Windows software that runs applications written for previous versions of Windows. Included in 32-bit versions of Windows, Windows on Windows 16 (WoW16) emulates the Windows 3.x operating system kernel and enables 16-bit Windows 3.1 apps to run. See NTVDM. WoW64 Included in 64-bit versions of Windows, Windows on Windows 64 creates a 32-bit environment that allows 32-bit apps to run. WoW64 handles the discrepancies between 32-bit and 64-bit versions of the Registry. It also allows a 32-bit app to use a full 4GB of virtual memory, which is more than it would have in 32-bit Windows. When running in 32-bit Windows, some of that address space is used by Windows itself. See binary values.