Microsoft source code from 1975—handwritten, byte-optimized, and punched into the history books—has been made public by Bill Gates himself. Exactly 50 years after it helped spark the personal computing revolution, the code that launched Microsoft is now open for the world to study. It’s not nostalgia—it’s a rare look under the hood of how one of the world’s most powerful companies got its start, at a time when computing wasn’t personal, and software didn’t ship with splashy UIs.
Gates released the complete Altair BASIC source in April 2025 to commemorate Microsoft’s 50th anniversary. The announcement came with a scanned 157-page PDF of the original handwritten and printed listings. It’s the same code that Gates called “the coolest I’ve ever written”—not because of its length or complexity, but because of how much it achieved with so little. The entire interpreter had to run in 4 KB of RAM. There were no code libraries, no IDEs, no debugging consoles—just raw assembly, a lot of arithmetic, and the stubborn resolve of three young programmers.
Paul Allen, Gates’ childhood friend and co-founder, had spotted an article in Popular Electronics about a new microcomputer called the MITS Altair 8800. The machine had no screen, no keyboard, and no software. But it was cheap enough for hobbyists to buy, and that was enough to spark an idea. What if they could write a version of BASIC for it?
At the time, Gates was still a Harvard student. Allen was working at Honeywell. Monte Davidoff, who joined them to help with floating-point math, was a fellow Harvard student. None of them had even touched an Altair, but that didn’t stop them from building the interpreter. Allen wrote a simulator for the Altair on a Harvard mainframe, and they wrote and tested the code on that—blindly hoping it would work once installed on real hardware.
That gamble paid off. Allen flew to Albuquerque, demoed the code to MITS, and secured their first licensing deal. Microsoft (originally spelled “Micro-Soft”) was born. Gates dropped out of Harvard, and software—previously considered a footnote to hardware—suddenly became a business of its own.
The code itself reads like a time capsule. Labels, loops, jumps, and comments crammed into every byte. At one point, a single typo or misplaced instruction could crash the whole system. Yet what stands out is not just the technical efficiency, but the human fingerprints throughout. You see the decisions made under memory constraints, the clever hacks used to squeeze performance, and the absolute obsession with getting things right.
For anyone used to modern programming environments, this is the equivalent of looking at cave drawings. But those drawings built the foundation. Gates has long said that writing Altair BASIC taught him more about coding—and discipline—than any class. The project was a crash course in optimization, team coordination, and hardware emulation, all done before the personal computer industry even existed.
There’s a subtle sadness to this release too. Gates acknowledged the moment as bittersweet, noting that Paul Allen, who passed away in 2018, wasn’t around to celebrate it. Allen was the one who saw the potential in the Altair article, the one who convinced Gates to act quickly, and the one who demoed the code in person when it mattered most. Without him, Microsoft may never have existed at all.
The anniversary comes at a time when Microsoft is again in the spotlight—this time for its leadership in AI. But this code reminds us that it all began with a lean, efficient interpreter written in machine code, for a machine that most people had never heard of. Back then, there were no billion-dollar valuations, no venture capital firms lining up to invest. Just a few college students, one mainframe, and a lot of sleepless nights.
And yet, the software they wrote could teach today’s coders a thing or two. Especially about writing lean code that does a lot with very little. Altair BASIC wasn’t fast by modern standards, but it was user-friendly. It gave instant feedback, made it easier for hobbyists to experiment, and most importantly, it worked. That balance—between accessibility and power—is something even modern tools still struggle with.
Releasing this code isn’t just an act of historical preservation. It’s a challenge. A quiet message from Gates to today’s developers: look at what we built when we had nothing. No GitHub. No Stack Overflow. Just handwritten notes, hardware specs, and a deep belief that computers would change everything.
In a world flooded with frameworks, APIs, and AI-generated code, the original Microsoft source code cuts through the noise. It’s not optimized for SEO, scalability, or multithreading. It’s optimized for necessity. And that’s what makes it timeless.