operating-systems computer-architecture
Definition
Privilege Ring
A privilege ring is a hierarchical level of privilege in a computer system’s architecture, enforced at the hardware level by some CPUs (for example, x86). Lower-numbered rings have more privilege and may access resources that higher-numbered rings cannot.
Rings
Ring levels
Ring Privilege Typical use -1 highest hypervisor — controls hardware access via virtualization instructions 0 high kernel — full access to CPU control registers and I/O controllers; also called supervisor mode 1 intermediate drivers (historical) 2 intermediate drivers (historical) 3 lowest applications — userspace code with restricted access
Hypervisor extension
Recent CPUs provide virtualization instructions that allow a hypervisor to control Ring 0 hardware access. These instructions operate only at the higher privilege level of Ring -1.