[syslinux] [PATCH] isohybrid: user options

[Thomas Schmitt]

Hi,

H. Peter Anvin wrote:
> Another option we probably should consider is the non-zero-offset
> option, since there are reported BIOSes that don't boot without it.  The
> somewhat obvious offset to use is 16 CD-ROM sectors (32K, 64 HD
> sectors) as that is the offset of the ISO 9660 filesystem superblock.
>
> The part that becomes really messy with that is that the resulting
> partition isn't a standard ISO 9660 filesystem anymore (if you can
> consider anything on a non-CD standard).

ECMA-119 puts much emphasis on "CD-ROM".
But the actual technical constraints are
about a block device with random read
access of 2 kB granularity.

So one mainly has to care for the right
appearance as block device.


> I *think* under Linux you can
> mount such a partition with the option "sbsector=0", or Linux can
> probably mount the raw device (/dev/sda as opposed to /dev/sda1).

On a media with no multiple logical tracks 
-o sbsector=0 should have the same effect
as mounting without sbsector option.
So if the byte address space of /dev/sda1 does
not start at media LBA 0, then sbsector=0 will
not help.

/dev/sda should be ok. I use my USB sticks
as /dev/sdb and /dev/sdc. Currently they
don't carry MBRs but i do not remember to
have had problems with mounting /dev/sdb
when it did have an MBR.


One could position the ISO image on media
so that it actually has the matching offset
without using it in its inner LBA addresses.
(I.e. the image may *not* be made aware of
 the offset by e.g. mkisofs option -C 0,offset)

The MBR would have to be copied to media LBA 0.

Of course then the image would not be mountable
via /dev/sda unless you give -o sbsector=offset.
But if /dev/sda1 has that same offset, then
everything would match again.

Question:
Which LBA would one have to write into El-Torito
record and into the boot image ?
The one for /dev/sda or the one for /dev/sda1 ?


Have a nice day :)

Thomas