As part of Microsoft's recent open source initiative, they have (re-)released the source code for MS-DOS 1.25 and 2.0 as open source. It's a "re-release" because the code was already released back in 2014 via the Computer History Museum, but they have recently added the code to their MS-DOS Github repo (Microsoft now owns Github of course ...) to make it more accessible.
Here are some of their notes regarding the code:
- All the source for MS-DOS 1.25 and 2.0 was written in 8086 assembly code
- The source code for the initial release of 86-DOS dates from around December 29th 1980
- The MS-DOS 1.25 code dates from around May 9th 1983, and is comprised of just 7 source files, including the original MS-DOS Command-Line shell - COMMAND.ASM!
- MS-DOS 2.0 dates from around August 3rd 1983, and grew considerably in sophistication (and team size), and is comprised of 100 .ASM files
- There are some interesting documentation (.TXT, .DOC) files interspersed with the source and object files - many are well worth a read, as are many of the source code comments!
View, download, or fork the MS-DOS ASM code at: https://github.com/microsoft/ms-dos