Ten years ago, Microsoft shared the source code for MS-DOS 1.25 and 2.0 with the Computer History Museum, now releasing MS-DOS 4.00 under the MIT license in collaboration with IBM. The history of DOS 4.0 versions involves Microsoft working with IBM and creating Multitasking DOS, an unreleased branch. Ray Ozzie discovered beta binaries of DOS 4.0, sparking interest in releasing the source code and documenting its link to OS/2. Original disks from the “Ozzie Drop” were imaged, including the ibmbio.com source. Microsoft, in partnership with IBM, finds this operating system history compelling and worthy of sharing. Additional beta binaries, documentation PDFs, and disk images are also included in the release.
https://www.hanselman.com/blog/open-sourcing-dos-4