The TPU$TRIGGER_ASYNC_ACTION routine, along with TPU$SPECIFY_
    ASYNC_ACTION routine allow applications to interrupt DECTPU after
    calling TPU$CONTROL. The command that was specified for this
    facility_index is put on the DECTPU queue of work items and is
    handled as soon as no other work items are present. This allows
    DECTPU to complete and stabilize its environment before executing
    the command. This routine must be called after control has been
    passed to DECTPU via the TPU$CONTROL routine.