How operating systems work and how to write one.
Getting started with bare-metal assembly
How to create a hello world program in assembly that runs directly on top of UEFI (no operating system!). By Jonas Hvid.
How Operating Systems Work: 10 Concepts you Should Know as a Developer
Helicopter view of various central operating system concepts.
Learning operating system development using Linux kernel and Raspberry Pi
Learn how to create an operating system from the ground up using a Raspberry Pi.
X Window System Basics
Under the hood explanation and walk through about how the basics of X Window system works.
Writing a Simple Operating System from Scratch
How-to book on implementing a basic operating system.