Win32s

Microsoft Win32s is an extension for the 16-bit Windows 3.x operating system allowing 32-bit applications compiled for a 32-bit operating system to be executed on 16-bit platforms. Although ostensibly compatible with early versions of Windows NT, many functions were not implemented—for example: threading and asynchronous I/O, the new serial port functions and many GDI extensions. This essentially limits it to applications specifically designed for the platform. In addition, Microsoft made a long series of mysterious and apparently rather pointless changes to Win32s which are widely regarded as a (highly successful) attempt to make it impossible for any third-party to market a compatible platform. (See OS/2.)

See also thunking—the mechanism by which 16-bit code calls 32-bit code and vice-versa in Microsoft Windows operating systems.