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Re: Using efi image on USB stick

From: Thomas Schmitt
Subject: Re: Using efi image on USB stick
Date: Tue, 18 May 2010 14:44:09 +0200


Colin Watson wrote:
> I don't know how well
> BIOSes handle MBR partition tables on CD-ROMs.

According to ECMA-119 (aka ISO 9660) the first
32 kB of an image are "System Area" with
arbitrary custom content.
El Torito specs mention that this area may
contain a bootable disk image. (Figure 1,
case "Multiple Boot-Image Configuration").
To my view, a bootable disk image may well begin
by an MBR which does not necessarily have to
describe the geometry of the entire CD.
The BIOS learns the positions of boot images from
the El Torito boot catalog. The position of the
boot catalog is given by the El Torito record in
sector 17 (decimal) of the CD.

So it seems natural that the BIOS should not
interpret the system area of a CD unless the
boot catalog points to it as boot image.
Currently the xorriso generated boot catalogs
do not point to the system area.
But even if they would, then the bytes at the
position of a partition table should not be
interpreted as partition table of the CD.

> :
"Many BIOS implementations, ... do not correctly
 handle the multiple El-Torito boot blocks"

El Torito specs (of 1995) talk of "BIOS with
Single Boot-Image capability" in contrast to
"BIOS with Multiple Boot-Image capability".
I interpret the following statement as a
prescription that single-boot BIOSes shall ignore
further boot images:
"Single-Image INT 19 knows nothing about
 multiple-images, nor does it know about their
 possible entries listed in the Booting Catalog."

> I guess all we can do is test ...

Maybe one should publish a little test image
which is alread yknown to work on some systems.
(I could test with a 64 bit AMD quadcore and an
 old 32 bit AMD of 2004.)

Have a nice day :)


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