Have a look at Minix.
Minix (from "mini-Unix") is a POSIX-compliant (since version 2.0),[9][10] Unix-like operating system based on a microkernel architecture.

Early versions of MINIX were created by Andrew S. Tanenbaum for educational purposes. Starting with MINIX 3, the primary aim of development shifted from education to the creation of a highly reliable and self-healing microkernel OS. MINIX is now developed as open-source software.

MINIX was first released in 1987, with its complete source code made available to universities for study in courses and research. It has been free and open-source software since it was re-licensed under the BSD-3-Clause license in April 2000.[5]
https://en.wikipedia.org/wiki/Minix