Microsoft's 6502 BASIC is now Open Source (2025)

https://opensource.microsoft.com/blog/2025/09/03/microsoft-open-source-historic-6502-basic/

by GTP • 2 months ago

92 29 2 months ago

opensource.microsoft.com

rhdunn2• 2 months ago

Ben Eater's 6502 series [1] uses MSBASIC for programming along with WozMon as the terminal interface.

[1] https://www.youtube.com/playlist?list=PLowKtXNTBypFbtuVMUVXN...

BeefySwain• 2 months ago

Is that the same BASIC as this?

rhdunn2• 2 months ago

From the video [1] that links to Ben Eater's fork with extensions and configuration specific to his 6502 breadboard computer [2]. That in turn is forked from `mist64/msbasic` which refers to a blog post [3] which states:

This episode of “Computer Archeology” is about reverse engineering eight different versions of Microsoft BASIC 6502 (Commodore, AppleSoft etc.), ...
This article also presents a set of assembly source files that can be made to compile into a byte exact copy of seven different versions of Microsoft BASIC, and lets you even create your own version.

So Ben Eater's version is based on a reverse engineered version of the same program. You should be able to adapt the code released here to run on Ben Eater's 6502 with a bit of work.

[1] https://www.youtube.com/watch?v=XlbPnihCM0E&list=PLowKtXNTBy...

[2] https://github.com/beneater/msbasic

[3] https://www.pagetable.com/?p=46

qingcharles2• 2 months ago

Sadly nothing in Scott's blog post about how they obtained the source. Was it still in Microsoft's archives? Did they happen upon some tractor-feed print-outs they had to type in by hand?

bdcravens2• 2 months ago

I assume today typing in by hand is no longer needed, with text parsing from images being table stakes for LLMs.

dhosek• 2 months ago

You don’t need an llm to do this.

qingcharles2• 2 months ago

No, there was a big post somewhere on the previous release two weeks ago of the DOS source showing just how incredibly hard of a challenge it was to properly input/OCR all the reams of source was.

This source is a lot smaller, but still annoying if they had to type it all in or OCR it.

chihuahua• 2 months ago

It would also be interesting why it was open-sourced now. I assume if they had done the same last year, the resulting loss of revenue would not have destroyed the plucky little $3T upstart.

urbnspacecowboy• 2 months ago

The source code Scott & co. released is word-for-word exactly the same as the pirated source code that's been floating around the Internet since at least 2009, and that pagetable.com wrote about in 2018:

https://www.pagetable.com/?p=774

This is what Microsoft slapped a license and an AI-slop README on. Sure, they can legally do that, they still own the copyright, but it's still pretty funny that they're essentially laundering pirated software.

The Zork I/II/III releases were even more blatant, with git commits adding license texts to existing pirate (or, if you prefer, archival) releases of the old source code:

https://github.com/historicalsource/zork1

https://github.com/historicalsource/zork2

https://github.com/historicalsource/zork3

It makes me wonder how many of Microsoft's other releases of old source code are laundered pirate releases, but I don't want to be too harsh. Microsoft's actions in these cases are considerably nicer than, say, what Warner Bros. did about pirated Mortal Kombat 2 source code (takedown), or Nintendo's continued legal hostility toward everyone everywhere. Maybe other companies could learn not to be so dog-in-the-manger about their precious Intellectual Property.

Smalltalker-80• 2 months ago

In 1979, I made a program called VisiBase in this BASIC. It's a visual database modeled after VisiCalc. That won me a joystick in at a competition by the local computer store. :-) Still have the source, that works in an Apple 2 emulator. It's 13 K in ASCII (untokenized).

homarp• 2 months ago

Please, put it in a public git somewhere!

rbanffy• 2 months ago

Maybe Apple can finally release MacBasic now that Microsoft can no longer stop licensing their Basic to the Apple II family.

WillAdams2• 2 months ago

I'll even send Bill Gates a dollar for that:

https://www.folklore.org/MacBasic.html

amichail2• 2 months ago

Do you think computing history would have been much different if Microsoft made a 6502 Pascal interpreter instead?

xxs• 2 months ago

Pascal is a lot broader language and won't fit in sub 16KB of ROM (even if you exclude monitor [call-151])

amichail2• 2 months ago

Maybe they could have implemented a useful subset of Pascal.

WillAdams2• 2 months ago

A subset of it?

I have a copy of "Tiny" Pascal by Supersoft from 1979 on a cassette tape which was licensed to Tandy Corp and which would load onto a 16KB TRS-80 Model III and allow a bit of room for programming.

One of the great regrets of my life is that when I was doing so and when it would have mattered, I was unaware of the patch for this which would have allowed it to be saved as an executable to a TRS-DOS disk....

SoftTalker• 2 months ago

They didn't invent the language. BASIC was already a popular language for beginners on microcomputers at that time.

bitwize2• 2 months ago

Microsoft itself popularized BASIC on microcomputers with its 8080 BASIC, starting on the Altair and ported to everything with A, B, C, D, E, H, and L registers since.

Before then, however, BASIC was already popular on minicomputers as both an introductory language for beginners and a business language; the various "Business BASIC" dialects providing a small-business alternative to COBOL on mainframes with their features for decimal math and ISAM database access.

dboreham• 2 months ago

Except that wasn't possible. Languages like BASIC and Forth exist because they were the only kind of language implementable in 4K with no disk. Pascal in its smallest form (UCSD p-system) still needed disk overlays. The smallest C compilers were poly phase, needing storage for intermediate state.

ofrzeta• 2 months ago

I am really torn about this. Sure Microsoft is doing a lot of open source today (.NET core, VS Code and a bit of historic curiosities such as this one) but the "open letter to the hobbyists" still stands :) Release the Windows source code then we are talking.

PxP_• 2 months ago

I doubt the .gitignore, README.md, and SECURITY.md files were created 49 years ago, as the GitHub repo indicates :D

zendist• 2 months ago

Ahead of their time ;-D

dang• 2 months ago

Previous discussion:

Microsoft BASIC for 6502 Microprocessor – Version 1.1 - https://news.ycombinator.com/item?id=45118392 - Sept 2025 (198 comments)

Related ongoing thread:

Microsoft open-sources "the earliest DOS source code discovered to date" - https://news.ycombinator.com/item?id=48253386 - May 2026 (110 comments)

rbobby• 2 months ago

The Apple ][ basic interpreter placed its "get next token" routine down in the first 255 bytes of the computer's memory. Being there allowed for the machine instructions to be fetched twice as fast as "high memory". And "get next token" is definitely a good guess for hotpath.

IcePic• 2 months ago

C64 also has a token parsing routine in zeropage.

timbit42• 2 months ago

I've benchmarked Applesoft and Commodore BASIC quite a bit and they benchmark exactly the same, so I'm not surprised. Both systems run at 1.023 MHz in NTSC. Of course, they are both derived from Microsoft's 6502 BASIC. I'm sure they're not cycle exact but they are close enough that BASIC doesn't show any difference.

Microsoft's 6502 BASIC is now Open Source (2025)

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