Bill Gates just published the ‘coolest code’ he ever wrote for Microsoft

“The coolest code I’ve ever written.” With these words, Bill Gates introduces a blog post that celebrates Microsoft’s 50th anniversary by looking back on how the company got started.

At the bottom of the that blog post, Gates published the original source code that he so fondly remembers—the very same code that ultimately led to the growth and success of Microsoft. You can download the original Microsoft source code here (PDF).

Microsoft began with a magazine cover

In January 1975, Bill Gates and Microsoft co-founder Paul Allen read an article in the magazine Popular Electronics about the Altair 8800 home computer by a small company named MITS. “When Paul and I saw that cover, we knew two things: the PC revolution was imminent, and we wanted to get in on the ground floor,” recalls Gates.

The plan was to create software that would allow Altair 8800 owners to develop their own programs using the easy-to-learn BASIC programming language. As such, Gates and Allen decided to develop a BASIC interpreter for the Altair 8800.

The interpreter would convert BASIC commands into machine code that the Altair 8800 could understand. A maximum of 4 kilobytes was available for the interpreter so that Altair owners still had enough memory available to run the other programs.

Microsoft’s first software was created in two months — for a PC they didn’t own

As they didn’t have an Intel 8080 chip—the one in the Altair 8800—Allen developed a simulator for it, which ran on a Harvard PDP-10 mainframe. Meanwhile, Gates programmed the main code for the BASIC interpreter, and another programmer named Monte Davidoff was responsible for the code for the so-called “math package.”

After around two months, they had finished programming and presented the BASIC interpreter to MITS, who licensed the software. Altair BASIC was thus the first product of the newly founded company Micro-Soft. (The hyphen was only later removed from the company name.)

“It’s amazing to think how this one piece of code led to half a century of innovation at Microsoft. Before there was Office or Windows 95 or Xbox or AI, there was this source code—and I still enjoy looking at it, even all these years later,” says Gates.

Again, you can download the source code here (PDF).