>I don't think NetBSD has ever handled shared IRQ on ISA, as this is not
>fully supported by the bus. Shared IRQ on PCI works fine, however.

Actually ... I am pretty sure that the code supports it just fine (it
will call all of the interrupt service routines for the devices that
are registered to use the same IRQ).

Whether or not the hardware supports it another question.  The case
where I used it successfully was a RS-485 board that had four 8250
UARTs that used one IRQ.  I just configured the kernel to point to
the right I/O addresses and told the kernel that the four UARTs
used the same IRQ, and it worked just fine.  Obviously there was
some glue logic on the board that made this sharing possible.

I doubt that two random devices on an ISA bus would be able to share
IRQs, but I'm not a PC expert, so don't listen to me :-)

--Ken