The first
operating system I know of written in a
high-level programming language (
PL/I).
(Actually
EPL).
Security in Multics was implemented with 64 rings.
In
The Aesthetics of Computing (page 62), David Gelernter compared the rings to those of Dante's hell.
The comparison was apt, in that moving to a higher-numbered ring gave one lower privileges.
You could say "abandon access, all ye who enter here" (my words, not Gelernter's).
Some canonical references are:
F. Corbató, V. Vyssotsky, "Introduction and Overview of the Multics System", AFIPS Conference 27,
1965, pp. 185-196.
F. Corbató, J. Saltzer, C. Clingen, "Multics - The First Seven Years", Spring Joint Computer Conference,
1972, pp. 571-583.
Organick, Elliott, The Multics System: an Examination of its Structure, MIT Press, Cambridge, MA, 1972. ISBN 0-262-15012-3