History of WinNT
Here is a brief history of WNT( VMS++) from winnetmag -
Windows NT and VMS: The Rest of the Story
Microsoft's internal project name for the new OS was OS/2 NT, because Microsoft's intention was for the new OS to succeed OS/2 yet retain the OS/2 API as its primary interface. The success of Windows 3.0 in April 1990 altered Microsoft's thinking and its relationship with IBM. Six weeks after Microsoft released Windows 3.0, Microsoft renamed OS/2 NT as Windows NT, and designated the Win32 API (a 32-bit evolution of Windows 3.0's 16-bit API) NT's official API.
Gates decided that compatibility with the 16-bit Windows API and the ability to run Windows 3.x applications unmodified were NT's paramount goals, in addition to support for portions of the DOS, OS/2, and POSIX APIs. From 1990 to NT's public release in August 1993, Cutler's team was in a mad dash to complete NT, and the project grew to involve more than 200 engineers and testers. Figure 1 shows a timeline of the major events in the history of NT.
"when Digital engineers noticed the similarities between VMS and NT, they brought their observations to senior management. Rather than suing, Digital cut a deal with Microsoft."
"Although Microsoft presents NT as a homegrown OS, NT is actually much older than its official 1993 birthdate. "
Interesting read - according to this article DOS is not an emulation, rather "support for portions of the DOS, OS/2, and POSIX APIs." Also, Cutler's team was in a mad dash to complete NT - no wonder so many buffer over flows/memory leaks.
The best example of OS Migration is from Apple - the Classic Mode.