DOS # Multitasking MS-DOS 4.00 and 4.10

(Ein Foto der originalen Beta-Disketten) Between MS-DOS 3.2 and MS-DOS 4.00, there was an exotic operating system called "Multitasking MS-DOS" (official spelling according to the beta documentation: M/T MS-DOS; but for simplicity, I will call it MTMS-DOS here) with version numbers 4.00 and 4.10. Microsoft developed it almost entirely anew based on MS-DOS 2.0, later 3.3, using the C programming language (the original MS-DOS was programmed primarily in assembler) and released it in 1986 in the form of OEM versions for IBM-compatible PCs in the US market.

(Zeitgenössischer Artikel aus Info World) Like Windows 1.0x and TopView, it natively mastered the ability to run multiple DOS applications (up to 6 via the Session Manager SM.EXE in the beta) in parallel using cooperative multitasking in real mode. For this purpose, it was expanded with a PIF editor, which allowed assigning a certain amount of memory to the various applications (this file format was introduced by TopView). Additionally, a SWAPPER.EXE (probably to swap programs from memory to the hard disk) and a QUEUER.EXE (a printer spooler) were part of the delivery package. Version 4.10 also came with a PS.EXE, an apparently Microsoft XENIX-derived process viewer.

(MS-DOS 4.00 mit dir Befehl)

Since it attempted to eliminate some shortcomings of the previous MS-DOS (but not the 640 KB limit, which naturally proves to be a bottleneck in multitasking), it was largely no longer compatible with applications of that time. Moreover, these would have had to be specifically rewritten for this system to allow parallel execution with other applications. Otherwise, they could still only be run alone.

(Kommandozeilen-PIF-Editor) Apart from bundling with some OEM systems, the system did not gain any widespread adoption. Microsoft's plan to release it as the direct successor to PC/MS-DOS 3.x (hence the name MS-DOS 4.00) and to develop a GUI similar to that of Windows 1.0x for it was quickly abandoned:

IBM expressed no interest in this, as they preferred to develop a PC-DOS successor system that actually took advantage of the benefits of the 286: We know this today as OS/2. Presumably, one reason for IBM was the experiences they had already made two years earlier with TopView.

Bowing to IBM's market power at the time, Microsoft agreed. However, ideas and parts of the MTMS-DOS code actually flowed into OS/2 1.x, as some discovered similarities and reports from former employees confirm. This makes MTMS-DOS the direct predecessor of OS/2.

After signing the OS/2 pact with IBM, Microsoft released a version of MS-DOS 4.00 that was based on the previous MS-DOS code and had a serious bug in the EMS memory management. Its bug-fix version 4.01 then finally gained wider distribution but had nothing in common with MTMS-DOS 4.00.

(Wechsel zwischen zwei laufenden Programmen via ALT-Taste)

A copy of the system can be obtained here: Multitasking MS-DOS Version 4.00 (5.25).7z.

On diskette 2 is KEYBGR.COM, the German keyboard driver. Hard disk installation via fdisk, format c: /s, sys c: and copy *.* C:.

(Die README Datei der Beta-Diskette 1) In 2024, in fact—and even with Microsoft's blessing—a beta version of MTMS-DOS (according to the README, based on MS-DOS 2.0) was released. The disk images can be found on Github.

Since the MTMS-DOS source code was no longer found in the Microsoft archive, which was actually being searched for, Microsoft decided instead to release the source code of the "normal" MS-DOS 4.00 as open source, which they found during their archive search.

They promised to supply the MTMS-DOS source code later, should they eventually find it.

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