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...

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.

Recent and related:

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

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?

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.

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.

Too little, too late.

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

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