| Microsoft Open Sources 6502 Basic |
| Written by Lucy Black |
| Friday, 05 September 2025 |
|
Microsoft has released the source code of Basic 1.1 for the 6502 microprocessor on GitHub. The page with the code says the assembly language source code represents one of the most historically significant pieces of software from the early personal computer era. The 6502 was one of the most important processors in the seventies and eighties. It was the CPU for computers that people learned to program on, played games on and ran businesses on, including the Commodore 64, BBC Micro, Atari 2600, and Nintendo Entertainment System. As people have pointed out, even the Terminator was programmed in 6502.
The language that was most prevalent for this powerhouse was Microsoft Basic. This was based on an earlier version of Basic that was written for the Intel 8080 by Bill Gates and Paul Allen for the Altair 8800. This had been adapted for other 8-bit CPUs including the MOS 6502, Motorola 6800, and 6809. In 1976, a version was ported for the 6502 by Bill Gates and Ric Weiland. This was licensed by Commodore for a flat fee of $25,000, meaning the Commodore PET computers and, later, the VIC-20 and Commodore 64 ran Microsoft Basic. The version that Microsoft has now made available as open source is Basic M6502 8K VER 1.1. The source code also contains adaptations for the Apple II. The original headers still read, "BASIC M6502 8K VER 1.1 BY MICRO-SOFT"—a time capsule from 1978. The source code consists of 6,955 lines of assembly language, and is available under the MIT License, meaning it has free unrestricted use. Writing about the release on the Microsoft Open Source blog, Scott Hanselman, Vice President, Developer Community and Stacey Haffner, Director, Microsoft OSPO, said: "The version we are releasing here—labeled "1.1"—contains fixes to the garbage collector identified by Commodore and jointly implemented in 1978 by Commodore engineer John Feagans and Bill Gates, when Feagans traveled to Microsoft's Bellevue offices. This is the version that shipped as the PET's "BASIC V2." It even contains a playful Bill Gates Easter egg, hidden in the labels STORDO and STORD0, which Gates himself confirmed in 2010." The release acknowledges the fact that this code is still of interest, with a number of projects and historians including it in museum archives and creating emulators and recreating the code: "Over the years, dedicated preservationists have reconstructed build environments and verified that the historical source can still produce byte-exact ROMs. This open-source release builds on that work, now with a clear, modern license. It follows Microsoft's earlier release of GW-BASIC, which descended from the same lineage and shipped in the original IBM PC's ROM." The source code is available now on GitHub.
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