“Everything starts with at the Kernel. Operating systems are
using Privilege Rings.
These rings are created by CPU and not by OS. Any OS kernel
operates in Ring 0 which is most privileged level and can
comunicate directly to the hardware and the CPU. Rings 1 and 2 are
commonly used for device drivers. And ring 3 is used for user-space
applications (media players, web servers and anything else user can
communicate to directly). Device drivers are a „bridge”
between user-space applications and hardware. You should note that
in Linux rings 1 and 2 are NOT used (at least this is what I found
out…), because Linux drivers are compilled directly into
kernel or as a dynamic kernel modules (in both cases drivers appear
in at a Ring 0).