Microsoft open-sources the source code for Bill Gates' 48-year-old '6502 BASIC'

Microsoft released the source code for ' Microsoft BASIC for 6502 Microprocessor - Version 1.1 ' as open source on September 3, 2025. While the 6502 BASIC source code had previously been distributed in fragments on the Internet, this open source release makes the official source code, which Microsoft has maintained, available for anyone to view and edit.

Microsoft Releases Historic 6502 BASIC - Microsoft Open Source Blog
https://opensource.microsoft.com/blog/2025/09/03/microsoft-open-source-historic-6502-basic/

6502 BASIC was a BASIC interpreter that ran on the MOS 6502 and was developed by Microsoft founder Bill Gates and Microsoft's second employee, Rick Weiland. 6502 BASIC was completed in 1976, and in 1977, a licensing agreement was signed with Commodore for $25,000 (approximately 3.7 million yen). Microsoft explained, 'As a result of this agreement with Commodore, Microsoft BASIC became the core of Commodore computers, providing learning opportunities for millions of novice programmers.'

The newly open-sourced 'Microsoft BASIC for 6502 Microprocessor - Version 1.1' includes a garbage collector fix that was implemented jointly by Gates and Commodore's John Fingers in 1978. Fingers implemented this fix during a visit to Microsoft's headquarters.

The source code for 'Microsoft BASIC for 6502 Microprocessor - Version 1.1' is available at the following link. It is licensed under the MIT License, and anyone can freely download, modify, and redistribute it.

GitHub - microsoft/BASIC-M6502: Microsoft BASIC for 6502 Microprocessor - Version 1.1
https://github.com/microsoft/BASIC-M6502

Microsoft has also made other older products open source, and in April 2024, it released the source code for MS-DOS 4.0 under the MIT License.

Microsoft makes 'MS-DOS 4.0' open source - GIGAZINE