Editing Win32s (section)

== Concept and characteristics ==

Win32s was intended as a partial implementation of the [[Windows API|Win32 Windows API]] as it existed in early versions of [[Windows NT]].<ref>{{cite web
 | title = General Overview of Win32s
  | publisher = Microsoft Corporation
  | archive-url = https://web.archive.org/web/20070820172339/http://support.microsoft.com/kb/83520
  | archive-date = 20 August 2007
  | url-status = dead
  | date = 12 April 2004
  | url = http://support.microsoft.com/kb/83520
  | accessdate = 2007-09-05 }}</ref>

The "s" in Win32s signifies ''subset'', as Win32s lacked a number of Windows NT functions, including [[thread (computing)|multi-threading]], [[asynchronous I/O]], newer [[serial port]] functions and many [[Graphics Device Interface|GDI]] extensions. This generally limited it to "Win32s applications" which were specifically designed for the Win32s platform,<ref>{{cite web
 | title = General limitations under Win32s
  | publisher = Microsoft Corporation
  | date = 22 November 2005
  | url = http://support.microsoft.com/kb/q131896/
  | accessdate = 2007-09-05 }}</ref> although some standard Win32 programs would work correctly, including Microsoft's ''[[Full Tilt! Pinball#Space Cadet|3D Pinball Space Cadet]]'' and some of Windows 95's included applets.<ref name="Win32s List"/> Early versions of Internet Explorer (up to Version 5) were also Win32s compatible, although these also existed in 16-bit format.<ref name="Win32s List">{{cite web|url=http://stephan.win31.de/w32slist.htm|title=The Win32s compatibility list|accessdate = 2008-11-30}}</ref> 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 (computing)|relocation information]].

A technique named [[thunk]]ing 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&mdash;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 [[NTVDM|a virtual machine]] for running Win16 applications, although [[Windows on Windows|this feature]] was removed from 64-bit versions of Windows.