Hi, Robert Heller wrote: > Originally, the Linux boot (live / install) images were just a raw ISO-9660 > file system image, normally ment to be "burned" onto an optical disk (write > once CD-ROM or one of the various flavors of writable DVDs). PC BIOS's (before > UEFI/Secure boot) would treat a USB drive with an ISO-9660 file system just > like a CD-ROM
It is a bit different. Legacy BIOS looks on CD, DVD, or Blu-ray for an El Torito Boot Record at block address 17 (decimal) which would point to a Boot Catalog which lists boot images. In a Debian amd64 ISO, there are two boot images. One for Legacy BIOS and one for EFI. Both are also accessible as files in the ISO: $ xorriso -indev debian-13.5.0-amd64-netinst.iso -report_el_torito plain ... El Torito images : N Pltf B Emul Ld_seg Hdpt Ldsiz LBA El Torito boot img : 1 BIOS y none 0x0000 0x00 4 2828 El Torito boot img : 2 UEFI y none 0x0000 0x00 7200 1028 El Torito img path : 1 /isolinux/isolinux.bin El Torito img opts : 1 boot-info-table isohybrid-suitable El Torito img path : 2 /boot/grub/efi.img But on USB stick a legacy BIOS will look for a Master Boot Record at the very beginning of the USB stick. The first 446 bytes are x86 machine code which is supposed to start up the operating system. The next 64 bytes are the MBR partition table. In case of a Debian amd64 ISO, the MBR code stems from the SYSLINUX project (department ISOLINUX). During ISO production it learns the block address of /isolinux/isolinux.bin, so that it can load and start that program. This is called "isohybrid". $ xorriso -indev debian-13.5.0-amd64-netinst.iso -report_system_area plain ... System area summary: MBR isohybrid cyl-align-on GPT APM ... MBR partition table: N Status Type Start Blocks MBR partition : 1 0x80 0x00 0 1546240 MBR partition : 2 0x00 0xef 4112 7200 MBR partition path : 2 /boot/grub/efi.img >From this point both storage media families are handled the same way. But the initial boot lures differ. > I think the newer images, which are meant to be imaged onto > flash drives, have a partituon table and a FAT32 UEFI partition, plus the > bootable ISO-9660 file system in a separate partition. isohybrid for legacy BIOS was introduced before EFI support. It brought a first partition table with one entry and "boot" flag in order to keep partition editors from re-using the block range of the ISO filesystem and to appease HP firmware which would not start MBR code without seeing such a partition. More or less like partition 1 in a modern Debian amd64 ISO. EFI support came a few years later. El Torito boot sector, catalog, and MBR partition table or GPT point to the same EFI System Partition / Boot image, which is a FAT filesystem with .efi programs. The partition layout of the contemporary Debian amd64 ISOs was invented for Fedora (http://mjg59.dreamwidth.org/11285.html , 2012) and implemented in the already existing ISOLINUX program named "isohybrid". It became one of the layouts which can be produced by xorriso. Dirty, hunchbacked, mildly specs violating ... but boots about everywhere. Other distros have abandoned it long ago in favor of neater partitioning at the price of leavng behind dirty, hunchbacked, mildly specs violating firmwares. How this all works on byte level is documented in https://dev.lovelyhq.com/libburnia/libisofs/raw/branch/master/doc/boot_sectors.txt Most of it is used in the Debian ISOs for the respective CPU architectures. (Some of them have possibly died out meanwhile ...) Have a nice day :) Thomas
