Rediscovering Computing’s Core: Microsoft 6502 BASIC Opened

Microsoft has released the source code for its 6502-based BASIC interpreter—BASIC for 6502 Microprocessor Version 1.1—under the MIT licence, inviting developers, historians and retro-enthusiasts to explore one of the earliest instances of personal computer software. The 6,955 lines of assembly code, originally written by Bill Gates and Ric Weiland in 1976 and later refined in 1978 with garbage-collector fixes by John Feagans and Gates, can now be downloaded, modified and redistributed via GitHub.

This version powered the BASIC interpreters in pioneering home computers including the Apple II, Commodore PET, VIC-20 and Commodore 64. The 1977 license deal with Commodore, struck for a flat fee of US $25,000, positioned Microsoft’s BASIC at the heart of early personal computing.

Preservationists had long reconstructed and verified historical build environments, successfully recompiling byte-exact ROMs—creating a foundation for credible retro-development and emulator projects. Microsoft has now formalised that work with this official open-source release, which is Rediscovering Computing’s Core: Microsoft 6502 BASIC Opened, offering a modern licence for continued exploration.

The open release is part of Microsoft’s ongoing effort to preserve its software heritage—preceded by similar releases of GW-BASIC and MS-DOS. Enthusiasts have welcomed the move as another valuable artifact now legally accessible for study and innovation.

Interest remains strong. The MOS 6502 CPU, first launched in 1975, powered household staples ranging from the Atari 2600 and the Nintendo Entertainment System to the Commodore 64. Its blend of low cost, efficiency and simplicity ignited the home computer boom, and the newly open-sourced BASIC engine that ran atop it remains central to that legacy.

For hobbyists and emulator developers, the release unlocks opportunities to examine original code in full fidelity, port it to contemporary platforms using tools like cc65, reconstruct alternate implementations or integrate it into FPGA-based re-creations. The essence of Rediscovering Computing’s Core: Microsoft 6502 BASIC Opened lies in restoring direct hands-on access to a foundational piece of computing history.

The officially released Version 1.1 includes a clever Easter-egg—labels “STORDO” and “STORD0”—confirmed by Gates in 2010, serving as a playful signature embedded in decades-old code.