Microsoft has decided to open source DOS 1.0 on the 45th anniversary. In 2018, Microsoft open-sourced MS‑DOS 1.25 and 2.11. In 2024 they made the source for MS‑DOS 4.0 available to the public as well. On DOS 1.00’s 45th anniversary, Microsoft has released  the earliest DOS 1.0 source code discovered to date.

Check out the Paterson listings on github. It has transcription of the listings, original files extracted from printouts of the source.