Nostalgia hits as Microsoft releases source code of MS-DOS 4

Microsoft, in partnership with IBM, recently announced the release of the source code for MS-DOS 4.00 to mark the spirit of open innovation. That means the source code is now publicly available for everyone to modify, study, or distribute.

The released package is available under the MIT license. Additionally, the package includes the OS’s source code, binary files, and original documentation.

The story behind the release of source code for MS-DOS 4.00

The decision to release the source code of MS-DOS 4.00 didn’t happen out of the blue. However, there’s a long story behind it. It all started after an interaction between a researcher named Connor “StarFrost” Hyde and Microsoft’s ex-CTO Ray Ozzie.

Interestingly, Hyde stumbled upon unreleased beta binaries of DOS 4.0 while exploring Ozzie’s old floppy disk collection. The collection that Ozzie first received when he was at Lotus Software.

As the discovery was quite intriguing, Hyde got in touch with Microsoft’s Open Source Programs Office (OSPO) to discuss the possibility of MS-Dos 4.00 source code’s release. Hyde aimed to document the relationship between DOS 4, Microsoft’s Multitasking DOS (MT-DOS), and IBM’s OS/2 operating system’s development.

Scott Hanselman collaborated with internet archivist Jeff Sponaugle, imaged the original disks, and scanned the original documentation from Ozzie’s old floppy disk collection, which now Microsoft refers to as Ozzie’s drop.

Microsoft’s Principal Engineer Jeff Wilcox and OSPO didn’t find the full source code of the MT-DOS. However, they did come across the MS-DOS 4.00 source code and eventually decided to publicly release it.

In the announcement post to open source MS-DOS 4.0, Microsoft wrote:

This code holds an important place in history and is a fascinating read of an operating system that was written entirely in 8086 assembly code nearly 45 years ago.

The discovered beta binaries didn’t see a widespread release back then

It is worth noting that in the 1980s, Microsoft and IBM collaborated on certain aspects of the DOS code. The partnership further resulted in the creation of a separate branch known as Multitasking DOS, which never saw mass rollout.

Although the later versions of the MT-DOS binaries are still traceable online, the beta binaries found in Ozzie’s collection were much earlier unreleased versions.

Microsoft says that it successfully ran the software on an original IBM PC XT, a newer Pentium, and via open-source emulators including PCem and 86box.

The company further promises to continuously explore its archives. And, if they find other relevant materials, they’ll update the release.

Undoubtedly, this discovery will offer valuable insight to those involved in digging into the history of the OS. You can access and use the released source code for MS-DOS 4.00 from GitHub.