Computer Science - Operating Systems Concepts - Discussion

Operating in one of two possible modes, or states: the user state, or the supervisor state.

The user state is the default (normal) state of operation, in which user programs are executed. The supervisor state is a special mode of operation to which the user has no access. When it is in the supervisor state, the processor and its actions are entirely controlled by the Operating System (OS). The supervisor state is entered only on special occasions not part of the regular processing work of the CPU, like start up, shut down, dealing with hardware or software errors etc. Those special occasions are called exceptions.

Typically associated with mainframes, it is a hardware mode in which the operating system executes instructions unavailable to an application program; for example, I/O instructions. Contrast with program state, which is an operating mode of the computer that executes instructions in the application program.

Supervisor mode: Usually what the OS runs in. It is entered when the processor encounters a software interrupt instruction. Software interrupts are a standard way to invoke operating system services on ARM.