Win32s

Win32s is an extension for the 16-bit Microsoft 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 including threading and asynchronous I/O, newer 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 unexplained 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.

A technique named thunking, the mechanism by which 16-bit code calls 32-bit code and vice-versa in Microsoft Windows operating systems, is fundamental to the implementation of Win32s.

See also: Windows API, OS/2