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El-Torito: Difference between revisions
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Corrected the statements about UEFI, MBR, and GPT
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(Mentioned the platform id number code of "80x86") |
(Corrected the statements about UEFI, MBR, and GPT) |
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an optical medium: CD, DVD, BD. For booting PC-BIOS and EFI from
USB stick or other hard-disk-like devices, there is need for an
[[MBR]] and, if desired, for a [[GPT]].
The MBR of SYSLINUX can be adjusted to jump with its execution
onto the El Torito boot image of ISOLINUX. This is known as "isohybrid".
grub-mkrescue offers the same capability by its MBR, too.
The relation of MBR and GPT is regulated by UEFI specs:
The most modern layout prescribes an MBR with a single partition which
starts at Logical Block Address 1 and covers the whole image size.
It shall have MBR partition type 0xee.
In this case there is a GUID Partition Table (GPT) which in one of
its partitions marks the EFI System Partition with the FAT filesystem.
Neither MBR partition nor GPT partitions can lead to an address where
the ISO filesystem can be mounted, unless the ISO offers a second
set of volume descriptors and file tree at a higher block address.
Script grub-mkrescue produces this layout for booting via EFI.
The legacy partition layout (not to be confused with Legacy BIOS mode)
prescribes that the EFI System Partition shall be marked by an MBR
partition of type 0xef. Other MBR partitions are permissible. E.g.
one which starts at Logical Block Address 0 to make the ISO mountable.
It is not allowed, nevertheless, to let the ISO partition enclose the
EFI System Partition.
Several popular Linux distributions offer a layout that does not comply
to either of the UEFI alternatives. The MBR marks the whole ISO by a
partition of type 0x00. Another MBR partition of type 0xef marks a
data file inside the ISO filesystem with the image of the EFI System
Partition FAT filesystem. Nevertheless there is a GPT which also marks
the EFI System Partition image file. This GPT is to be ignored by any
UEFI compliant firmware. The nesting of the MBR partitions is made
acceptable by giving the outer MBR partition the type 0x00, which UEFI
specifies to be ignored.
Debian used for its installation image debian-7.1.0-amd64-netinst.iso
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