Microsoft has released the source code for MS-DOS 4.00, a historic operating system written in 8086 assembly code. The release is a collaboration with IBM and part of Microsoft's open innovation initiative. The source code was discovered by researcher Connor "Starfrost" Hyde in unreleased beta binaries sent to Ray Ozzie. Microsoft and IBM believe this release is a valuable piece of operating system history. The release includes the source code for MS DOS 4.00, beta binaries, documentation, and disk images. Microsoft and IBM continue to explore the archives for additional materials. The source code can be accessed on GitHub. The software can be run on original IBM PC XT, Pentium computers, or within PCem and 86box emulators. The release acknowledges the contributions of the original authors, some of whom still work at Microsoft and IBM.