Runtime config

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SeaBIOS can read several configuration items at runtime. On coreboot the configuration comes from files located in CBFS. When SeaBIOS runs natively on QEMU the files are passed from QEMU via the fw_cfg interface.

This page documents the user visible configuration and control features that SeaBIOS supports.

LZMA compression

On coreboot, when scanning files in CBFS, any filename that ends with a ".lzma" suffix will be treated as a raw file that is compressed with the lzma compression algorithm. This works for option ROMs, configuration files, floppy images, etc. . (This feature should not be used with embedded payloads - to compress payloads, use the standard section based compression algorithm that is built into the payload specification.)

For example, the file pci1106,3344.rom.lzma would be treated the same as pci1106,3344.rom, but will be automatically uncompressed when accessed.

A file is typically compressed with the lzma compression command line tool. For example:

lzma -zc /path/to/somefile.bin > somefile.bin.lzma

However, some recent versions of lzma no longer supply an uncompressed file size in the lzma header. (They instead populate the field with zero.) Unfortunately, SeaBIOS requires the uncompressed file size, so it may be necessary to use a different version of the lzma tool.

File aliases

It is possible to create the equivalent of "symbolic links" so that one file's content appears under another name. To do this, create a links file with one line per link and each line having the format of "linkname" and "destname" separated by a space character. For example, the links file may look like:

pci1234,1000.rom somerom.rom
pci1234,1001.rom somerom.rom
pci1234,1002.rom somerom.rom

The above example would cause SeaBIOS to treat "pci1234,1000.rom" or "pci1234,1001.rom" as files with the same content as the file "somerom.rom".

Option ROMs

SeaBIOS will scan all of the PCI devices in the target machine for option ROMs on PCI devices. It recognizes option ROMs in files that have the form pciVVVV,DDDD.rom. The VVVV,DDDD should correspond to the PCI vendor and device id of a device in the machine. If a given file is found then SeaBIOS will deploy the file instead of attempting to extract an option ROM from the device. In addition to supplying option ROMs for on-board devices that do not store their own ROMs, this mechanism may be used to prevent a ROM on a specific device from running.

SeaBIOS always deploys the VGA rom associated with the active VGA device before any other ROMs.

In addition, SeaBIOS 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. The ROMS in vgaroms/ are run immediately after running the option ROM associated with the primary VGA device (if any were found), and the genroms/ ROMs are run after all other PCI ROMs are run.

Bootsplash images

SeaBIOS can show a custom JPEG image or BMP image during bootup. To enable this, add the JPEG file to flash with the name bootsplash.jpg or BMP file as bootsplash.bmp.

The size of the 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 ESC 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 via a configuration parameter.

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.

Payloads

On coreboot, SeaBIOS will treat all files found in the img/ directory as a coreboot payload. Each payload file will be available for boot, and one can select from the available payloads in the bootmenu. SeaBIOS supports both uncompressed and lzma compressed payloads.

Floppy images

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

Using LZMA file compression with the .lzma file suffix is a useful way to reduce the file size. 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

The bootorder file may be used 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 debug 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.

SeaBIOS also supports a special "HALT" directive. If a line that contains "HALT" is found in the bootorder file then SeaBIOS will (by default) only attempt to boot from devices explicitly listed above HALT in the file.

Other Configuration items

There are several additional configuration options available in the etc/ directory.

Filename Description
show-boot-menu Controls the display of the boot menu. Set to 0 to disable the boot menu.
boot-menu-message Customize the text boot menu message. Normally, when in text mode SeaBIOS will report the string "\nPress ESC for boot menu.\n\n". This field allows the string to be changed. (This is a string field, and is added as a file containing the raw string.)
boot-menu-key Controls which key activates the boot menu. The value stored is the DOS scan code (eg, 0x86 for F12, 0x01 for Esc). If this field is set, be sure to also customize the boot-menu-message field above.
boot-menu-wait Amount of time (in milliseconds) to wait at the boot menu prompt before selecting the default boot.
boot-fail-wait If no boot devices are found SeaBIOS will reboot after 60 seconds. Set this to the amount of time (in milliseconds) to customize the reboot delay or set to -1 to disable rebooting when no boot devices are found
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 laptops that emulate PS2 keyboards don't respond to keyboard commands immediately after powering on. One may specify the amount of time (in milliseconds) here to allow as additional time for the keyboard to become responsive. When this field is set, SeaBIOS will repeatedly attempt to detect the keyboard until the keyboard is found or the specified timeout is reached.
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.
pci-optionrom-exec Controls option ROM execution for roms found on PCI devices (as opposed to roms found in CBFS/fw_cfg). Valid values are 0: Execute no ROMs, 1: Execute only VGA ROMs, 2: Execute all ROMs. The default is 2 (execute all ROMs).
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.
advertise-serial-debug-port If using a serial debug port, one can set this file to a zero value to prevent SeaBIOS from listing that serial port as available for operating system use. This can be useful when running old DOS programs that are known to reset the baud rate of all advertised serial ports.
sercon-port Set this to the IO address of a serial port to enable SeaBIOS' VGA adapter emulation on the given serial port.
floppy0 Set this to the type of the first floppy drive in the system (only type 4 for 3.5 inch drives is supported).
floppy1 The type of the second floppy drive in the system. See the description of floppy0 for more info.
threads By default, SeaBIOS will parallelize hardware initialization during bootup to reduce boot time. Multiple hardware devices can be initialized in parallel between vga initialization and option rom initialization. One can set this file to a value of zero to force hardware initialization to run serially. Alternatively, one can set this file to 2 to enable early hardware initialization that runs in parallel with vga, option rom initialization, and the boot menu.
sdcard* One may create one or more files with an "sdcard" prefix (eg, "etc/sdcard0") with the physical memory address of an SDHCI controller (one memory address per file). This may be useful for SDHCI controllers that do not appear as PCI devices, but are mapped to a consistent memory address. If this option is used then SeaBIOS will not scan for PCI SHDCI controllers.
usb-time-sigatt The USB2 specification requires devices to signal that they are attached within 100ms of the USB port being powered on. Some USB devices are known to require more time. Prior to receiving an attachment signal there is no way to know if a USB port is empty or if it has a device attached. One may specify an amount of time here (in milliseconds, default 100) to wait for a USB device attachment signal. Increasing this value will also increase the overall machine bootup time.