Ultrix tape job is unkillable!

[John B. Nagle]

      Lost interrupts tend to be more of a problem on systems with interrupts
as edges rather than levels.  PDP11s and VAXen are in the former category, and
Motorola M680x0 machines are in the latter.  On Motorola iron, and on the
buses usually used with it, controllers raise an interrupt line when they
want attention, and the interrupt will recur until the controller is made
happy.  Thus, interrupts tend not to be lost; even if the CPU, bus, or
driver loses one, it will recur until serviced.

      On the other hand, obscure errors in driver interrupt processing in
level-triggered interrupt systems tend to result in the system hanging
in a tight loop incorrectly servicing the interrupt.  In systems with
edge-triggered interrupts, one needs a timer to detect lost interrupts.
In systems with level-triggered interrupts, one may need a counter to
detect ones that won't clear.

					John Nagle