The technology age doesn’t feel like it’s much older than its twenties, but software stalwart Microsoft has just celebrated its 50th birthday. Besides its various conferences and events, former head Bill Gates has marked the occasion a little differently.

Gates, in a blog post, gave some background on the company’s earliest days, ending by offering up the original Microsoft operating system source code as a PDF download.

Happy birthday, Microsoft

There’s very little you can do with the code unless you intend to replicate an archaic operating system, but it is an interesting glance at the history of mass-market software. Originally written for the Intel-powered Altair 8800 (after Gates had promised Micro Instrumentation and Telemetry Systems’ CEO that the code was already written), it formed the basis for the foundation of Microsoft in the form it takes today.

According to Gates himself, “The code remains the coolest I’ve ever written to this day”.

It’s not an exaggeration to say that the source code led directly to the age of personal computers, though if Gates and Paul Allen hadn’t performed the task, someone else would have gotten around to it. Whether the world would look the same as it currently does is up for debate.

“It’s amazing to think about how this one piece of code led to half a century of innovation by Microsoft. Before there was Office, or Windows 95, or Xbox, or AI, there was the original source code — and I still get a kick out of seeing it, all these years later,” finished Gates.