Microsoft has open-sourced another bit of computing history this week: The company teamed up with IBM to release the source code of 1988's MS-DOS 4.00, a version better known for its unpopularity, bugginess, and convoluted development history than its utility as a computer operating system.

The MS-DOS 4.00 code is available on Microsoft's MS-DOS GitHub page along with versions 1.25 and 2.0, which Microsoft open-sourced in cooperation with the Computer History Museum back in 2014. All open-source versions of DOS have been released under the MIT License. //

The publicly released version of MS-DOS 4.00 is known less for its new features than for its high memory usage; the 4.00 release could consume as much as 92KB of RAM, way up from the roughly 56KB used by MS-DOS 3.31, and the 4.01 release reduced this to about 86KB. The later MS-DOS 5.0 and 6.0 releases maxed out at 72 or 73KB, and even IBM's PC DOS 2000 only wanted around 64KB.

These RAM numbers would be rounding errors on any modern computer, but in the days when RAM was pricey, systems maxed out at 640KB, and virtual memory wasn't a thing, such a huge jump in system requirements was a big deal. //

Microsoft has open-sourced some other legacy code over the years, including those older MS-DOS versions, Word for Windows 1.1a, 1983-era GW-BASIC, and the original Windows File Manager. While most of these have been released in their original forms without any updates or changes, the Windows File Manager is actually actively maintained. It was initially just changed enough to run natively on modern 64-bit and Arm PCs running Windows 10 and 11, but it's been updated with new fixes and features as recently as March 2024.