After nearly half a century of waiting, Microsoft has decided to make public one of the most precious treasures in the history of computing: the source code of the BASIC interpreter for the 6502 processor, which powered home computers of the 1980s.
A historic decision that changes everything
Microsoft’s move on September 3, 2025 marks a turning point for retrocomputing enthusiasts and software historians. By publishing the complete code on GitHub under the MIT license, the Redmond company has finally put an end to decades of unofficial copies and scattered fragments circulating within the community.
“From the blinking cursor of 1977 to the FPGA builds of 2025, BASIC is still within everyone’s reach,” Microsoft emphasized in its official statement, highlighting how this language continues to inspire new generations of programmers.
The birth of a computing icon
It all began in 1975, when Bill Gates and Paul Allen developed their first BASIC for the Altair 8800. But it was the adaptation for the MOS 6502 chip—completed the following year with the help of Ric Weiland—that changed the course of home computing history.
The real breakthrough came with the 1977 Commodore deal: a license sold for just $25,000 that turned out to be one of the most influential transactions of the digital era. This agreement brought Microsoft BASIC into the heart of the Commodore PET, VIC-20, and the legendary Commodore 64, introducing millions of people to the world of programming.
An ecosystem that defined an era
The influence of this interpreter extended far beyond Commodore. The same codebase powered Applesoft BASIC on the Apple II, becoming a bridge that connected home and professional users through a common, accessible language.
The newly released version 1.1 represents the culmination of this development: it includes garbage collector optimizations implemented in 1978 through a direct collaboration between Commodore engineer John Feagans and Bill Gates at Microsoft’s Bellevue offices.
Hidden secrets in the code
Among the 6,955 lines of assembly lies a small treasure: a personal easter egg by Bill Gates hidden in the labels STORDO and STORD0. This detail, confirmed by Gates himself in 2010, reflects the personal, artisanal approach to software development of that era.
The code also reveals the elegant architecture that enabled floating-point arithmetic, arrays, strings, and input/output operations to fit within just 8KB of memory—a true engineering feat given the hardware constraints of the time.
The 6502 renaissance
Microsoft’s decision comes at a moment of renewed interest in the 6502 architecture. The retrocomputing community is more vibrant than ever, with FPGA projects like the new official Commodore 64 promising to revive the original experience using modern technology.
Michael Steil, one of the main figures behind the reconstruction of historical build environments, had already demonstrated that the original code could still produce ROMs identical byte-for-byte to the originals. Now, with the official release, these digital preservation efforts receive the recognition and support they deserve.
Beyond nostalgia: modern applications
The release is not just a tribute to the past but also opens up new opportunities for the future. Educators, makers, and embedded systems developers can now tap directly into the original source, freely adapting and modifying a piece of software that taught entire generations how to program.
The MIT license ensures maximum freedom of use, enabling both open-source projects and commercial applications without legal restrictions.
A heritage finally made free
What for years circulated as an urban legend among collectors and enthusiasts is now common property of the digital world. Microsoft BASIC 6502 thus joins other historical preservation projects from the company, following in the footsteps of the earlier release of GW-BASIC.
Scott Hanselman, Microsoft’s Vice President of Developer Community, spearheaded this initiative, which acts as a bridge between past and future, showing how modern technology can honor and preserve its roots.
The publication of this code is not only a goodwill gesture toward the community but also an acknowledgment of the debt the computer industry owes to those pioneers who, with just a few thousand lines of assembly, lit the spark of the home digital revolution.
