3C905B-TX Boot Prom Mapping?

[Dan Marks]

I am interested in using the NetBoot package to create a BOOTP/TFTP image
for my 3Com 3C905B-TX ethernet card.  However, it seems that the "standard"
type ROM images do not work.  I have the version of the card that has the
vertical ROM socket, not the horizontal one.  What I am calling the "standard"
ROM type is the type that maps the entire ROM contents (usually 16k or 32k)
into the upper conventional memory area.  It appears that this version of
the 3C905B I have does not do this.  The 3C905B takes either a 64k or 128k
part (the ATMEL 29C512 or 29C010).  It seems that in order to avoid mapping
in such a large ROM in its entirety, 3Com decided to have only the first
2 kilobytes of the ROM visible in the conventional memory area.  The code
in this 2K copies the remaining 62K off of the ROM into conventional memory
(I think) where it is executed and the actual boot loader code runs.

This means that the conventional boot ROMs that can be built by the NetBoot
package (http://www.han.de/~gero/netboot/) won't work because they expect
to be mapped in their entirety into conventional memory.  I do not have
the 3C905B-TX technical (register level type) documentation.  But if there
were a register on the 3Com board that controls how much of the ROM is
mapped into conventional memory (such as in the EEPROM), then perhaps it
could be set to allow the entire memory to be mapped.

Alternatively, if the registers on the 3C905B-TX were known that controls
the memory mapping into extended memory of the ROM, then a similar boot
loader snippet could be written for the visible 2K of the 3C905B boot ROM
that would similarly copy the Netboot code into conventional memory and
allow it to function normally.

Another snag I've found is that it seems there may be a special "signature"
needed to make the ROM visible that I do not know.  This "signature" if it
existed would be above and beyond the "55AA" signature for any ROM in the
PC architecture, and is also not the "$Pnp" or "PCIR" signature blocks that
are present in the ROMS for PNP and PCI cards (or perhaps it may be a
nonstandard version of these signature blocks).  I have attempted to build
a NetBoot ROM on a 29C512 part with the PCIR and $Pnp signatures that
signify a network bootable ROM, but the 3Com card refuses to even map it
at all and does not allocate any conventional memory space for it.  I think
the 3C905B may be looking for this signature and not mapping the card at
all if it does not find it.  All I can really find out about the signature
from 3Com is that, because they have acquired a company called Lanworks
to write their boot roms for them, they have decided to make all of the
boot ROMs proprietary and the information about such a signature would not
be available from 3Com.  This is especially disappointing because it would
probably mean that future 3Com cards would require proprietary boot PROMs.

This new ROM format is part of the 3Com "Tri-ROM" MBA ROM format (I guess
so named because it supports TCP/IP, Netware, and PXI), which is what
necessitated such a large ROM part to be used.

So far this is what I know.  If someone knows any technical information
about the ROMs or their mapping registers it would be much appreciated.

Dan Marks
dmarks@uiuc.edu