The Ring Model: 4 ring model that separates Users (Untrusted) from the Kernel (Trusted). The full
model is slow and rarely used; most OS’ only use rings 0 and 3. The applications are at layer 3.
There is a new addition to the Ring Model: Hypervisor mode is called Ring -1 and is for VM Hosts.
Ring -1 sits below the Client kernel in Ring 0.