A really simple and lightweight chat server.
It's so lightweight, it can run on your aging Raspberry Pi.
Raspberry Pi OS is a step-by-step guide that teaches how to create a simple operating system from scratch. The OS source code is largely based on Linux kernel, but it has very limited functionality and supports only Raspberry PI 3. The main goal is not only to teach OS development, but also to use the project as a starting point to learn Linux.