User and Supervisor Processes

POSIX processes are divided into two types, user and supervisor processes. The main differences between user and supervisor processes are outlined below:

• Supervisor processes run in the same address space as the system. User processes run in their own private address space so that if they attempt to reference a memory address that is not valid in their address space, they encounter a fault and, by default, are deleted.

• Supervisor processes run in privileged mode. This is architecture-specific, and means that certain privileged instructions are available to supervisor processes which are not available to user-mode applications.

• User applications must trap into the system, whereas supervisor applications do not since they operate in privileged mode.

• Supervisor applications have only one stack (the system stack), whereas user applications have one stack in user mode and another stack to execute when they trap into privileged mode.

• Some libraries are different between supervisor applications and user applications, since supervisor applications can execute privileged instructions, and user applications trap to access system services.