Microsoft has officially released the code for its 6502 BASIC version under an open-source license. According to the tech giant’s blog post published on Sept. 3, version 1.1 of its BASIC code for MOS 6502 is now available on GitHub under an MIT license.

The 6502 port became a core element of Commodore’s offerings, helping to popularize early home computers. It played a significant role during the initial onset of coding, as it enabled many of the first developers to learn programming through the Commodore machines and led to the standardization of programming language implementations.

After years of demand for copies of Microsoft’s 6502 BASIC, developers, historians, and retro-computing fanatics alike can explore the officially released code.

Back to the BASICs

The BASIC source code originally developed for the MOS 6502 processor was fundamental to the early era of home computing as the foundation of many of Commodore’s computers, including the PET, VIC-20, and Commodore 64 machines.

Microsoft’s first product, a BASIC interpreter for the Altair 8800 with the Intel 8080 processor, was developed beginning in 1975 using code written by Bill Gates and Paul Allen. Gates and Ric Weiland later adapted this codebase language to run on other 8-bit CPUs, such as the MOS 6502 processor, and it was later licensed by Commodore.

“In 1977, Commodore licensed it for a flat fee of $25,000, a deal that placed Microsoft BASIC at the heart of Commodore’s PET computers and, later, the VIC-20 and Commodore 64,” Microsoft’s blog post described.

Microsoft’s 6502 BASIC CPU powered other adaptations that developed from its core BASIC source, including the Apple II (“Applesoft BASIC”), Commodore 8-bit series, Nintendo Entertainment System, and Atari 2600.

Microsoft’s version 1.1 contains amendments implemented to the original version in 1978, then shipped as the PET’s “BASIC V2.” The code consists of 6,955 lines of assembler and includes functions for 8-bit systems and support for several classic computers, such as the Commodore PET, Apple II, MOS KIM-1, and Ohio Scientific.

In April 2025 during Microsoft’s 50th anniversary celebrations and reflections, Bill Gates shared details about the company’s original source code and more about the genesis of what became a tech giant.