> Please try to add some printfs in the siop_pci and siop_pci_common code to
> try to find where it dies exactly.

Done. The following line in siop.c:siop_attach() appears to cause the problem

	sc->sc_chan.chan_id = bus_space_read_1(sc->sc_rt, sc->sc_rh, SIOP_SCID);

I can't give you a line number just now because my source has lots of extra lines in it (with printf on each one).

I'm rebuilding my test kernel with DDB switched on so that I can try and narrow it down to the instruction sequence generated for that line. Compiling with optimisation switched off didn't help. 

Cheers,
Lloyd