This article is about the Win16 subsystem in 32-bit Windows NT. For the 32-bit compatibility layer in 64-bit Windows, see WOW64.
Windows on Windows, commonly referred to as WOW or WoW, is a critical component for backward compatibility of legacy code in Windows NT-based operating systems. WoW supports applications using the Win16 API and relies on NTVDM (NT Virtual DOS Machine).
Win16 applications run as-is without preemptive multitasking or long filename support. These programs do not take advantage of the 32-bit version of Windows NT.
The Windows 9x series of operating systems were hybrid 16/32-bit systems, and thus did not need WoW to run Win16 applications, since they contained all the necessary components in the core of the operating system itself.
64-bit versions of Windows do not include the 16-bit WoW subsystem or NTVDM and therefore cannot run 16-bit Windows or DOS applications without the use of third-party emulators.
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