SeaBIOS

SeaBIOS is an open-source legacy BIOS implementation which can be used as a coreboot payload. It implements the standard BIOS calling interfaces that a typical x86 proprietary BIOS implements.

This page describes using SeaBIOS with coreboot. SeaBIOS can also run natively in QEMU and bochs — see the SeaBIOS README file for information on non-coreboot uses.

Use cases

Any software requiring 16-bit BIOS services benefits from SeaBIOS (eg, Windows and DOS). SeaBIOS also enables booting Linux out of the box (using standard boot-loaders like GRUB and Syslinux).

SeaBIOS supports booting from ATA hard drives, ATAPI CDROMs, USB hard drives, USB CDROMs, payloads in flash, and from Option ROMs (eg, SCSI or network cards). SeaBIOS can initialize and use a PS/2 keyboard or USB keyboard.

Windows

SeaBIOS has been tested with Windows XP, Windows 2008, Windows Vista (64/32 bit), Windows 7 (32 bit and 64 bit).

However, Windows has a very strict ACPI interpreter, and many coreboot boards do not have a complete ACPI definition. As a result, many coreboot boards will fail during Windows boot (eg, it may fail with a STOP 0xA5 code).

So far ASUS M2V-MX SE and GIGABYTE GA-M57SLI-S4 have known good working ACPI and are able to boot XP/Vista/Windows 7. Please ask on the mailing list for the status of other boards/chipsets.

Linux

SeaBIOS has been tested with GRUB, LILO, and Syslinux. Linux booting works well.

Other

SeaBIOS has also been tested with FreeDOS, NetBSD, and OpenBSD.

Because SeaBIOS implements the standard x86 BIOS interfaces, it is expected many other operating systems and boot-loaders will work.

Building

Building via coreboot's menuconfig

Probably the easiest way to use SeaBIOS as coreboot payload is to simply use the coreboot build process, which downloads and builds SeaBIOS as payload by default nowadays. You just have to run the following in your coreboot checkout:

<source lang="bash"> $ make menuconfig $ make </source>

Both SeaBIOS and coreboot will be built, and SeaBIOS will be added as payload to the coreboot.rom image that is being built.

Manual build

One can download the latest version of SeaBIOS through a git repository:

<source lang="bash"> $ git clone git://git.seabios.org/seabios.git seabios $ cd seabios </source>

There's also a gitweb facility to browse the latest source code online.

Run make menuconfig and set the following variables:

• CONFIG_COREBOOT 1
• CONFIG_DEBUG_SERIAL 1

Then:

<source lang="bash"> $ make </source>

The final SeaBIOS payload file is out/bios.bin.elf.

coreboot

For best results, configure coreboot with CONFIG_WRITE_HIGH_TABLES and CONFIG_VGA_BRIDGE_SETUP both enabled, and CONFIG_VGA_ROM_RUN and CONFIG_PCI_ROM_RUN both disabled.

Finally, configure the SeaBIOS out/bios.bin.elf file as the coreboot payload and build coreboot. The resulting coreboot.rom file will contain both SeaBIOS and coreboot, and it can be flashed to a ROM chip.

SeaBIOS and CBFS

SeaBIOS can read the coreboot flash filesystem and extract files.

When SeaBIOS scans the target machine's PCI devices, it will recognize option ROMs in CBFS that have the form pciVVVV,DDDD.rom. It will also run any file in the directory vgaroms/ as a VGA option ROM not specific to a device and files in genroms/ as a generic option ROM not specific to a device. In the above cases, SeaBIOS will recognize files with a .lzma suffix, and automatically decompress them (eg, pci1106,3344.rom.lzma and vgaroms/sgabios.bin.lzma).

SeaBIOS can also load a graphical bootsplash image from bootsplash.jpg, payloads found in the CBFS directory img/, and floppy images found in the floppyimg/ directory.

Further, SeaBIOS can obtain configuration information from CBFS. A file bootorder determines the order of devices and methods to attempt to boot the system from. Additional configuration items may be found in the CBFS etc/ directory.

The examples below show some common uses of these features.

Adding a VGA option ROM

It is frequently necessary to add a VGA option ROM to CBFS in order to use a VGA adapter that is built-in to a motherboard. Note, VGA adapters on external cards (PCI, AGP, PCIe) do not require this step as SeaBIOS will automatically extract the VGA BIOS directly from the card. For machines without a VGA adapter, please follow the sgabios instructions below.

The first step is to find the vendor and device ID of the built-in VGA adapter. This information can be found from lspci:

<source lang="bash"> $ lspci -vnn ... 01:00.0 VGA compatible controller [0300]: VIA Technologies, Inc. UniChrome Pro IGP [1106:3344] (rev 01) (prog-if 00 [VGA controller]) </source>

In the above example, the VGA vendor/device ID is 1106:3344. Obtain the VGA ROM (eg, vgabios.bin) and add it to the ROM with:

<source lang="bash"> $ ./build/cbfstool coreboot.rom add /path/to/vgabios.bin pci1106,3344.rom raw $ ./build/cbfstool coreboot.rom print </source>

Alternatively, SeaBIOS supports LZMA compressed option ROMs. Use the following to add a compressed option ROM instead:

<source lang="bash"> $ lzma -zc /path/to/vgabios.bin > vgabios.bin.lzma $ ./build/cbfstool coreboot.rom add vgabios.bin.lzma pci1106,3344.rom.lzma raw $ ./build/cbfstool coreboot.rom print </source>

After the above is done, one can write the coreboot.rom file to flash. SeaBIOS will extract the VGA ROM and run it during boot.

Adding gpxe support

A gpxe option ROM can nicely complement SeaBIOS and coreboot by adding network boot support. Adding gpxe is similar to #Adding a VGA option ROM. The first step is to find the Ethernet vendor/device ID. For example:

<source lang="bash"> $ lspci -vnn ... 00:09.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL-8110SC/8169SC Gigabit Ethernet [10ec:8167] (rev 10) </source>

Then one can build a gpxe option ROM. For example:

<source lang="bash"> $ cd /path/to/gpxe/src/ $ make bin/10ec8167.rom </source>

And add it to the coreboot image. For example:

<source lang="bash"> $ ./build/cbfstool coreboot.rom add /path/to/gpxe/src/bin/10ec8167.rom pci10ec,8167.rom raw $ ./build/cbfstool coreboot.rom print </source>

As with VGA option ROMs, the gpxe option ROM may be compressed with LZMA. However, compression won't significantly reduce gpxe's size as it implements its own compression.

In addition to gpxe, other option ROMs can be added in the same manner.

Adding sgabios support

An sgabios option ROM can forward many VGA BIOS requests and keyboard events over a serial port. One can deploy it in addition to the primary VGA BIOS or by itself.

If the target machine does not have a VGA adapter, then one should install sgabios. Most bootloaders (eg, GRUB) require a VGA BIOS in order to function properly — the sgabios ROM can fill this requirement.

Place the sgabios ROM file in the vgaroms/ directory of CBFS. For example:

<source lang="bash"> $ ./build/cbfstool coreboot.rom add /path/to/sgabios.bin vgaroms/sgabios.bin raw $ ./build/cbfstool coreboot.rom print </source>

When using sgabios, all the characters that SeaBIOS writes to the screen will be seen twice — once from SeaBIOS sending the character to the serial port and once from sgabios forwarding the character. To prevent the duplicates unset CONFIG_SCREEN_AND_DEBUG in make menuconfig or (on newer versions of SeaBIOS) set the config file etc/screen-and-debug to zero.

Adding a graphical "bootsplash" image

SeaBIOS can show a custom JPEG image during bootup. To enable this, add the JPEG file to flash with the name bootsplash.jpg. For example:

<source lang="bash"> $ ./build/cbfstool coreboot.rom add /path/to/image.jpg bootsplash.jpg raw $ ./build/cbfstool coreboot.rom print </source>

The size of the JPEG image determines the video mode to use for showing the image. Make sure the dimensions of the image exactly correspond to an available video mode (eg, 640x480, or 1024x768), otherwise it will not be displayed.

SeaBIOS will show the image during the wait for the boot menu (if the boot menu has been disabled, users will not see the image). The image should probably have "Press F12 for boot menu" embedded in it so users know they can enter the normal SeaBIOS boot menu. By default, the boot menu prompt (and thus graphical image) is shown for 2.5 seconds. This can be customized by modifying SeaBIOS' CONFIG_BOOTMENU_WAIT in make menuconfig.

The JPEG viewer in SeaBIOS uses a simplified decoding algorithm. It supports most common JPEGs, but does not support all possible formats. Please see the Trouble reporting section if a valid image isn't displayed properly.

Adding payloads

Most payloads can also be launched from SeaBIOS. To add a payload, build the corresponding .elf file and then add it to the coreboot.rom file in the img/ directory. For example:

<source lang="bash"> $ ./build/cbfstool coreboot.rom add-payload /path/to/payload.elf img/MyPayload l $ ./build/cbfstool coreboot.rom print </source>

During boot, one can press the F12 key to get a boot menu. SeaBIOS will show all files in the img/ directory, and one can instruct SeaBIOS to run them.

SeaBIOS supports both uncompressed and LZMA compressed payloads.

Adding a floppy image

It is possible to embed an image of a floppy in flash. SeaBIOS can then boot from and redirect floppy BIOS calls to the flash image. This is mainly useful for legacy software (such as DOS utilities). To use this feature, place a floppy image into the CBFS directory floppyimg/. For example:

<source lang="bash"> $ lzma -zc /path/to/myfloppy.img > myfloppy.img.lzma $ ./build/cbfstool coreboot.rom add myfloppy.img.lzma floppyimg/MyFloppy.lzma raw $ ./build/cbfstool coreboot.rom print </source>

Both uncompressed and LZMA compressed images are supported. Several floppy formats are available: 360K, 1.2MB, 720K, 1.44MB, 2.88MB, 160K, 180K, 320K.

The floppy image will appear as writable to the system, however all writes are discarded on reboot.

When using this system, SeaBIOS reserves high-memory to store the floppy. The reserved memory is then no longer available for OS use, so this feature should only be used when needed.

Configuring boot order

Place a file in CBFS with the name bootorder to configure the boot up order. The file should be ASCII text and contain one line per boot method. The description of each boot method follows an Open Firmware device path format. SeaBIOS will attempt to boot from each item in the file — first line of the file first.

The easiest way to find the available boot methods is to look for "Searching bootorder for" in the SeaBIOS serial output. For example, one may see lines similar to:

Searching bootorder for: /pci@i0cf8/*@f/drive@1/disk@0
Searching bootorder for: /pci@i0cf8/*@f,1/drive@2/disk@1
Searching bootorder for: /pci@i0cf8/usb@10,4/*@2

The above represents the patterns SeaBIOS will search for in the bootorder file. However, it's safe to just copy and paste the pattern into bootorder. For example, the file:

/pci@i0cf8/usb@10,4/*@2
/pci@i0cf8/*@f/drive@1/disk@0

will instruct SeaBIOS to attempt to boot from the given USB drive first and then attempt the given ATA harddrive second.

Once a file has been created, add it to CBFS with the name bootorder. For example:

<source lang="bash"> $ ./build/cbfstool coreboot.rom add mybootlist.txt bootorder raw $ ./build/cbfstool coreboot.rom print </source>

Other Configuration items

Additional configuration options are available in the CBFS etc/ directory. For example, to set the duration of the boot menu to five and a half seconds, one would do the following:

<source lang="bash"> $ /path/to/seabios/tools/encodeint.py boot-menu-wait 5500 $ ./build/cbfstool coreboot.rom add boot-menu-wait etc/boot-menu-wait raw $ ./build/cbfstool coreboot.rom print </source>

The SeaBIOS tool tools/encodeint.py will create a litte-endian encoded binary integer which can be placed into a CBFS file.

Configuration variables

Filename	Description
boot-menu-wait	Amount of time (in milliseconds) to wait at the boot menu prompt before selecting the default boot.
extra-pci-roots	If the target machine has multiple independent root buses set this to a positive value. The SeaBIOS PCI probe will then search for the given number of extra root buses.
ps2-keyboard-spinup	Some PS2 keyboards don't respond to commands immediately after powering on. Specify a positive value here to allow additional time for the keyboard to become responsive.
optionroms-checksum	Option ROMs are required to have correct checksums. However, some option ROMs in the wild don't correctly follow the specifications and have bad checksums. Set this to a zero value to allow SeaBIOS to execute them anyways.
s3-resume-vga-init	Set this to a non-zero value to instruct SeaBIOS to run the vga rom on an S3 resume.
screen-and-debug	Set this to a zero value to instruct SeaBIOS to not write characters it sends to the screen to the debug ports. This can be useful when using sgabios.

Trouble reporting

If you are experiencing problems with SeaBIOS, it's useful to increase the debugging level. This is done by running make menuconfig and setting CONFIG_DEBUG_LEVEL to a higher value. A debug level of 8 will show a lot of diagnostic information without flooding the serial port (levels above 8 will frequently cause too much data).

To report an issue, please collect the serial boot log with SeaBIOS set to a debug level of 8 and forward the full log along with a description of the problem to the coreboot mailing list.