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When IBM introduced the PC, they used the 8088 processor from Intel. Intel had developed the 8088 and 8086 (used by the IBM XT) as cut down versions of the 80286 protected mode processor that they wanted to build.
DOS was not designed to run in protected mode so IBM and Microsoft decided to develop a new operating system to take advantage of the protected mode capabilities of the 80286 (IBM AT). The advantage of a protected mode operating system is that each program runs in its own protected area of memory and cannot interfere with any of the other programs you have running on your system. OS/2 version one was the result of this effort.
The IBM ET (386) computer introduced 32 bit capability. Prior to this all computers had been either 8 or 16 bit. Version two of OS/2 was intended to take advantage of this capability which would enable much faster processing at the same processor speed.
It was at about this time that Microsoft finally came up with a version of their graphical User Interface (GUI) called Windows which became far more popular than the other GUIs available at the time (eg. GEM, Deskview, Topview). Version two of Windows had been intended as an interim step between DOS and OS/2 (which has its own GUI integrated into the operating system). Now with the success of Windows 3, Microsoft questioned the arrangement that they had formed with IBM five years earlier and decided to go it alone. Each company retained the right to use and distribute the operating systems and GUIs developed up until that time (hence the inclusion of Windows 3.1 in OS/2).
IBM has continued to develop OS/2 and soon released OS/2 version 2.0. Microsoft decided to cash in on the popularity of their GUI and renamed their copy of OS/2 to Windows NT 3.1. Subsequent development by Microsoft has gone in a different direction than that followed by IBM and Windows XP (as version 5 is now known) is a very different operating system from eComStation (or eCS) as version 5 of OS/2 is now known.