Syslinux - Jun 2014 - How is a isohybrid --partok image to be used ?

Hi,

i am trying to document the MBR selection by isohybrid option --partok.

I assume it is related to "altmbr.bin" as described in
http://www.syslinux.org/wiki/index.php/Mbr
But there i read
"altmbr.bin [...] boots from a partition with fixed number, see
below."

The "see below" probably refers to
"The altmbr.bin (all 3 variants) is only 439 bytes long, -- one extra
 byte should be set to the number of partition to boot from."

I do not see in isohybrid.c any provisions to write that number.
There is option -e to choose a partition entry, but this only
causes a particular partition table entry to mark the overall
ISO filesystem. It does not cause special manipulation of the MBR.

Do i get it right that the MBR of a --partok isohybridized image
shall become the VBR of a partition, which the user has to create
by other means before copying the ISO image into it ?

If yes, how should the MBR of the overall disk be created resp.
manipulated to boot the ISO ?

Are there examples known of bootable ISOs which where treated
with isohybrid --partok ?


Have a nice day :)

Thomas

On 06/25/2014 01:16 AM, Thomas Schmitt wrote:
> Hi,
> 
> i am trying to document the MBR selection by isohybrid option --partok.
> 
> I assume it is related to "altmbr.bin" as described in
> http://www.syslinux.org/wiki/index.php/Mbr
> But there i read
> "altmbr.bin [...] boots from a partition with fixed number, see
below."
> 
> The "see below" probably refers to
> "The altmbr.bin (all 3 variants) is only 439 bytes long, -- one extra
>  byte should be set to the number of partition to boot from."
> 
> I do not see in isohybrid.c any provisions to write that number.
> There is option -e to choose a partition entry, but this only
> causes a particular partition table entry to mark the overall
> ISO filesystem. It does not cause special manipulation of the MBR.
> 
> Do i get it right that the MBR of a --partok isohybridized image
> shall become the VBR of a partition, which the user has to create
> by other means before copying the ISO image into it ?
> 
> If yes, how should the MBR of the overall disk be created resp.
> manipulated to boot the ISO ?
> 
> Are there examples known of bootable ISOs which where treated
> with isohybrid --partok ?
> 
It means that the .iso can be booted from a partition using any of the
standard MBRs (not necessarily altmbr), or via chainloading, as long as
the MBR handover is respected by the chainloader.

One of the main uses is for backup partitions -- often distros want a
filesystem that cannot be written for that, and iso9660 fits the bill.

	-hpa

Hi,

H. Peter Anvin:
> It means that the .iso can be booted from a partition using any of the
> standard MBRs (not necessarily altmbr),
But the --partok MBRs (isohdpp*) stem from altmbr ?

If so: don't they need the partition number patched in ?

(If not: i have to rethink the link to article "Mbr" at
  http://www.syslinux.org/wiki/index.php/Isohybrid#MBR_selection
)

> One of the main uses is for backup partitions -- often distros want a
> filesystem that cannot be written for that, and iso9660 fits the bill.
Are examples known ?

Up to now i have to stay quite vague in
  http://www.syslinux.org/wiki/index.php/Isohybrid#Images_prepared_with_--partok

"==Images prepared with --partok= ISO images, which were prepared by
--partok resp. with
 MBR isohdppx*.bin, have to be copied to a partition device.
 E.g. /dev/sdb1 rather than /dev/sdb.
 Such an ISO partition may then be booted by chainloading or
 by a suitable MBR at the start of the base disk device.
"

I hope this is at least not wrong.
Whatever, it would not suffice for myself to really create such
a disk or USB stick. (Courageous experimenters wanted.)

---------------------------------------------------------------

Further i wonder how --partok does play with --gpt.
Afaik, there is no equivalent to VBR in GPT.
So can it be that GPT in the ISO image is useless if the image is
put into a partition ?
Does the installing user have to compute an own GPT entry from a
listing of the GPT in the image ? (Or from the MBR patition table
in the image.)


Have a nice day :)

Thomas