DOS 4.0 Source Code Released Under MIT License

Main discussion here: https://news.ycombinator.com/item?id=40163405

Looking at the pdf folder, this is the "Multitasking MS-DOS" product which is not the same as the "DOS 4" that most people think of. That "DOS 4" is actually IBM Personal Computer DOS 4, the successor to MS-DOS/PC-DOS 3.3 (with > 32 meg filesystem support borrowed from Compaq DOS 3.31) and predecessor to MS-DOS 5. Microsoft did later, quite confusingly, also release an MS-DOS 4 based on PC-DOS 4.

This is not that. This is the road not taken, at least not by IBM and compatibles.

At the time, Microsoft assumed that DOS as we know it would never live past version 3. This was their idea of what DOS 4 should be. A new major version breaking strict compatibility. What we know as OS/2 1.x was originally named DOS 5, another new major "DOS" version breaking strict compatibility.

This Multitasking MS-DOS never took off except in niche applications, and OS/2 eventually got its own product name which is why there were then regular DOS releases 4 and 5 followed by 6 and eventually 7 become part of Windows 95.

Old code should be open sourced to learn from.

Looking at \src\H\VERSIONC.H, this seems to be DOS 4.0

...which was buggy and quickly followed by 4.01

Are they releasing the source to 4.01 too? Would be neat to see a diff of those.