Abstract: A memory management and protection mechanism in which access to protected entitites is controlled. The protected entities are represented by descriptors. Each protected entity is accessed via a selector which comprises an index integer assigned to the descriptor at the time of its creation. Tasks are active entities which may perform accesses and therefore are subject to control. A task has certain access rights. Each protected entity is assigned a specific privilege level. Each task within the system operates at one and only one privilege level at any instant in time. Protected entities which reside at a privilege level which is equal or less privileged than the current privilege level (CPL) of the task are generally accessible. The effective privilege level (EPL) of an access to a protected entity is defined as the numeric maximum of the CPL and the requested privilege level (RPL) present in the selector pointing to the memory segment to be accessed.