Microsoft made a significant contribution to computer history ten years ago by releasing the source code for the operating system MS-DOS 1.25 and 2.0 to the Computer History Museum. This system, written in assembler code 8086 nearly 45 years ago, holds a crucial place in the development of computing. The source code was later “reprinted” and shared on GitHub for educational purposes.
Recently, on April 25, Microsoft made another historic move by publishing the source code for MS-DOS version 4.0. This disclosure, made possible through cooperation with IBM, was carried out under the MIT license as part of an initiative to support open innovation. Enthusiasts are particularly interested in this release as it was developed in conjunction with IBM. Historical materials for MS-DOS 4.0, including source code, binary files, and documentation, are now also available on GitHub.
The discovery of the MS-DOS 4.0 source code was made possible by the efforts of researcher Connor Haide, who uncovered unpublished beta versions of DOS 4.0 during correspondence with former Microsoft technical director Ray Ozzie. Scott Hanselman and internet archivist Jeff Sponagl successfully restored data from original floppy disks, known as the Ozzie Drop, which included rare versions of software like “Ibmbio.com”.
The OSPO team, led by Jeff Wilcox, delved into the Microsoft archives in search of the complete MT-DOS code but instead found the source code for MS-DOS 4.0. Alongside the code, beta versions, documentation in PDF format, and other files were published. The software was successfully launched on original IBM PC XT machines, newer Pentium processors, and open emulators like PCEM and 86Box to confirm its functionality.
Microsoft and IBM plan to continue exploring their archives for additional materials that can enrich the current release. Both companies extend their gratitude to all project participants, including those involved in the digital archeology of this software and the original code authors.