Syslinux - Mar 2015 - isohybrid warning about 1024 cylinders

Hello,

The recent discussion about the size of the image produced by isohybrid not
being a multiple of 2048 when people use -h 255 -s 63, made me wonder of whom
should be worried by this more than 1024 cylinders warning.

Are there any technicals details available on this "not all BIOSes will be
able to boot this device"? After all, any hard drive now a days cannot be
represented under CHS as having less than 1024 cylinders (as they are all beyond
8G in size) and those drive boot fine.

So is this warning intended to people which may have to boot the hybrid ISO
under old BIOS which doesn't support the extended INT13, forcing isohdpfx.S
to use CHS instead of LBA when reading isolinux.bin? And possibly (I didn't
had time yet to read this code section) the same for isolinux.asm when it
attempt to read the configuration files and the kernel.

And would this exhibit an issue only if any of these files (isolinux.bin, and
maybe isolinux.cfg, kernel) ends up beyond cylinder 1023; as once the kernel is
loaded one would hope that the BIOS INT13 is no longer used and relied upon.


Regards,
Dany St-Amant

On 03/01/2015 07:28 AM, Dany St-Amant via Syslinux
wrote:
> Hello,
> 
> The recent discussion about the size of the image produced by
> isohybrid not being a multiple of 2048 when people use -h 255 -s 63,
> made me wonder of whom should be worried by this more than 1024
> cylinders warning.
> 
> Are there any technicals details available on this "not all BIOSes
> will be able to boot this device"? After all, any hard drive now a
> days cannot be represented under CHS as having less than 1024
> cylinders (as they are all beyond 8G in size) and those drive boot
> fine.
> 
> So is this warning intended to people which may have to boot the
> hybrid ISO under old BIOS which doesn't support the extended INT13,
> forcing isohdpfx.S to use CHS instead of LBA when reading
> isolinux.bin? And possibly (I didn't had time yet to read this code
> section) the same for isolinux.asm when it attempt to read the
> configuration files and the kernel.
> 
> And would this exhibit an issue only if any of these files
> (isolinux.bin, and maybe isolinux.cfg, kernel) ends up beyond
> cylinder 1023; as once the kernel is loaded one would hope that the
> BIOS INT13 is no longer used and relied upon.
> 
Yes, this is about booting the hybrid ISO on an old BIOS.  Unfortunately
some idiot BIOS vendors decided that USB sticks didn't need EDD support.

And yes, it only affects files touched by the bootloader, of course.

	-hpa