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ZDNET Highlights
- Microsoft will become the top dog of computing because of PC-DOS 1.00
- Microsoft continues to embrace open source.
- Source code and annotations provide information about the early days of the operating system.
Before “MicroSoft” became Microsoft, Bill Gates wrote BASIC interpreters. Microsoft’s first shipping operating system was a Unix distro called Xenix. Then, in 1980, Microsoft got its big opportunity: IBM needed an operating system for its planned IBM PC and asked Gates if he could provide one. you betcha! the rest is history.
Now, Microsoft releases source code and notes for PC-DOS 1.00First DOS release for the IBM PC.
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Microsoft’s AT&T Unix license did not allow the company to port Xenix to the x86 IBM PC. From then on, a very different world would be born with Unix as the top desktop operating system. In another reality, Linus Torvalds could have been Microsoft Unix CTO.
Microsoft purchased 86‑DOS for $100,000
In the real world, Gates and company had to create an operating system as quickly as possible. He didn’t have time to develop his own, so he purchased 86‑DOS, aka QDOS, from Seattle Computer Products and its inventor, Tim Patterson, for just under $100,000. What a steal! DOS would become the program that would lead Microsoft on its path to becoming one of the top companies in the technology industry for the next 50 years and beyond.
IBM wanted a CP/M-like operating system, but CP/M owner Digital Research faltered, so Big Blue turned to Microsoft. Microsoft adapted 86-DOS, with a CP/M‑style application programming interface (API), which IBM shipped as PC‑DOS 1.0 in August 1981. Microsoft retained the right to sell it as MS‑DOS if there were other PC‑compatible manufacturers. This would set the stage for Microsoft’s dominance after 1981.
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However, it was quite a big bet at the time. That first release was extremely limited by modern standards. It ran from a 160KB floppy disk but provided no subdirectory or hard-disk support. Nevertheless, it became the foundation of the MS‑DOS line that dominated PC operating systems in the 1980s and early 1990s.
Until now, the earliest DOS sources widely accessible to developers were MS‑DOS 1.25 and 2.0, which Microsoft originally released through the Computer History Museum in 2014 and then republished on GitHub in 2018. Those GitHub releases, along with the recent publication of the joint Microsoft-IBM MS‑DOS 4.00 sources, indicated that Microsoft was increasingly comfortable with how it treated its once-proprietary DOS code. As an educational and historical resource.
When Microsoft and the Computer History Museum first published an early MS‑DOS source in 2014, it came under a strictly restrictive license, which permitted only “non-commercial research, experimentation, and educational purposes” and explicitly prohibited re-use in other projects. That approach made the code readable but not actually usable. Subsequent GitHub re-releases of MS‑DOS 1.25 and 2.0 under the MIT License changed this, adopting a permissive license that the Free Software Foundation describes as GPL‑compliant and allowing nearly unrestricted reuse, modification, and redistribution.
Bring in DOS 1.0 in the same license The story takes place at the beginning of the PC era. Instead of being stuck in a repository, the code is now a browsable Git tree. With this code, systems programmers, teachers, and retrocomputing fans can clone, build, and experiment using the contemporary toolchain.
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It’s not just the DOS source code that Microsoft is sharing. Microsoft explained, “These materials are not just operating system releases in the traditional sense. In many cases, the listings represent point-in-time working states and hand-written notes preserved by Tim Paterson himself. Think of them as the printed commit history of a Git repository.”
How were operating systems developed?
Nobody will use these releases for real work. However, they are still remarkably instructive for those who want to understand how operating systems were structured on the first generation 8086 hardware. DOS 1.0’s small size and feature limitations make it an understandable codebase that can be understood almost from beginning to end, especially compared to today’s massive operating systems.
As Microsoft stated, “The listings include the sources of the x86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel, and some well-known utilities like CHKDSK. Not only were these assembler listings, but also listings of the assemblers themselves! This work provides rare insight into how MS-DOS/PC-DOS came to be, and how the operating system was developed at the time, as it was not rebuilt later. Gone.”
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Open-sourcing also helps clarify long-standing versioning questions. No MS‑DOS 1.0 product was ever sold under that exact name, and historians have had to reconcile IBM’s PC‑DOS 1.0, internal Microsoft version numbers, and OEM releases such as MS‑DOS 1.25. Having a clearly labeled DOS 1.0 code drop, dating back to the original IBM PC era, gives researchers a solid reference point for that tangle of early DOS builds.
So, if you want to enjoy the past, give the code a try. If nothing else, it will help you realize what a long, strange journey it has been from the early days of the PC to today’s world, where you have more computing power in your pocket than Gates and an entire company combined.
