OS 01/24

Operating System Note 01/24

Two abstractions: processes, files.
Process is a program in execution.

Virtualize CPU

  • Challenges
    Efficiency
  • Mechanism
    Limited Directed Execution
  • User mode, Kernel mode
    • User mode can execute a system call to perform restricted operation
    • Execute a trap instruction, jumps into kernel and raise privilege level, when finished, OS calls a return-from-trap instruction, reduce privilege level
  • Switching between processes
    How to regain control of the CPU? (Since OS is not running when a process running on the CPU)
    • Wait for system calls (Cooperative approach)
    • OS takes control (Non-cooperative): A timer interrupt
  • Saving and Restoring Context
    Decision made by scheduler. If decide to switch, executes context switch.
    • Context switch
      OS saves register values for the current running process, and restore for the soon-to-be-executing process.
      For timer interrupt, the user registers are implicitly saved by hardware.
      For OS switch, the kernel registers are explicitly saved by software.

Between OS & Hardware

  • Protection
    1. CPU modes - User, Kernel (privileged instruction) (monitor, supervisor)
    2. Base + limit registers base <= address < limit base
    3. Interrupts (timer) /Allow sys do sth else/
      • stop current execution
      • handle interrupt
      • resume current execution
      • e.g while (1); How do sys regain control?
        Set a timer, when counts to 0, kernel sends an interrupt
      • Can masking certain interrupts
      • CPU can have status disable&enable
      • System calls -> Software interrupt
  • Interrupt - btw instructions
  • Exception - within an instruction
  • Trap - go to the OS