Ah, the PC in the spare room. Back when “going on the computer” was an event, not a daily necessity, I cut my teeth on MS-DOS games. I didn’t really know how they worked, and I wasn’t very good, but there a hardware and gaming enthusiast was born. Now Microsoft has released the source code for MS-DOS 4.0 in partnership with IBM, and apparently, all it takes is the logo to make me go all mushy.

This isn’t the first time Microsoft has open-sourced MS-DOS, as its GitHub repository already has versions 1.25 and 2.0, which were originally shared at the Computer History Museum back in 2014. Now a decade later it’s MS-DOS 4.0’s turn to get the source code release treatment.

Microsoft’s version of MS-DOS 4.0 was originally released back in 1986 after a joint development with IBM for portions of the code, and a somewhat difficult relationship between the two at the time led to two branches eventually being released, MS-DOS 4.0 and what is now referred to as IBM DOS 4.0.

The Microsoft version, sometimes referred to as European MS-DOS 4.0, featured support for the New Executable file format, and pre-emptive multitasking. However, it retained many of the limitations of MS-DOS 3.0, including the inability to use memory above 640 KB.

While neither version set the world on fire—and perhaps overall both releases should be viewed more as Vista than Windows 7—I’m pretty sure it was one of these versions I used on our rubbish spare room machine, and that it allowed me to play Doom and F-15 Strike Eagle II. 

That was good enough for little ol’ me. Simpler times, ey?

The open-sourcing of this version of MS-DOS was made possibly by a researcher, who got in touch with Microsoft regarding some unreleased beta binaries of DOS 4.0 they’d found on a stack of old floppies from their time working at Lotus. These disks were imaged and scanned to reveal that they were earlier, unreleased versions of the OS.

This discovery, along with some included printed documents, kicked off a hunt through the Microsoft Archives where the original full source code of MS-DOS 4.0 was found.

Microsoft says that they’ve successfully run it on an original IBM PC XT, a “newer” Pentium, and within open source PCem and 86box emulators, so if you’re the sort of person that likes messing around with a bit of PC archaeology, here’s another old-school curiosity to enjoy.