]] calls made by legacy applications that were written with a different set of assumptions and operating system best practices in mind.<ref>{{cite web|website=TechNet|publisher=[[Microsoft]]|url=https://technet.microsoft.com/en-us/library/ee461265(v=ws.10).aspx|title=Application Compatibility|access-date=February 7, 2017}}</ref> These fixes are updated from time-to-time as issues are discovered in popular legacy applications that are still in use.<ref>{{cite web|url=http://support.microsoft.com/kb/2272691|title=Application Compatibility Update for Windows 7 and Windows Server 2008 R2: August 2010|website=Support|publisher=[[Microsoft]]|access-date=February 7, 2017|date=August 24, 2010}}</ref>
A considerable number of shims are present in the [[compatibility layer|application compatibility layer]] of later versions of Windows to intercept and modify [[Application programming interface|API]] calls made by legacy applications that were written with a different set of assumptions and operating system best practices in mind.<ref>{{cite web|website=TechNet|publisher=[[Microsoft]]|url=https://technet.microsoft.com/en-us/library/ee461265(v=ws.10).aspx|title=Application Compatibility|access-date=February 7, 2017}}</ref> These fixes are updated from time-to-time as issues are discovered in popular legacy applications that are still in use.<ref>{{cite web|url=http://support.microsoft.com/kb/2272691|title=Application Compatibility Update for Windows 7 and Windows Server 2008 R2: August 2010|website=Support|publisher=[[Microsoft]]|access-date=February 7, 2017|date=August 24, 2010}}</ref>
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==See also==
==See also==
* [[Wine (software)]]
* [[Wine (software)]]
* [[Virtual DOS machine#WineVDM|OTVDM]], a third-party project based on code from Wine which runs 16-bit Windows programs on 64-bit versions of Windows.


==References==
==References==
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{{DEFAULTSORT:Windows On Windows}}
{{DEFAULTSORT:Windows On Windows}}
[[Category:Windows components]]
[[Category:Discontinued Windows components]]
[[Category:1993 software]]
[[Category:1993 software]]
[[Category:Compatibility layers]]

Latest revision as of 12:09, 6 October 2024

This article is about the 16-bit subsystem in the 32-bit editions of Windows NT. For the 32-bit compatibility layer in the 64-bit editions, see WoW64.
Windows on Windows
Other namesWOW
Developer(s)Microsoft
Initial releaseJuly 27, 1993; 31 years ago (1993-07-27)
Operating systemMicrosoft Windows
PlatformIA-32
TypeCompatibility layer
LicenseProprietary commercial software

In computing, Windows on Windows (commonly referred to as WOW)[1][2][3] is a discontinued compatibility layer of 32-bit versions of the Windows NT family of operating systems since 1993 with the release of Windows NT 3.1, which extends NTVDM to provide limited support for running 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 has since been discontinued as of 2021. The last version of Windows to include this subsystem is Windows 10, as Windows 11 (and Windows Server 2008 R2 and later) dropped support for 32-bit processors and therefore cannot run 16-bit software without third-party emulation software (e.g. DOSBox).

Background

[edit]

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 3.1x 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] 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 runtime limitations). They are called thunking and shimming.

Thunking

[edit]
Main article: Thunk § Interoperability

The WOW subsystem of the operating system thunks legacy 16-bit APIs to their newer 32-bit equivalents[clarification needed] in order to provide support for 16-bit pointers, memory models and address space.

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.

The WOWEXEC.EXE process on a Windows NT system facilitates Windows-on-Windows.[4][5] 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 NTVDM, which runs the 16-bit DOS-based Windows applications on Windows NT.

Shimming

[edit]
Main article: Shim (computing)

Application compatibility issues, notably around long filenames, multiple users and the concept of least privilege, may prevent some applications from working. For example, they may incorrectly assume full write access to the whole file system whereas NTFS security is in place.

When the Windows 95 line of operating systems was designed, a key requirement was for the file system to keep backward compatibility with 8.3 filenames to allow legacy applications to continue to work on the platform. Windows 95 and later operating systems therefore support a compatibility mode whereby both a long filename and a short filename are stored in the directory entry.

Furthermore, legacy applications that attempt to access hardware directly cannot do so in user mode. Legacy applications may also fail if system configuration files from the DOS and Windows 9x era are not present in Windows NT based kernels, hence the reason for zero-length versions of files like AUTOEXEC.BAT and CONFIG.SYS having to be carried forward on operating systems that do not use them.

A considerable number of shims are present in the application compatibility layer of later versions of Windows to intercept and modify API calls made by legacy applications that were written with a different set of assumptions and operating system best practices in mind.[6] These fixes are updated from time-to-time as issues are discovered in popular legacy applications that are still in use.[7]

See also

[edit]
  • Wine (software)
  • OTVDM, a third-party project based on code from Wine which runs 16-bit Windows programs on 64-bit versions of Windows.

References

[edit]
  1. ^ "WOW Environment Remains in Memory After Quitting 16-Bit Program". Support. Microsoft. February 22, 2007. Archived from the original on October 23, 2007. Retrieved February 7, 2017.
  2. ^ "Starting 16-Bit WOW Subsystem on Windows NT Server". Support. Microsoft. November 1, 2016. Archived from the original on May 9, 2007. Retrieved February 7, 2017.
  3. ^ "Disabling the MSDOS and WOWEXEC Subsystems on Terminal Server". Support. Microsoft. November 1, 2006. Archived from the original on January 13, 2008. Retrieved February 7, 2017.
  4. ^ "Windows NT Subsystems and Associated Files". Support. Microsoft. October 31, 2006. Archived from the original on March 16, 2007. Retrieved February 7, 2017.
  5. ^ "PRB: Relocation of Ntvdm.exe Fails on Multiprocessor Computers". Support. Microsoft. November 21, 2006. Archived from the original on February 22, 2009. Retrieved February 7, 2017.
  6. ^ "Application Compatibility". TechNet. Microsoft. Retrieved February 7, 2017.
  7. ^ "Application Compatibility Update for Windows 7 and Windows Server 2008 R2: August 2010". Support. Microsoft. August 24, 2010. Retrieved February 7, 2017.
[edit]
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