Microsoft open-sources "the earliest DOS source code discovered to date" Old 86-DOS source code dates back to the time before Microsoft bought it. https://arstechnica.com/gadgets/2026/04/microsoft-open-sources-the-earliest-dos-source-code-discovered-to-date/?utm_brand=arstechnica&utm_social-type=owned&utm_source=mastodon&utm_medium=social