Context Switch

• Switching between processes is termed a context switch. When the CPU switches to another process, the system must save the state of the old process and load the saved state for the new process;
  • process table keeps track of processes,
  • context information stored in PCB,
  • process suspended: register contents etc stored in PCB,
  • process resumed: PCB contents loaded into registers
• Context-switch time is overhead; the system does no useful work while switching.
• Context switching can be critical to performance,
• Dealing with multiple processes is difficult;
  • Synchronization ensure a process waiting for an I/O device receives the signal, signals may be lost or duplicated.
  • Failed mutual exclusion attempt to use a shared resource at the same time.
  • Non-deterministic program operation; program should only depend on input to it, not relying on common memory areas.
  • Deadlocks.

  Figure 11: CPU Switch From Process to Process

  
• OS requirements for multiprogramming;
  • Policy to determine which process to schedule (Scheduler).
  • Mechanism to switch between processes (Low-level code that implements the decision Dispatcher).
  • Methods to protect processes from one another (memory system).