[syslinux] Isohybrid wiki page and UEFI

[Ady]

> Hi,
> 
> Ady wrote:
> > So, someone could assert the following reasoning steps:
> 
> > 1_ We can workaround the Sector Count limitation of 32MiB for 
> > no-emulation methods.
> 
> If the EFI firmware programmers obey version 2.4, yes.
> 
> > 2_ The ESP is considered a no-emulation method.
> 
> From the view of UEFI 2.4 it is a partition.
> 
> Partitions may be marked by MBR partition table or by GPT
> on hard disk (rotating or not), and by EL Torito Boot
> Catalog on CD, DVD, or BD. In the El Torito catalog, only
> the entries with platform id 0xEF and with emulation 0
> "no emulation" are candidates for EFI System Partition.
> 
> (For BIOS the platform id is 0x00 and emulations may range
>  from 0 to 4. 3 sizes of floppy and one hard disk.
>  I assume the hard disk is supposed to have a MBR partition
>  table telling its size.)
> 
> 
> > 3_ From the above assertions, we could conclude that the limitation of 
> > 32MiB is not really relevant for the ESP.
> 
> It needs to be tested.
> 
> 
> > To be clear, with "workaround" I don't mean the "size of 0 or 1"; I 
> > mean similar workarounds as for no-emulation methods under BIOS.
> 
> Not like in BIOS, because the approach is totally different.
> BIOS via  El Torito "no emulation" loads program code and executes it.
> EFI via the same form finds the block range of a partition which
> hosts a FAT filesystem.
> 
> Maybe the Sector Count is irrelevant for EFI implementations,
> because FAT can tell its size without a 16 bit limitation.
> 
> But the small load size of isolinux.bin is not the way how to
> negotiate with EFI.
> 
> 
> > The assumed size limitations for each El Torito 
> > "emulation" methods are not really there, as many real-life BIOSes will 
> > be able to boot other sizes, even bigger than 32MiB. Even VMs can boot 
> > such ISO images!
> 
> It is not about the size of the ISO filesystem. Debian has 25 GB
> bootable ISOs.
> It is about the size of the FAT image which serves as EFI System
> Partition inside the ISO. And it is about an unsurpassable limit
> of 16 bit numbers: If i add 1 to 65535 i get 0 as result.
> 
> We have two chances with real firmware:
> - The Sector Count is ignored and FAT filesystem size is used
>   insteadi as partition size.
> - The official values 0 or 1 are interpreted correctly and
>   the partition spans up to the end of the medium. No problem then.
>   FAT knows what's its own range and will not read trailing data.
> 
> Of course the real firmware could bail out on Sector Count 0
> or if FAT size is larger than perceived partition size.
> We will have to test.
> Needed is a EFI FAT image > 32 MiB and a new xorriso (available
> on demand).
> 
> 
> Have a nice day :)
> 
> Thomas
> 
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I have been misunderstood in several ways. Perhaps my writing was not 
as clear or as easy to read, but, at this moment, I'm not sure I am 
capable of making my prior email more clear. Considering that there has 
not been replies from Bruno, for now I'll leave the topic as it is.

Regards,
Ady.