Re: Unable to boot a raw kernel image :??

Edgar Toernig
2002-12-06T11:37:55Z

DervishD wrote:
>
> Hi all :))
>
> A time ago I was able to generate bootable Linux CD's just by
> dd'ing a floppy containing a raw kernel image:
>
> dd if=/dev/fd0 of=eltorito
>
> After that, mkisofsing, toasting and booting. But now, depending
> on the machine, I have 'invalid compressed format' errors, or even
> ye olde register dump when the image was damaged :(
>
> Booting directly from the floppy works, but booting with that
> same image from a CD does not :(( I now I can use LILO, or better
> yet, Syslinux or isolinux, but I'm just curious why I cannot boot raw
> image-based CD's anymore.
>
> Anyone knows what's happenning here?

I had this problem a while ago. It turned out to be a (widespread)
BIOS bug triggered be the disk-size probe of the kernel's boot loader.

The crude fix which worked for me: disable the probing for 2.88MB disks
by changing the first byte of the disksizes table at the end of bootsect.S
from 36 to 18.

The long explanation: The BIOS allows bigger-than-track-size reads
in El-Torito mode which confuses the probe routine which then assumes
a 2.88MB disk when the BIOS is actually emulating a 1.44MB disk.
In LBA mode that would be no problem but in CHS mode (which is used
by the loader) it does not work.

Ciao, ET.