Microsoft’s 6502 BASIC Goes Open Source

Microsoft has officially open-sourced its 6502 BASIC, the interpreter that powered early home computers. For decades, fans only had fragments and unofficial copies. Now, the original code is finally available under a modern license.

For the uninitiated, this release dates back to the early days of Microsoft. In 1975, Bill Gates and Paul Allen built their first BASIC interpreter for the Altair 8800. That foundation was soon adapted for the MOS 6502, a processor that became central to personal computing.

By 1977, Commodore licensed the interpreter for just $25,000, embedding it into the PET, VIC-20, and the iconic Commodore 64. Generations of programmers learned by typing loops like “10 PRINT HELLO.”

The version published today is BASIC M6502 8K Version 1.1. It includes bug fixes written in 1978 by Gates and Commodore engineer John Feagans. There’s even a playful Easter egg from Gates inside the code, preserved for nearly five decades.

So, why did Microsoft release it now, you may ask, right? Well, MOS 6502 powered not only Commodore’s lineup but also the Apple II, Atari 2600, and Nintendo’s NES. Its simplicity keeps attracting educators, hobbyists, and hardware tinkerers. FPGA recreations, emulators, and retro projects remain active, while Commodore has even announced a new FPGA-powered Commodore 64.

Preservationists have rebuilt this source over the years, proving it can still generate exact ROMs. Moreover, Microsoft’s decision builds on that work and follows its earlier open-sourcing of GW-BASIC, an ancestor of QBASIC and Visual Basic.