Processor Modes •Mode bit used to distinguish between execution on behalf of OS & execution on behalf of user. •Supervisor mode: processor executes every instruction in its hardware repertoire. •User mode: can only use subset of instructions
Processor Modes •Instructions that can be executes in supervisor mode are supervisor, privileges, or protection instruction –I/O instructions are protected. If an application needs to do I/O, it needs to get the OS to do it on its behalf. –Instructions that can change the protection state of the system are privileges (e.g., process’ authorization status, pointers to resources, etc)
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