Microsoft open sources 6502 BASIC after almost 50 years

Microsoft has finally open-sourced one of its oldest products: 6502 BASIC. The source code for Microsoft BASIC Version 1.1 for the 6502 microprocessor is now available on the Redmond giant's GitHub repo, comprising 6,955 lines of code.

6502 BASIC is among the most historically significant pieces of software from Microsoft in the early computer era. It even pre-dates the decades-old MS-DOS and Windows, which became the company's identity in later years.

Microsoft has previously open-sourced the code for the GW-BASIC interpreter originally released in 1983. But up until now, only fragments and unofficial copies of 6502 BASIC had circulated online, mirrored on retrocomputing sites, and preserved in museum archives.

Built for the MOS Technology 6502 8-bit microprocessor, 6502 BASIC is an adaptation of the BASIC-80 interpreter Microsoft built for Intel 8080 processors. 6502 offered support for full BASIC implementation, floating-point arithmetic, arrays, string handling, and input/output operations, among its features.

The programming language interpreter was important because it introduced millions of users to personal computing and became the de facto standard. Microsoft notes that its design patterns and conventions influenced programming languages and development tools in later years.

Bill Gates and Ric Weiland completed 6502's port in 1976 and licensed it to Commodore for a $25,000 flat fee in 1977. The deal paved the way for the Commodore BASIC dialect on 8-bit computers like the PET, VIC-20, and Commodore 64. 6502 BASIC also made its way to the Apple II as an adaptation known as Applesoft BASIC.

Microsoft said that the open-sourced version "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."

It also includes a Bill Gates easter egg hidden in the labels STORDO and STORD0, which was confirmed by the Microsoft co-founder in 2010.

6502 BASIC has powered a wide variety of hardware, including the Atari 2600 and the Nintendo Entertainment System. Its return in an open source avatar could entice hobbyists, tech historians, and hardware tinkerers, who might want to experiment with it on modern hardware. It's complemented by the rising retro-computing trend with emulator projects and FPGA-based recreations like the Commodore 64 Ultimate.

Source

Hope you enjoyed this news post. Feedback welcome.

Posted Friday 5 September 2025 at 3:36 am AEST (my time).

News posts... 2023: 5,800+ | 2024: 5,700+ | 2025 (till end of August): 4,048

RIP Matrix