A Hardware Architecture for Implementing Protection Rings
Protection of computations and information
is an important aspect of a computer utility.  In 
a system which uses segmentation as a memory addressing
scheme, protection can be achieved in part by 
associating concentric rings of decreasing access privilege
with a computation.  This paper describes 
hardware processor mechanisms for implementing these rings
of protection.  The mechanisms for implementing 
these rings of protection. The mechanisms allow cross-ring
calls and subsequent returns to occur without 
trapping to the supervisor.  Automatic hardware validation
of references across ring boundaries is also 
performed.  Thus, a call by a user procedure to a protected
subsystem (including the supervisor) is identical 
to a call to a companion user procedure.  The mechanisms
of passing and referencing arguments are the 
same in both cases as well.
CACM March, 1972
Schroeder, M. D.
Saltzer, J. H.
