[W/K] :: multitasking

  multitasking
       
           (Or "multi-tasking", "multi-processing",
          "multiprogramming", "concurrency", "process scheduling") A
          technique used in an operating system for sharing a single
          processor between several independent jobs.  The first
          multitasking operating systems were designed in the early
          1960s.
       
          Under "{cooperative multitasking" the running task decides
          when to give up the CPU and under "{pre-emptive multitasking"
          (probably more common) a system process called the
          "{scheduler" suspends the currently running task after it has
          run for a fixed period known as a "{time-slice".  In both
          cases the scheduler is responsible for selecting the next task
          to run and (re)starting it.
       
          The running task may relinquish control voluntarily even in a
          pre-emptive system if it is waiting for some external event.
          In either system a task may be suspended prematurely if a
          hardware interrupt occurs, especially if a higher priority
          task was waiting for this event and has therefore become
          runnable.
       
          The scheduling algorithm used by the scheduler determines
          which task will run next.  Some common examples are
          round-robin scheduling, priority scheduling, shortest job
          first and guaranteed scheduling.
       
          Multitasking introduces overheads because the processor
          spends some time in choosing the next job to run and in saving
          and restoring tasks' state, but it reduces the worst-case time
          from job submission to completion compared with a simple
          batch system where each job must finish before the next one
          starts.  Multitasking also means that while one task is
          waiting for some external event, the CPU to do useful work
          on other tasks.
       
          A multitasking operating system should provide some degree of
          protection of one task from another to prevent tasks from
          interacting in unexpected ways such as accidentally modifying
          the contents of each other's memory areas.
       
          The jobs in a multitasking system may belong to one or many
          users.  This is distinct from parallel processing where one
          user runs several tasks on several processors.  Time-sharing
          is almost synonymous but implies that there is more than one
          user.
       
          Multithreading is a kind of multitasking with low
          overheads and no protection of tasks from each other, all
          threads share the same memory.
       
          (1998-04-24)