Background
Many Win16 applications can run without changes on 32-bit editions of Windows, complete with the limitations of such applications compared with applications written for Win32. The operating system thunks 16-bit APIs to their underlying 32-bit equivalents in order to provide support for 16-bit pointers, memory models and address space. 32-bit Windows shortens long filenames into 8.3 filenames to allow their use by legacy applications. Application-compatibility issues, notably around multiple users and the concept of least privilege, may prevent some applications that incorrectly assume write access to the whole file system from working on newer platforms. Furthermore, legacy applications that attempt to access hardware directly cannot do so in user mode.
The Windows 9x series of operating systems functioned as hybrid 16/32-bit systems in the sense that the underlying operating system was not truly 32-bit, and therefore could run Win16 applications natively without the use of a distinct WoW component.
64-bit versions of Windows do not include the WoW Win16-support subsystem and therefore cannot run Win16 applications, nor do they provide the NTVDM DOS PC emulator. DOS applications therefore cannot run on a 64-bit edition of Windows unless a system administrator has installed third-party emulator-software (e.g. DOSBox) or set up virtual machine with either a 32-bit version of Windows, Windows XP Mode, or DOS itself.
The WIN.COM file in a Windows NT System32 folder facilitates Windows-on-Windows. In addition to Windows-on-Windows emulating the Windows 95, Windows NT 4.0, 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. In 64-bit Windows NT environments Microsoft has removed both NTVDM and the emulated Windows 3.x kernel; thus 64-bit Windows-on-Windows only emulates the kernels of Windows 95, Windows NT 4.0, Windows 98, and later (where the emulated kernel of Windows Vista and later were only 32-bit).
Read more about this topic: Windows On Windows
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