[syslinux] Isohybrid wiki page WAS: Is efiboot.img required?

Ady ady-sf at hotmail.com
Mon Apr 6 05:47:51 PDT 2015

> I am still interested in improving the isohybrid wiki
> and will watch for pointers to descriptions of the FAT
> production process (with GRUB/GRUB2 if not with SYSLINUX).
> If no such descriptions appear within the next weeks,
> then i plan to state in
>   http://www.syslinux.org/wiki/index.php/Isohybrid#UEFI
> "The SYSLINUX project cannot yet provide a FAT filesystem
>  image which would work with the isohybrid feature.
>  Known isohybrid ISOs for UEFI employ FAT images which
>  contain software from bootloader projects GRUB or GRUB2.
>  This software boots by own non-SYSLINUX configurations."
> It would be nice, of course, if the statement could present
> SYSLINUX UEFI isohybrid as being more autonomous.
> Have a nice day :)
> Thomas
IMHO, this seems somewhat inaccurate, or at least not entirely clear 
for users that don't already have the necessary knowledge about what we 
are talking about. This could be misinterpreted by some users as: "if 
you want to build an isohybrid image, then you cannot use 
'syslinux.efi'". This would be incorrect.

There is no incompatibility between 'syslinux.efi' and isohybrid 
images. The apparent limitation, if someone wants to call it that way, 
is that 'syslinux.efi' does not make use of "efiboot.img" as GRUB2 
does. Currently (as of v.6.03) 'syslinux.efi' is not capable of booting 
optical media (or any media using ISO9660) in UEFI mode.

But, if I take an isohybrid image already containing 'syslinux.efi' 
(and related files) and I copy the contents onto a FAT partition of a 
USB drive formatted with a GPT partition table, I could then boot this 
USB drive in UEFI mode. If, instead of copying the content of the 
isohybrid image I would use a dd-like method, then the device won't 
boot in UEFI mode since the (ISO9660) file system is not currently 
supported by 'syslinux.efi'.

I am still of the opinion that the whole FAT "efiboot.img" issue is not 
about _isohybrid_, but about how GRUB2 does it. For the GRUB2 method, 
this "efiboot.img" is equivalent to an "El Torito no-emulation" method 
valid for UEFI. This is the relevant information that concerns the 
isohybrid tools.

(In fact, the UEFI specs are not very clear for common users about the 
"no-emulation" part. The UEFI specs categorize it as "no-emulation", 
except that it is not a bootloader as 'isolinux.bin' is (located in the 
same ISO9660 filesystem) but a superfloppy image, which is closer to a 
floppy-emulation type (but not exactly the same). In any case, this is 
more technical than what most users want to know. Users usually want to 
know which steps to perform and move on.)

I said it before: frequently the confusion starts with "guides" making 
a "salad", by not distinguishing between UEFI specs, GRUB2's methods 
(and its features) and (buggy) firmware. Then users attempt to follow 
these "guides" and try to apply the same methods that GRUB2 uses to 
other bootloaders such as Syslinux.

Regarding the characteristics of GRUB2's "efiboot.img", that's about 
GRUB2, not isohybrid. If a day comes when such image (type) is relevant 
for Syslinux, the discussion of its characteristics might be relevant 

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