How to modify your PR440FX's BIOS images for netbooting
Our cluster "Ecgtheow" has 40 nodes with Intel
"Providence" PR440FX motherboards.
These are very nice for
clustering since they have everything we need, including an Intel
EtherExpress Pro, right on the motherboard. A serious defect of these
boards is that it's impossible to boot off the built in ethernet
interface. There is no socket for a netboot ROM and only
4k of available user flash area. It appears the only real use Intel
has in mind for this user flash area is store OEM logos. (Sheesh)
This pages gives the details of my solution to this problem and links
to everything you'll need to do it yourself.
*** WARNING ***
THE PROCEDURES OUTLINED ON THIS PAGE INVOLVE WRITING MODIFIED BIOS
IMAGES INTO YOUR BIOS. IT IS POSSIBLE THAT THIS WILL RENDER YOUR
MACHINE UNABLE TO BOOT. NOT ALL MOTHERBOARDS HAVE A BIOS RECOVERY
JUMPER. DO NOT ATTEMPT THIS IF YOUR BOARD DOES NOT HAVE SUCH A JUMPER
OR YOU DO NOT HAVE THE NECESSARY DISKS TO RECOVER.
YOU HAVE BEEN WARNED
The Solution Overview
The Flash BIOS on the PR440FX has a 4K user or logo area which it can
for option ROMs (such as netboot ROMs). The rest of the flash area is
reserved for System BIOS and other things. 4K is far too small for
every netboot ROM image that I've ever seen. (It also seems that the
BIOS doesn't call the option ROM images correctly but that's another
It turns out that the System BIOS doesn't actually use all 224K that
are reserved for it. It would be surprising if it did fill up the
space allocated for it perfectly. Fortunately the spots that are not
occupied are easy to spot.
My solution places fragments of the netboot ROM image into unused
spots in the System BIOS image. Then I use another Option ROM which
is small enough to fit into the user flash area to reassemble and run
the fragments of the netboot ROM at boot time.
The Solution Details
The interesting part of the boot process is what happens when no
bootable devices are found. In the original IBM PC's the BIOS started
the BASIC interpreter built into ROM by calling interrupt 18h. Modern
PC's don't have this anymore but most BIOSes still call interrupt 18h
if no bootable devices are found. (Interrupt 18h simply points to an
IRET instruction in this case.)
One way Neboot ROMs can work is by hooking interrupt 18h and running
the netboot code when no other bootable devices are found. (Every ROM
has an initialization routine called by ROM before attempting to
boot. The "hooking" is done by this routine.)
Since we have no socket for a boot rom and only 4k of available user
flash area, storing a 32k ROM image is going to be a problem. It
turns out that the system BIOS doesn't use all the flash memory that's
available on the motherboard. Luckily, it's easy you spot these
stretches by looking at hex dump. It turns out that you write data
into most of these sections without causing any problems. This leaves
three problems to be solved:
This is where the "loader" ROM that I've written comes in. The
initialization for the loader is the same as a netboot ROM, hooking
interrupt 18h. When the boot process finally comes to it, it scans
the entire system BIOS image for fragments of the Netboot rom.
(Storage of these fragments into the system BIOS image is done by the
"fillgaps" program.) These fragments are assembled into a segment in
low memory. This segment is then checked to see if it contains a
valid option ROM image. If so, the initialization function is
called. Then the loader calls interrupt 18h which the netboot ROM
should have hooked. That should commence netbooting. If not, the
loader will print an error message and return to the BIOS.
- The system BIOS area is not scanned for option ROMs.
- The BIOS image generally contains many small holes. There may
not be a single hole large enough to hold the entire option ROM
image. (Which means scanning the system BIOS area for option
ROMs wouldn't even be useful.)
- The Flash images loaded with the flash utilities are not mapped
into low memory.
How to install the loader ROM and the netboot ROM
Step by step instructions
The only board that I have done this with is the Intel "Providence"
PR440FX. This technique is probably portable to other boards using
AMI bios that will scan the user flash (logo) area for option ROMs.
- Netboot ROM
- I used netboot ROM images created by the netboot package for Linux to
boot Linux. I have not tried booting DOS or windows with theses ROMs
although it appears to be possible.
- Build Environment
- The build environment used was Red Hat Linux
with the 8086 tools from the TSX-11 (see below for links). The C
programs to generate ROM checksums and the like should compile fine in
UNIX environment and maybe even Windows.})
- Verify that the Flash BIOS recovery mechanism for you motherboard
works and that you have everything you need to use it. I really
- Make yourself a working netboot rom image.
- Grab Gero Kuhlmann's netboot utilities for Linux. (See
- Build a working ROM image. (Just try out the floppy image.
If that one works, so will the ROM version.)
Note: You need to build a ROM that hooks
interrupt 18h. To do this make sure you answer yes to the
question "Do you want the bootrom to look for boot drives
first?" Otherwise the ROM will hook interrupt 19h instead.
- Obtain the latest ROM image for your motherboard. (at the time this was
written, the latest version available was 1.00.08DI0)
- Take the System BIOS image files from the BIOS update package.
Use the "fillgaps" utility to place the netboot image in the
system BIOS image.
Important note: Not all the gaps in the BIOS image
are useable. On the Intel Providence board, the tail portion of
the last segment of the system BIOS image contains the user flash
area (which we don't want to overwrite with the netboot rom
image) and an area (which is reserved for some other purpose)
which gets overwritten.
Therefore you should specify the BIOS image files in order on the
fill gaps command line. Here is the command that I used to pack my BIOS:
fillgaps image.rom 1008di0_.bio 1008di0_bi1 1008di0_.bi2
- Using the Flash Utilities, install these new BIOS images.
- Install the "loader" ROM image into the user flash area.
- Using the Flash Utilites, save a copy of the user flash
- Place a copy of loader ROM image. If the user flash image
from the previous step was saved into "loader.usr" and the
loader ROM image is in loader.rom, you could use the
dd if=loader.rom of=loader.usr bs=1 seek=128 conv=notrunc
- Use the Flash Utilites to load the modifidieplace the new userflash image back
- Go into the BIOS setup and go to the boot options menu.
- Set your boot sequence to floppy disk, network, hard disk.
(You can set it to something else, just remember that the
loader ROM appears as the "network" boot device and it
should appear other boot devices.
- Enable "Scan User Flash Area". (This is under "Boot Options")
- Remove all floppies and reboot. After trying the floppy disk you
should see the "Flash ROM loader" banner. After that, if all is
well, you should see the startup messages from the netboot ROM.
Loader ROM Error Messages and other trouble shooting
- Failed to determine Flash ROM size
- The loader uses BIOS interrupt 15h, service=db01 to ask the
BIOS how big the flash images is. If you see this message
you're probably out of luck.
- Flash ROM read failed.
- BIOS interrupt 15h service=db00 failed to read a piece of
the flash BIOS. If this fails, you're definitely out of luck.
- No boot ROM found in Flash ROM
- The loader assembles all the fragments it finds into a
segment of low memory. If it doesn't find a valid option
ROM after it's done assembling segments, it prints this
- ROM image failed to boot.
- The loader expects the netboot ROM's initialization routine
to hook interrupt 18h. The loader then calls interrupt 18h
and doesn't expect it to return. If you see this message, it
returned and probably means the netboot rom image didn't hook
interrupt 18h. Make sure you answer "y" to "Do you want the
bootrom to look for boot drives first?" when building the
netboot ROM image.
- The system fails to boot.
- Whoops! I guess you screwed up your BIOS image. You might try
specifying the System BIOS images in a different order when
running "fillgaps". In any case, you get to try out your
recovery jumper and recovery disk!
- The system boots ok - selecting network as a boot option doesn't
- This probably means that your BIOS isn't running the loader
option ROM. Make sure that "Scan User Flash Area" is enabled in
the boot options.
- The system hangs at boot time but only when network is selected
as a boot device.
- Congratulations, you've found a bug in the boot ROM.
- The source package for the loader ROM and utilities.
- This package contains binaries for a boot ROM appropriate for the
PR440FX, the loader ROM and a Linux ELF binary for the "fillgaps"
utility. If you have a PR440FX, this package and the BIOS upgrade
from Intel (version 1.00.08DI0) should be enough to generate the
modified BIOS images.
- The Netboot Page
- This is the home page for Gero Kuhlmann's very excellent Netboot
- Linux 8086 Development Tools
- The assembler (as86) and linker (ld86) needed to build the ROM image.
- Ralf Brown's Interrupt List
- A great big heap of information about PC interrupts including
lots of undocumented stuff. (It's even got documentation for the
installation check that a bunch of viruses do. That's awesome.)
- PR440FX Documentation
- This documentation doesn't actually contain much useful
information for coding. It features mainly market-drone type
statements of the form "product X supports feature Y".
Contact: Erik Hendriks