[syslinux] "isolinux.bin missing or corrupt" when booting USB flash drive in old PC

[Thomas Schmitt]

Hi,

Ady wrote:
> We seem to be suggesting to just dd' the first 432 bytes of their USB 
> devices with Martin's isohybrid

Yes. As long as it is not about the old Macs which the APM of Fedora
and some Gentoo ISOs wants to make boot, and not about ISOs made
with isohybrid options --partok -ctrlhd0 --forcehd0.


> which would be the step-by-step instructions for users of other
> isohybrid images?

Copy at most 432 bytes to the start of USB stick or ISO image file:
  
  iso=/dev/sdc
or
  iso=image.iso

  dd conv=notrunc if=isohdpfx.bin.170324 bs=1 count=432 of="$iso"

  
> Would they differ according to some/any parameter?

We do not yet have downloadable binaries of the other five isohdp*
variants as listed in
  http://www.syslinux.org/wiki/index.php?title=Isohybrid#MBR_selection
with --partok -ctrlhd0 --forcehd0.
I doubt that any distro uses them.


If one cares for Macs which were old in 2010, then one can keep
the first 32 bytes of the ISO:

  dd conv=notrunc if=isohdpfx.bin.170324 bs=1 skip=32 seek=32 count=400 of="$iso"

but only after having verified that this is really a pseudo APM:

  dd if="$iso" bs=1 count=8 | od -t x1

must report

  0000000 45 52 08 00 00 00 90 90

(... or the start bytes of isohdpfx.bin without APM patched in:
  0000000 33 ed 90 90 90 90 90 90
But then one would not need to preserve them.
)

Else the old MBR is supposed to be older than the preparation for
mjg layout on 2011-08-24:
  https://git.kernel.org/pub/scm/boot/syslinux/syslinux.git/commit/mbr/isohdpfx.S?id=2c3a24e5f4b807ec31595227afa59a818c060ca9
Thus it would be dangerous to let its first 32 bytes survive.
After all they get executed as x86 instructions when BIOS starts
the MBR.

As long as no old Mac is around, one should not take that risk.


To my knowledge the APM in the Debian and Ubuntu ISOs is useless.
The APM of Fedora and some Gentoo ISOs points to a HFS+ filesystem
and thus might be of interest for the Macs.

In any case the APM Block0 is a dummy with always the same content,
regardless of the size of the ISO or the position of the HFS+ partition.
Its presence without further APM blocks would not harm BIOS booting
but could mislead the old Macs for which APM matters.


> With the variety of potential combinations, is there really one generic 
> "receipt" that can be provided so as to help users (other than those 
> using the specific Debian ISO image that was tested by David)?

Above dd run should match any isohybrid which was made with SYSLINUX
version 3.82 or later.

The MBR has to properly work in itself, which Martin's hopefully does.

And it has to properly prepare the run of isolinux.bin, of which it
finds the block address in byte 432 to 435.
Martin's MBR prepares better for the expectations of isolinux.bin of
version 3.82 or later, than the old isohdpfx.bin did.


> independently of:
> _ the original size of the original isohybrid image?

Yes. This is mentioned only in the MBR partition table. Bytes 446 to 510.

> _ whichever CHS geometry that was used for the isohybrid tool (or 
> xorriso's equivalent) when the original isohybrid image was created?

Yes. C/H/S addresses are only recorded in the MBR partition table.

(Anyways, only the heads/cylinder and sectors/head ideas of the BIOS
 matter. One should never believe the C/H/S addresses in the partition
 table, but rather convert the LBAs of the partition table to C/H/S by
 the numbers inquired from the BIOS.)

> _ whichever combination of boot entries in the El Torito boot catalog?

Yes. El Torito does not matter if the MBR gets started.

(That's why i do not generally propose to patch in Martin's MBR by
 program isohybrid and its option --mbr.
 The isohybrid program reads El Torito with some special expectations.
 If all works well it puts the LBA of isolinux.bin to byte 432 to 435
 of the MBR.
 Above dd run stops overwriting exactly before byte 432.)


> @Thomas, in "CHS", the "S" is not really "Sectors per head" but rather 
> "Sectors per Track".

C/H/S = Cylinder/Head/Sector.

The two conversion factors HpC and SpH are needed for the formula
to convert C/H/S to LBA or vice versa:

  LBA = C * HpC * SpH + H * SpH + S - 1

Introducing the term "track" here seems surplus.
It would become confusing as soon as we speak of optical media too.
There "track" is a consequtive sequence of some normally quite large
number of blocks.


Have a nice day :)

Thomas