de0 recognized but not configurable

[Gary Aitken]

I'm building a kernel with two ethernet devices, an ed0 and a de0.
Both devices are recognized during the hardware probe at system startup:
  de0: <Digital 21041 Ethernet> irq 9 at device 18.0 on pci 0
device_probe_and_attach: de0 attach returned 6
  ed0: <Netgear EA201 Ethernet Card> at port 0x240-0x25f irq 5 on isa 0

Both cards are good, as I've tested them in a different system (also fbsd).

However, the de0 card is not configurable in this system, although the 
ed0 device is:
  ifconfig de0 a.b.c.d netmask 255.255.255.248
  ifconfig: interface de0 does not exist

I presume the "device_probe_and_attach: de0 attach returned 6" has 
something to do with this;
my archive search didn't find anything, however.  
The irqs appear to be allocated as follows:
  irq1   atkbd0
  irq3   sio1
  irq4   sio0
  irq5    ed0
  irq6    fdc0
  irq9    de0
  irq11  bt0
  irq15  pci0 <S3 Trio Graphics Accelerator>

On the (other) system where the device is configurable successfully, the 
graphics card is at 9 and the de device is at 15.

My kernel config specifies
  device        de

There is an if_de.o in the compile/KERNELNAME directory.  An nm on the 
kernel shows
  d    if_de_pci_driver_list
  d    if_de_pci_driver_mod
  d    if_de_pci_mod
  d    if_de_pcimodule_sys_init

What determines the order of irq assignment?
What does the "device_probe_and_attach" message mean?  Device not 
present / responding?
Any hints on how to get this device to pay attention?
What's the syntax for pci device assignment in rc.config when specifying 
irqs?  I tried permutations on the isa version and got nowhere.

Thanks for any hints.

Gary