Microsoft Releases Earliest Known DOS Source Code, Including 86-DOS 1.00 Kernel

For decades, Microsoft has periodically released source code from its original MS-DOS operating system, which became the foundation of its consumer PC dominance. This week, the company has gone further back in time than ever before, unveiling what it calls "the earliest DOS source code discovered to date."

The newly released source code includes:

• 86-DOS 1.00 kernel source code
• Several development snapshots of the PC-DOS 1.00 kernel
• Well-known utilities such as CHKDSK

This release is significant because it predates the MS-DOS branding entirely, offering a rare glimpse into the operating system’s earliest iterations.

How 86-DOS Became MS-DOS: A Brief History

To understand the context of this release, it’s helpful to revisit the origins of what would later become MS-DOS:

1. Tim Paterson’s 86-DOS: Programmer Tim Paterson originally developed 86-DOS (initially called QDOS, for "quick and dirty operating system") for an Intel 8086-based computer kit sold by Seattle Computer Products.
2. Microsoft’s Licensing Deal: Microsoft needed an operating system for the upcoming IBM PC 5150. The company licensed 86-DOS from Seattle Computer Products and hired Paterson to continue its development. Microsoft later purchased the rights to 86-DOS outright.
3. PC-DOS and MS-DOS: Microsoft licensed the operating system to IBM as PC-DOS while retaining the right to sell it to other companies. The version sold by Microsoft was renamed MS-DOS. The rise of third-party IBM PC clones in the 1980s and 1990s made MS-DOS the dominant operating system for most users.

Why This Release Matters

The release of this source code is more than a historical curiosity—it provides invaluable insights into the technical foundations of early personal computing. Developers, historians, and enthusiasts can now examine the code that powered the first wave of IBM-compatible PCs, which shaped the entire PC industry.

Microsoft’s Stacey Haffner and Scott Hanselman, who co-authored the announcement post, emphasized the rarity of such a release:

"Today's source release is so old that it predates the MS-DOS branding, and it includes sources to the 86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel, and some well-known utilities such as CHKDSK."