This one is maybe a bit theoretical, but... How the heck can X11 touch the video hardware? As I understand it, X11 is an unprivileged user-mode program. But only kernel-mode software can access the hardware. So... how?
(It's a simple enough question, but I haven't been able to find any documentation that explains this simple point. There's a lot of documentation about how to set up X11, or how the X11 client/server arrangement works, but not much about how it drives the hardware...)
Basically I'm interested in knowing how much of the work is X11, and how much of it is the kernel, and where the two meet.