• Process: An instance of a program in execution.  Or the “execution context” of a running program.
  • Traditionally a process executes a single set of instructions in its allotted address space
    • Modern OSs allow processes with multiple execution flows (multiple sequences of instructions executed in the same address space)
  • Now in multiprocessing/multiprogramming capable computers (2 or more CPUs) all CPUs share system resources such as RAM.
    • Therefore it is possible for multiple CPUs to execute code from the same address space
  • On uniprocessor systems only one process can hold the CPU and only one execution flow can occur at any time
    • Scheduler: Chooses the process which can progress
      • If the processes is “nonpreemptable” it means the scheduler is only invoked when the process voluntarily stops using CPU
      • All processes in multiuser systems must be preemptable
        • The scheduler will be invoked periodically
  •  Unix is multiprocessing and uses preemptable processes and adopts a process/kernel model
  • Process/kernel Model:
    • Each process believes its the only process on the machine and has exclusive rights to all resources
    • System calls change process from user to kernel mode allowing access to only required resources for execution and then reverts back to user mode

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