Extracting BIOS images and tools from ThinkPad update ISOs

With my old ThinkPad, Lenovo provided BIOS updates in the form of Windows executables or ISO images for a bootable CD.  Since I had wiped Windows partition, the first option wasn’t an option.  The second option didn’t work either, since it expected me to be using the drive in the base I hadn’t bought.  Luckily I was able to just copy the needed files out of the ISO image to a USB stick that had been set up to boot DOS.

When I got my new ThinkPad, I had hoped to do the same thing but found that the update ISO images appeared to be empty when mounted.  It seems that the update is handled entirely from an El Torito emulated hard disk image (as opposed to using the image only to bootstrap the drivers needed to access the CD).

So I needed some way to extract that boot image from the ISO.  After a little reading of the spec, I put together the following Python script that does the trick:

import struct
import sys


def find_image(fp):
    # el-torito boot record descriptor
    fp.seek(0x11 * SECTOR_SIZE)
    data = fp.read(SECTOR_SIZE)
    assert data[:0x47] == b'\x00CD001\x01EL TORITO SPECIFICATION' + b'\x0' * 41
    boot_catalog_sector = struct.unpack('<L', data[0x47:0x4B])[0]

    # check the validation entry in the catalog
    fp.seek(boot_catalog_sector * SECTOR_SIZE)
    data = fp.read(0x20)
    assert data[0:1] == b'\x01'
    assert data[0x1e:0x20] == b'\x55\xAA'
    assert sum(struct.unpack('<16H', data)) % 0x10000 == 0

    # Read the initial/default entry
    data = fp.read(0x20)
    (bootable, image_type, load_segment, system_type, sector_count,
     image_sector) = struct.unpack('<BBHBxHL', data[:12])
    image_offset = image_sector * SECTOR_SIZE
    if image_type == 1:
        # 1.2MB floppy
        image_size = 1200 * 1024
    elif image_type == 2:
        # 1.44MB floppy
        image_size = 1440 * 1024
    elif image_type == 3:
        # 2.88MB floppy
        image_size = 2880 * 1024
    elif image_type == 4:
        # Hard disk image.  Read the MBR partition table to locate file system
        data = fp.read(512)
        # Read the first partition entry
        (bootable, part_type, part_start, part_size) = struct.unpack_from(
            '<BxxxBxxxLL', data, 0x1BE)
        assert bootable == 0x80 # is partition bootable?
        image_offset += part_start * 512
        image_size = part_size * 512
        raise AssertionError('unhandled image format: %d' % image_type)

    return fp.read(image_size)

if __name__ == '__main__':
    with open(sys.argv[1], 'rb') as iso, open(sys.argv[2], 'wb') as img:

It isn’t particularly pretty, but does the job and spits out a 32MB FAT disk image when run on the ThinkPad X230 update ISOs. It is then a pretty easy task of copying those files onto the USB stick to run the update as before. Hopefully owners of similar laptops find this useful.

There appears to be an EFI executable in there too, so it is possible that the firmware update could be run from the EFI system partition too.  I haven’t had the courage to try that though.

5 thoughts on “Extracting BIOS images and tools from ThinkPad update ISOs”

  1. Hi James,

    there is a much much simpler solution to boot this ISO image without a windows partition. I was in the same situation, since
    I bought a lenovo x121 which only has an SSD and no optical drive. Space on the SSD being scarce, I promptly wiped the windows
    partition. To boot on a bootable iso without optical drive:

    (1) install grub-imageboot
    (2) copy your iso file in /boot/images/
    (3) run update-grub2
    (4) profit! (that is reboot and you will get a grub entry to boot your iso as-if it had been loaded from a CD).

    I updated my bios this way, worked flawlessly.
    You can put all sorts of images in that directory (e.g. bootable floppy images with the .img suffix are also recognized).



  2. @Marius Gedminas: that geteltorito script looks like it would have done what I wanted. It was still fun working out how to pull the image apart though.

    I’m still somewhat interested in whether it would be possible to run the BIOS updates from the pre-OS EFI shell though. If it is, it probably isn’t well tested though, since the installed copy of Windows was set to boot in legacy BIOS mode despite the machine having a UEFI BIOS.

  3. hi James,

    Can you give me some hints of how to execute this script on Linux.
    I am complete newbie in programming, scripting and linux.
    Can I copy the content in a file with extension .py and run it in terminal typing python script.py for instance.

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