[syslinux] isohybrid and ISO images whose size is not a multiple of 2048 bytes vs. VirtualBox

[Thomas Schmitt]

Hi,

Martin Str|mberg wrote:
> First I want to hear if anyone has failed to boot such non-even-
> multiple-of-2048-sectors on real hardware?

This is (theoretically) impossible because a real DVD
has a block size of 2048. When an image gets burned onto
a real DVD, the burn program has to align to block size.
It has no means to address or transmit data units smaller
than one block.


I have meanwhile adopted in xorriso-1.4.0 the workaround of
intrigery's team mate anonym. I.e. if the user forces a
cylinder size that is not divisible by 2048 and the number
of cylinders does not heal this problem, then 1536, 1024
or 512 padding bytes get added. (Previous versions of
xorriso did not align to cylinders but to 2048 in such a
case.)

I have read in the web that contemporary hard disks (as of
year 2000 ?) are not aligned to what BIOSes perceive as
cylinder size. Only partitions are expected to be aligned.
Further i learned that BIOSes and partition editors may have
different perceptions of cylinder size.
So a few 512-byte blocks after the end of the last partition
are supposed to be normal on a real hard disk.


> It seems to me from Thomas mails that Debian has
> (inadvertedly) tested this successfully.

Debian 8.0.0 DVD-1 for i386 is a flypaper to which any
vulnerable BIOS should stick. (Vulnerable to non-traditional
hard disk cylinder sizes, not to misalignment of DVDs.)

Everybody is invited to put
http://cdimage.debian.org/debian-cd/8.0.0/i386/iso-dvd/debian-8.0.0-i386-DVD-1.iso
on a USB stick or hard disk with at least 4 GB and to test it
with the oldest x86 machine in reach.
(Failures to boot should be reported as Debian bugs against
 package "debian-cd".)


Ady wrote:
> The suggested patch [of adjusting cylinder count] is simple-enough, 
> and the "cost" is, IMO, not at all excessive.

Somebody will have to clone the current git, apply the patch,
give isohybrid a thorough test, and create a git-ly patch
for submission.
I am ready to diagnose any problems which might show up.


Have a nice day :)

Thomas