NAME
       Cause -- cause a software interrupt

   SYNOPSIS
       Cause(interrupt)
             A1

       void Cause(struct interrupt *);

   FUNCTION
        This function causes a software interrupt to occur.  If it is
        called from user mode (and processor level 0), the software
        interrupt will preempt the current task.  This call is often used
        by high-level hardware interrupts to defer medium-length processing
        down to a lower interrupt level.  Note that a software interrupt is
        still a real interrupt, and must obey the same restrictions on what
        system function it may call.

        Currently only 5 software interrupt priorities are implemented:
        -32, -16, 0, +16, and +32.  Priorities in between are truncated,
        values outside the -32/+32 range are not allowed.

   NOTE
        When setting up the interrupt structure, set the node type to
        NT_INTERRUPT, or NT_UNKOWN.

   IMPLEMENTATION
        1> Checks if the node type is NT_SOFTINT.  If so does nothing since
           the softint is already pending.  No nest count is maintained.
        2> Sets the node type to NT_SOFTINT.
        3> Links into one of the 5 priority queues.
        4> Pokes the hardware interrupt bit used for softints.

        The node type returns to NT_INTERRUPT after removal from the list.

   INPUTS
        interrupt - pointer to a properly initialized interrupt node

   BUGS
        Unlike other Interrupts, SoftInts must preserve the value of A6.