It was a co-development deal and they fought a lot; IBM wanted to take out desktop-centric features (printer drivers, etc) and pack in terminal emulation features.
The split did not result in NT spinning out, rather two versions of OS/2, initially.
When the final schizm played out MS kept all rights to Windows and DOS, and OS/2 1.3 APIs. IBM got the unfinished OS/2 2.x that became warp eventually.
NT is an entirely different creature architected by David Cutler (previously of DEC and VMS) for 90's hardware. Processor independent, it was built around a microkernel design, not a monolithic kernel, with installable OS subsystems.
http://en.m.wikipedia.org/wiki/WindowsNT
Originally NT carried a 16-bit Windows subsystem, the core Win 32bit subsystem, an OS/2 1.3 text only system (Presentation Manager was all IBM and one of the causes of friction--MS wanted Windows APIs instead of OS/2 APIs) and a barebones Posix subsystem. The latter two were dropped with WinXP.
http://en.m.wikipedia.org/wiki/WindowsNT
Since then both the microkernel and installable systems have been modified and replaced repeatedly and very little original code likely remains.
Also of note:
http://en.m.wikipedia.org/wiki/Archi..._of_Windows_NT
And
http://en.m.wikipedia.org/wiki/Windo...Virtualization
Elements of both were used to create the XBOX OSes but they are not really Windows.
Word is that the XBOX One will be getting an installable Win10 environment. No details yet.
As to OS/2, the reasons for the split were cultural:
http://en.m.wikipedia.org/wiki/OS/2
MS coders coming from the OC side wrote terse code with minimal documentation, IBMers from the mainframe side, used to more resources were... less economical.
Quote:
OS/2 1.x targets the 80286 processor and DOS fundamentally doesn't. IBM insisted on supporting the Intel 80286 processor, with its 16-bit segmented memory mode, due to commitments made to customers who had purchased many 80286-based PS/2's because of IBM's promises surrounding OS/2.[18] Until release 2.0 in April 1992, OS/2 ran in 16-bit protected mode and therefore could not benefit from the Intel 80386's much simpler 32-bit flat memory model and virtual 8086 mode features. This was especially painful in providing support for DOS applications. While, in 1988, Windows/386 2.1 could run several cooperatively multitasked DOS applications, including expanded memory (EMS) emulation, OS/2 1.3, released in 1991, was still limited to one 640 kB "DOS box".
Given these issues, Microsoft started to work in parallel on a version of Windows which was more future-oriented and more portable. The hiring of Dave Cutler, former VMS architect, in 1988 created an immediate competition with the OS/2 team, as Cutler did not think much of the OS/2 technology and wanted to build on his work at Digital rather than creating a "DOS plus". His "NT OS/2," was a completely new architecture.[19]
The OS/2 2.0 upgrade box
IBM grew concerned about the delays in development of OS/2 2.0 and the diversion of IBM funds earmarked for OS/2 development towards Windows. Initially, the companies agreed that IBM would take over maintenance of OS/2 1.0 and development of OS/2 2.0, while Microsoft would continue development of OS/2 3.0. In the end, Microsoft decided to recast NT OS/2 3.0 as Windows NT, leaving all future OS/2 development to IBM.
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