Ever wondered what Microsoft looked like before Windows, Office, Xbox - or even MS-DOS?
Microsoft began with a tiny, mighty piece of software called Altair BASIC. Yup, that was Microsoft’s very first product.
And now? You can scroll through the original Microsoft source code — all 157 pages of it — thanks to the team at the Computer History Museum.
Altair BASIC: Microsoft’s First Product
Before Microsoft became a global tech giant, it was just two young coders — Bill Gates and Paul Allen — burning the midnight oil.
Their mission? Write a BASIC interpreter for a brand-new microcomputer called the Altair 8800.
And get this: the entire program had to fit into just 4 kilobytes of memory.
That’s smaller than most emojis.
The Birth of Microsoft (In a 4K World)
To celebrate 50 years of Microsoft, Gates shared a personal post looking back at the project that started it all.
He called the Altair BASIC source code the “coolest code I’ve ever written.”
And if you’ve ever tried cramming software into a 4K memory limit, you know that’s not just nostalgia talking.
Back in 1975, Gates, Allen, and Monte Davidoff built the interpreter for the Altair 8800 — a microcomputer featured on the cover of Popular Electronics.
Fun fact? They didn’t even have an Altair to test it on.
They wrote the entire thing on a PDP-10 mainframe, crossed their fingers, and sent Paul to demo it live at MITS.
Spoiler: it worked.
That moment landed them Microsoft’s first licensing deal — and the rest is history.
A 3-Person Team That Launched Microsoft
This wasn’t a huge dev team with layers of QA.
It was just:
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Bill Gates writing the core interpreter
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Paul Allen adapting it to Altair’s hardware
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Monte Davidoff building the floating-point math routines
They crammed the entire program — all written in assembly language — into 4KB.
Then Allen flew to MITS, the Altair’s manufacturer, demoed it, and...
It worked. First try.
That moment led to Microsoft’s first licensing deal.
That tiny piece of code became Microsoft’s first product.
“The Coolest Code I’ve Ever Written”
That’s how Gates describes this lean, mean, 157-page program written entirely in assembly language.
Altair BASIC wasn’t just some weekend project.
It was the code that launched Microsoft — and now, the original source code is available to the public, line by line.
Thanks again to the Computer History Museum for making it accessible online.
Gates Releases the Source Code
To celebrate 50 years of Microsoft, Gates made the entire Altair BASIC source code available to the public for the first time ever.
You can now explore all 157 pages of code on the Computer History Museum’s website.
And here’s why he did it:
“I hope people will find it as fascinating as I do.
It reminds me how far computing has come—and how exciting it is to keep building what’s next.”
Why BASIC? Why Altair?
Here’s the deal:
- BASIC was already popular among students and hobbyists.
- It was interactive — perfect for learning and experimenting.
- It ran well on minicomputers but hadn’t hit personal PCs yet.
Until the Altair 8800 came along.
When Gates saw it on the cover of Popular Electronics, he and Allen knew it was their chance.
They got to work coding the BASIC interpreter — without even touching the machine.
Yep. The whole thing was written and debugged remotely on a PDP-10.
Now You Can Explore the Original Microsoft Source Code
For the first time ever, the Altair BASIC source code is public.
You can now explore the original files — complete with retro assembly syntax and handwritten comments — over at the Computer History Museum website.
Gates hopes releasing the code will inspire a new generation of coders.
Because it shows what’s possible when you mix curiosity, creativity, and a whole lot of hustle.
A Tiny Team. A Huge Impact.
Only three people built this revolutionary software:
- Bill Gates wrote the interpreter.
- Paul Allen adapted it for Altair hardware.
- Monte Davidoff handled the math routines.
They squeezed every last byte into that 4K limit, flew to demo it, and nailed it on the first try.
That code became Microsoft’s first product.
That demo became Microsoft’s first deal.
That moment became Microsoft’s origin story.
Lessons From a Legacy
Gates says Altair BASIC taught him three things that still hold true:
- Be ruthlessly efficient with your code.
- Build tools that help people learn and create.
- A small, focused team can absolutely change the world.
In a world full of AI tools, massive frameworks, and endless abstraction, sometimes it’s refreshing to remember where it all started:
Just a few lines of clean, clever code.
Want to See the Code That Started It All?
Bill Gates has made it public.
👉 Check out the full story on GatesNotes
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