Microsoft has open-sourced the version of BASIC it created in 1976 for the MOS 6502 processor used in many early microcomputers.

As the software colossus explained in a Wednesday [1]post , Microsoft founders Bill Gates and Paul Allen wrote the company’s first product, [2]BASIC for the Altair 8800 microcomputer and the Intel 8080 processor that powered it, in 1975.

A year later Gates and Ric Weiland, Microsoft’s second employee, ported Microsoft BASIC to the 6502 processor.

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In 1977, Commodore Computer licensed it for $25,000 and used Microsoft BASIC in its PET, VIC-20, and Commodore 64 machines. Commodore sold millions of the latter two machines, which helped bring computing to a mass market.

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The code Microsoft has released is version 1.1, which apparently contains fixes to the garbage collector identified by Commodore and jointly implemented in 1978 by Commodore engineer John Feagans and Bill Gates. Commodore PET users would know it as BASIC V2.

The release is assembly language source code – 6,955 lines of it – and Microsoft placed it [6]on GitHub , under the MIT License that allows free unrestricted use, and even resale.

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In a nice touch, time stamps for commits in the repo record its creation as having taken place “48 years ago”.

The code includes what Microsoft describes as “conditional compilation support for multiple pioneering computer systems,” including the Apple II, Commodore PET, Ohio Scientific, and the MOS Technology KIM-1.

[8]Microsoft drops a little surprise thank-you gift for sitting through Build: The source for GW-BASIC

[9]Classic Psion fan releases proof-of-concept language server for OPL

[10]Microsoft developer ported vector database coded in SAP’s ABAP to the ZX Spectrum

[11]BASICally still alive: Classic language celebrates 60 years with new code and old quirks

If you get it running, Microsoft advertises the code’s main features as:

Full BASIC language implementation

Floating-point arithmetic

String handling and manipulation

Array support (both integer and string arrays)

Mathematical functions and operators

Input/output operations

You’ll also have the chance to enjoy “Efficient memory utilization for 8-bit systems” plus “String garbage collection” and “Dynamic variable storage.”

The repo includes some notes on Microsoft history, stating that BASIC established the company “as a dominant force in personal computer software before MS-DOS or Windows” and “the licensing of this BASIC interpreter to multiple computer manufacturers was crucial to Microsoft's early business model.”

That rather glosses over how Microsoft later, ahem, drew inspiration from CP/M when creating MS-DOS, and bullied OEMs with Windows licenses designed to snuff IBM’s OS/2.

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But for now let’s celebrate the release of undoubtedly important code, and that this is Microsoft’s second such effort after the 2020 release of source code for [13]GW-BASIC . ®

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[1] https://opensource.microsoft.com/blog/2025/09/03/microsoft-open-source-historic-6502-basic/

[2] https://www.theregister.com/2025/07/24/50y_of_gates_allen_mits_basic/

[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/front&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2aLljOL6Z1kHBdbAQgqxReQAAAM4&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0

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[6] https://github.com/microsoft/BASIC-M6502

[7] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/front&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44aLljOL6Z1kHBdbAQgqxReQAAAM4&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0

[8] https://www.theregister.com/2020/05/22/gw_basic/

[9] https://www.theregister.com/2025/08/27/vintage_computing_boffin_resurrects_opl/

[10] https://www.theregister.com/2025/07/08/sap_abap_db_spectrum_port/

[11] https://www.theregister.com/2024/05/03/basic_60th_birthday_releases/

[12] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/front&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33aLljOL6Z1kHBdbAQgqxReQAAAM4&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0

[13] https://www.theregister.com/2020/05/22/gw_basic/

[14] https://whitepapers.theregister.com/