[syslinux] isohybrid question

[Shao Miller]

Thomas Schmitt wrote:
> Hi,

Thanks for the quick response. :)

> This is well possible.
> Needed is just a way to make the generator
> do that.
>
> In general one may write data blocks which are
> not referred by the directory tree of the image.
> Or one may write data files with synthetic
> content.

How about zeroes?  Or is it ones?  Which burn better?

> Isohybrid operates on an already reserved space
> at the very start of the image, the System Area.
> It resides in block 0 to 15 (block size 2K).
>
> The ISO generator does not have to prepare the
> image for the isohybrid script but only has to
> prepare it for ElTorito booting.
> The isohybrid script reads a few parameters
> from the image, patches them into an MBR template
> and writes that MBR to the first 512 bytes of
> the System Area.

Ah yes.

> The reason why xorriso strives for own capability
> to write an MBR is that the isohybrid script
> needs an overwriteable image, whereas xorriso can
> write to sequential optical media directly.
> (It should be well possible to apply the script
>  to written DVD+RW, DVD-RAM or BD-RE media, but
>  not to CD, DVD-R, DVD+R, BD-R.)

Wait a minute...  Will optical media with an MBR ever be useful?  I 
guess the idea is that someone could later DD that optical media 
directly to a disk?  If so, I guess that just a DD would be simpler than 
DDing the optical media to the disk, followed by the isohybrid 
post-processing?  People could exclude the post-processor and MBR 
collection from their file set with using just a simple DD...  Is that 
part of the benefit?  A distribution could churn out batches of optical 
media with xorriso directly without having ever prepared an .ISO image, 
for example?

> I am not aware of a problem with fractional
> cylinder boundaries at the end of partitions.

Good point.  It's probably the partition start that's more important for 
flaky C/H/S access.  I was thinking of a second partition following 
exactly on the heels of the first, but that's not actually required.

> ...
> To allow the 2 KB address granularity of ISO 9660,
> partition 1 would have to start at a multiple of
> 2048 bytes on the raw device. Else we would not
> be able to address the same data content from
> both volume descriptor sets and trees.
> If we want cylinder alignement in the partition
> table we would either need an unusual cylinder
> size or more than one cylinder as unpartioned
> space at the device start. Max. 4 cylinders.

Where does this maximum of 4 cylinders come from, if you recall?

> ...

Ok.  Good info. :)

- Shao Miller