how BIOS boot no-emulation iso image

El Torito booting begins by the Boot Record of the ISO 9660 filesystem at block address 0x11. See also article ISO 9660. This Boot Record points to a Boot Catalog which is stored in one or more blocks inside the ISO 9660 filesystem.

The content of Boot Record and Boot Catalog is created during filesystem production by the ISO 9660 producing software.

The Boot Catalog lists the available boot images which may be prepared for multiple system architectures, called "platforms". These images are marked either as emulated floppies, or as emulated hard disks, or as no-emulation images. In any case they are the first stage in the boot process where custom code from the ISO filesystem can be executed.

While the emulated boot images are to be interpreted by the firmware as is supposed for floppies and hard disks, the no-emulation boot images are on their own:

PC-BIOS reads from the Boot Catalog the number of blocks to load, loads them (usually to segment 07c0) and then executes them as code. As with a normal floppy or hard disk, the DL register contains the BIOS drive number.

EFI interprets the boot image as FAT filesystem and looks up a standardized file path for further processing. The file name depends on the processor type. E.g. /EFI/BOOT/BOOTIA32.EFI for 32 bit x86 and /EFI/BOOT/BOOTX64.EFI for 64 bit x86.

The original El Torito specification mentions platforms "80x86", "PowerPC", and "Mac". Boot setups based on GRUB2 and ISOLINUX use 0x00 = "80x86" for PC-BIOS, and platform id 0xef for EFI which is defined by the UEFI specification.