perhaps this instructions/script is of interest / use
https://github.com/patatetom/isohybrid-bios-uefi - 'how to build from Linux an ISO hybrid image bootable from BIOS or UEFI. '
https://raw.githubusercontent.com/patatetom/isohybrid-bios-uefi/master/README.md# isohybrid-bios-uefi
Or how to build from Linux an ISO hybrid image bootable from BIOS or UEFI.
> *An ISO image is an archive file of an optical disc, a type of disk image composed of the data contents from every written sector on an optical disc,
>including the optical disc file system. ISO image files usually have a file extension of .iso.
>The name ISO is taken from the ISO 9660 file system used with CD-ROM media, but what is known as an ISO image might also contain a UDF (ISO/IEC 13346) file system (commonly used by DVDs and Blu-ray Discs).* [»](https://en.wikipedia.org/wiki/ISO_image)
> *Universal Disk Format (UDF) is a profile of the specification known as ISO/IEC 13346 and ECMA-167 and is an open vendor-neutral file system for computer data storage for a broad range of media.
>In practice, it has been most widely used for DVDs and newer optical disc formats, supplanting ISO 9660.* [»](https://en.wikipedia.org/wiki/Universal_Disk_Format)
> *The El Torito Bootable CD specification is an extension to the ISO 9660 CD-ROM specification. It is designed to allow a computer to boot from a CD-ROM.* [»](https://en.wikipedia.org/wiki/El_Torito_(CD-ROM_standard))
> *The ISO hybrid feature enhances ISO 9660 file system by a Master Boot Record (MBR) for booting via BIOS from disk storage devices like USB flash drives.* [»](http://www.syslinux.org/wiki/index.php?title=Isohybrid)
[Core Linux](http://tinycorelinux.net/) 7.2, a minimal Linux operating system focusing on providing a base system using BusyBox, is provided on a small ISO image only bootable from BIOS :
```makefile
# Core-7.2.iso is available in this repo
stat -c%s Core-7.2.iso
11116544
md5sum Core-7.2.iso
77bf8cceacd2110120451f3f22f85156 Core-7.2.iso
# modprobe kvm-intel or modprobe kvm-amd before using -enable-kvm qemu option
qemu -version
QEMU emulator version 2.7.0,
Copyright (c) 2003-2016 Fabrice Bellard and the QEMU Project developers
# starting qemu with Core ISO image as cdrom under BIOS firmware
qemu -enable-kvm -m 2048 -machine q35 -cdrom Core-7.2.iso -snapshot
# Core displays its start menu :-)
# starting qemu with Core ISO image as hard disk under BIOS firmware
qemu -enable-kvm -m 2048 -machine q35 -hda Core-7.2.iso -snapshot
# no bootable device :-(
# nightly builds UEFI firmware can be found @ https://www.kraxel.org/repos/jenkins/edk2/
# uefi.md (OVMF-pure-efi.fd) is available in this repo
# starting qemu with Core ISO image as cdrom under UEFI firmware
qemu -enable-kvm -m 2048 -machine q35 -cdrom Core-7.2.iso -bios uefi.fd -snapshot
# no bootable device :-(
# starting qemu with Core ISO image as hard disk under UEFI firmware
qemu -enable-kvm -m 2048 -machine q35 -hda Core-7.2.iso -bios uefi.fd -snapshot
# no bootable device :-(
```
The objective is now to reconstruct an ISO hybrid image bootable from BIOS or UEFI from the Core ISO image.
## Tree
First, we extract files from Core ISO image :
```make
7z x Core-7.2.iso -oCore/
tree Core/
Core/
├── [BOOT]
│ └── Boot-NoEmul.img
└── boot
├── core.gz
├── isolinux
│ ├── boot.cat
│ ├── boot.msg
│ ├── f2
│ ├── f3
│ ├── f4
│ ├── isolinux.bin
│ └── isolinux.cfg
└── vmlinuz
```
Next, we remove unnecessry files and move `isolinux` folder at the root :
```make
rm -rf Core/\[BOOT\]/
rm -f Core/boot/isolinux/boot.cat
mv Core/boot/isolinux/ Core/
tree Core/
Core/
├── boot
│ ├── core.gz
│ └── vmlinuz
└── isolinux
├── boot.msg
├── f2
├── f3
├── f4
├── isolinux.bin
└── isolinux.cfg
```
## Old BIOS side
> *ISOLINUX is a boot loader for Linux/i386 that operates off ISO 9660/El Torito CD-ROMs in "no emulation" mode.
>This avoids the need to create an "emulation disk image" with limited space (for "floppy emulation") or compatibility problems (for "hard disk emulation").* [»](http://www.syslinux.org/wiki/index.php?title=ISOLINUX)
First, we override the embedded bootloader with our current one and add `ldlinux.c32` :
```make
syslinux --version
syslinux 6.03 Copyright 1994-2014 H. Peter Anvin et al
cp /lib/syslinux/bios/isolinux.bin Core/isolinux/
overwrite 'Core/isolinux/isolinux.bin'? y
cp /lib/syslinux/bios/ldlinux.c32 Core/isolinux/
```
Next, we build our first ISO image bootable from BIOS and test it with qemu :
```make
mkisofs -version
mkisofs 3.02a06 (x86_64-unknown-linux-gnu)
Copyright (C) 1993-1997 Eric Youngdale (C) 1997-2016 Joerg Schilling
mkisofs -output Core.iso \
-eltorito-boot \
isolinux/isolinux.bin -no-emul-boot -boot-load-size 4 -boot-info-table \
-eltorito-catalog isolinux/boot.cat \
Core/
# starting qemu with new Core ISO image as cdrom under BIOS firmware
qemu -enable-kvm -m 2048 -machine q35 -cdrom Core.iso -snapshot
# Core displays its start menu :-)
```
Now, we enhance the ISO image with the isohybrid feature to be able to boot Core from a USB flash drive :
```make
isohybrid -version
isohybrid version 0.12
isohybrid Core.iso
qemu -enable-kvm -m 2048 -machine q35 -cdrom Core.iso -snapshot
# Core displays its start menu :-)
# starting qemu with new Core ISO image as hard disk under BIOS firmware
qemu -enable-kvm -m 2048 -machine q35 -hda Core.iso -snapshot
# Core displays its start menu :-)
```
## New UEFI side
> *The EFI system partition (ESP) is a partition on a data storage device (usually a hard disk drive or solid-state drive) that is used by computers adhering to the Unified Extensible Firmware Interface (UEFI).
>When a computer is booted, UEFI firmware loads files stored on the ESP to start installed operating systems and various utilities.* [»](https://en.wikipedia.org/wiki/EFI_system_partition)
The « new » UEFI boot is based on the presence of a specific EFI System Partition (ESP) formated with FAT file system.
First, to adjust the size of the ESP image, we prepare the content of this partition in a folder :
```make
mkdir -p Image/{efi/boot/,syslinux}
# because syslinux can't access files outside of the image
# core.gz and vmlinux must be embedded
cp -a Core/boot/ Image/
cp Core/isolinux/{boot.msg,f*} Image/syslinux/
cp Core/isolinux/isolinux.cfg Image/syslinux/syslinux.cfg
cp /lib/syslinux/efi64/ldlinux.e64 Image/syslinux/
cp /lib/syslinux/efi64/syslinux.efi Image/efi/boot/bootx64.efi
tree Image/
Image/
├── boot
│ ├── core.gz
│ └── vmlinuz
├── efi
│ └── boot
│ └── bootx64.efi
└── syslinux
├── boot.msg
├── f2
├── f3
├── f4
├── ldlinux.e64
└── syslinux.cfg
```
Next, we build the ESP image from the previous folder :
```make
du -s Image/
10796 Image/
mkdir Core/efi/
# 128Kb for FAT data and 128Kb for syslinux bios files
truncate -s $((10796+128+128))k Core/efi/esp.img
# can also be FAT12 or FAT32 according volumetry
mkfs.msdos -F 16 -f 1 -M 0xF0 -r 112 -R 1 Core/efi/esp.img
# mount for you the ESP image on a ready mount point
sudo mount Core/efi/esp.img mount.point/ -o uid=you
cp -av Image/* mount.point/
sudo umount mount.point/
syslinux --install --directory syslinux/ Core/efi/esp.img
# starting qemu with ESP image under BIOS firmware
qemu -enable-kvm -m 2048 -machine q35 -hda Core/efi/esp.img -snapshot
# Core displays its start menu :-)
# starting qemu with ESP image under UEFI firmware
qemu -enable-kvm -m 2048 -machine q35 -hda Core/efi/esp.img -bios uefi.fd -snapshot
# Core displays its start menu :-)
```
Now, we insert the ESP image as a second El Torito boot entry :
```make
tree Core/
Core/
├── boot
│ ├── core.gz
│ └── vmlinuz
├── efi
│ └── esp.img
└── isolinux
├── boot.msg
├── f2
├── f3
├── f4
├── isolinux.bin
├── isolinux.cfg
└── ldlinux.c32
mkisofs -output Core.iso \
-eltorito-boot \
isolinux/isolinux.bin -no-emul-boot -boot-load-size 4 -boot-info-table \
-eltorito-alt-boot -eltorito-platform efi -eltorito-boot \
efi/esp.img -no-emul-boot \
-eltorito-catalog isolinux/boot.cat \
Core/
isohybrid Core.iso
qemu -enable-kvm -m 2048 -machine q35 -cdrom Core.iso -snapshot
# Core displays its start menu :-)
qemu -enable-kvm -m 2048 -machine q35 -hda Core.iso -snapshot
# Core displays its start menu :-)
# starting qemu with new Core ISO image as cdrom under UEFI firmware
qemu -enable-kvm -m 2048 -machine q35 -cdrom Core.iso -bios uefi.fd -snapshot
# KVM internal error :-(
# starting qemu with new Core ISO image as hard disk under UEFI firmware
qemu -enable-kvm -m 2048 -machine q35 -hda Core.iso -bios uefi.fd -snapshot
# Core displays its start menu :-)
```
## Links
- [https://en.wikipedia.org/wiki/ISO_image](https://en.wikipedia.org/wiki/ISO_image)
- [http://www.syslinux.org/wiki/index.php?title=Isohybrid](http://www.syslinux.org/wiki/index.php?title=Isohybrid)
- [https://en.wikipedia.org/wiki/El_Torito_(CD-ROM_standard)](https://en.wikipedia.org/wiki/El_Torito_(CD-ROM_standard))
- [http://www.syslinux.org/wiki/index.php?title=ISOLINUX](http://www.syslinux.org/wiki/index.php?title=ISOLINUX)
- [https://en.wikipedia.org/wiki/EFI_system_partition](https://en.wikipedia.org/wiki/EFI_system_partition)
- [http://www.rodsbooks.com/gdisk/hybrid.html](http://www.rodsbooks.com/gdisk/hybrid.html)
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