If you look back 30 or so years ago, it wasn’t clear what was going to happen with personal computers. One thing most people would have bet on, though, was that CP/M — the operating system from Digital Research — would keep growing and power whatever new machines were available. Except it didn’t. MS-DOS took over the word and led — eventually — to the huge number of Windows computers we know today. Microsoft has released the source code to MS-DOS 1.25 and 2.0 on GitHub.
Microsoft — then another fledgling computer company — had written some BASIC interpreters and wanted in on the operating system space. They paid the princely sum of $75,000 to Seattle Computer Products for something called QDOS written by [Tim Paterson]. Rebranded as MS-DOS, the first version appeared in late 1981 and version 1.25 was out about a year later.
While you might not think having MS-DOS source code is a big deal, there’s still a lot of life left in DOS and it is also interesting from an educational and historical perspective. If you don’t want to read x86 assembly language, there’s also the BASIC source for the samples (paradoxically, in the bin subdirectory) along with compiled COM files for old friends like EDLIN and DEBUG.
Of particular interest is the very small assembler source code. There’s also the source for something that would convert Z80 code to x86 which could be fun to pick apart. Be warned, though. There are not many comments in the file’s 1,200 lines.
The source code for version 2.0 has a lot more files including the source for things like EDLIN and DEBUG. We wonder if the version 1.25 files were lost, too ugly to show, or if the COM files were hand-coded?
If you had told us in 1990 that Microsoft would open source MS-DOS we’d have had you committed. They did show their sense of humor with this little bit in the README file:
The source files in this repo are for historical reference and will be kept static, so please don’t send Pull Requests suggesting any modifications to the source files,…