Re: [Bug-xorriso] mkisofs emulation

[Thomas Schmitt]

Hi,

Alexandros Alexandrou wrote:
> I can build a quick test ISO to in order to demonstrate how this would work.
> However, I think it wouldn't hurt to wait for a more official ISO image.

The effect of the new program behavior should be the same as if you
use option -boot-load-size with the byte count of the boot image file
divided by 512 and rounded up to multiples of 4. (Or divide by 2048,
round up to next integer, multiply by 4).
Like:

  N=$(expr '(' $byte_count + 2047 ')' / 2048 '*' 4)

  xorriso -as mkisofs ... -boot-load-size $N ...

At least that's my plan after reading man genisoimage about -boot-load-size.

A boot image with size not divisible by 2048 will nevertheless be stored
so that the rounded number of 512-blocks is readable. (That's because the
block size of the ISO is 2048.)

Be aware that the number N may not be larger than 65535, because the field
in the boot catalog is only 16 bit wide.


> I however wasn't aware that there are BIOSes that don't support loading more
> than 2048 bytes. I think I need to investigate this issue as well.

Still no success with finding the statement in my mail archives. :(

My memory is that i asked why ISOLINUX, GRUB, and other boot loaders
all let BIOS load only 2048 bytes and take effort to load the rest
of the boot image by code in those 2048 bytes.
Typically i would have asked that at syslinux or at grub-devel mailing
lists.

Google found me an old statement by H.Peter Anvin, the founder of
SYSLINUX/ISOLINUX:
  http://www.syslinux.org/archives/2003-September/002559.html
  "The reason is simply that not all BIOSes will handle any other size than 
   4.  The quality of some El Torito BIOSes is absolutely atrocious.
  "
But that was five years before i began to watch that mailing list.
Mine must have been after october 2008.
Whatever: Probably i got no more detailed answer than above one.


Have a nice day :)

Thomas