E.g., a system where users are isolated from each other but some pieces of information are shared by everyone. A user can implement in her program ways of restricting access not provided by the system. Protection mechanisms provides ways of permitting access. Knowledge of how the protection mechanism works should not compromise it. A system with completely isolated users See Fig.
Memory is divided into variable-size segments.
Each segment contains a user program. One segment contains the supervisor program which performs CPU scheduling. One descriptor is associated with each user program. The processor has a privileged state bit. If it is on, the currently running program can load the descriptor register and also modify this bit.