OS/2

OS/2 (Operating System/2) was an operating system originally developed jointly by Microsoft and IBM, in development was codenamed "CP/DOS". Between the release of Windows 3.0 and OS/2 1.3, Microsoft stopped all cooperation with IBM, due to a feud between the companies' significantly different cultures and visions. IBM later on handled the development of the operating system by itself.

OS/2 have features like a virtual DOS machine, the WinOS/2 subsystem (Windows 3.0/3.1x inside OS/2), Presentation Manager, and more.

Microsoft later used OS/2 code to develop Windows NT 3.1. OS/2 code was later used by Serenity Systems to develop eComStation in 2001, which in turn was used by Arca Noae LLC to develop ArcaOS in 2017.

Warp era

 * OS/2 Warp 5
 * Version 5.0
 * Codename Gandalf or Merlin 2
 * Developed 2003, probably scheduled for release in 2005.
 * Development started but project was later scrapped.