Microsoft, founded in April 1975, is celebrating 50 years this Friday. The company’s journey began with a pivotal computer code written by Bill Gates, then a Harvard freshman, and his late friend Paul Allen. Gates recently reflected on the moment in a blog post, calling it a key milestone in personal computing history.

A breakthrough in computing

The inspiration for Microsoft came from a 1975 Popular Electronics magazine article about the Altair 8800, a minicomputer powered by a microchip from Intel. Gates and Allen saw an opportunity and contacted Altair’s manufacturer, Micro Instrumentation and Telemetry Systems (MITS), offering software that would allow users to control the hardware. However, they had not yet developed the promised program.

Using the BASIC programming language, first developed at Dartmouth College in 1964, the duo worked tirelessly for two months without access to an Altair prototype. Eventually, Gates completed the software, which became the foundation for Altair’s first operating system.

The start of a revolution

“That code remains the coolest I’ve ever written,” Gates wrote in his blog post, where he also made the original program available for download. The software laid the groundwork for Microsoft’s growth, leading to the creation of Windows, Word, Excel, and PowerPoint—programs that dominate personal computing today.

“That was the revolution,” Gates said in an accompanying video. “That was the thing that ushered in personal computing.”

A nostalgic look back

As he approaches his 70th birthday in October, Gates has been reflecting on his early years. In February, he released a memoir detailing his childhood and career, including his complicated relationship with Apple co-founder Steve Jobs. This year also marks the 25th anniversary of the Bill & Melinda Gates Foundation, which he established after stepping down as Microsoft’s CEO in 2000.

Microsoft, now led by CEO Satya Nadella, has reached a market value of approximately $2.8 trillion. Gates, whose fortune is estimated at $108 billion, remains amazed by the journey.