Lecture6 - System Call Programming & Debugging Week 6...

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System Call Programming & Debugging Week 6
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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
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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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Processor Modes • Mode bit may define areas of memory to be used when the processor is in supervisor mode vs user mode User Space Supervisor Space Supervisor Process User Process
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The Kernel • Part of system software executing in supervisor state • Trusted software: implements protection mechanisms that could not be changed through actions of untrusted software in user space
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trap call(…); return User Space Supervisor Space Trap: exception or fault Execute a trap handler inside kernel
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This note was uploaded on 11/16/2011 for the course CS35L 187105201 taught by Professor Pope during the Spring '10 term at UCLA.

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Lecture6 - System Call Programming & Debugging Week 6...

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