What Happens When Microsoft Open‑Sources Its 1976 6502 BASIC Interpreter?

For many years, unofficial copies of Microsoft’s 6502 BASIC have circulated online; now the software giant has released the code under an open‑source license.

6502 BASIC, one of Microsoft’s earliest software projects, was adapted in 1976 by co‑founder Bill Gates and early employee Rick Weiland, and runs on the 6502 CPU of Apple II, Atari 2600, Nintendo Entertainment System and Commodore 8‑bit series.

The open‑source release of Microsoft’s 6502 BASIC reminds us of the early days of coding, as the software helped standardize language implementation and allowed early developers on Commodore machines to learn programming by typing commands such as 10 PRINT “HELLO” and 20 GOTO 10.

In the blog post announcing the open‑source release, Microsoft explained that in 1977 Commodore paid a fixed fee of $25,000 for the license, making Microsoft BASIC the core of the Commodore PET and later the VIC‑20 and Commodore 64.

Although 6502 BASIC and other early Microsoft projects helped drive the development of MS‑DOS and established Microsoft’s software licensing model, interest in the early code and the MOS 6502 CPU remains strong fifty years later, with preservationists and hobbyists building FPGA recreations and emulators, and the code appearing in museum archives.

Microsoft said that dedicated preservationists have rebuilt the build environment over the years and verified that the historic source still generates a byte‑accurate ROM; the open‑source release builds on this work under a clear, modern license and continues the lineage of Microsoft’s early GW‑BASIC, which was included in the original IBM PC ROM.

If you are interested in exploring the assembly‑language source of Microsoft’s 6502 BASIC, the full code is now available on GitHub.

Author: 洛逸 6502 Basic: https://github.com/microsoft/BASIC-M6502 GWBase: https://github.com/microsoft/GW-BASIC