Microsoft's 6502 BASIC is now Open Source (2025)
Recorded: May 24, 2026, 2:59 p.m.
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Microsoft Releases Historic 6502 BASIC | Microsoft Open Source Blog
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Project updates • September 3, 2025 2 min read Bringing BASIC back: Microsoft’s 6502 BASIC is now Open Source By Stacey Haffner
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Content type Project updates Topic IT Trends Programming Languages Tools Tags GitHub Windows Today, we’re opening the vault—for real. Discover the newly open-sourced Microsoft 6502 BASIC code Microsoft’s first products: From the Altair to the Commodore 64 10 PRINT “HELLO” This is BASIC M6502 8K VER 1.1, the 6502 BASIC lineage that powered an era of home computing and formed the foundation of Commodore BASIC in the PET, VIC-20, and the legendary Commodore 64. This very source tree also contains adaptations for the Apple II (“Applesoft BASIC”), built from the same core BASIC source. The original headers still read, “BASIC M6502 8K VER 1.1 BY MICRO-SOFT”—a time capsule from 1978. Start exploring the newly open-sourced Microsoft 6502 BASIC code Listing of Altair BASIC on display at the New Mexico Museum of Natural History and Science. “Altair Basic Sign” by Swtpc6800, public domain via Wikimedia Commons.
Scott Hanselman VP, Member of Technical Staff, Microsoft/GitHub Scott Hanselman is a VP, Member of Technical Staff, Microsoft/GitHub working on Open Source, Windows, and mentoring early in career engineers as we navigate this AI augmented moment. As a programmer, teacher, and speaker for 30+ years, he’s worked on .NET, Azure, and Developer Tools. He hosts Hanselminutes, blogs at hanselman.com, and champions open source, inclusion, and human-centered tech. See more articles from this author
Stacey Haffner Director, Microsoft OSPO Stacey Haffner is the Director of Microsoft's OSPO and a developer advocate, where she focuses on open source, AI, and the future of developer tools. For over a decade she's been a product leader, builder, and teacher, working across startups and big tech on .NET, Xbox, Unity, and much more. She's also an independent game developer and teaches on YouTube at youtube.com/@staceyhaffner. See more articles from this author Related posts
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Microsoft has officially open-sourced the historic Microsoft 6502 BASIC code, making this code publicly accessible for exploration, modification, and sharing. This release addresses a long-standing presence of fragmented and unofficial copies of the code that had circulated in online communities, museum archives, and among hobbyists. The development of Microsoft BASIC began in 1975 as the company's initial product, an interpreter for the Intel 8080 written by Bill Gates and Paul Allen for the Altair 8800, which was later adapted to run on various 8-bit CPUs, including the MOS 6502, Motorola 6800, and 6809. The 6502 port was finalized in 1976 by Bill Gates and Ric Weiland. This legacy code formed the basis of Commodore BASIC in the PET, VIC-20, and the iconic Commodore 64, establishing Microsoft BASIC as a foundational element in early home computing. The specific version being released is labeled 1.1 and contains refinements to the garbage collector that was jointly implemented in 1978 by Commodore engineer John Feagans and Bill Gates. This specific iteration also includes a playful Easter egg, hidden in the labels STORDO and STORD0, which Bill Gates confirmed in 2010. This version corresponds to the code that shipped as the PET’s "BASIC V2" and represents a significant historical artifact from 1978. The underlying 6502 CPU, known for its simplicity and efficiency, remains highly influential, powering numerous systems including the Apple II, the Commodore 8-bit line, the Atari 2600, and the Nintendo Entertainment System. The significance of this open-source release is amplified by the ongoing interest in retrocomputing, which is currently thriving with projects involving FPGA-based recreations and emulator development. Dedicated preservationists have contributed to reconstructing build environments and verifying that the historical source can generate byte-exact ROMs. For instance, Michael Steil documented and rebuilt the original BASIC process for multiple targets, porting the code to modern assemblers such as cc65, enabling its execution on contemporary systems. This open-source release builds upon earlier milestones, tracing a lineage that includes the earlier release of GW-BASIC, which derived from the same source and was shipped in the original IBM PC ROM, eventually evolving into QBASIC and Visual Basic, which remain relevant for Windows application development. This action aligns with Microsoft's broader commitment to open source, which serves as a foundation for the security and predictability required for scaling artificial intelligence workloads and developing agentic systems. |