In computing, Windows on Windows (commonly referred to as WOW), was a 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 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). Windows 10 is the final version of Windows to include this subsystem. This subsystem has since been discontinued, as Windows 11 dropped support for 32-bit processors. 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. 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, 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 The WOW subsystem of the operating system 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. In addition to Windows-on-Windows emulating the Windows 95 and