The Process/Kernel Model

  • A program usually executes in user mode and switches to kernel when requesting kernel services using system calls.
  • Kernel creates, eliminates, and syncs processes.  It is a process MANAGER not a process itself.
  • Kernel Threads: Run in kernel address space, do not interact with users, created during startup

Example of execution flow on a uniprocessor system

  • System call by user program runs something in kernel mode
  • Scheduler interrupts process 1 because it ran too long and allows process 2 to run
  • An interrupt was received by the system (eg. keyboard press) and processes tat before returning to process 2


Ways to activate kernel routines

  • System calls invoked by user programs
  • CPU raises an exception (error)
  • Interrupt by something like an I/O device (processed by interrupt handler)
  • Kernel threads

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