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Operation System - Coggle Diagram

- - - - Fetch the next instruction from memory : take much time than other step
      - Decode the instruction to figure out what to do
      - Execute the instruction and store the result
- - - - "Stealing" memory space for device registers has 2 functions:
        
        Allows protected access --- only the OS can access the device.
        
        User programs must trap into the OS to access I/O devices because of the
        
        normal protection mechanisms in the processor
- - - - Only OS code is executed in system mode
      - Only OS can modify its memory
      - The mode register can only be modified in system mode
    - - Mode register provided
        
        zero = CPU is executing the OS (in system mode)
        
        one = CPU is executing in user mode