Excerpt. The complete work is at the source.
The oldest MS-DOS source Microsoft has published is version 1.25, from March 1982, and its main file still opens with the name the system had before Microsoft bought it. Tim Paterson wrote 86-DOS at Seattle Computer Products in 1980 and 1981; Microsoft licensed it in December 1980, bought it outright in July 1981, sixteen days before the IBM PC launched, and shipped it with that machine. The file begins not with code but with a changelog.
Every line of the history is a decision a working operating system had to make: 32-byte directory entries, Ctrl-C handling, hidden files, a date and time on each file, a separate file allocation table. Version 1.13 fixes a "classic" buffer bug; 1.23 un-does a change in the IBM version "back to 1.01!". The instruction at the top, that every change must be noted, is version control by discipline. Microsoft first released this code through the Computer History Museum in 2014 under a licence that allowed study only, then put it on GitHub in 2018 under the MIT licence, where anyone can read all 4,030 lines.