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Re: Debian installation issues




Hi,

by mistake Fungi4All and i exchanged a few mails in private.
I put this back to the list (although it could deserve a new topic).

He showed me a xorriso report of a MS-Windows "ISO" (which we now
know is actually an UDF filesystem), and i stated the same as with
my reply here to Dan Ritter:

> > it is only prepared for booting from CD, DVD, or BD but not from
> > hard disk or USB stick.

Fungi4All wrote:
> So BIOS must still be able to act smart to pass it on for execution?

I would have to act against the specs.
To boot from hard disk or USB stick, there must be a Master Boot Record.

BIOS loads the 512 bytes of this first block of the disk and executes them
as 16 bit x86 program. This program then has to do what is necessary to
start the boot loader and later the operating system.

EFI only looks at the partition table. There are two kinds of table in
use: MBR based or GPT. In case of a MBR partition table, EFI looks for
a partition with type 0xef. If the MBR partition table only contains only
one partition starting at block 1, there might be a GUID Partition Table
where EFI looks for a partition with type GUID
C12A7328-F81F-11D2-BA4B-00A0C93EC93B.
In the EFI partition there must be a FAT filesystem which contains
EFI boot programs like
  /EFI/BOOT/BOOTX64.EFI for amd64
  /EFI/BOOT/BOOTIA32.EFI for i386
  /EFI/BOOT/BOOTAA64.EFI for arm64

The EFI System Partition is in the MS-Windows image. But it is not
advertised by a partition table. Only by El Torito, which is to be
interpreted only if presented on CD, DVD, or BD media.


> Are you saying that the boot part of what I send you is pretty much the
> same as that of win10ent you downloaded? [...] that image is for a
> version of the system 3 generations back, [...]

The boot equipment is the same. El Torito for BIOS and EFI. No MBR.


> > The size 1 of the EFI System Partition says that it might extend
> > up to the end of the image or storage device.

> So whether it is on a 4gb medium or 256gb medium it wouldn't matter as
> it can not read that far.

El Torito can store only block counts up to 65535 which means 32 MiB minus
1 block. If the EFI partition is larger, then UEFI specs prescribe to set
the size field to 0 or 1, which both mean that it might reach up to the
end of the medium.
The FAT filesystem inside the EFI partition has its own size fields.
So a reader of the FAT filesystem will not be lured into reading blocks
which do not belong to the FAT filesystem.

In the case of Win10_1607_English_x64.iso there is really no need to
announce 0 or 1. The FAT filesystem is far smaller than 32 MB.


> > There seems to be not more than a single file in the ISO.
> > So possibly all the show happens inside the FAT filesystem of the
> > EFI System Partition.

> I suppose the installer and data is all compressed in one file

At least in case of Win10_1607_English_x64.iso my suspicion was wrong.
The show happens in the UDF filesystem. The EFI partition is too small
to contain much brain.


Have a nice day :)

Thomas