Microsoft Open Sources 6502 BASIC
- Paul Thurrott
- Sep 04, 2025
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Microsoft announced that it has open sourced the source code for 6502 BASIC, one of first ports of its original BASIC. 6502 BASIC was written by Microsoft co-founder Bill Gates and Ric Weiland in 1976, and it was licensed to Commodore for a flat $25,000 fee that remains a unique deal in Microsoft history.
“This is BASIC M6502 8K VER 1.1, the 6502 BASIC lineage that powered an era of home computing and formed the foundation of Commodore BASIC in the PET, VIC-20, and the legendary Commodore 64,” Microsoft’s Scott Hanselman and Stacey Haffner write. “This very source tree also contains adaptations for the Apple II (“Applesoft BASIC”), built from the same core BASIC source. The original headers still read, “BASIC M6502 8K VER 1.1 BY MICRO-SOFT”—a time capsule from 1978.”
As many know all too well, Gates and partner Paul Allen wrote the original Microsoft BASIC, called Altair BASIC, for the first personal computer in 1975. This led to the creation of the company and to subsequent ports of the language to 8-bit processors that were popular at the time, like the MOS Technologies 6502, Motorola 6800, and Motorola 6809. But none had as much impact as the 6502 port, as it became central to the user experience enjoyed by millions via the Commodore PET, VIC-20, and C-64.
The version Microsoft is open sourcing is 1.1, as it contains fixes to the garbage collector, and it was called BASIC V2 on Commodore computers like the PET. It includes a fun Easter Egg secretly added by Bill Gates. And this marks the first time it’s been released with a “clear, modern” license.
This release follows the open sourcing of GW-BASIC, a later implementation for the IBM PC that later evolved into Quick BASIC and then Visual Basic, Altair BASIC, and MS-DOS 4.0.