Most people know about Microsoft’s official timeline for its operating-system like products
1. Xenix - Microsoft’s first operating system, which was a version of UNIX that we did for microprocessors.
2. MS-DOS/PC-DOS, a 16 bit operating system for the 8086 CPU
3. Windows (not really an operating system, but it belongs in the timeline).
4. OS/2, a 16 bit operating system written in joint development with IBM.
5. Windows NT, a 32 bit operating system for the Intel i386 processor, the Mips R8800 and the DEC Alpha
But most people don’t know about Microsoft’s other multitasking operating system, MS-DOS 4.0 (not to be confused with PC-DOS 4.0)
MS-DOS 4.0 was actually a version of MS-DOS 2.0 that was written in parallel with MS-DOS 3.x (DOS 3.x shipped while DOS 4 was under development, which is why it skipped a version).
DOS 4 was a preemptive real-mode multitasking operating system for the 8086 family of processors. It had a boatload of cool features, including movable and discardable code segments, movable data segments (the Windows memory manager was a version of the DOS 4 memory manager). It had the ability to switch screens dynamically – it would capture the foreground screen contents, save it away and switch to a new window.
Bottom line: DOS 4 was an amazing product. In fact, for many years (up until Windows NT was stable), one of the DOS 4 developers continued to use DOS 4 on his desktop machine as his only operating system.
We really wanted to turn DOS 4 into a commercial version of DOS, but... Microsoft at the time was a 100% OEM shop – we didn’t sell operating systems, we sold operating systems to hardware vendors who sold operating systems with their hardware. And in general the way the market worked in 1985 was that no computer manufacturer was interested in a version of DOS if IBM wasn’t interested. And IBM wasn’t interested in DOS. They liked the idea of multitasking however, and they were very interested in working with that – in fact, one of their major new products was a product called “TopView”, which was a character mode window manager much like Windows. The wanted an operating system that had most of the capabilities of DOS 4, but that ran in protected mode on the 286 processor. So IBM and Microsoft formed the Joint Development Program that shared development resources between the two companies. And the DOS 4 team went on to be the core of Microsoft’s OS/2 team.
But what about DOS 4? It turns out that there WERE a couple of OEMs that had bought DOS 4, and Microsoft was contractually required to provide the operating system to them. So a skeleton crew was left behind to work on DOS and to finish it to the point where the existing DOS OEM’s were satisfied with it.
Edit: To fix the title which somehow got messed up.