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I checked the Hex code to know the information about the boot catelog sector, boot catelog entry...

I have obtained information such as Start LBA and Sector count of first entry in boot catelog but with last entry, it have only Start LBA and Hex code of Sector count is 00 00.

So I need to check the total size, then subtract the previous size. How to check the size of it?

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If the last entry points to a EFI System Partition, then size 0 means "up to the end of the ISO". UEFI specs prescribe this if the the partition image is larger than 32 MiB.

You will have to inquire the size of the FAT filesystem in the partition image or you may look in the ISO whether there is a data file pointing to the start address of the boot image.

xorriso -indev my.iso -report_el_torito plain

might tell it by e.g.

El Torito img path : 2 /boot/grub/efi.img

Else you may find it by the checksum of the first few blocks and comparing it with all files in the ISO.

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