Monthly Archives: November 2012

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

SECTOR_SIZE = 2048

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
        fp.seek(image_offset)
        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
    else:
        raise AssertionError('unhandled image format: %d' % image_type)

    fp.seek(image_offset)
    return fp.read(image_size)

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

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.