Dispatcher

• Dispatch Mechanism OS keeps system-wide list of processes. Each process in one of three modes; Running: On the CPU (only one in uniprocessor system), Ready: Waiting for the CPU, Blocked: Waiting for I/O or synchronization with another thread. Dispatch loop:

  while (1) {
  run a process for a while
  stop process and save its state     context-switch
  load state of another process       context-switch
  }
  

• How does Dispatcher gain Control?
  • must change from user mode to system mode; the CPU can only do one thing at a time. While a user process is running, dispatcher cannot run, thus the operating system may lose control
  • two ways operating system gains control;
    • Traps: Events internal to user process (System calls, Errors, Page faults)
    • Hardware interrupts: Events external to user process (Character typed at terminal, Completion of disk transfer, Control given to OS interrupt service routine (ISR))
• Dispatcher must track state of process when not running; On every trap or interrupt, save process state in Process Control Block (PCB)