1 PartitionOffset
Thomas Schmitt edited this page 10 months ago

The partition offset feature of libisofs can produce ISO 9660 images which bear a quite conventional partition table if copied onto a USB stick. The first partition marks the size of the ISO image but starts at a non-zero address. Thus it marks a small part of the device as unclaimed by partitions and available for storing boot loader code.

Nevertheless the USB stick is mountable via its overall device file as well as via the partition device file. E.g. on GNU/Linux: /dev/sdb and /dev/sdb1. This is achieved by two distinct sets of meta-data which refer to the same file content.

The dual-mount feature supports Rock Ridge and Joliet too. It is capable of multi-session.

Currently only offset 32 kB seems to make sense. Smaller offsets are prohibited by fundamental assumptions of libisofs and libisoburn. Larger offsets would extend the unclaimed area into vital blocks of the ISO image.

According to a thread of march 2011 on Syslinux mailing list this enabled booting of a Kontron CG2100 server from USB stick, which otherwise failed.

Regrettably the feature seems to prevent mounting of ISO 9660 images on Apple "Snow Leopard" systems. At least this is the outcome of a debian-cd thread of april 2011.


Testing mountability and ISOLINUX bootability from USB stick and CD.


The test image was derived from one year old RIPLinux-9.3-non-X.iso which has an isohybrid MBR. Syslinux version seems to be 3.82. That MBR and the file tree from the mounted RIPLinux image was used to build a new ISO image with 16 * 2kB partition offset. Isohybrid MBR patching was done by xorriso.


The first 32 kB of an ISO 9660 image are called System Area and may host any byte pattern. The first 512 bytes of RIPLinux-9.3-non-X.iso contain the isohybrid capable MBR, which will be re-used in this example.

  dd if=RIPLinux-9.3-non-X.iso bs=512 count=1 of=RIPLinux-9.3-non-X.mbr

Normally the isohybrid MBR is provided by the Syslinux installation under the name isohdp[fp]x*.bin . E.g. /usr/lib/syslinux/isohdpfx.bin

The files of the image are made accessible for reading

  mount -o loop RIPLinux-9.3-non-X.iso /mnt

A new ISO image gets composed. The first three lines of arguments are taken from the prescriptions of ISOLINUX wiki and adapted to the names used in RIPLinux-9.3-non-X.iso. Option -isohybrid-mbr imports the copied MBR and patches it according to rules published by hpa on Syslinux mailing list. Option -partition_offset 16 causes the first partition to start at 2 kB block number 16. It also prepares the image to be mountable by this partition, too.

  xorriso -as mkisofs \
     -o new_image.iso \
     -b boot/isolinux/isolinux.bin -c boot/boot.cat \
     -no-emul-boot -boot-load-size 4 -boot-info-table \
     -isohybrid-mbr RIPLinux-9.3-non-X.mbr \
     -partition_offset 16 \

The image was copied onto a USB stick

  dd if=new_image.iso of=/dev/sdc

and plugged into a Debian system.

  fdisk -lu /dev/sdb


   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *          64      120831       60384   17  Hidden HPFS/NTFS

I can mount /dev/sdb and /dev/sdb1 alike:

  mount /dev/sdb1 /mnt1
  mount -o loop /dev/sdb /mnt

-o loop avoids failure with "mount: /dev/sdb already mounted or /mnt busy". A comparison by

  diff -r /mnt /mnt1

reports no difference. Human readable files look ok. Test-reading all content by

  tar cf - /mnt | wc

yields a reasonable byte count of 60743680 and no errors.

The machine boots RIPLinux from this USB stick with no visible problems. It can then mount /dev/sdb as well as /dev/sdb1. The ISO image boots from CD too.

Mounting the partition can be simulated with an image file on hard disk by cutting off the first partition_offset blocks of 2 KB:

  dd if=new_image.iso of=partition_image.iso bs=2048 skip=16
  mount -o loop partition_image.iso /mnt1

Another test was made with GRUB 2 by downloading

  bzr branch http://bzr.savannah.gnu.org/r/grub/trunk/grub

Before building GRUB 2, the file


was edited to replace in the options of the xorriso command:



  -partition_offset 16 -no-pad

Then GRUB 2 was built and installed.

The resulting image from

  ./grub-mkrescue -o image.iso

was put onto USB stick. It passed the same tests on Debian as above RIPLinux example. It boots to a GRUB prompt.

Due to option -no-pad the image is about 250 kB smaller than the image produced by original grub-mkrescue. Else it would have grown by about 50 kB.

Unpadded ISO images are safe except for burning on CD in TAO mode. In this case problems may occur with reading the last few data blocks. So when burning onto CD make sure to require SAO mode and/or to require padding by 300 kB. Burning on DVD or BD needs no such caution. Neither does copying on USB stick or hard disk.

Program fdisk will complain about "different physical/logical" addresses. This can be silenced by adding option

 -partition_cyl_align on

at the cost of image padding up to the next full MB. E.g. by 402 kB to 2 MB.

Open questions:

  • Shall the partition of an isohybrid image be marked bootable ? Currently xorriso keeps the 0x80 mark of an imported MBR and the 0x80 mark which xorriso sets by its own MBR preparations.
    • If not to be marked bootable: What equipment would the partition need to justify having the mark ?


The partition offset feature can be controlled by libisofs API calls

int iso_write_opts_set_part_offset(IsoWriteOpts *opts,
                                   uint32_t block_offset_2k,
                                   int secs_512_per_head,
                                   int heads_per_cyl);

int iso_write_opts_set_system_area(IsoWriteOpts *opts, char data[32768],
                                   int options, int flag);

or by libisoburn calls

int isoburn_igopt_set_part_offset(struct isoburn_imgen_opts  *opts,
                                  uint32_t block_offset_2k,
                                  int secs_512_per_head, int heads_per_cyl);

int isoburn_igopt_get_part_offset(struct isoburn_imgen_opts *opts,
                                  uint32_t *block_offset_2k,
                                  int *secs_512_per_head, int *heads_per_cyl);

int isoburn_igopt_set_system_area(struct isoburn_imgen_opts *o,
                                  char data[32768], int options);

int isoburn_igopt_get_system_area(struct isoburn_imgen_opts *o,
                                  char data[32768], int *options);

or by xorriso options

  -boot_image any partition_offset=(2kb_block_adr)
  -boot_image any partition_sec_hd=(number)
  -boot_image any partition_hd_cyl=(number)
  -boot_image any partition_cyl_align(on|auto|off)

  -as mkisofs ... -partition_offset (2kb_block_adr) \
                  -partition_hd_cyl (number) \
                  -partition_sec_hd (number) \
                  -partition_cyl_align (on|auto|off) ...

As stated above, an offset larger than 16 would expose vital parts of the ISO image as unclaimed space. Values smaller than 16 are not accepted. So use either an offset of 16 blocks or keep the feature disabled by offset 0.