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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 allow cross-ring calls and subsequent returns to occur without trapping to the supervisor. Automatic hardwaredoi:10.1145/800212.806498 dblp:conf/sosp/SchroederS71 fatcat:f3fxqui7bvbvhm7u7ve2rdcgaq