Windows 3.1 had cooperative multitasking for Window applications. It only had pre-emptive multitasking for DOS programs. See: retrocomputing.stackexchange.com/questions/791/…

There might be some mistakes in my post but why did they have two OSes at the same time? Doesn't make sense to me.

Do the answers to your earlier question not explain this? Stephen Kitt's, especially, gives “to ensure Microsoft would stay relevant in the face of the emerging RISC platforms and the perceived threat of Unix” as the explanation.

You have two OSes when one is plainly unsuitable for the applications you want. The NT project was started in order to get a modern (for the time) OS base. "Single user on a desktop" is a subset of the applications for NT.

As for "two at the same time" - well, that's been answered now, hasn't it? I'm not sure when the plan for NT everywhere was formulated, but a good rule of thumb is that an OS takes a decade to mature. The switch of the consumer base to NT (in the form of Windows XP) more or less conforms to that.

NT was a pilot project for swapping the MS-DOS / Windows hierarchy. When MS "shelved" it, it was because "NT is dead, long live XP!".

@WeatherVane - huh? XP is just NT V5.1 with a silly marketing name.

@another-dave that was what I said. It wasn't "shelved" but reborn. AFAIR Microsoft announced that NT was finished.

I don't recall that, and I was writing code on NT from 3.1 through to about 2008, whatever that was called.

I think the real problem with this question is the claim of "roughly the same". The systems are entirely different; there is a similar UI, and there is a Windows-like API on top of the NT system calls. But the underlying system structure is entirely different. NT was designed for pre-emptive multitasking on multiprocessor systems with object and user based security, packet-driven asynchronous I/O, and independent of any particular hardware. Porting between hardware is straightforward if you design with that intent; forcing it on a system that was never designed to be portable is much harder.

You could have done just a little more research before confidently asserting that Windows 1.0 had the Win32 API…

@user3840170 I'm sorry I thought it has it because Win95 has.

"Why was the existing Windows not ported [...]?" - Windows 3.x and DOS were tied to the the x86 platform. A project to modify the Win16 API into something more complete with portability, virtual memory, etc. would look a lot like OS/2 or Windows NT.

"In 1985 Windows 1.0 was released. AFAIK it had everything..." - great punchline to start with!

@user3840170 You changed the wording "synonym question" to "related question". It's a little sad because the question is indeed a synonym.

The other question is not actually equivalent in meaning (consider: ‘Why did Microsoft start Windows NT at all [instead of keeping Windows anchored to x86 forever]?’ is asking something different from ‘Why was the existing Windows not ported to the Intel i860?’), so it’s not a synonym. Never mind that the term ‘synonym’ applies to words and idioms forming an established lexicon of a language (dialect), not entire sentences with sum-of-parts meaning spoken just for the occasion.

@Steve It might not have had everything, but at least it had Reversi - which more than you can say for Windows 11.

@user3840170 If they had ported the existing Windows to i860 then there would have been no reason to create NT. The questions are identical. Not related.

Have you actually used Windows NT? It was such a step forward from Windows 3.11, it's hard to imagine how this question can even arise.

@AndreKR No I didn't (I already said this in the question). Even if it was a big step it doesn't answer my question which could also be said like this: Why did Microsoft have two OSes at the same time for roughly 7 years?

@zomega Because Windows 9x was architectually dependent on DOS, and MS-DOS is written almost entirely in x86 assembly so it not able to be feasibly ported-over to a non-x86 machine. Everyone at Microsoft knew that Windows 9x needed to die because it was effectively impossible to port it to other platforms which would be an albatross around the company's neck. By analogy, it's like asking why we couldn't simply use actual horses to power a Ferrari instead of spending time and effort developing internal combustion engines.

@Dai Yes but why keep it alive for 7 years and not let it immedietly die with the release of NT?

@zomega Industrial inertia: Microsoft's customers (you and me, and companies and enterprises worldwide) already had billions of dollars invested in software that only runs on DOS/9x - Microsoft needed to keep 9x alive until Windows NT attained runtime binary compatibility for that huge body of existing software (i.e. Windows XP) - otherwise all that trust/goodwill that the world had in the Microsoft brand ("we won't break your software!") would be gone and everyone would be using some non-Microsoft OS by now, and Bill Gates wouldn't be trying to cure malaria.

@Dai Wasn't it possible to run DOS/9x on NT? In my opinion the assumption that all actions by Microsoft were always logical and wise is.... WRONG!

@zomega "Wasn't it possible to run DOS/9x on NT?" - In short, no, not really: Windows NT could not reliably run software-that-was-built-for-9x until 2001, and even then there's still plenty of 9x software titles that XP, Vista, etc still can't run correctly (Command & Conquer titles come to mind...), though to be fair that's more the fault of third-party game devs than Microsoft's fault specifically.

@JohnDallman - fair enough; I'm firmly in the programmer camp!

@zomega NT did not provide compatibility with third-party DOS drivers, with software dependent on them, or with DOS applications that required being run in real mode for any other reason. There were even some differences in the Win32 API of Windows 9x and NT (although Microsoft did their best to keep them from getting in the way).

I recommend the book "ShowStopper", which goes into details of the why's and hows of NT.

Preemptive multitasking. What more needs to be said?

@WalterMitty Windows 95 also provided that, between 32-bit tasks; and earlier versions of Windows preemptively multitasked DOS boxes.