[syslinux] Isohybrid wiki page

[Thomas Schmitt]

Hi,

Ady wrote:
> 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.

I don't know whether syslinux.efi can be used for booting
from ISO 9660 via UEFI. Currently there is no example
around which would succeed without GRUB or GRUB2.

When describing the UEFI aspects of isohybrid in the wiki,
i left this question open in the hope that somebody would
show a pure SYSLINUX example of UEFI isohybrid.

The questions of piranna were not the first time that we
had to explain our non-knowledge around ISOLINUX and UEFI.
The wiki needs clarification.


> I am still of the opinion that the whole FAT "efiboot.img" issue is not 
> about _isohybrid_, but about how GRUB2 does it.

It is about what UEFI expects (without an UEFI boot manager
like rEFInd involved). It wants an UEFI image, i.e. a *.efi
file. Its own means of finding it are the EFI System
Partitions of the devices, the NVRAM variables (e.g.
manipulated by efibootmgr(8)), and the fallback paths
(e.g. /efi/boot/bootx64.efi).


> In fact, the UEFI specs are not very clear for common users about the 
> "no-emulation" part

You mean "no emulation" in the sense of El Torito ?
(As opposed to "1.2 MB diskette", "1.44 MB diskette",
"2.88 MB diskette", and "hard disk drive".)

This will not be noted by EFI from USB stick. Only with
EFI from CD-ROM (and DVD or BD).
I assume most EFI implementations ignore the Boot Media
Type fields in the El Torito boot catalog and only look
for Platform ID 0xef.

In UEFI specs, El Torito just serves as substitute for
a partition table. 


> a superfloppy image,

It's more like a twenty years old hard disk.
UEFI 2.6, 12.3 "File System Format":
"EFI encompasses the use of FAT32 for a system partition
 [...] The definition of the EFI file system will be
 maintained by specification and will not evolve over
 time to deal with errata [...]"

When used with El Torito, the image should not be larger
than 32 MiB. That's because the Sector Count fields in
the boot catalog have only 16 bits.


> 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 
> then.

Well, since all traditional users of ISOLINUX isohybrid enhance
their ISO production by a FAT image, modules, and configuration
from GRUB or GRUB2, in order to make use of isohybrid option --uefi
... i would say it is relevant.

Anybody show me a pure SYSLINUX isohybrid ISO that boots via EFI
and i will be able to dissect it and hopefully derive knowledge
for the wiki.


Have a nice day :)

Thomas