Microsoft has released the source code for its historic 6502 BASIC interpreter, marking the first time this foundational code is officially available under an open-source license. The interpreter began in 1975 as Microsoft's very first product, a BASIC for the Altair 8800 written by Bill Gates and Paul Allen. That codebase was adapted for multiple 8-bit CPUs including the MOS 6502, which Gates and Ric Weiland completed in 1976.

In 1977, Commodore licensed this 6502 BASIC for a flat fee of $25,000, placing Microsoft's interpreter at the heart of the PET computer and later the VIC-20 and Commodore 64. The released version, labeled "1.1," contains fixes from 1978 implemented by Commodore engineer John Feagans and Gates during Feagans' visit to Microsoft's Bellevue offices. This is the exact code that displayed the famous prompt: "BASIC M6502 8K VER 1.1 BY MICRO-SOFT".

The 6502 CPU powered iconic systems including the Apple II, Atari 2600, and Nintendo Entertainment System, making this release significant for retro computing enthusiasts. Modern developers can now study, modify, and build upon the interpreter that taught millions to program. The release builds on preservation work by Michael Steil and follows Microsoft's earlier GW-BASIC open-source release.