OS/2

OS/2 (Operating System/2) is 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 has features like a virtual DOS machine, the WIN-OS/2 subsystem (Windows 3.0/3.1x inside OS/2), Presentation Manager, and more.

Microsoft later used the OS/2 code to develop Windows NT 3.1. The 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.

NEC sold a port of the Japanese localized version for their PC-9801 series of computers. This port features NEC copyrights as well as IBM and/or Microsoft copyrights along with NEC's own modified version of DOS.