Microsoft opens source historic 6502 BASIC interpreter powering Commodore legacy
The release of version 1.1, featuring 1978 garbage collector fixes, marks a significant milestone in retro-computing preservation and follows Microsoft’s previous open-source initiatives for MS-DOS and GW-BASIC.
Microsoft has officially released the source code for its 6502 BASIC interpreter under a formal open-source licence, marking the first time the historic software has been made available through such a channel. For decades, fragments and unofficial copies of the code circulated online, mirrored on retrocomputing sites and preserved in museum archives, but this release provides a definitive, licensed version for the first time.
Originally developed in 1976 by Bill Gates and Ric Weiland, the interpreter was adapted to run on the MOS 6502 processor, a chip that powered a generation of iconic home computers. In 1977, Commodore licensed the code for a flat fee of $25,000, a deal that placed Microsoft BASIC at the core of the Commodore PET, VIC-20, and the legendary Commodore 64. This decision helped millions of new programmers learn to code during the early days of home computing.
The specific version released is labelled 1.1, which includes garbage collector fixes implemented in 1978. These fixes were jointly developed by Gates and Commodore engineer John Feagans, who travelled to Microsoft’s Bellevue offices to collaborate on the updates. The original headers in the code read "BASIC M6502 8K VER 1.1 BY MICRO-SOFT", dating the iteration to that period.
This source tree also contains adaptations for the Apple II, known as Applesoft BASIC, built from the same core source. The release builds upon preservation work by Michael Steil, who documented the build process and ported the code to modern assemblers like cc65. Steil’s efforts allowed the historical source to produce byte-exact ROMs, a feat now supported by Microsoft’s clear licensing.
The announcement coincides with a resurgence in interest in retro-computing, including FPGA-based recreations and the recent announcement of a new FPGA-powered Commodore 64. This initiative follows Microsoft’s previous open-source releases of MS-DOS and GW-BASIC, continuing a tradition of sharing foundational code. On the 45th anniversary of 86-DOS 1.00, Microsoft also released the earliest DOS source code discovered to date.
