Attached is the modified file.  It only required two bytes to be
changed. If you already have the Phobos driver software installed,
just replace the original if_fe.o file in /var/sysgen/boot with this
one, do an autoconfig -v (BTW, I'm using IRIX 6.5.11), and it should
work. Then you can use FEControl to set the interface speed (auto
detect didn't seem to work for me, I forced it to 100Mbit/Full
Duplex). Don't forget to change /etc/config/netif.options to use fe0
instead of ec0 if you want this to be the primary adapter.

I get this to work, first I did a disassembly of the if_fe.o file
(using dis). The code is located in a function called if_feedtinit.
After looking through the code, I calculated a few offsets so I could
get a feel for something to look for. Anyways, to make a long story
short, I was laying down half asleep last night looking at this thing
when it just popped out--  I was trying to find where the program
made the call to print the text where it said unknown adapter, which
I calculated at offset 208 in the text section. I saw two calls that
used offset 208 as a parameter, and they were the only ones in the
whole file.

MIPS assembly is very "between the lines" and the compiled C code
looks very unfriendly-- I'm an assembly language guy from the old
days (6502, Z80), and this was definitely a change from what I was
used to but basically here it is:

There is some kind of call to some other routine, upon return
(located at 0x404 in the disassembly):

    sd    v0,40(sp) sh    v0,24(sp) lhu    a3,24(sp) li    a4,96
    beq    a3,a4,0x48c nop li    a0,4 lui    a1,0 addiu    a1,a1,208
    lw    a2,0(sp) jal nop b    0x9d4


I had previously determined that the code at 0x9d4 exits back to the
OS. As you can see when it returns, it compares some value in
register a3 with 96. If it's equal, it will continue at 0x48c which
contains another dozen or so lines that do another system call. If
it's not equal, it moves that magic offset 208 into the register,
calls a print routine (jal) and then exits. I changed the
"offending" beq to bne (branch if not equal) to ignore it (this also
has the side effect of making it think a Phobos card is an unknown
adapter! hehe).

The other check ANDs some result from another call with 0xff00 and
ORs it with 0xf500 (at least that's what it looks like):

    sd    v0,40(sp) sh    v0,24(sp) lhu    a7,24(sp) andi
    a7,a7,0xff00 ori    t0,zero,0xf500 beq    a7,t0,0x548 nop li
    a0,4 lui    a1,0 addiu    a1,a1,208 lw    a2,0(sp) jal nop b
    0x9d4

Again, bne was replaced with beq (0x17). I used a hexeditor to do
all this.

Good luck, at least now you know it's not a myth! The beauty of this
is that the 3Com card cost me US $20!

Take care, Sergio