You Can Now Download the Operating System That Inspired Windows

Would you forget the code you wrote years ago if it helped build the foundation for an operating system as massive as Windows is today? Bill Gates hasn’t. In fact, he’s decided to release it, and you can try it too.

Bill Gates Releases Altair BASIC’s Source Code

Microsoft is celebrating its 50th anniversary on Friday, April 4, 2025, and Bill Gates decided there’s no better way to mark the occasion than by returning to his roots and sharing the source code that started it all.

He took to his Gates Notes blog to release a 157-page document containing the source code for Altair BASIC, which was written in 1975 using assembly language. You can download the entire source code PDF for yourself by scrolling to the bottom of the blog page.

Altair BASIC, which Gates calls “the coolest code” he’s ever written, was inspired by an article in Popular Electronics magazine about the Altair 8800 microprocessor powered by Intel’s 8080 chip.

The article inspired Gates, who was a freshman at Harvard at the time, and Microsoft co-founder Paul Allen to contact Ed Roberts, the founder of Micro Instrumentation and Telemetry Systems (MITS), the Altair 8800’s manufacturer.

They claimed they had created a BASIC interpreter for the Altair 8800 using assembly language that would allow users to write their own programs in BASIC. This would allow users to write their own programs using the BASIC language that was easier to use than assembly code, making computers more accessible at the time. The catch? They were bluffing and hadn’t actually created what they claimed.

Related

10 Interesting Facts About Microsoft You Probably Didn’t Know

Microsoft is known worldwide, and yet there are still parts of the company that people know nothing about. Here are some of the rarer facts.

Over the next two months, they then worked day and night to create the software they’d promised. Allen wrote a program to simulate the Intel 8080 chip on a Harvard PDP-10 mainframe, which allowed them to test their software without needing an actual Altair. Meanwhile, Gates focused on writing the main code for the program, and Gates’ friend Monte Davidoff worked on the math package.

Eventually, they successfully created Altair BASIC. Once they presented it to Ed Roberts, MITS agreed to license the software, marking the first official product of Microsoft (then called Micro-Soft). In his blog post, Bill Gates acknowledged that, before there was Office, Windows 95, Xbox, or AI, there was Altair BASIC, something he’s still incredibly proud of today.