[hga]

Initial software based Multics had 64 rings, and as I recall only 8 in the hardware versions. No more than 4 were needed in practice: 0 for root, 1 for mail (e.g. you could delete mail you'd sent to other people from their mailboxes if they'd not read it yet), 4 for normal users, and 5 for some stuff that e.g. allowed anyone to use, but was restricted at touching anything deeper in the system.

AMD dropped rings in their 64 bit architecture which Intel was forced to adopt, so they're becoming a historical curiosity.

[creshal]

> AMD dropped rings in their 64 bit architecture which Intel was forced to adopt, so they're becoming a historical curiosity.

Not quite. Hypervisors are operating in Ring -1, SMM is equivalent to another ring above that, and I can't find anything about AMD64 dropping Ring 1/2? Ring 0/3 at least are still in use.

[e12e]

AFAIK 64-bit linux has kernel in 0 and userspace in 3?

http://wiki.osdev.org/SYSENTER

http://www.x86-64.org/pipermail/discuss/2000-October/001068....

I tried looking at the latest 4.2 kernel tree - but the assembler/c-code that sets up and deals with sys-calls has been rather re-factored, so I'm not entirely adamant it's ring 0 and ring 3 for both 32bit and 64bit - but I think so? (From a quick glance, no sections stand out as calling out ring 3 explicitly when talking about returning to user-land -- granted I didn't do any searching over the code).

[hga]

I submit that "supervisor/user" isn't an implementation of "rings", plural (and it's got to predate Multics by a lot, but I can't quickly prove that), and that SMM is something entirely different. Hypervisors and there use really aren't comparable to Multics Rings.

A couple of minutes with Google only found hints that confirm my memory WRT to AMD64 and rings, and/or Intel not copying a segmentation feature added to later versions of AMD's chips.