Microsoft open-sources "the earliest DOS source code discovered to date"

It is rare that I say this but, thanks MS! Arguably just as, if not more, important is the BASIC that they wrote. That was what they actually wanted to do. DOS just got them the contract with IBM. For decades MS was really a developer tools company with a side biz of writing operating systems and other misc software. They also open sourced that BASIC code too [1].

[1] https://opensource.microsoft.com/blog/2025/09/03/microsoft-o...

I cannot describe to you how jealous I am of the fact that back then writing a few thousand lines of assembly was what it took to launch a successful software company.

Discussion, on the source, at the time (79 points, 24 days ago, 19 comments) https://news.ycombinator.com/item?id=47957494

Or on the GitHub clone (162 points, 15 comments) https://news.ycombinator.com/item?id=47946813

wow, they had to OCR it back in from paper printouts

> This source code is old enough that it hadn’t been stored digitally. “A dedicated team of historians and preservationists led by Yufeng Gao and Rich Cini,” calling itself the “DOS Disassembly Group,” painstakingly transcribed and scanned in code from paper printouts provided by Paterson. This process was made even more difficult because modern OCR software struggled with the quality of the decades-old printout.

Interesting story of how MS got into the operating system business. IBM wanted the CPM operating system, but Digital Research wouldn’t sign ibms NDA… really a pivot point in computing history.

From “Triumph of the Nerds” tv transcript:

https://www.pbs.org/nerds/part2.html

Jack Sams (IBM) was looking for a package from Microsoft containing both the BASIC computer language and an Operating System. But IBM hadn't done their homework.

Steve Ballmer: They thought we had an operating system. Because we had this Soft Card product that had CPM on it, they thought we could licence them CPM for this new personal computer they told us they wanted to do, and we said well, no, we're not in that business.

Jack Sams (IBM); When we discovered we didn't have - he didn't have the rights to do that and that it was not...he said but I think it's ready, I think that Gary's got it ready to go. So I said well, there's no time like the present, call up Gary.

Steve Ballmer: And so Bill right there with them in the room called Gary Kildall at Digital Research and said Gary, I'm sending some guys down…. Treat them right, they're important guys.

Looking through the source is a great reminder of how constrained early computing was. It's amazing how much of this architecture still influences modern systems.

I wonder how long it'll be before they release the source for the earliest Windows versions. The fact that they still have the source for this very old DOS at least gives hope that they also do for old Windows.

Recent and related:

Microsoft open sources DOS 1.00 on 45th anniversary - https://news.ycombinator.com/item?id=47957494 - April 2026 (19 comments)

While oldest source of it, note that the 86-DOS v0.1-C binaries are even earlier (and v0.34 has also been found) than this v1.00 source and can be downloaded and used in an emulator. :-)

https://arstechnica.com/gadgets/2024/01/the-oldest-known-ver...

It is wonderful how early years of modern computing was brilliant. We treated machines as they really are: machines. Performance, creativity, science..., all possible to make a 386 machine work. Nowadays is all about libraries, virtualization, [bad] code over [bad] code over [bad] code..., I dont like it.

At some point, we'll probably have a new field in history for digital archeology, and I'm really envious for those future historians! They'll be getting to sleuth around old datasets, trying to reconstruct the history of computing, understand long-forgotten file formats to preserve data, use statistical methods to analyse binary backups, and trace for specific documentation versions to crack old encryption formats...

Related ongoing thread:

Microsoft's 6502 BASIC is now Open Source (2025) - https://news.ycombinator.com/item?id=48257058

Fascinating piece of computing history. Preserving early DOS source code gives a lot of context to the structural choices that stuck around in x86 architecture for decades.

Time to find vulnerabilities!

I remember in the naughts, coming across a dos machine that was quite out of time… even for the university basement it was living in next to a pile of lead brick. Its only job was to run an instrument via an home-built ISA card and write data out to 5.25” floppies.

What uses would this code have in 2026?

THANK YOU!

Can we now have all the Infocom games owned by Activision (which is yours) now? Pretty please? I know the source is available, but we'd like them with a MIT license (including the manuals, artwork etc).

PS: a couple of them could be harder, like Shogun, but it's okay to skip these.

I’m sure this is better software than Windows Millenium Edition

readable plain text plus boring metadata still ages better than most clever archival systems

More code that copilot can be trained on.

How about Microsoft fixes npm, github, and vscode

Back when it was all written by hand and optimized well.

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Too little, too late.

in the words of mr. mitch-hedburg “here, you throw this away“

I'm wondering whether ReactOS can exploit Claude et. al. to their fullest and "recreate" Windows 2000/95. I may donate some tokens for that cause.