Implementing Protection Rings.pdfVIP

A Hardware Architecture for Implementing Protection Rings Michael D. Schroeder and Jerome H. Salt zer Massachusetts Institute of Technology* 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 hardware validation of references across ring boundaries is also performed. Thus, a call by a user procedure to a protected subsystem (including the 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. Key Words and Phrases: protection, protection rings, protection hardware, access control, hardware access control, computer utility, time-sharing, shared information, segmentation, virtual memory, Multics CR Categories: 4.32, 6.21 Copyright ? 1972, Association for Computing Machinery, Inc. General permission to republish, but not for profit, all or part of this material is granted, provided that reference is made to this publication, to its date of issue, and to the fact that reprinting privileges were granted by permission of the Association for Com- puting Machinery. Introduction The topic o f this paper is the control o f access to stored informat ion in a computer utility. The paper describes a set o f processor access control mechanisms that were devised as part of the second i teration o f the hardware base for the Multics system. These mecha- nisms provide a hardware implementat ion of protect ion rings which limit the access privileges o f an executing program. °Mult ics is a genera