Win32s. Win32s is a 32-bit application runtime environment for the Microsoft Windows 3.1 and 3.11 operating systems.  It allowed some 32-bit applications to run on the 16-bit operating system using call thunks. A beta version of Win32s was available in October 1992. Version 1.10 was released in July 1993 simultaneously with Windows NT 3.1.

Concept and characteristics 

Win32s was intended as a partial implementation of the Win32 Windows API as it existed in early versions of Windows NT.

The "s" in Win32s signifies subset, as Win32s lacked a number of Windows NT functions, including multi-threading, asynchronous I/O, newer serial port functions and many GDI extensions. This generally limited it to "Win32s applications" which were specifically designed for the Win32s platform, although some standard Win32 programs would work correctly, including Microsoft's 3D Pinball Space Cadet and some of Windows 95's included applets. Early versions of Internet Explorer (up to Version 5) were also Win32s compatible, although these also existed in 16-bit format. Generally, for a 32-bit application to be compatible with Win32s, it had to not use more than 16MB of memory or any extended features such as DirectX.

Win32s inherits many of the limitations of the Win16 environment. True Win32 applications execute within a private virtual address space, whereas Windows 3.x used an address space shared among all running applications. An application running on Win32s has the shared address space and cooperative multitasking characteristics of Windows 3.1. Consequently, for a Win32 application to run on Win32s, it must contain relocation information.

A technique named thunking is fundamental to the implementation of Win32s as well as Chicago-kernel operating systems, which are Windows 95, Windows 98, and Windows ME. However, allowing user-level thunking greatly complicates attempts to provide stable memory management or memory protection on a system-wide basis, as well as core or kernel security—this allows poorly written applications to undermine system stability on Win32s, as well as the Chicago-kernel systems. 32-bit versions of Windows NT transparently provide a virtual machine for running Win16 applications, although this feature was removed from 64-bit versions of Windows.

Compatibility 
Win32s-compatible applications could be built using Microsoft's development tools, as well as at least Borland C++ 4.x and Symantec C++ (now Digital Mars C++). However, several program compilation options (such as EXE relocation information) and DLLs which were implicit in Windows NT 3.1 have to be included with the application in Win32s. Microsoft ceased support for Win32s in Visual C++ version 4.2 released in late 1996.

Win32s included an implementation of the card game FreeCell as an example application, a couple of years before the game became famous with its inclusion in Windows 95.

Warcraft II: Tides of Darkness, a DOS game, included a level editor that required Win32s to run. The editor used a Windows GUI for displaying the large maps because high-resolution graphic modes were already handled by Windows. The NCSA Mosaic web browser version 2 also required Win32s.

C# applications can be compiled by the CoreRT compiler into Win32 code that is accepted by the 1994 Visual C++ linker. This combination produces Win32 executables that can run on Windows 3.1 with Win32s.

Sources and installation 

Win32s can still be found using web search engines; PW1118.EXE is generally the installation file used.
$ sha512sum pw1118.exe
31b532f324a1eebd1073e043e0c81cb131ad40681b1a7e5e73f881369eafa38ba9d91a541d99fd8b96e937f0e93bfd17cf9c26cc77daf8c77d7c9df803b3aef9
Developers should ensure they are installing OLE if they require consistent clipboard handling. Many applications which need to be made OLE aware (i.e. setting up their OLE server) must be reinstalled. Win32s was also included with some early Win32 programs.

References

External links 
Win32 Tutorial by Graeme Chandler
Win32s Compatibility List
Download Win32s from Microsoft 
Download Win32s from mirror
Archive of almost every version of Win32s
Andreas C. Hofmann: Win32 subset / Win32s (1993 to 1995), in: Computing History Dictionary (german) [⁴11.10.2021].

Microsoft application programming interfaces
Compatibility layers

de:Windows Application Programming Interface#Win32s