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| Windows on Windows | |
|---|---|
| Other names | WOW |
| Developer | Microsoft |
| Initial release | July 27, 1993; 32 years ago (1993-07-27) |
| Operating system | Microsoft Windows |
| Platform | IA-32 |
| Type | Compatibility layer |
| License | Proprietarycommercial software |
Incomputing,Windows on Windows (commonly referred to asWOW)[1][2][3] is a discontinuedcompatibility layer of32-bit versions of theWindows NT family ofoperating systems. Since 1993, with the release ofWindows NT 3.1, WoW extendsNTVDM to provide limited support for runninglegacy16-bit programs written forWindows 3.x or earlier. There is a similar subsystem, known asWoW64, on64-bit Windows versions that runs 32-bit programs.
This subsystem was retired with theend of support ofWindows 10 in October 14, 2025. The last version of Windows to include this subsystem is Windows 10, asWindows 11 (andWindows Server 2008 R2 and later) only run thex86-64 processor inlong mode and therefore cannot run 16-bit software without emulation software.
Many 16-bit Windows legacy programs can run without changes on newer32-bit editions of Windows. The reason designers made this possible was to allow software developers time to remedy their software during the industry transition fromWindows 3.1 toWindows 95 and later, without restricting the ability for the operating system to be upgraded to a current version beforeall programs used by a customer had been taken care of.
TheWindows 9x series of operating systems, reflecting their roots inDOS, 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 someruntime limitations). They are calledthunking andshimming.
The WOW subsystem of the operating systemthunks legacy 16-bit APIs to their newer 32-bit equivalents[clarification needed] in order to provide support for 16-bitpointers, memory models andaddress space.
All 16-bit programs run by default in a singlevirtual 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 dedicatedvirtual 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-bitinter-process communication and increased memory utilization.
TheWOWEXEC.EXE process on aWindows NT system facilitates Windows-on-Windows.[4][5] In addition to Windows-on-Windows emulating theWindows 95 andWindows 98kernels, theWIN.COM file emulates aWindows 3.x kernel forNTVDM, which runs the 16-bit DOS-based Windows applications on Windows NT.
Application compatibility issues, notably aroundlong filenames, multiple users and the concept ofleast privilege, may prevent some applications from working. For example, they may incorrectly assume full write access to the wholefile system whereasNTFS security is in place.
When the Windows 95 line of operating systems was designed, a key requirement was for the file system to keepbackward compatibility with8.3 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 thedirectory entry.
Furthermore, legacy applications that attempt to access hardware directly cannot do so inuser 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 likeAUTOEXEC.BAT andCONFIG.SYS having to be carried forward on operating systems that do not use them.
A considerable number of shims are present in theapplication compatibility layer of later versions of Windows to intercept and modifyAPI 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]
