Ring 0 used to be the privileged mode

Then came Ring -1

Then Ring -2, then Ring -3

See the work by Christopher Domas

DEF CON 26 - The Ring 0 Facade Awakening the Processors Inner Demons

Your computer is not yours. You may have shelled out thousands of dollars for it. It may be sitting right there on your desk. You may have carved your name deep into its side with a blowtorch and chisel. But it's still not yours. Some vendors are building secret processor registers into your system's hardware, only accessible by shadowy third parties with trusted keys. We as the end users are being intentionally locked out and left in the dark, unable to access the heart of our own processors, while select organizations are granted full control of the internals of our CPUs. In this talk, we'll demonstrate our work on how to probe for and unlock these previously invisible secret registers, to break into all-powerful features buried deep within the processor core, to finally take back our own computers.