Microsoft Open-Sources 86-DOS 1.00 on Its 45th Anniversary, Releasing Code Transcribed From Tim Paterson's Original Printouts

Overview

Microsoft has released what it describes as the earliest known DOS source code as open source, publishing the 86-DOS 1.00 kernel along with development snapshots of the PC-DOS 1.00 kernel and utilities such as CHKDSK. The materials were posted to a GitHub repository named DOS-History/Paterson-Listings on April 28, 2026, the 45th anniversary of 86-DOS 1.00, according to the Microsoft Open Source Blog.

The release closes a roughly half-decade gap in Microsoft’s DOS preservation efforts. The company previously open-sourced MS-DOS 1.25 and 2.11 in 2018 and MS-DOS 4.0 in 2024, as the Microsoft Open Source Blog notes. The new repository pushes the publicly accessible source code further back in time, into the pre-PC-DOS era when 86-DOS was still being developed by Tim Paterson at Seattle Computer Products.

What We Know

The announcement post, titled “Continuing the story of early DOS development,” was authored by Stacey Haffner, Director of OSPO at Microsoft, and Scott Hanselman, VP, Member of Technical Staff, Microsoft/GitHub. They credit a team “led by Yufeng Gao and Rich Cini” with the recovery work that made the release possible.

The source did not arrive at Microsoft as a clean archive. According to It’s FOSS, “Tim did not hand over a tidy source archive; instead, what he kept were physical assembler printouts and stacks of continuous-feed paper from 1981.” The historians had to “locate, scan, and transcribe the DOS-related portions into compilable code,” the same outlet reports. Tom’s Hardware describes the printouts as having been found in a garage and quotes a Microsoft VP saying the transcribed document “is perfect and recompiles byte for byte to the original binaries.”

The GitHub repository is described as a “Transcription of Tim Paterson’s DOS printouts” and is organized into three top-level directories: 1_transcription, 2_printed_files, and 3_source_code. Its README states the repository contains source code for the 86-DOS 1.00 kernel, various PC-DOS 1.00 pre-release kernels and utilities, and the Microsoft BASIC-86 Compiler runtime library. The compilable sources target Seattle Computer Products’ ASM assembler. The accompanying LICENSE file declares the MIT License with copyright held by Microsoft Corporation, matching the Microsoft blog’s confirmation that the materials were “licensed under MIT via pull request.”

Open Source For You reports that Microsoft framed the release as intended “to support study, preservation, and exploration of computing history.” The same outlet confirms the released materials include the 86-DOS 1.00 kernel, development snapshots of the PC-DOS 1.00 kernel, utilities such as CHKDSK, and assembler listings and related tools.

What We Don’t Know

The public record on the financial history surrounding 86-DOS remains imprecise. It’s FOSS writes that “Microsoft bought the rights to 86-DOS for just under $100,000, shipped it to IBM as PC DOS 1.0 in August 1981,” while PCWorld cites a figure of about $75,000 for the same acquisition. The two figures cannot both be exact, and neither outlet documents an underlying contract reference; the question of the precise sum Microsoft paid is left to historians working from primary documents.

The completeness of the transcription effort is also bounded. The repository README indicates that some bundles of original printouts remain untranscribed and that the project accepts pull requests for further transcription work — a signal that more material may surface as the community contributes.

Why It Matters

For a system that defined the early personal computing era and seeded a software lineage that runs through MS-DOS into Windows, 86-DOS has until now been visible to the public mostly through later iterations and second-hand accounts. Releasing the kernel sources that pre-date PC-DOS 1.0 — which It’s FOSS and PCWorld note shipped to IBM in August 1981 — gives historians, educators, and curious developers a primary-source view into the assembly-language design choices that shaped the platform.

It also reflects a quieter kind of open-source work: not new code under active development, but old code preserved, attributed, and licensed cleanly enough for downstream study. The MIT License declaration in the Paterson-Listings repository, with Microsoft as copyright holder, makes the material redistributable in ways the original 1981 printouts never were.