Microsoft releases MS-DOS 4.0 source code and floppy images through an open-source license

TL;DR: Microsoft will likely never release the original source code of Windows into the wild, but the company is clearly interested in sharing important episodes of its software development history. MS-DOS 4.0 has now become part of this shared history.

Ten years after releasing the source code of MS-DOS 1.25 and 2.0, Microsoft is making yet another contribution to the world of open-source software preservation. Working in partnership with IBM and "in the spirit of open innovation," the company has released the source code of MS-DOS 4.00 under an MIT license.

Earlier MS-DOS editions held an important place in the history of software as an operating system written entirely in 8086 assembly code 45 years ago, Microsoft remarks. MS-DOS 4 has a "somewhat complex" and fascinating history as well, because Microsoft partnered with IBM to create the OS while working on a completely different branch ...