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. protection, protection rings, protection hardware, access control, hardware access control, computer utility, time-sharing, shared information, segmentation, virtual memory, Multics 4.32 6.21 CA720304 JB January 31, 1978 2:58 PM 1458 4 2377 1523 4 2377 1603 4 2377 1698 4 2377 1747 4 2377 1748 4 2377 1753 4 2377 1828 4 2377 1854 4 2377 1854 4 2377 1877 4 2377 1877 4 2377 1960 4 2377 1960 4 2377 2150 4 2377 2317 4 2377 2319 4 2377 2358 4 2377 2377 4 2377 2377 4 2377 2377 4 2377 2377 4 2377 2378 4 2377 2372 4 2377 2342 4 2377 2376 4 2377 2379 4 2377 2424 4 2377 2482 4 2377 2497 4 2377 2558 4 2377 2618 4 2377 2625 4 2377 2625 4 2377 2626 4 2377 2626 4 2377 2632 4 2377 2632 4 2377 2669 4 2377 2704 4 2377 2723 4 2377 2738 4 2377 2740 4 2377 2741 4 2377 2840 4 2377 2840 4 2377 2867 4 2377 2941 4 2377 3105 4 2377 3144 4 2377 3184 4 2377 1471 5 2377 1746 5 2377 1749 5 2377 2358 5 2377 2377 5 2377 2377 5 2377 2377 5 2377 2436 5 2377 2626 5 2377 2868 5 2377 2870 5 2377 953 6 2377 1746 6 2377 2080 6 2377 2358 6 2377 2377 6 2377 2377 6 2377 2377 6 2377 2377 6 2377 2380 6 2377 2558 6 2377 2625 6 2377 2625 6 2377 2629 6 2377 2632 6 2377 2632 6 2377