[syslinux] How is a isohybrid --partok image to be used ?

[H. Peter Anvin]

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