2840	 Protection in Operating Systems	 A model of protection mechanisms in computing systems is presented and its appropriateness is argued. The safety problem for protection systems under this model is to determine in a given situation whether a subject can acquire a particular right to an object. In restricted cases it can be shown that this problem is decidable i.e. there is an algorithm to determine whether a system in a particular configuration is safe. In general and under surprisingly weak assumptions it cannot be decided if a situation is safe. Various implications of this fact are discussed. protection protection system operating system decidability Turing machine
