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.