Transfer of control(interrupt)
  Interrupts - transfers execution to code external to program.
    May be requested by program (standard system functions)

    May be triggered by external events. (printer out of paper).

    Should preserve PC, CCR, and other registers.
      May be responsibility of cpu 
      or may have to be coded into the software invoked by the interrupt.

    When returning from an interrupt, the cpu should be restored to
    its original state, although exceptions exist.