After some more help from uwe, he pointed out that revision 1.169 of locore.s
is almost certainly the culprit.  After the change to use separate software
interrupt handler chains, it's not possible to share hardware and software
interrupts at the same level on sun4 and sun4c (so hardware level 1, 4 and
6 won't work).  So, on the 4/300 with:

  dma0 at obio0 addr 0xfa001000 level 4: DMA rev 0
  le0 at obio0 addr 0xf9000000 level 6: address 08:00:20:09:6c:af
  esp0 at obio0 addr 0xfa000000 level 4: ESP100A, 24MHz, SCSI ID 7

neither ethernet nor SCSI will work.  Might this apply to other sun4 machines
too (4/1xx, 4/2xx and 4/4xx)?

Should sun4 go back to having shared software and hardware interrupt handler
chains?

Thanks,

J

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