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

https://arstechnica.com/gadgets/2026/04/microsoft-open-sources-the-earliest-dos-source-code-discovered-to-date/

517 197 2 months ago

https://opensource.microsoft.com/blog/2026/04/28/continuing-...

dang2• 2 months ago

Recent and related:

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

gnabgib• 2 months ago

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

userbinator2• 2 months ago

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.

teamsolid2• 2 months ago

I am sure that there is a lot good material to take inspiration and learning even from the early Windows 3.11.

SoftTalker4• 2 months ago

/s ?

mycall• 2 months ago

Do a deep dive into how OS/360 formalized to having DOS.

throwaway27448• 2 months ago

They waited a couple decades too long for this to be of interest.

GaryBluto• 2 months ago

The day they would make Windows 2000 codebase open source (or source available) would be the day I could die happy (although I'd probably be long dead anyways by the time there's a glimmerof chance of it happening). What a beautiful, smooth-running operating system it was.

londons_explore• 2 months ago

There is a mostly complete leak of it...

optymizer• 2 months ago

Agreed. It's still my favorite Windows version.

NitpickLawyer• 2 months ago

Wasn't there a 2000 source leak a while ago? I remember some exploits coming out after the leak.

greenbit7• 2 months ago

Except for "the hive". Remember the hive? Sort of an alternate registry, in addition to the actual registry. Granted, it was pretty invisible, until it got corrupted.

I had a win2k machine that was my daily (at home) that was fine until idk about 2006, at which point something happened (muons?) and it would go into some kind of panic state just after bringing up the desktop. Hive corruption. I tried on and off for a couple of years to repair it, no luck. It wasn't just about the files on the HD, it was easy enough to transplant the drive and read/write anything, it was that I really liked the way I had the environment configured. Sure, it was all kind of moot, but it became a kind of personal windmill to resurrect this old thing. In the end, I booted an XP CD in it, and selected 'upgrade', and voila, it was Duncan Idaho, back from the dead.

Anyway.. loved win2k, but not a fan of the hive.

ndiddy2• 2 months ago

They will never release the code for anything that new because at that point, there's tons of licensed third-party code and the codebase is so large that going through everything to verify ownership would not be feasible. The code to NT 4 and XP have been leaked though.

ThrowawayB7• 2 months ago

And what would you do with the Windows 2000 source? It's 32 bit x86 code and the driver model for Windows has changed significantly since those days so it wouldn't run on modern x86-64 hardware anyway. Maybe it would run in a VM but I wouldn't care to bet on it.

fredoralive2• 2 months ago

I suspect Windows 2000 (and NT based stuff in general) is too near to modern Windows to be (officially) released. DOS is long dead, but modern Windows still uses the NT kernel and Win32 and so on, and they probably don't want to give an official peek behind the curtain, even if it's an over a quarter century old version.

protocolture• 2 months ago

I imagine its not far off. I get the impression they are almost done with windows as a platform.

WalterBright6• 2 months ago

It shouldn't be hard to disassemble it.

teamsolid2• 2 months ago

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.

dhosek• 2 months ago

I sometimes think that my mental model of a computer is still an Apple ][+ with 48K of RAM leads to my writing better code.

stevesimmons• 2 months ago

And mine is a Commodore Vic-20 circa 1981, with 3583 bytes of free RAM. Programmed in 6502 assembler. Can't get much closer to the CPU than that.

WalterBright6• 2 months ago

While I did a few 10 line programs in BASIC in high school on punch cards, when things really started was a freshman class on semiconductors. The class started with diodes and quantum mechanics, then onto transistors, then flip flops, then registers, then ALUs. Then it was on to designing/building a digital clock (which never worked right), and later designing/building/programming single board computers (6802 chip).

It was fun knowing everything about a computer. That's long gone!

aenis3• 2 months ago

For a very long while now, we had programmers who never understood any low level concepts at all. They have started with js or python, and never looked 'down'. There are no limits to monstrosities they will consider normal.

Linus Torvalds, a few months ago, said something to this effect when discussing AI coding tools. That his (also, mine) generation was lucky to have started with low level stuff and managed to retain the understanding of the whole stack - and kids these days don't get that. Good luck acquiring this level of feel for computers, algorithms, data structures today, when a kid's first experience with coding will be a seemingly genius chatbot.

charcircuit• 2 months ago

and managed to retain the understanding of the whole stack

No one understands the whole stack. There is too much specialized information.

goodpoint• 2 months ago

DOS and brilliant in the same sentence...

signa11• 2 months ago

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

TedDoesntTalk• 2 months ago

He could have sold those printouts instead of giving them away.

froyooh• 2 months ago

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

imoverclocked2• 2 months ago

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?

FarmerPotato4• 2 months ago

To see what decisions they made. Like any historical document. Aim to understand the people of the time.

yjftsjthsd-h• 2 months ago

It's a single user OS that runs everything in ring zero by design. I'm not sure, definitionally, that it can have security vulnerabilities. I... guess maybe code execution on exposure to an untrusted floppy disk filesystem?

aflag2• 2 months ago

Even then, what would that accomplish in most systems? That same disk would most likely be the only permanent storage available to the system when it's inserted. Maybe if you've got two drives and have two floppies inserted at the same time?

greenbit7• 2 months ago

Look closely, you'll notice there's no network interface. The only vulnerability in a system like that is physical access by malicious individuals.

About the worst mal-ware it can have is a boot sector that installs a "terminate, stay resident" (TSR) that copies itself onto any floppy that gets inserted.

locusofself• 2 months ago

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.

SoftTalker4• 2 months ago

Yet another case where text printed on paper outlived any digital storage.

jshier• 2 months ago

Seems like it was never digitally stored in the first place, and the printed text was barely readable due to age. Not really a big win for paper.

petcat• 2 months ago

struggled with the quality of the decades-old printout.

barely

It sounds like this printout has deteriorated badly and was barely readable.

FarmerPotato4• 2 months ago

I'd like to hear more about what works in OCR of dot-matrix fonts.

I've been able to OCR letter-quality printer output to 97% (mostly Os and Xs problems).

But it seems that machine-learning text-recognition is also now biased to reject computer code because it doesn't look like human language.

embedding-shape3• 2 months ago

Boring reply perhaps, but I've had wild success with adding even a tiny LLM afterwards to do "fixups" over OCRd text, works great for the typical O/0 issues and similar, just pass it the scrambled OCRd text together with the text around it, and even dumb and tiny 7b models running on CPU do a pretty fine job.

bob778• 2 months ago

ABBYY has a specific module for dot matrix printouts so I’m surprised it was a struggle for them but every document is different

ndiddy2• 2 months ago

There's a writeup here from one of the people on the team about the work it took to go from the listings to source code. http://cini.classiccmp.org/recoveryblog.htm

With less-than-satisfactory OCR output, I resorted to a process I used many years ago when converting scans made of old Commodore ROM dumps printed on a Commodore 1515 dot-matrix printer. The process relies on the ASCII OCR output having the same repetitive errors. "B" and "8", "S" and "5" are good examples, as are "l" and "1", and "O" and "0". There are many other similar single-character errors and, when working with x86 code, there are similar errors with instructions like "MOV". This process naturally works better if the output file is monolithic rather than single-page OCR conversions because you can do substitutions across the entire converted printout and not 75 separate files.
The next formatting hassle was the spacing. This required repetitive substitutions of a descending numbers of spaces to tabs (i.e., replace 8 spaces with a tab, 7, 6, etc.). Then if you want to return it to fixed spaces (which is likely how the original printer printed it -- spaces and not vertical tabs), you can. For pure re-creation work, spaces produce absolute column formatting while tabs can move around depending on the program displaying the file.
Once you run thought the 15 or so common global substitutions and tab conversion, it's a lot easier to work with the file to fix formatting and perform other cleanup. This is then followed by a line-by-line comparison against the original printouts. Overall I'd say the conversion output quality with this method is very good.

WalterBright6• 2 months ago

I've recovered some ancient software I wrote via scanning in listings I found among my dad's papers.

jmward01• 2 months ago

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...

vee-kay• 2 months ago

What's interesting is that Microsoft BASIC itself was derived from BASIC-PLUS which itself was derived from Dartmouth BASIC (which evolved into a structured programming language called SBASIC (Structured BASIC). But the popularity of Microsoft BASIC, actually halted the standardisation of SBASIC as an ANSI standard.

https://en.wikipedia.org/wiki/Microsoft_BASIC

The Altair BASIC interpreter was developed by Microsoft founders Paul Allen and Bill Gates using a self-written Intel 8080 emulator running on a PDP-10 minicomputer.[1] The MS dialect is patterned on Digital Equipment Corporation's BASIC-PLUS on the PDP-10, which Gates had used in high school.

https://en.wikipedia.org/wiki/Dartmouth_BASIC

Dartmouth BASIC is the original version of the BASIC programming language. It was designed by two professors at Dartmouth College, John G. Kemeny and Thomas E. Kurtz. With the underlying Dartmouth Time-Sharing System (DTSS), it offered an interactive programming environment to all undergraduates as well as the larger university community.

Dartmouth also introduced a dramatically updated version known as Structured BASIC (or SBASIC) in 1975, which added various structured programming concepts. SBASIC formed the basis of the American National Standards Institute (ANSI) "Standard BASIC" efforts in the early 1980s.

In contrast to the Dartmouth compilers, most other BASICs were written as interpreters. This decision allowed them to run in the limited main memory of early microcomputers. Microsoft's Altair BASIC is one example: it was designed to run in only 4 KB of memory (interestingly, it was delivered on paper tape).

Kemeny became involved in an effort to produce an ANSI standard BASIC in an attempt to bring together the many small variations of the language that had developed through the late 1960s and early 1970s. This effort initially focused on a system known as Minimal BASIC that was similar to earliest versions of Dartmouth BASIC, while later work was aimed at a Full BASIC that was essentially SBASIC with various extensions.

But by the late 1980s, tens of millions of home computers were running some variant of the MS BASIC interpreter. It had become the de facto standard for BASIC, which eventually led to the abandonment of the ANSI SBASIC efforts.

Kemeny and Kurtz, however, decided to continue their efforts to introduce the concepts from SBASIC and the ANSI Standard BASIC efforts. This became True BASIC.

https://en.wikipedia.org/wiki/True_BASIC

There are versions of the True BASIC compiler for MS-DOS, Microsoft Windows, and Classic Mac OS. At one time, versions for TRS-80 Color Computer, Amiga and Atari ST computers were offered, as well as a UNIX command-line compiler.

After several years of inactivity, as of February 2026, the TrueBASIC website is officially closed.

BobbyTables22• 2 months ago

Ah, the good ol’ “Embrace, Extend, … Extinguish”

dboreham2• 2 months ago

Nit: the pdp-10 is generally considered a mainframe not a minicomputer.

pjmlp2• 2 months ago

Kudos for the compilers reference, this is something that many to this day aren't aware, thinking that BASIC started as interpreted and compilers came later, when it was actually an engineering decision to make it fit into microcomputers.

ramon156• 2 months ago

I dont think I've ever seen a commit that says "49 years ago". Damn.

formerly_proven• 2 months ago

https://github.com/dspinellis/unix-history-repo

pletnes• 2 months ago

1968 https://www.netlib.org/go/fft.f

RobotToaster• 2 months ago

Not quite as old, but brl-cad is still in active development and has commits from 1983. https://github.com/BRL-CAD/brlcad/graphs/contributors?all=1

mapM• 2 months ago

It's funny that the .gitignore is marked as being from 49 years ago ...

nextaccountic2• 2 months ago

Even the README here is (erroneously?) timestamped as 1978 https://github.com/microsoft/BASIC-M6502

Which is unlikely since it seems to be written by AI

steve1977• 2 months ago

I remember when I realized I had been using Microsoft all along through my Commodore 64.

jimt1234• 2 months ago

The Commodore 64 got me started in computers, back when I was in grade school. Countless late nights programming in BASIC. All this time I never knew BASIC was licensed from Microsoft. Thanks!

"Commodore licensed BASIC from Microsoft in 1977 on a 'pay once, no royalties' basis after Jack Tramiel turned down Bill Gates' offer of a $3 per unit fee, stating, 'I'm already married,' and would pay no more than $25,000 for a perpetual license."

https://en.wikipedia.org/wiki/Commodore_BASIC

dooosss• 2 months ago

Too little, too late.

gnarlouse• 2 months ago

How about Microsoft fixes npm, github, and vscode

theanonymousone2• 2 months ago

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.

CursedSilicon• 2 months ago

That sounds like a terrifying legal minefield that they would not want to tread

theanonymousone2• 2 months ago

Is it not safe to assume Window source code is not present in the LLM training data?

leni536• 2 months ago

But surely anything the LLM outputs is clear of licensing requirements /s

Or would Microsoft like to argue otherwise in court?

xandrius2• 2 months ago

Slap a fair use on it and call it a day.

leobuskin• 2 months ago

I've used Claude to fix/reconstruct & build leaked Win2k3 on Linux with original toolchain via Wine. This approach included full gdi sources reconstruction. I just don't know what to do with this, it's kinda difficult to "wash" on this scale

hulitu• 2 months ago

Run it as your daily driver and trust your data to it. /s

okandship• 2 months ago

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

xandrius2• 2 months ago

In this case a paper printout.

hackerqwe• 2 months ago

More code that copilot can be trained on.

nananana9• 2 months ago

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.

avadodin2• 2 months ago

More than a few people would rather die in poverty than put in the effort today even if you offered to time-machine them back with their finished product.

yokoprime• 2 months ago

To be fair, i think you needed a cutthroat businessman leading the company. Which i guess is more or less the same today

justsomehnguy3• 2 months ago

This too but early MS to their employs was closer to a hipster SV vibe coding in a coffee shop a decade ago.

themafia• 2 months ago

a cutthroat businessman leading the company

I'm sure his family connections aided him significantly.

greenbit7• 2 months ago

And for such simple processors and systems no less! No descriptor tables to deal with, no memory management to configure. These days it takes a little processor inside the main processor, just to get things started. Those were golden times.

embedding-shape3• 2 months ago

Replace Assembly with TypeScript/Rust/Go/whatever and as long as the idea is good and useful, same thing applies today.

risyachka• 2 months ago

Except the competition was essentially non existent and no one would copy your product with llm in a day

curiousObject• 2 months ago

writing a few thousand lines of assembly was what it took to launch a successful software company.

Yes, but that assembly was not DOS, and it wasn’t easy.

Microsoft purchased the DOS code, they didn’t write it. Of course, they did develop and modify DOS. But that was a clever (and lucky) business deal, not a technological accomplishment.

The real beginning of Microsoft was earlier, with Allen, Gates and Davidoff writing the Altair BASIC interpreter. That was a serious achievement.

They had never seen the computer they were writing that assembly code for. They did not even own any computers. It took them 8 weeks on a university computer they were not supposed to be using for that

“Altair agreed to meet them to possibly buy a BASIC interpreter… Gates and Allen had neither a BASIC interpreter nor even an Altair system on which to develop and test one. However, Allen had written an Intel 8008 emulator that ran on a PDP-10 time-sharing computer. Allen adapted this emulator based on the Altair programmer guide, and they developed and tested the interpreter on Harvard's PDP-10.

The finished interpreter, including its own I/O system and line editor, fit in only four kilobytes of memory, leaving plenty of room for the interpreted program. In preparation for the demo, they stored the finished interpreter on a punched tape that the Altair could read, and Paul Allen flew to Albuquerque to meet with Altair…

While on final approach into the Albuquerque airport, Allen realized that they had forgotten to write a bootloader to read the tape into memory. Writing in 8080 machine language, Allen finished the program before the plane landed. Only when they loaded the program onto an Altair and saw a prompt asking for the system's memory size did Gates and Allen know that their interpreter worked on the Altair hardware.”

https://en.wikipedia.org/wiki/Altair_BASIC

BobbyTables22• 2 months ago

Imagine if the University had sued for their share of the IP and that was created using their resources…

It’s funny because I thought Jobs/Wozinak got their initial funding from selling phreaking boxes. And more recently, Anthropic engaged in criminal copyright violations with only a slap on the wrist.

Feels like a common theme of every “great” company having its origins from a “boost” resulting from criminal activity. (After all, that’s where the money is!)

Just imagine the criminal penalties possible for pirating and selling one copy of a movie or making one long distance phone call with phreaking.

Andrex• 2 months ago

My take-away is kind of the opposite. You literally have to be the right person in the right place at the right time.

Scroll_Swe2• 2 months ago

I find younger zoomers say this a lot and even to me in my 30s "oh you had it so easy!" because they think the knowledge today, we would have had back then. This is false.

Loser today, you would have been a loser back then too.

utopiah• 2 months ago

No, it wasn't the case then, it's not the case now :

"Beyond the Seattle area, Gates [Bill's mother] was appointed to the board of directors of the national United Way in 1980, becoming the first woman to lead it in 1983. Her tenure on the national board's executive committee is believed to have helped Microsoft, based in Seattle, at a crucial time. In 1980, she discussed her son's company with John Opel, a fellow committee member, and the chairman of International Business Machines Corporation (IBM). Opel, by some accounts, mentioned Mrs. Gates to other IBM executives. A few weeks later, IBM took a chance by hiring Microsoft, then a small software firm, to develop an operating system for its first personal computer." from https://en.wikipedia.org/wiki/Mary_Maxwell_Gates

It's as if your mother now was at the board of NVIDIA (because IBM then was quite the powerhouse). Sure, you still need to write "a few thousands lines of assembly" or equivalent... or find someone else who would, but you still need the connection to sell "it".

So... is your mom at such a famous board? Actually ANY board? If not you might have to write a LOT more and still a successful software company, sorry.

danborn262• 2 months ago

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.

jug• 2 months ago

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...

rvnx2• 2 months ago

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

acomjean2• 2 months ago

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.

bragr• 2 months ago

Eh, basically all facts in this story are disputed by all sides. Aside from general gist that there was some meeting that didn't go well.

chuckadams2• 2 months ago

Whether Kildall actually blew IBM off at that meeting or not, what was definitely the case was that CP/M didn't have a 16-bit version ready to meet IBM's schedule, and that's what ultimately took them out of the running.

9dev2• 2 months ago

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...

giobox• 2 months ago

This field already is alive and well in the gaming community. Games companies are notorious for not spending money on keeping their old code around, which is why it's been at the forefront of digital archaeology efforts a lot of the time to preserve the industry's history.

I'd also throw the wayback when machine and the internet archive into this bucket.

EvanAnderson• 2 months ago

The term "programmer-archaeologist" was coined by the author Vernor Vinge in his 1999 "A Deepness in the Sky"[0] (a pretty great read and definitely recommended) and the field is arguably a real thing now[1].

[0] https://en.wikipedia.org/wiki/A_Deepness_in_the_Sky

[1] https://en.wikipedia.org/wiki/Software_archaeology

9dev2• 2 months ago

Thank you for the recommendation! I'm definitely aware of many initiatives into that direction, but I'm still curious when it'll invariably arrive as a real discipline not in computer science, but humanities.

dang2• 2 months ago

Related ongoing thread:

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

gxd• 2 months ago

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.

danborn262• 2 months ago

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.

chorlton2080• 2 months ago

This is nearly a month old

KellyCriterion• 2 months ago

haha, rise your hand if you are getting old when reading this :-D

wpnx• 2 months ago

I got it up and running in dos box - it still runs smooth as butter: https://github.com/phyous/DOS-1.0

miohtama• 2 months ago

At this stage, was Microsoft already a well-established company if they had money and resources to contract and then license the 86-DOS?

fredoralive2• 2 months ago

Yes, Microsoft had established themselves as supplier of BASIC interpreters to most of the US microcomputer manufacturers. There initial contact with IBM was to provide a version of Microsoft BASIC for the new computer.

OrangeMusic• 2 months ago

Wait this is from April 30th. Pretty sure this was posted on HN then.

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

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