Microsoft has published the assembly source for “BASIC for 6502 Microprocessor — Version 1.1” on GitHub under a modern permissive license, making the exact code that powered a generation of home computers readable, buildable, and reusable by anyone — hobbyists, historians, educators, and commercial developers alike. (github.com)
Microsoft’s BASIC was the company’s first commercial product. The original Altair/8080 BASIC (1975) — written by Bill Gates and Paul Allen — was quickly ported to other CPUs as the personal-computing market took off. One of the earliest and most consequential ports targeted the MOS Technology 6502, a low-cost, high-value 8‑bit CPU that became the heart of machines such as the Apple II, Commodore PET, VIC‑20 and, by lineage, the Commodore 64. The public 1.1 assembly snapshot being distributed today is the same family of code that underpinned many of those devices. (theregister.com) (tomshardware.com)
That lineage is not academic: Commodore licensed Microsoft’s 6502 BASIC in the late 1970s for a flat fee (commonly reported as $25,000), a deal that placed Microsoft’s interpreter in front of millions of new programmers and helped crystallize Microsoft’s long-term licensing model. (theverge.com) (theregister.com)
Another example of where caution is warranted is the popular anecdote about the garbage-collector fixes. Multiple contemporary reports state that Commodore engineer John Feagans and Bill Gates collaborated on collector fixes in 1978; the released source claims such fixes are present in the 1.1 tree. Those technical claims are verifiable by reading the routines in the assembly, but colorful personal narratives about “who met whom in which office” remain oral history unless backed by dated corporate documents or corroborating primary materials. In short: the code validates the technical claims; the precise chronology of in-person events remains best-effort historical reconstruction. (tomshardware.com, theregister.com)
Microsoft’s publication of 6502 BASIC Version 1.1 is both a historian’s gift and a practical tool for today’s retro‑computing communities. It turns a decades‑old interpreter into an active teaching resource, restores technical context to ROM images that have long circulated as binaries, and brings a pivotal piece of software history under an explicit modern license. At the same time, responsible reuse demands that engineers, historians and vendors verify repository provenance, treat colorful personal anecdotes with appropriate skepticism unless corroborated, and observe best practices for supply-chain hygiene when moving historic code into production or commercial projects. (github.com, theregister.com)
Source: Tom's Hardware Bill Gates’ 48-year-old Microsoft 6502 BASIC goes open source
Background
Microsoft’s BASIC was the company’s first commercial product. The original Altair/8080 BASIC (1975) — written by Bill Gates and Paul Allen — was quickly ported to other CPUs as the personal-computing market took off. One of the earliest and most consequential ports targeted the MOS Technology 6502, a low-cost, high-value 8‑bit CPU that became the heart of machines such as the Apple II, Commodore PET, VIC‑20 and, by lineage, the Commodore 64. The public 1.1 assembly snapshot being distributed today is the same family of code that underpinned many of those devices. (theregister.com) (tomshardware.com)That lineage is not academic: Commodore licensed Microsoft’s 6502 BASIC in the late 1970s for a flat fee (commonly reported as $25,000), a deal that placed Microsoft’s interpreter in front of millions of new programmers and helped crystallize Microsoft’s long-term licensing model. (theverge.com) (theregister.com)
What Microsoft released — the essentials
- The release is labeled Version 1.1 and is distributed on GitHub under a permissive MIT-style license in the published repository. Multiple contemporary reports confirm Microsoft placed a modern license on the code to make reuse straightforward. (computerworld.com, hackaday.com)
- The assembly source in the published snapshot totals roughly 6,955 lines and contains a complete interpreter: tokeniser, parser/editor, runtime, floating-point arithmetic routines, string handling and a garbage collector, array support, I/O hooks, and vendor-targeted conditional builds. (theregister.com, github.com)
- The codebase includes conditional compilation and configuration symbols to generate variants for several period platforms (Apple II / AppleSoft, Commodore PET, Ohio Scientific machines, KIM‑1, AIM‑65, and others). That’s how Microsoft maintained one core interpreter while producing many OEM ROM images. (github.com)
Why this matters now — preservation, education, and authenticity
This release matters for at least three distinct communities:- For historians and software archaeologists, it’s an original, compact artifact that reveals how early language runtimes were implemented under extreme constraints. The code lays bare design trade-offs — memory layout, floating-point format, compact parsing strategies — that are difficult to recover from binary ROM images alone. (theregister.com)
- For retro-computing hobbyists and emulator authors, having the original assembly makes it significantly easier to produce faithful ROMs, validate emulator behavior, and correct decades-old discrepancies between ROM dumps and emulated behavior. Community build trees already demonstrate practical paths from source to ROM to emulator. (github.com, hackaday.com)
- For educators and systems programmers, the interpreter is a pristine teaching tool: a full language implementation in a few thousand lines of hand-crafted assembly that remains readable enough to teach interpreter structure, low-level arithmetic, and memory-management strategies. (github.com)
Technical deep dive — what’s in the code
The published 6502 assembly exhibits several interesting technical features that reflect both the limitations and ingenuity of late‑1970s microcomputer software engineering.Core interpreter components
- Line editor and program storage: A classic line-numbered BASIC kernel with routines for entering, editing, storing and tokenizing program lines, and for managing the tokenised program memory area.
- Parser / runtime: Compact, stack-light evaluation routines for BASIC expressions, control flow, and statement execution optimized for a 1‑MHz 6502.
- Numeric format: A floating-point implementation (often a 40‑bit format in Microsoft’s early BASICs) that supports a surprisingly wide dynamic range for an 8‑bit environment.
- String handling and GC: Dynamic string allocation and a garbage collector to compact fragmented string heaps — a memory-efficient approach that also created well-documented pauses on constrained machines. (github.com, theregister.com)
Portability and OEM hooks
The codebase uses compile-time defines and I/O vector abstractions so vendor-specific I/O, screen, and disk handlers could be injected at build time. That modularity explains how the same core could become Commodore BASIC, AppleSoft, or OSI BASIC with modest local changes. The repository’s README and build scripts show these configuration options explicitly. (github.com)Notable quirks and Easter eggs
The published sources contain small historical artifacts — labels and comments that enthusiasts have flagged as playful or autobiographical (so-called “Easter eggs”) — which add cultural value to the technical material. Microsoft’s retrospective notes mention a few of these human touches. (computerworld.com, tomshardware.com)Provenance and caution — commit dates, oral history, and verification
Several reports note that some file commits in the published tree bear historic dates (for example, a commit date shown as July 27, 1978). Those timestamps are evocative, but they are not definitive proof of an immutable archival lineage by themselves. Git records contain both author and committer dates, and those dates can be explicitly set or rewritten when a repository is created or migrated. In other words, commit timestamps are valuable context but not absolute evidence about when a file was first published. Treat commit dates as historical clues that should be corroborated against other documentary evidence (original memos, physical media images, dated press, or Microsoft’s archival notes). (alexpeattie.com, slingacademy.com)Another example of where caution is warranted is the popular anecdote about the garbage-collector fixes. Multiple contemporary reports state that Commodore engineer John Feagans and Bill Gates collaborated on collector fixes in 1978; the released source claims such fixes are present in the 1.1 tree. Those technical claims are verifiable by reading the routines in the assembly, but colorful personal narratives about “who met whom in which office” remain oral history unless backed by dated corporate documents or corroborating primary materials. In short: the code validates the technical claims; the precise chronology of in-person events remains best-effort historical reconstruction. (tomshardware.com, theregister.com)
Licensing and legal implications
Microsoft published this snapshot under a permissive open-source license (reported across outlets as the MIT License). That license allows broad reuse, modification and even commercial redistribution — but two pragmatic cautions apply:- Always verify the authoritative license in the live repository you intend to clone. Multiple unofficial mirrors of historic BASIC source have circulated under different license texts (Unlicense, archival disclaimers, or no license at all). The canonical repo’s LICENSE file is the legal touchstone. (computerworld.com, theregister.com)
- Even when the core interpreter is clearly covered by a permissive license, there can be surrounding artifacts worth auditing (third‑party drivers, vendor binary patches, or customer-supplied code). For this particular core interpreter that risk is low, but prudent legal hygiene is mandatory before commercial distribution. (theregister.com)
How to get it running — practical starter steps
For readers who want to reconstruct ROMs or run the interpreter in emulation, community build trees show a straightforward path:- Clone the authoritative repository (or a well-maintained community fork) that contains the M6502 assembly and build scripts. (github.com)
- Install a 6502 toolchain and assembler (project READMEs commonly recommend cc65 or other 6502 toolchains). (github.com)
- Run the included make/build script (many repositories include sh/make recipes that produce multiple ROM builds and compare them to historic ROM images). (github.com)
- Load the produced ROM into an emulator (VICE for Commodore targets, Apple II emulators for Apple targets, or dedicated KIM‑1/OSI emulators) or flash it to a suitable FPGA/board with a 6502 core. (hackaday.com, github.com)
- If you only want to experiment quickly, prebuilt community ports exist (for example portable reimplementations that emulate Commodore BASIC behavior as a native program). These are useful for rapid exploration before moving to authentic ROM building. (github.com)
- Expect to tweak zero-page layout and I/O vectors for non-standard hardware; the repository’s configuration switches are designed to help with this. (github.com)
Opportunities unlocked by the release
- Accurate emulation and ROM restoration: historians and emulator authors can now produce true-to-original ROM images and reconcile long-running differences between binaries and circulating source dumps. (github.com)
- Educational syllabi for low-level systems: university courses and bootcamps can use the code as a compact, realistic case study in interpreter design and low-level optimization. (github.com)
- FPGA and hardware recreation projects: authentic ROMs make it simpler to boot original firmware on recreated hardware, which helps museums, collectors, and preservationists. (hackaday.com)
- Creative reuse and commercial products: the MIT license allows companies to build derivative products — from teaching tools to commercial retro-enabled devices — provided they verify the repository’s license and any added artifacts. (computerworld.com)
Risks, limitations, and responsible reuse
While the release is broadly beneficial, there are real caveats and responsibilities:- Provenance hygiene — Treat commit timestamps and oral recollections with care. Git makes it easy to set author/committer dates, so a repository’s visible history is not a substitute for corroborating evidence. Researchers should cross-check the code with independent ROM images, dated press, or original vendor materials where possible. (alexpeattie.com, slingacademy.com)
- Fragmentation hazard — The permissive license invites forks and commercial derivatives; that’s beneficial but also risks a fragmented ecosystem where incompatible variants proliferate. For preservation projects, canonical archives and institutional mirrors are preferable to ephemeral forks. (github.com)
- IP edge cases — Although unlikely for this core interpreter, caution is warranted if anyone bundles the code with later vendor-specific patches or binary blobs that may have different provenance. Always check LICENSE and README files in the exact repository being used. (theregister.com)
- Security and supply-chain concerns — Reintroducing assembly that predates modern secure-coding practices has minimal direct risk for modern systems, but developers reusing old code in new products should be mindful of supply-chain hygiene (signed commits, verified provenance, and CI-based build reproducibility). (blog.zsec.uk)
Cultural and corporate context — why Microsoft is doing this
This release follows a measured trend: major vendors periodically publish vintage source code for scholarly, preservation, and goodwill reasons. Microsoft previously released GW‑BASIC and has been active in curated historical releases. These steps serve several corporate and civic goals simultaneously: they support preservation, demonstrate an open‑source posture, and provide marketing-soft power through cultural stewardship of software history. The practical effect is to empower a global community of retro-developers and historians while reinforcing Microsoft’s public narrative about its origins and evolution. (theregister.com, computerworld.com)Verification summary — what’s been cross-checked
- The existence of a public 6502 BASIC source labeled Version 1.1 and posted to GitHub under a permissive license is corroborated by multiple independent outlets and community repositories. (computerworld.com, theregister.com)
- The 6,955 lines figure for the assembly snapshot appears consistently in reporting and matches counts visible in published source files. (theregister.com, github.com)
- The Commodore $25,000 license fact is a long-reported historical datum; it is cited in Microsoft’s historical notes and in contemporary reporting tied to the company’s early OEM licensing strategy. (theverge.com, theregister.com)
- The garbage-collector fixes credited to John Feagans and Bill Gates appear in the 1.1 release notes and are consistent with technical traces visible in the source; the human narrative describing in-person collaboration is plausible and frequently repeated in oral histories, but precise dating or private meeting details are best regarded as anecdotal unless supported by primary internal documents. (tomshardware.com, theregister.com)
- The presence of historically dated commit timestamps is accurate in the repository view, but such timestamps can be set manually in Git; readers should therefore regard them as informative but not conclusive evidence of chronology without corroboration. (alexpeattie.com, slingacademy.com)
Final analysis — strengths and potential risks
Strengths- Transparency and pedagogy: This is a rare, production-quality piece of vintage software exposed as readable source code. It’s ideal for teaching, preservation, and faithful emulation.
- Practical reuse: The MIT license and included build scripts lower the bar for hobbyist, academic, and commercial projects that want to recreate authentic ROMs or port the interpreter into new form-factors.
- Cultural value: Making code written by early Microsoft engineers publicly available is a tangible act of software preservation that enriches computing history.
- Provenance confusion: The existence of multiple mirrors and the manipulability of Git metadata means that provenance must be checked carefully.
- Fragmentation and commercial misuse: Permissive licensing enables many derivative works; that’s mostly good, but it can complicate preservationists’ efforts to identify canonical versions.
- Legal/operational due diligence: Anyone intending to ship products containing this code should verify the repository license and audit any third-party additions.
Microsoft’s publication of 6502 BASIC Version 1.1 is both a historian’s gift and a practical tool for today’s retro‑computing communities. It turns a decades‑old interpreter into an active teaching resource, restores technical context to ROM images that have long circulated as binaries, and brings a pivotal piece of software history under an explicit modern license. At the same time, responsible reuse demands that engineers, historians and vendors verify repository provenance, treat colorful personal anecdotes with appropriate skepticism unless corroborated, and observe best practices for supply-chain hygiene when moving historic code into production or commercial projects. (github.com, theregister.com)
Source: Tom's Hardware Bill Gates’ 48-year-old Microsoft 6502 BASIC goes open source
