Microsoft Disk Operating System (MS DOS) was a very popular Command Line Interface (CLI) Operating System in the late 1980's and early 1990's. It offered very basic File, Input/Output, Process and Memory management capabilities with a very low memory footprint. It was widely used back in the inception days of Microsoft and was licensed out to very famous hardware vendors such as IBM, HP, Compaq, etc.
Now Microsoft has released the source code of this vintage and legendary Operating System by releasing it in Github. The Source code is in Assembly Language which is readable but you may still find it a bit difficult if you are only familiar with high level programming languages. Nevertheless it is good through the source codes of the commands you might have used once in your life like COPY, DIR, COMMAND, etc. The versions 1.25 and 2.0 are released in both Binary and Assembly Language.
You can find the source here.