Source code for MS-DOS 4.00 is released by IBM and Microsoft.

In collaboration with IBM, Microsoft has made the source code for MS-DOS 4.00 available in an effort to celebrate innovation and preserve computing history. The source code, binary files, and original documentation of the operating system are all included in this release, which is made available under the permissive MIT license. Recent correspondence between former Microsoft CTO Ray Ozzie and researcher Connor "Starfrost" Hyde led to the decision to release MS-DOS 4.00 as open source. As Hyde was going through Ozzie's old floppy disk collection, he came upon unpublished DOS 4.0 beta binaries that Ozzie had obtained from Lotus Software. This discovery intrigued Hyde, so he contacted Microsoft's Open Source Programs Office (OSPO) to inquire about the prospect of making the DOS 4 source code available. His goal was to record the connection between DOS