Ring-protection mechanisms: general properties and significant implementations

A protection mechanism has the role of controlling different forms of interaction among computations of different users and among different portions of the computation of a same user. In the class of protection schemes, the ring-protection mechanism is the one that, without significant loss of programming generality, allows the interaction to be controlled, mainly within the same computation, in a simple and economic way. In the paper the most relevant characteristics of the ring model are first pointed out. Next, the architecture requirements of the model implementation are described, and finally, some relevant computer systems provided with a ring-protection mechanism are analysed, such as MULTICS, VAX11, iAPX286, ICL 2900 and MARA.