OS/2 is an operating system from IBM and Microsoft, that was intended as the successor to MS-DOS and Microsoft Windows.
OS/2 1.0 edit
The first version of OS/2 uses a command line interface similar to that of MS-DOS. Unlike MS-DOS, it uses a protected mode kernel and dynamic linking and supports multitasking of native OS/2 applications, which use the same "New Executable" as 16-bit Windows applications, although the two APIs are incompatible. It also can run a single MS-DOS application at a time. This involves forcing the processor back into real mode.
OS/2 1.1, OS/2 1.2, OS/2 1.3 edit
OS/2 1.1 added the Presentation Manager, a graphical user interface visually similar to Windows 2.0. Presentation Manager also expanded the OS/2 API.
OS/2 1.2 added the High Performance File System (HPFS), which allows filenames up to 255 characters and eliminated several other filename restrictions of the older File Allocation Table (FAT) file system. OS/2 1.2 also updated the appearance of Presentation Manager, creating the visual style more commonly associated with the later, more popular Windows 3.0. OS/2 1.3 is a bug-fix release.