User Mode and Kernel Mode

Mode Bit

  • A bit, called mode bit, is added to the hardware of the computer to indicate current mode.
    • 0: kernel mode
      • When a task is executed on behalf of the operating system
    • 1: user mode
      • When a task is executed on behalf of the user

Switch

  • Whenever a trap or interrupt occurs, the hardware switches from user mode to kernel mode
    • i.e., change mode bit to 0
  • When a user application requests a service from the operating system (via a system call)
    • It must transition from user to kernel mode to fulfill the request
  • Privileged Instructions
    • The hardware allow privileged instructions to be executed only in kernel mode. 
    • The instruction to switch to kernel mode is an example of a privileged instruction. Some other example include I/O control, timer management, and interrupt management. 

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