libisofs/doc/boot_sectors.txt

            Collection of Boot Sector Formats for ISO 9660 Images


            by Thomas Schmitt    - scdbackup@gmx.net
            Libburnia project    - bug-xorriso@gnu.org
                                   pkg-libburnia-devel@lists.alioth.debian.org

This information is collected from various sources. Some is backed by
specifications, some is just rumor which happens to work (maybe not even that).


                                  Content

EL Torito CD booting, for PC-BIOS x86, PowerPC, (old) Mac, EFI.

Boot Info Table and GRUB2 Boot Info

Master Boot Record (MBR), for PC-BIOS x86 from (pseudo-) hard disk

Apple Partition Map (APM), for more modern Mac

GUID Partition Table (GPT), for EFI from (pseudo-) hard disk

MIPS Volume Header, for MIPS Big Endian, e.g. SGI Indigo2.

DEC Boot Block, for MIPS Little Endian , e.g. DECstation.

SUN Disk Label and boot images, for SUN SPARC
GRUB2 SUN SPARC Core File Address

PowerPC Reference Platform (PReP), for IBM PowerPC

Common Hardware Reference Platform (CHRP), for IBM PowerPC

HP-PA via PALO header version 4
HP-PA via PALO header version 5

DEC Alpha SRM boot sector, for Alpha architecture

Combinations of boot mechanisms:
- SYSLINUX isohybrid MBR
- SYSLINUX isohybrid for MBR, UEFI and x86-Mac
- GRUB2 grub-mkrescue MBR
>>> Mac and/or PowerPC bootable GRUB2 image with HFS+/FAT, APM,
    EFI GPT partition, PreP MBR partition, mountable FAT partition

------------------------------------------------------------------------------


                         EL Torito CD booting
                 for PC-BIOS x86, PowerPC, (old) Mac, EFI

Sources:
    El Torito, Bootable CD-ROM Format Specification, Version 1.0, 1995
    which refers to ECMA-119, the standard for ISO 9660 filesystems.
    libisofs/eltorito.[ch] by Vreixo Formoso.
    http://www.uefi.org/sites/default/files/resources/2_4_Errata_B.pdf


ECMA-119 prescribes that the first 32 kB of an ISO 9660 image are System Area
with arbitrary content. This prescription is obeyed by PC-BIOS systems only
if the ISO 9660 image is presented on CD, DVD or BD media.
In this case the El Torito Boot record is the starting point of booting.

After the System Area, an ISO 9660 image usually has three distinct block
intervals for:
- Volume descriptors (Primary Volume Descriptor, Boot Record, Joliet, ...)
- Directory trees, tables, boot catalog, embedded partitions and filesystems.
- Data file content, including content of El Torito boot images.

The Boot Record is an ECMA-119 Volume Descriptor which is located at 2 kB block
number 17 (decimal), if present at all. Its content points to the location of
the Boot Catalog.
The format is described in part by ECMA-119 8.2 "Boot Record" and further
specified by El Torito figure 7.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   0 |          0 | Volume Descriptor Type. 0= Boot record
   1 -   5 |    "CD001" | Standard Identifier
   6 -   6 |          1 | Volume Descriptor Version
   7 -  38 |  el_torito | Boot System Identifier
  39 -  70 |          0 | Boot Identifier
           |            |
  71 -2047 | ========== | Boot System Use
           |            |
  71 - 74  | cataloglba | The 2 kB block number of the Boot Catalog
           |            | as little-endian 32 bit number.
           |            |
  75 -2047 |          0 | Unused
---------- | ---------- | ----------------------------------------------------

el_torito is the constant string "EL TORITO SPECIFICATION" padded by 9 zeros.
cataloglba has to be provided by the file system generator.


The Boot Catalog lists the available boot images which may be prepared for
multiple system architectures, called "platforms".
It consists of one or more 2 kB blocks. The content is a sequence of fixed
format entries, 32 bytes each.
The entries are grouped in sections, which assign the entries to a particular
system architecture. The booting system will then choose an entry from an
appropriate section.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -  31 | ========== | Validation Entry
           |            | begins the first section, specifies an architecture
  32 -  63 | ========== | Initial/Default Entry
           |            | points to a boot image for given architecture
---------- | ---------- | ----------------------------------------------------
Optional: 
---------- | ---------- | ----------------------------------------------------
  64 -  95 | ========== | Section Header entry
           |            | begins new section, specifies an architecture
  96 - 127 | ========== | Section Entry
           |            | points to a boot image for given architecture
    ...    | .......... | Optional more Section Entries
    ...    | .......... | Optional more Section Headers and their Section
           |            | Entries
---------- | ---------- | ----------------------------------------------------

An architecture is referred by a Platform Id number.
Defined by El Torito are:
    0 = "80x86" which is used for standard PCs with Intel x86 or compatible CPU
    1 = "PowerPC" (possibly for IBM machines with PowerPC CPU)
    2 = "Mac" (possibly for Apple computers with MC68000 or PowerPC CPU)
UEFI 2.4 specifies in 12.3.2.1 "ISO-9660 and El Torito":
 0xef = EFI, a competitor and successor to PC-BIOS, further in use with
             Intel ia64 Itanium and newer Apple machines.

Words and numbers are represented as little-endian. 

Validation Entry:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   0 |          1 | Header Id
           |            |
   1 -   1 | platform_id| Platform Id. One of: 0, 1, 2, 0xef. See above.
           |            |
   2 -   3 |          0 | Reserved
   4 -  27 |  manuf_dev | ID string identifies the manufacturer/developer
           |            | (no non-zero examples known yet)
           |            |
  28 -  29 |   checksum | Checksum Word for the Validation Entry. 
           |            | The sum of all words in the entry has to be 0.
           |            |
  30 -  30 |       0x55 | 
  31 -  31 |       0xaa | 
---------- | ---------- | ----------------------------------------------------


Initial/Default Entry:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   0 | boot_indct | Boot Indicator: 0x88 = bootable, 0x00 = not bootable
           |            |
   1 -   1 | boot_media | Boot Media Type (i.e. media emulated by boot image):
           |            | 0= no emulation , 1= 1.2 MB diskette, 2=1.44 MB,
           |            | 3= 2.88 MB , 4= hard disk
           |            | (About everybody uses 0 = no emulation)
           |            |
   2 -   3 |   load_seg | Load Segment. (meaning unclear)
           |            | "If this value is 0 the system will use the
           |            |  traditional segment of 7C0."
           |            | libisofs default is 0
           |            |
   4 -   4 |   sys_type | System Type.
           |            | "Must be a copy of byte 5 from the partition table
           |            |  found in the boot image."
           |            | libisofs reads the start the boot image as MBR
           |            | if  boot_media == 4. This emulated MBR has a
           |            | partition table from where a byte gets copied.
           |            | Else this byte is 0.
           |            |
   5 -   5 |          0 | Unused
           |            |
   6 -   7 |  sec_count | Sector Count. Sector size 512:
           |            | "the number of virtual/emulated sectors the system
           |            |  will store at Load Segment during the initial boot
           |            |  procedure."
           |            | libisofs stores 1 for emulated boot_media and a
           |            | user defined value for boot_media == 0. Often: 4.
           |            |
   8 -  11 |  load_rba  | Load RBA. The 2 kB block address where the boot
           |            | image file content is located in the ISO 9660 image.
           |            |
  12 -  31 |          0 | Unused
---------- | ---------- | ----------------------------------------------------


Section Header Entry:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   0 |   head_ind | Header Indicator: 0x90 = more headers follow
           |            |                   0x91 = final header, last section
           |            |
   1 -   1 | platform_id| Platform Id. One of: 0, 1, 2, 0xef. See above.
           |            |
   2 -   3 | num_entries| Number of entries to follow in this section
           |            |
   4 -  31 |            | ID string identifies the manufacturer/developer
---------- | ---------- | ----------------------------------------------------


Section Entry:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   0 | boot_indct | Boot Indicator: 0x88 = bootable, 0x00 = not bootable
           |            |
   1 -   1 | boot_media | Boot Media Type (i.e. media emulated by boot image):
           |            | Bit 0 to 3 govern emulation
           |            |   0= no emulation , 1= 1.2 MB diskette, 2=1.44 MB,
           |            |   3= 2.88 MB , 4= hard disk
           |            |   (About everybody uses 0 = no emulation)
           |            | Bit 4 is reserved and must be 0
           |            | Bit 5 "Continuation entry follows" (meaning unclear)
           |            |       Might be the indicator for Extension Entries,
           |            |       which are not described here.
           |            | Bit 6 "Image contains an ATAPI driver"
           |            | Bit 7 "Image contains SCSI drivers"
           |            |
   2 -   3 |   load_seg | Load Segment. (meaning unclear)
           |            | See above Initial/Default Entry
           |            | libisofs default is 0.
   4 -   4 |   sys_type | System Type.
           |            | See above Initial/Default Entry
           |            | 0 if not emulation == 4.
   5 -   5 |          0 | Unused
           |            |
   6 -   7 |  sec_count | Sector Count. Sector size 512.
           |            | See above Initial/Default Entry
           |            | libisofs stores 1 for emulated boot_media and a
           |            | user defined value for boot_media == 0. Often: 4.
           |            |
   8 -  11 |  load_rba  | Load RBA. The 2 kB block address where the boot
           |            | image file content is located in the ISO 9660 image.
           |            |
  12 -  31 |   sel_crit | "Vendor unique selection criteria."
---------- | ---------- | ----------------------------------------------------

------------------------------------------------------------------------------


                      Boot Info Table and GRUB2 Boot Info

Sources:
    man mkisofs by Joerg Schilling
    Mail conversations with Vladimir Serbinenko.


The boot image file content is mostly opaque to the ISO 9660 image generator.
Nevertheless there is a tradition named "Boot Info Table" which prescribes
to write information into byte fields of the boot image file content.
Recent versions of GRUB2 expect a similar patching which has no name yet.
For now let's call it "GRUB2 Boot Info"
 
There are no general means known how a producer of ISO 9660 images could
detect the need for Boot Info Table production.
It rather needs a hint from the user who has to know whether the boot image
expects a Boot Info Table.
The Boot Info Table begins at byte 8 of the boot image content.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   8 -  11 |    pvd_lba | Block address of the Primary Volume Descriptor.
           |            | This is the session start LBA + 16.
           |            |
  12 -  15 |   file_lba | Block address of the start of the boot image file
           |            | content. Block size is 2048.
           |            |
  16 -  19 |   file_len | Number of bytes in boot image file content.
           |            |
  20 -  23 |   checksum | The sum of all 32-bit words of the file content
           |            | from byte 64 to file end.
           |            |
  24 -  63 |          0 | Reserved
---------- | ---------- | ----------------------------------------------------
All numbers are stored little-endian.

GRUB2 Boot Info represents a particular block address inside the boot image.
It may well be combined with Boot Info Table. See GRUB2 script grub-mkrescue
use of xorrisofs options -boot-info-table and --grub2-boot-info.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
2548 -2555 |  grub2_adr | Block address of the start of the boot image file
           |            | content plus 5. Block size is 512.
           |            | 64 bit Little-endian.
---------- | ---------- | ----------------------------------------------------

------------------------------------------------------------------------------


                           Master Boot Record (MBR)
                  for PC-BIOS x86 from (pseudo-) hard disk

Sources:
    http://en.wikipedia.org/wiki/Master_boot_record
    https://en.wikipedia.org/wiki/INT_13H
    Mailing list conversations with H. Peter Anvin and Vladimir Serbinenko.
    Mail conversations with Natalia Portillo.


The candidates for MBR booting will normally use El Torito rather than MBR
if the ISO image is presented on CD, DVD, or BD media.
The MBR comes into effect if the image is on a media that is interpreted by
the BIOS as some kind of hard disk. Usually real hard disks, floppy disks,
USB sticks, memory cards.

An important part of an MBR is the DOS style partition table. It describes up
to four primary partitions. There are two formats used for block address:
Cylinder/Head/Sector (C/H/S) and Logical Block Address (LBA). Both are based
on units of 512 bytes. So MBR_LBA = ISO_LBA * 4.

Contemporary x86 BIOS normally supports LBA addressing directly.
If INT 0x13 AH 0x41 returns with CX bit0 set, then INT 0x13 AH 0x42 may be used
for reading. (Sometimes even if the bit is not set to indicate the capability.)

For C/H/S, the sector address is broken up into whole cylinders, remaining
heads, and remaining sectors + 1. The nomenclature seems to stem from antique
drum storage.
There are two parameters, sectors_per_head and heads_per_cylinder which are not
stored in the MBR. So at ISO production time it is more or less arbitrary how
to convert a LBA into a C/H/S address and vice versa.
At boot time the x86 BIOS decides about the two parameters. The boot loader may
inquire these values by INT 0x13 AH 0x08 and use them to convert LBA to C/H/S
for the read operation INT 0x13 AH 0x02. So the C/H/S values in an ISO's
partition table are quite fictional and of few impact on boot loaders.

More important seems to align partition ends to a consistent cylinder size,
because some partition editors deduce their idea of disk geometry from there
and raise protest if they deem it inconsistent.
For maximum range of C/H/S addresses one may use sectors_per_head = 63 ,
heads_per_cylinder = 255. But that is not divisible by 4 and imposes alignment
problems with ISO 9660 filesystems. So (32,64) for images up to 1 GiB
or (63,252) for larger images are better.

Words are composed little-endian style.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 - 439 | = opaque = | Code Area filled with bytes for some boot system,
           |            | typically machine code.
           |            |
 440 - 443 | disk_sgntr | Disc signature: An individual disk id. Some software
           |            | might use it to recognize the same storage medium
           |            | at different device addresses.
           |            | (The Code Area might extend up to this field.)
           |            |
 444 - 445 |          0 | "usually nulls"
           |            | (The Code Area might extend up to this field.)
           |            |
 446 - 461 | ========== | Partition Table Entry for partition 1
           |            |
 446 - 446 |     status | For some generic MBRs this marks the one partition
           |            | from which the MBR should load and run more code.
           |            | 0x80 = bootflag/active , 0x00 = noboot/inactive
           |            | Some BIOSes ignore MBRs with no bootflag in any of
           |            | their partition table entries.
           |            |
 447 - 449 | ========== | C/H/S address of partition start
 447 - 447 | start_head | Heads part of start address.
 448 - 448 |  start_c_s | Bits 0 to 5 : Sectors part of start address.
           |            | Bits 6 to 7 : Bits 8 to 9 of cylinders part.
 449 - 449 |  start_cyl | Lower 8 bits of cylinders part of start address
           |            |
 450 - 450 |  part_type | Partition type indicates the purpose or kind of
           |            | filesystem in the partition.
           |            |
 451 - 453 | ========== | C/H/S address of last absolute sector in partition
 451 - 451 |   end_head | Heads part of end address.
 452 - 452 |    end_c_s | Bits 0 to 5 : Sectors part of end address.
                        |               Values: 1 to 63, not 0.
           |            | Bits 6 to 7 : Bits 8 to 9 of cylinders part.
 453 - 453 |    end_cyl | Lower 8 bits of cylinders part of end address
           |            |
 454 - 457 |  start_lba | LBA of first absolute sector in partition. 
           |            | Block size is 512. Counting starts at 0.
           |            |
 458 - 461 | num_blocks | Number of sectors in partition.
           |            |
 462 - 477 | ========== | Partition Table Entry for partition 2
           | part_entr2 | 16 bytes. Format as with partition 1.
           |            | All 0 means that partition is unused/undefined.
           |            |
 478 - 493 | ========== | Partition Table Entry for partition 3
           | part_entr3 | 16 bytes. See above.
           |            |
 494 - 509 | ========== | Partition Table Entry for partition 4
           | part_entr4 | 16 bytes. See above.
           |            |
 510 - 510 |       0x55 | MBR signature
 511 - 511 |       0xaa | MBR signature
           |            |
---------- | ---------- | ----------------------------------------------------


By tradition the MBR itself and possibly more blocks are not claimed by any
partition. But starting the first partition at a non-zero block address causes
on Linux a partition device file (e.g. /dev/sdb1) which cannot be used to mount
the ISO filesystem.

libisofs is able to produce a second set of trees and meta data which is
suitable for being mounted at start block 16 (ISO) which is block 64 in MBR.
See <libisofs/libisofs.h> for call iso_write_opts_set_part_offset()
and http://libburnia-project.org/wiki/PartitionOffset for examples with
program xorriso.

------------------------------------------------------------------------------


                           Apple Partition Map (APM) 
    for Apple Macs introduced since 2000 and more computer-like than iPad
                  from CD and often from (pseudo-) hard disk

Sources:
    http://mjg59.dreamwidth.org/11285.html
    http://opensource.apple.com/source/IOStorageFamily/IOStorageFamily-116/IOApplePartitionScheme.h (typedef struct Block0)
    http://www.informit.com/articles/article.aspx?p=376123&seqNum=3
    syslinux-4.05/utils/isohybrid.c
    Mail conversations with Vladimir Serbinenko.
    Mail conversations with Natalia Portillo of DiskImageChef,
                            who quoted "Inside Macintosh" Volumes IV and V.


APM has an adjustable block size. Because the ISO images shall always work
on optical media, and in order to make room for the header block of an
additional GPT, only block size 2048 is considered here.
The role of APM in the boot process is to guide the firmware to a
HFS+ filesystem.

Block0 (aka Driver Descriptor Map) of an APM begins at byte 0 of the medium.
Thus it collides with MBR and other boot sector formats. By lucky coincidence
it is possible to compose a mock-up of a Block0 which is acceptable to firmware
which expects APM, and is also harmless x86 machine code with no negative
side effects. So it is possible to combine APM with an especially prepared MBR.

Block0 is optional. But in the context of bootable hybrid ISOs it is not only
needed to announce block size 2048, but also it is much better suited for
staging as harmless x86 machine code than is an APM partition entry.

Usually there is no Device Partition Map block (signature "TS"), although it
is demanded by the book "Inside Macintosh". It would sit where GPT has its
header block. So DPM is not described here.

The layout of a Block0 of an APM is:

Byte Range | Value      | Meaning (all numbers are stored big endian)
---------- | ---------- | ----------------------------------------------------
   0 -   1 | sig        | Signature 0x45 = 'E' , 0x52 = 'R'
   2 -   3 | block_size | 0x0800 = 2048
   4 -   7 | block_count| Number of blocks covered by APM
           |            | Often some x86-harmless dummy. E.g. 0x9090 = 37008
           |            | or 0xeb02ffff = 3,942,842,367                         
   8 -   9 | dev_type   | Device type: The id of the Mac driver which is in
           |            | charge of the storage device.
  10 -  11 | dev_id     | Device id: Address in an i/o bus system.
  12 -  15 | drv_data   | Driver data: Not used.
  16 -  17 | drv_count  | Driver count: Count of entries in drv_map.
  18 - 505 | drv_map    | Driver descriptor table:
           |            | Up to 61 entries of 8 bytes each.
           |            | They contain the 32 bit 512-byte LBA of the driver's
           |            | storage location, its 16 bit size in 512-byte blocks,
           |            | and value 0x0001.
 506 - 511 | reserved   |
---------- | ---------- | ----------------------------------------------------

The SYSLINUX program isohybrid.c overwrites the first 32 bytes of this
layout by its dummy values. It uses the small block_count 0x00009090 and
sets all bytes up to 31 to 0.
The libisofs HFS+ extension by Vladimir Serbinenko overwrites only the
first 8 bytes. It uses the large block_count 0xeb02ffff.

Block0 and the following APM entries each occupy 1 block of the announced size.

The first APM entry describes the range from its own start to the end of the
last APM entry. Each of the other APM entries describes a partition.

The layout of an Apple partition map entry is:

Byte Range | Value      | Meaning (all numbers are stored big endian)
---------- | ---------- | ----------------------------------------------------
   0 -   1 | sig        | Signature 0x50 = 'P' , 0x4d = 'M'
   2 -   3 | reserved   |
   4 -   7 | map_entries| Number of partition entries.
           |            | All entries show the same number.
   8 -  11 | start_block| "physical block start of partition"
  12 -  15 | block_count| "physical block count of partition"
  16 -  47 | name       | Partition name 
  48 -  79 | type       | Type string
  80 -  83 | lb_start   | Logical block start = 0
  84 -  87 | lb_count   | Logical block count (same as block_count)
  88 -  91 | flags      | Status flags
           |            | bit0= entry is valid
           |            | bit1= partition is allocated
           |            | bit2= partition is in use
           |            | bit3= partition contains valid boot information
           |            | bit4= partition is readable
           |            | bit5= partition is writable
           |            | bit7= boot code is position independent
           |            | bit30= automatic mount (legacy Mac)
  92 -  95 | boot_block | Logical start block number of boot code = 0
  96 -  99 | boot_bytes | Number of bytes in boot code = 0
 100 - 119 |            | More boot code stuff = 0
 120 - 135 | processor  | "processor type" = 0
 136 - 511 | reserved   |
---------- | ---------- | ----------------------------------------------------

For the first APM entry (byte 0x0800), the following values apply:
map_entries = number of APM entries, including itself. E.g. 4.
start_block = 1
block_count = map_entries
name        = "Apple"
type        = "Apple_partition_map"
flags       = 3

libisofs uses APM to mark a HFS+ filesystem partition within an ISO 9660 image.
Usually the APM has 3 more entries after the first entry:

Entry 2 (byte 0x1000) describes the block interval from ISO image start to
the start of the HFS+ filesystem meta data.
start_block = 16
block_count = start_of_hfs - 16
name        = "Gap0"
type        = "ISO9660_data"
flags       = 0x13

Entry 3 (byte 0x1800) describes the interval from the start of the HFS+ meta
data to the end of the HFS+ data at the end of its partition. This includes all
content blocks of the data files in the ISO image.
start_block = start_of_hfs
block_count = end_of_hfs - start_of_hfs
name        = "HFSPLUS_Hybrid"
type        = "Apple_HFS"
flags       = 0x40000013

Entry 4 (byte 0x2000) describes the interval from the end of the HFS+
partition to the end of the ISO image. It is possible that this interval is
empty. In this case, no fourth APM entry will be written.
start_block = end_of_hfs
block_count = end_of_iso - end_of_hfs
name        = "Gap1"
type        = "ISO9660_data"
flags       = 0x13


>>> Open questions:
>>> What HFS+ blessings are needed for booting ?
>>> What files need what HFS creator and type settings ?


------------------------------------------------------------------------------


                        GUID Partition Table (GPT)
                    for alternative mountability paths
      and for EFI booting of some Apple Macs from (pseudo-) hard disk

Sources:
  http://mjg59.dreamwidth.org/11285.html
  http://mjg59.fedorapeople.org/Fedora-LiveCD.iso
  http://en.wikipedia.org/wiki/GUID_Partition_Table
  http://en.wikipedia.org/wiki/GUID
  http://www.uefi.org/sites/default/files/resources/2_4_Errata_B.pdf


GPT is the partition map format of EFI, a successor of PC-BIOS.
Block size is always 512. GPT consists of a header block at block address 1 and
a partition table near the start of the medium. This is called the primary GPT.
There is a backup copy of header and table near the end of the medium.

GPT is particularly designed to co-exist with MBR. Officially only with a
Protective MBR which covers the whole medium (except the MBR itself) by
a single partition of type 0xee. Inofficially often with filesystem partitions
marked in both, GPT and MBR. In the latter case the booting firmware may
or may not prefer GPT over the MBR partition table.
GPT can co-exist with APM if APM block size is at least 1024. In this case,
the primary partition table will begin after the last APM entry block.

The header block format is:
Byte Range | Value      | Meaning (little endian numbers, LBA unit is 512 byte)
---------- | ---------- | ----------------------------------------------------
   0 -   7 | sig        | Signature "EFI PART" (with no trailing zero)
   8 -  11 | revision   | Revision = {0x00, 0x00, 0x01, 0x00} meaning "1.0"
  12 -  15 | head_size  | Header size = 0x5c = 92
  16 -  19 | head_crc   | CRC-32 of this header while head_crc is 0
  20 -  23 | reserved   | = 0
  24 -  31 | curr_lba   | Location of this header block = 1
  32 -  39 | backup_lba | Location of header backup block. See below.
  40 -  47 | first_lba  | First usable LBA for partitions
  48 -  55 | last_lba   | Last usable LBA for partitions
  56 -  71 | guid       | Disk GUID, Random
  72 -  79 | part_start | Partition entries start
           |            | Normally this is 2. But to co-exist with APM, it
                        | might become some other number up to 62.
  80 -  83 | entry_count| Number of partition entries
  84 -  87 | entry_size | Size of a partition entry = 0x80 = 128
  88 -  91 | p_arr_crc  | CRC-32 of the partition array
  92 - 511 | reserved   | Must be 0
---------- | ---------- | ----------------------------------------------------

The CRC-32 algorithm can be characterized as follows:
The generating polynomial has the bit representation 0x104c11db7.
The seed value for a bit shifting division algorithm is 0x46af6449. It is
chosen so that the CRC of 0 bytes of input is 0x00000000.
The least significant bits of input bytes get processed first. I.e. bit0 of
the last input byte gets mapped to x exp (7 + 32), bit7 of this byte gets
mapped to x exp (0 + 32).
The resulting division residue gets bitwise mirrored. E.g. bit0 becomes bit31,
bit1 becomes bit30, and so on. Further it gets exored with 0xffffffff.

A GUID consists of a 32-bit integer, two 16-bit integers, and an array of
8 bytes. The integers are to be stored big-endian.
A globally registered class of GUID are the partition type GUIDs:
Basic data partition:  a2 a0 d0 eb , e5 b9 , 33 44 , 87 c0 68 b6 b7 26 99 c7
HFS+ partition      :  00 53 46 48 , 00 00 , aa 11 , aa 11 00 30 65 43 ec ac
EFI System partition:  28 73 2a c1 , 1f f8 , d2 11 , ba 4b 00 a0 c9 3e c9 3b
Note that the wikipedia list shows the first 32-bit word and the next two
16-bit words in little-endian interpretation.

The partition table is an array of entries. Each has a size of 128 bytes.
A partition table entry looks like:

Byte Range | Value      | Meaning (numbers are stored little endian)
---------- | ---------- | ----------------------------------------------------
   0 -  15 | type_guid  | Partition type GUID
  16 -  31 | part_guid  | Unique partition GUID, Random
  32 -  39 | start_lba  | First LBA
  40 -  47 | end_lba    | Last LBA (inclusive)
  48 -  55 | flags      | Attribute flags
           |            | bit0= "System Partition" Do not alter.
           |            | bit2= Legacy BIOS bootable (MBR partition type 0x80)
           |            | bit60= read-only
  56 - 127 | name       | Characters encoded as UTF-16LE. Padded by 0-bytes.
---------- | ---------- | ----------------------------------------------------


About header field "Location of header backup block":

Near to the end of the image, after any data blocks which might be of interest
for the filesystems covered by GPT partitions, there is a backup of partition
table and header block.
The header block is supposed to mark the end of the usable medium. But libisofs
may have the need to add more data.
The partition table is stored directly before the header block. So it will
normally not begin at a 2 KiB block start.

The content of the backup partition table is the same as the one of the
primary table.
The backup header differs from the primary header by 
Byte Range | Value      | Meaning (little endian numbers, LBA unit is 512 byte)
---------- | ---------- | ----------------------------------------------------
  16 -  19 | head_crc   | CRC-32 of this header while head_crc is 0.
           |            | Is recomputed after the following changes.
  24 -  31 | curr_lba   | Location of this header block.
           |            | Shows own block address.
  32 -  39 | backup_lba | Location of header backup block.
           |            | Points to primary header block = 1
  72 -  79 | part_start | Partition entries start.
           |            | Points to start of backup partition table.
---------- | ---------- | ----------------------------------------------------


An EFI System partition usually contains the same data blocks as the El Torito
boot image for EFI. It is used for booting some Macs from (pseudo-) hard disk.


------------------------------------------------------------------------------


                               MIPS Volume Header
                       for MIPS Big Endian, e.g. SGI Indigo2

Sources:
   cdrkit-1.1.10/genisoimage/boot-mips.c
   by Steve McIntyre <steve@einval.com>
   which refers to
      genisovh by Florian Lohoff <flo@rfc822.org>
              and Thiemo Seufer <seufer@csv.ica.uni-stuttgart.de>
      who seem to have learned parameter settings from IRIX CD media
      There are traces in the web which relate this to specs by
               MIPS Computer Systems, Inc. , 1985
               Silicon Graphics Computer Systems, Inc. , 2000
   Mail conversations with Natalia Portillo.


The first 512 bytes of the media constitute the Volume Header.
Words are composed big-endian style.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   3 | 0x0be5a941 | Magic number
   4 -   5 |          0 | Root partition number
   6 -   7 |          0 | Swap partition number
   8 -  23 |          0 | One of the boot_name items from the Volume Directory
           |            | may be put here to choose for booting the entry with
           |            | that name.
           |            | (Obviously it may be empty if only one non-zero entry
           |            |  exists in the Volume Directory.)
           |            |
  24 -  71 | ========== | Device Parameters
           |            |
  24 -  24 |          0 | Spiral addressing skew (unclear what this means)
  25 -  25 |          0 | Words of 0 before header
  26 -  26 |          0 | Words of 0 between hdr and data
  27 -  27 |          0 | Spare sectors per cylinder
  28 -  29 |  num_cyl_l | Number of usable cylinder, lower two bytes
           |            |  ((iso_size + BYTES_PER_SECTOR - 1) /
           |            |   (SECTORS_PER_TRACK * BYTES_PER_SECTOR)) & 0xffff
  30 -  31 |          0 | Starting head of volume 0 
  32 -  33 |          1 | Number of tracks per cylinder
  34 -  34 |          0 | Depth of CTQ queue (unclear what this means)
  35 -  35 |  num_cyl_h | Number of usable cylinders, high byte 
           |            |  ((iso_size + BYTES_PER_SECTOR - 1) /
           |            |   (SECTORS_PER_TRACK * BYTES_PER_SECTOR)) >> 16
  36 -  37 |          0 | unused
  38 -  39 |         32 | SECTORS_PER_TRACK
  40 -  41 |        512 | BYTES_PER_SECTOR
  42 -  43 |          0 | Sector interleave (unclear what this means)
  44 -  47 | 0x00000034 | Controller characteristics composed from
           |            | DP_RESEEK 0x00000020 /* recalibrate as last resort */
           |            | DP_IGNOREERRORS 0x00000010
           |            |              /* transfer data regardless of errors */
           |            | DP_TRKFWD 0x00000004
           |            |                    /* forward to replacement track */
  48 -  51 |          0 | Bytes/sec for kernel stats
  52 -  55 |          0 | Max num retries on data error
  56 -  59 |          0 | ms per word to xfer, for iostat
  60 -  71 |          0 | 6 parameter words for xylogics controllers
           |            |
  72 - 311 | ========== | Volume Directory with 15 entries of 16 bytes each
           |            |
  72 -  87 | ========== | Volume Directory Entry 1
  72 -  79 |  boot_name | Boot file basename, eventually padded by 0 to length 8
  80 -  83 | boot_block | ISO 9660 LBA of boot file * 4, i.e. in blocks of 512
  84 -  87 | boot_bytes | File length in bytes
           |            |
  88 - 311 | see above  | Volume Directory Entries 2 to 15
           |            |
 312 - 504 | ========== | Partition Table with 16 entries of 12 bytes each
           |            |
 312 - 407 |          0 | Unused partition entries 1 to 8
           |            |
 408 - 419 | ========== | Partition Table Entry 9 for Volume Header
 408 - 411 |  part_blks | Number of 512 byte blocks in partition
           |            |(iso_size + (BYTES_PER_SECTOR - 1)) / BYTES_PER_SECTOR
 412 - 415 |          0 | Start block of partition
 416 - 419 |          0 | PTYPE_VOLHDR = Partition is volume header
           |            |
 420 - 431 |          0 | Unused partition entry 10
           |            |
 432 - 443 | ========== | Partition Table Entry 11 for Volume
 432 - 435 |  part_blks | Number of 512 byte blocks in partition
           |            |(iso_size + (BYTES_PER_SECTOR - 1)) / BYTES_PER_SECTOR
 436 - 439 |          0 | Start block of partition
 440 - 443 |          6 | PTYPE_VOLUME = Partition is entire volume
           |            |
 444 - 503 |          0 | Unused partition entries 12 to 16
           |            |
 504 - 507 |   head_chk | Volume header checksum
           |            | The two's complement of bytes 0 to 503 read as big
           |            | endian unsigned 32 bit: sum(words) + head_chk == 0
           |            |
 508 - 511 |          0 | Volume header end padding
           |            |
up to 2048 |          0 | ISO 9660 Block end padding
---------- | ---------- | ----------------------------------------------------


------------------------------------------------------------------------------


                               DEC Boot Block
                    for MIPS Little Endian , e.g. DECstation

Sources:
   cdrkit-1.1.10/genisoimage/boot-mipsel.c
   by Steve McIntyre <steve@einval.com>
   which refers to
      delo   by Florian Lohoff <flo@rfc822.org>
            and Thiemo Seufer <seufer@csv.ica.uni-stuttgart.de>
   cdrkit-1.1.10/include/glibc_elf.h
   by Steve McIntyre
   which is based on
      <elf.h> from GNUC C Library by Free Software Foundation, Inc. 


There seems to be only one boot file possible.
Some information needs to be read out of the ELF headers of this boot file.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   7 |          0 | Padding
           |            |
   8 -  11 | 0x0002757a | Magic number
           |            |
  12 -  15 |          1 | Mode /* 0: Single extent, 1: Multi extent boot */
           |            |
  16 -  19 |   load_adr | Load address /* Load below kernel */
           |            | Stems from ELF header of boot file.
           |            | See below Elf32_Phdr field p_vaddr.
           |            |
  20 -  23 |   exec_adr | Execution address /* And exec there */
           |            | Stems from ELF header of boot file.
           |            | See below Elf32_Ehdr field e_entry.
           |            |
  24 -  31 | ========== | Boot Map Entry 1
           |            |
  24 -  27 |   seg_size | Segment size in file. Blocks of 512 bytes.
           |            | Stems from ELF header of boot file.
           |            | (Elf32_Phdr field p_filesz + 511) / 512;
           |            |
  28 -  31 |  seg_start | Segment file offset. Blocks 512 bytes.
           |            | ISO 9660 LBA of boot file * 4 plus offset which
           |            | stems from ELF header of boot file:
           |            | (Elf32_Phdr field p_offset + 511) / 512;
           |            |
  32 - 431 | ========== | Boot Map Entries 2 to 51
           |          0 |
           |            |
---------- | ---------- | ----------------------------------------------------


Elf32_Ehdr gets loaded from boot file byte address 0:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -  23 |            | ( Magic number, file information )
           |            | 
  24 -  27 |    e_entry | /* Entry point virtual address */
           | = exec_adr | Needed for exec_adr
           |            |
  28 -  31 |    e_phoff | /* Program header table file offset */
           |            | Byte address of Elf32_Phdr
           |            |

Elf32_Phdr gets loaded from boot file byte_address Elf32_Ehdr.e_phoff :
Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   3 |            | ( Segment type )
           |            |
   4 -   7 |   p_offset | /* Segment file offset */
           |-> seg_start| Needed for seg_start
           |            |
   8 -  11 |    p_vaddr | /* Segment virtual address */
           |  =load_adr | Needed for load_adr
           |            |
  12 -  15 |            | (Segment physical address)
           |            |
  16 -  19 |   p_filesz | /* Segment size in file */
           |-> seg_size | Needed for seg_size
           |            |
---------- | ---------- | ----------------------------------------------------


------------------------------------------------------------------------------


                          SUN Disk Label and boot images
                                 for SUN SPARC

Sources:
   cdrtools-2.01.01a77/mkisofs/sunlabel.h
   cdrtools-2.01.01a77/mkisofs/mkisofs.8
   by Joerg Schilling


The Disk Label is written to the first 512 bytes of the image. It can mark
8 partitions (slices ) of which the first contains the ISO image. The other
7 may contain boot images.
Words are composed big-endian style. Block size is 512.

Boot images are provided externally. mkisofs arranges them after the end of
the ISO image so that each starts at a cylinder boundary (320 kB).

There is a mechanism in mkisofs which fills unused partitions by copies of
their predecessor in the partition table:
"If the special filename ... is used, the actual and all following
 boot partitions are mapped to  the  previous  partition.
 If mkisofs is called with -G image -B ... all boot partitions are
 mapped to the partition that contains the ISO9660 filesystem."

Disk Label components:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 - 127 |      label | ASCII Label
           |            | "CD-ROM Disc with Sun sparc boot created by ..."
           |            | mkisofs option -sparc-label
           |            |
 128 - 263 | ========== | /* vtoc inclusions from AT&T SVr4 */
           |            |
 128 - 131 |          1 | Layout version
 132 - 139 |          0 | /* volume name           */
 140 - 141 |          8 | Number of partitions
           |            |
 142 - 173 | ========== | 8 partition entries of 4 bytes
           |            |
 142 - 145 | ========== | Entry for partition 1
 142 - 143 |          4 | ID tag of partition: 4 = User partition
 144 - 145 |       0x10 | Permissions: 0x10 = read-only
           |            |
 146 - 149 | ========== | Entry for partition 2 
 146 - 147 |    id_tag2 | ID tag of partition:
           |            | 0 = unused
           |            | 2 = Root partition with boot image
 148 - 149 |      perm2 | Permissions:
           |            | 0 = unused
           |            | 0x10 = read-only (if used)
           |            |
 150 - 173 | ========== | Entries for partition 3 to 8.
           |            | See above: Entry for partition 2
           |            |
 174 - 175 |          0 | Padding
           |            |
 176 - 187 |          0 | /* info for mboot       */
           |            |
 188 - 191 | 0x600ddeee | /* to verify vtoc sanity */
           |            |
 192 - 231 |          0 | Reserved
           |            |
 232 - 263 |          0 | 8 Timestamps of yet unknown format
           |            |
 264 - 419 |          0 | Padding
           |            |
 420 - 443 | ========== | Disk properties
           |            |
 420 - 421 |        350 | Rotations per minute
 422 - 423 |       2048 | Number of physical cylinders (fixely 640 MB)
 424 - 425 |          0 | /* alternates per cylinder */
 426 - 429 |          0 | /* obsolete */
 430 - 431 |          1 | /* interleave factor */
 432 - 433 |       2048 | Number of data cylinders (fixely 640 MB)
 434 - 435 |          0 | /* # of alternate cylinders */
 436 - 437 |          1 | Number of heads per cylinder (i.e. 1 cyl = 320 kB)
 438 - 439 |        640 | Number of sectors per head (i.e. 1 head = 320 kB)
 440 - 443 |          0 | /* obsolete */
           |            |
 444 - 507 | ========== | Partition table
           |            |
 444 - 451 | ========== | Partition table entry #1
           |            |
 444 - 447 |  start_cyl | Start cylinder
           |            |
 448 - 451 | num_blocks | Number of 512-byte blocks in partition
           |            |
 452 - 507 | ========== | Partition table entries #2 to #8
           |        ... | See above Partition table entry #1
           |            |
 508 - 509 |     0xdabe | Magic Number
           |            |
 510 - 511 |   checksum | The result of exoring 2-byte words 0 to 254
           |            |
---------- | ---------- | ----------------------------------------------------

------------------------------------------------------------------------------


                      GRUB2 SUN SPARC Core File Address

Sources:
    Mail conversations with Vladimir Serbinenko.

GRUB2 lets libisofs write after the disk label block the address and size of a
data file in the ISO image. E.g. of /boot/grub/sparc64-ieee1275/core.img.
This is combined with a SUN Disk Label which exposes only the single partition
describing the overall ISO filesystem size.

  Byte Range | Value      | Meaning
------------ | ---------- | --------------------------------------------------
 512 -   551 |     opaque | Code and data provided by GRUB2
             |            |
 552 -   559 |     offset | Start byte number of the file. 64-bit big-endian.
             |            |
 560 -   563 |      size  | Number of bytes in the file. 32-bit big-endian.
             |            |
 564 - 32767 |     opaque | Code and data provided by GRUB2
             |            |
------------ | ---------- | --------------------------------------------------

------------------------------------------------------------------------------


                      PowerPC Reference Platform (PReP)
                              for IBM PowerPC

Sources:
    Mail conversations with Vladimir Serbinenko.


PReP boots via a MBR partition containing only raw ELF and having type 0x41.


------------------------------------------------------------------------------


                 Common Hardware Reference Platform (CHRP)
                              for IBM PowerPC

Sources:
    Mail conversations with Vladimir Serbinenko.
    http://stuff.mit.edu/afs/sipb/contrib/doc/specs/protocol/chrp/chrp1_7a.pdf
    https://www.ibm.com/developerworks/community/wikis/home?lang=en#!/wiki/W51a7ffcf4dfd_4b40_9d82_446ebc23c550/page/PowerLinux%20Boot%20howto

CHRP is marked by an MBR partition entry of type 0x96 spanning the whole
ISO 9660 image.

The specs in chrp1_7a.pdf promise that CHRP also recognizes ISO 9660 file
systems on unpartitioned disks. (See 11.1.1. Media Layout Format)

The firmware looks up a file /ppc/bootinfo.txt which in SGML-ish tag
<boot-script> contains firmware commands.
E.g. to execute the binary /boot/grub/powerpc.elf as first stage of GRUB2:
  <boot-script>boot &device;:\boot\grub\powerpc.elf</boot-script> 

Vladimir Serbinenko stated:
PReP boot may be preferable. At least it can co-exist with other partitions
in the ISO image [without causing overlapping between partitions].


------------------------------------------------------------------------------

                      HP-PA via PALO header version 4
                              for HP PA-RISC  

Sources:
   cdrkit-1.1.10/genisoimage/boot-hppa.c
   by Steve McIntyre <steve@einval.com>
      who states "Heavily inspired by palo"

This format is expected by PALO versions before 1.92. Their source code defines
PALOHDRVERSION as 4. The format also serves as fallback for newer versions,
which expect header version 5, if a 0-byte is found at byte position 1024.

There are five parameters which get encoded into the first 248 bytes of the
System Area: cmdline, bootloader, 32-bit kernel, 64-bit kernel, and ramdisk.
They are all mandatory.
While cmdline is simply a string of at most 127 characters, the other four
point to data files inside the ISO image.

All numbers are recorded big endian.

Boot sector components:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   1 |     0x8000 | Magic
           |            |
   2 -   6 |     "PALO" | Zero terminated string
           |            |
   7 -   7 |          4 | Version
           |            |
   8 -  11 | kern32_adr | Byte address of the "HPPA 32-bit kernel" file
           |            | genisoimage option -hppa-kernel-32
  12 -  15 | kern32_len | Byte count of the "HPPA 32-bit kernel" file
           |            |
  16 -  19 | ramdsk_adr | Byte address of the "HPPA ramdisk" file
           |            | genisoimage option -hppa-ramdisk
  20 -  23 | ramdsk_len | Byte count of the "HPPA ramdisk" file
           |            |
  24 - 151 |    cmdline | "Command line"
           |            | genisoimage option -hppa-cmdline
           |            |
 232 - 235 | kern64_adr | Byte address of the "HPPA 64-bit kernel" file
           |            | genisoimage option -hppa-kernel-64
 236 - 239 | kern64_len | Byte count of the "HPPA 64-bit kernel" file
           |            |
 240 - 243 | bootld_adr | Byte address of the "HPPA bootloader" file
           |            | genisoimage option -hppa-bootloader
 244 - 247 | bootld_len | Byte count of the "HPPA bootloader" file
           |            |
---------- | ---------- | ----------------------------------------------------


------------------------------------------------------------------------------

                      HP-PA via PALO header version 5
                              for HP PA-RISC  

Sources:
   Public mail conversations with Helge Deller, beginning with
      https://lists.debian.org/debian-hppa/2014/01/msg00016.html
   http://git.kernel.org/cgit/linux/kernel/git/deller/palo.git/tree/lib/
      (especially struct firstblock in common.h and struct partition in part.h)

This format is expected by PALO versions 1.92 or higher. They fall back to
header version 4 if a 0-byte is found at byte position 1024.
Their source code defines PALOHDRVERSION as 5.

There are five parameters which get encoded into the first 2048 bytes of the
System Area: cmdline, bootloader, 32-bit kernel, 64-bit kernel, and ramdisk.
They are all mandatory.
While cmdline is simply a string of at most 1023 characters, the other four
point to data files inside the ISO image.

Several fields of the firstblock shall be hardcoded to 0, on advise of
Helge Deller. Their description is shown in round brackets.

All numbers are recorded big endian.
Except flags, all 4-byte integers are signed.

Boot sector components:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   1 |     0x8000 | Magic
           |            |
   2 -   6 |     "PALO" | Zero terminated string
           |            |
   7 -   7 |          5 | Version
           |            |
   8 -  11 | kern32_adr | Byte address of the 32-bit kernel file
           |            |
  12 -  15 | kern32_len | Byte count of the 32-bit kernel file
           |            |
  16 -  19 | ramdsk_adr | Byte address of the ramdisk file
           |            |
  20 -  23 | ramdsk_len | Byte count of the ramdisk file
           |            |
  24 - 141 |          0 | All 0s. Old command line of version 4.
           |            |
           |            |
 220 - 223 |          0 | (Length of uncompressed 32-bit kernel)
           |            |
 224 - 227 |          0 | (Length of uncompressed 64-bit kernel)
           |            |
 228 - 231 |          0 | (flags)
           |            |
 232 - 235 | kern64_adr | Byte address of the 64-bit kernel file
           |            |
 236 - 239 | kern64_len | Byte count of the 64-bit kernel file
           |            |
 240 - 243 |    ipl_adr | Byte address of the bootloader file
           |            |
 244 - 247 |    ipl_len | Byte count of the bootloader file
           |            |
 248 - 251 |          0 | (ipl_entry: offset to first command in bootloader)
           |            |
 446 - 511 |          0 | (MBR partition table and signature)
           |            |
1024 -2047 |    cmdline | Zero terminated command line of up to
           |            | 1023 characters
           |            |
---------- | ---------- | ----------------------------------------------------


------------------------------------------------------------------------------

                          DEC Alpha SRM boot sector
                           for Alpha architecture

Sources:
    http://www.tldp.org/HOWTO/text/SRM-HOWTO
      SRM Firmware Howto - Rich Payne, and David Huggins-Daines
    cdrkit-1.1.10/genisoimage/boot-alpha.c
      by Steve McIntyre
      who states "Heavily inspired by isomarkboot by David Mosberger in 1996"
    mail conversations with Helge Deller
    
The SRM firmware expects a Secondary Bootstrap Loader program, which usually
is a data file of the ISO filesystem. This loader is announced by size and
block address in the first 512 bytes of the System Area.
SRM accepts the boot sector and executes the loader if the checksum matches.

All numbers are recorded as unsigned 64 bit little endian.

Boot sector components:

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 -   ? | boot_string| genisoimage writes
           |            | "Linux/Alpha aboot for ISO filesystem." 
           |            | with terminating zero byte.
           |            |
   ? - 479 |          0 | Unused / undefined.
           |            |
 480 - 487 |     length | Size of boot loader file in units of 512 bytes.
           |            |
 488 - 495 |    address | LBA of the boot loader file in units of 512 bytes.
           |            |
 496 - 503 |       flag | "Always 0"
           |            |
 504 - 511 |   checksum | Sum of 64 bit words 0 to 63 (bytes 0 to 503).
           |            |
---------- | ---------- | ----------------------------------------------------


------------------------------------------------------------------------------

                         Combinations of boot mechanisms

------------------------------------------------------------------------------


                             SYSLINUX Isohybrid MBR

Sources:
    syslinux-3.72/utils/isohybrid , a perl script by H. Peter Anvin = hpa.
    Mailing list conversations with hpa.


An isohybrid MBR directs the booting BIOS to an ISOLINUX boot image which
is also the target of an El Torito boot catalog entry.
For that purpose one has to take an MBR template and has to set a few bytes
to values which sufficiently describe the ISO image and the boot image file.

Words are composed little-endian style.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 - 431 | = opaque = | Syslinux machine code provided by MBR template
           |            |
 432 - 439 | hd_bootlba | Address of the ISOLINUX boot image file in the
           |            | ISO image. Counted in 512 byte blocks.
           |            |
 440 - 443 |     mbr_id | Random number
 444 - 445 |          0 | Padding
           |            |
 446 - 509 | ========== | Partition table
           |            |
 446 - 461 | part_entry | Partition table entry 1 describing ISO image size
           |            | rounded up to the next full MiB. The partition starts
           |            | at LBA 0. (I.e. contrary to tradition.)
           |            | See above for partition table entry format.
           |            |
 462 - 509 |          0 | Unused partition entries 2 to 4
 510 - 511 |     0xaa55 | MBR signature
---------- | ---------- | ----------------------------------------------------

The ISO image file gets padded up to the next full MiB.

hpa about MBR templates and partition table filesystem types:

"[MBR templates] are available in the Syslinux build tree under the names:
   mbr/isohdp[fp]x*.bin
 The default probably should be mbr/isohdppx.bin, but it's ultimately up
 to the user.
 [...]
 Note: the filesystem type is largely arbitrary, in theory it can be any
 value other than 0x00, 0x05, 0x0f, 0x85, 0xee, or 0xef.  0x17 ("Windows
 IFS Hidden") seems safeish, some people believe 0x83 (Linux) is better.
"


>>>                SYSLINUX isohybrid for MBR, UEFI and x86-Mac

Sources:
  http://mjg59.dreamwidth.org/11285.html
  http://mjg59.fedorapeople.org/Fedora-LiveCD.iso
  http://opensource.apple.com/source/IOStorageFamily/IOStorageFamily-116/IOApplePartitionScheme.h (typedef struct Block0)
  http://www.informit.com/articles/article.aspx?p=376123&seqNum=3
  http://en.wikipedia.org/wiki/GUID_Partition_Table
  http://en.wikipedia.org/wiki/GUID
  syslinux-4.05/utils/isohybrid.c

>>> Motivation: What systems will use the additional data ? amd64 UEFI ?

This is a very condensed format which exposes a lot of entry points for boot
firmware. It disobeys some of the prescriptions in the previous chapter.

  Byte Range   | Value      | Meaning
-------------- | ---------- | -------------------------------------------------
     0 -   511 | mbr        | Isohybrid MBR pointing to x86 boot image file,
               |            | with three entries in its partition map.
               |            | It shares its first 32 bytes with apm_head.
     0 -    31 | apm_head   | Mock-up of the start of the first block of
               |            | an Apple Partition Map (APM).
   446  -  461 | mbr_entry1 | Partition table entry 1 describing the size of
               |            | the ISO image plus the backup GPT, padded up to
               |            | the next full MiB.
   462  -  477 | mbr_entry2 | Entry 2 describing the EFI VFAT boot image.
   478  -  493 | mbr_entry3 | Entry 3 describing the HFS+ boot image.
               |            |
   512 -  1023 | gpt_head   | GPT header describing the GPT partition array.
  1024 -  2047 | unused     |
  2048 -  4095 | apm_entry1 | APM entry 1 describing APM entries 1 to 3.
  4096 -  6143 | apm_entry2 | APM entry 2 describing the EFI VFAT boot image.
  6144 -  8195 | apm_entry3 | APM entry 3 describing the HFS+ boot image.
  8192 -  8319 | gpt_entry1 | GPT partition entry 1 for the ISO image size.
  8320 -  8447 | gpt_entry2 | GPT partition entry 2 for EFI VFAT boot image,
  8448 -  8575 | gpt_entry3 | GPT partition entry 3 for the HFS+ boot image.
  8576 - 24575 | gtp_empty  | Empty GPT partition entries 4 to 128.
 24576 - 32767 | unused     |
 32768 - 34815 | iso_pvd    | ISO 9660 superblock
 34816 - 36863 | el_torito  | EL Torito boot block pointing to a boot catalog
               |            | with entries for the MBR boot image (platform id
               |            | 0x00), and for the two other boot images
               |            | (platform id 0xef)
-------------- | ---------- | -------------------------------------------------


For the purpose of booting, there may be two EFI boot image files in the
ISO image. A VFAT image and a HFS+ image. The content of both is not in the
scope of this document.
These boot images get announced by EL Torito boot catalog entries with
Platform Id 0xef.


Newer SYSLINUX MBR templates begin by 32 bytes of machine code which are
intentionally non-essential:
   33  ed  90  90  90  90  90  90  90  90  90  90  90  90  90  90
   90  90  90  90  90  90  90  90  90  90  90  90  90  90  90  90
They may be overwritten by other bytes which must not produce errors or
undesirable side effects when executed as x86 machine code.
The following 32 bytes from block 0 of an Apple Partition Map (APM) are such
harmless code. They stem from Fedora-LiveCD.iso by Matthew Garrett:
   45  52  08  00  00  00  90  90  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
They do not depend on any properties of the ISO image or the information
that is described in the following text.

The layout of Block0 is constant:

Byte Range | Value      | Meaning (all numbers are stored big endian)
---------- | ---------- | ----------------------------------------------------
   0 -   1 | sig        | Signature 0x45 = 'E' , 0x52 = 'R'
   2 -   3 | block_size | 0x0800 = 2048
   4 -   7 | block_count| 0x9090 = 37008
   8 -   9 | dev_type   | obscure: "device type" = 0
  10 -  11 | dev_id     | obscure: "device id"   = 0
  12 -  15 | drv_data   | obscure: "driver data" = 0
  16 -  17 | drv_count  | obscure: "driver descriptor count" = 0
  18 -  81 | drv_map    | obscure: "driver descriptor table"
           |            |          with 8 entries of 16 bytes each
           |            |          first 14 bytes are 0
---------- | ---------- | ----------------------------------------------------

The block_count of 0x9090 is pure fantasy. It shall only mark the disc as
not empty.

The data block ranges of the two EFI boot images get described by MBR
partition entries 2 and 3, which thus claim blocks inside of partition 1.
This partition nesting is acceptable to some EFI implementations only if the
type of partition 1 is 0x00.

The MBR partition entry number 1 is
   80  00  01  00  00  3f  a0  89  00  00  00  00  00  50  14  00
It marks the partition as bootable, starting at block 0, with a size that
is the smallest full MiB not smaller than the ISO image size + 18 KiB.
(The 18 KiB are needed for the GPT backup.)
The ISO image has a size of 332362 blocks of 2K = 1329448 * 512 = 649.14 MiB.
The partition size is 0x145000 = 1331200 * 512 = 650 MiB.
Start C/H/S = 0/0/1, type is 0x0 ("Empty"), end C/H/S is 649/63/32.

Partition 2:
   00  fe  ff  ff  ef  fe  ff  ff  a4  00  00  00  70  04  00  00
Start block is 0xa4 = 164 * 512 = 41 * 2048. The VFAT image file.
Partition size is 0x0470 = 1136 * 512. The size of that file.
Start C/H/S = 1023/254/63, type 0xef (fdisk says: "EFI (FAT-12/16/"),
end C/H/S =  1023/254/63.

Partition 3:
   00  fe  ff  ff  00  fe  ff  ff  44  05  00  00  c0  08  00  00
Start block is 0x0544 = 1348 * 512 = 337 * 2048. The HFS+ image file.
Partition size is 0x08c0 = 2240 * 512. The size of that file.
Start C/H/S = 1023/254/63, type 0x00 ("Empty"), end C/H/S =  1023/254/63.


The second 512-block in the ISO image is the GPT header.

   45  46  49  20  50  41  52  54  00  00  01  00  5c  00  00  00
    E   F   I       P   A   R   T
   13  db  71  5d  00  00  00  00  01  00  00  00  00  00  00  00
   fe  4f  14  00  00  00  00  00  30  00  00  00  00  00  00  00
   de  4f  14  00  00  00  00  00  73  23  c8  79  19  e6  97  4d
   95  17  69  30  c5  38  e2  99  10  00  00  00  00  00  00  00
   80  00  00  00  80  00  00  00  5b  6b  8a  65

Byte Range | Value      | Meaning (little endian numbers, LBA unit is 512 byte)
---------- | ---------- | ----------------------------------------------------
  12 -  15 | head_size  | Header size = 0x5c = 92
  24 -  31 | curr_lba   | Location of this header block = 0x1
  32 -  39 | backup_lba | Location of header backup block = 0x144ffe = 1331198
           |            | This is 1 KiB before the end of MBR partition 1
           |            | (but should be 512 bytes).
           |            | (Potential isohybrid.c bug #1:
           |            |  Backup GPT is dislocated by 512 bytes.)
  40 -  47 | first_lba  | First usable LBA for partitions = 0x30 = 48
  48 -  55 | last_lba   | Last usable LBA for partitions  = 0x144fde = 1331166
  56 -  71 | guid       | Disk GUID
           |            | Random, produced by uuid_generate(),
           |            | 32,16,16 byte swapped
  72 -  79 | part_start | Partition entries start LBA = 0x10 = 16 = byte 0x2000
           |            | (This is unusual. It leaves room for the Apple
           |            |  partition map entries.)
  80 -  83 | entry_count| Number of partition entries 0x80 = 128
  84 -  87 | entry_size | Size of a partition entry = 0x80 = 128
---------- | ---------- | ----------------------------------------------------


Because the block size was announced as 2048, the first Apple partition map
entry is located at byte 0x800 = 2048.
It describes the partition map itself:
   50  4d  00  00  00  00  00  03  00  00  00  01  00  00  00  10
    P   M
   41  70  70  6c  65  00  00  00  00  00  00  00  00  00  00  00
    A   p   p   l   e
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   41  70  70  6c  65  5f  70  61  72  74  69  74  69  6f  6e  5f
    A   p   p   l   e   _   p   a   r   t   i   t   i   o   n   _
   6d  61  70  00  00  00  00  00  00  00  00  00  00  00  00  00
    m   a   p
   00  00  00  00  00  00  00  0a  00  00  00  03  00  00  00  00


Byte Range | Value      | Meaning (all numbers are stored big endian)
---------- | ---------- | ----------------------------------------------------
   4 -   7 | map_entries| "number of partition entries" = 3
   8 -  11 | start_block| "physical block start of partition" = 1
  12 -  15 | block_count| "physical block count of partition" = 16
           |            | (Potential isohybrid.c bug #2:
           |            |  The value of 16 claims the ISO 9660 PVD as part of
           |            |  the partition table. It should be 4 instead.)
  16 -  47 | name       | Partition name = "Apple"
  48 -  79 | type       | Type string = "Apple_partition_map"
  80 -  83 | lb_start   | Logical block start = 0
  84 -  87 | lb_count   | Logical block count = 10
  88 -  91 | flags      | Status flags = 3 (valid, allocated)
  92 -  95 | boot_block | Logical start block number of boot code = 0
  96 -  99 | boot_bytes | Number of bytes in boot code = 0
 100 - 119 |            | More boot code stuff = 0
 120 - 135 | processor  | "processor type" = 0
 136 - 511 | reserved   |
---------- | ---------- | ----------------------------------------------------

(Potential isohybrid.c bug #2:
 Apple partition map entries bear the block count for blocks of 512 bytes
 whereas Apple Block0 announces blocks of 2048 bytes.)

The next Apple partition map entry is at byte 0x1000 = 4096:

   50  4d  00  00  00  00  00  03  00  00  00  29  00  00  04  70
   45  46  49  00  00  00  00  00  00  00  00  00  00  00  00  00
    E   F   I
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   41  70  70  6c  65  5f  48  46  53  00  00  00  00  00  00  00
    A   p   p   l   e   _   H   F   S
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  04  70  00  00  00  33  00  00  00  00

3 partitions, start block is 0x29 = 41,
block count is 0x0470 = 1136 (should be 284), name is "EFI",
type is "Apple_HFS" (although this is a FAT image),
logical block start = 0, lb_count = 1136 (should be 284),
flags = 0x33 : valid, allocated, readable, writable.
This points to file /isolinux/efiboot.img in the ISO image.
(Potential isohybrid.c bug #2:
 Apple partition map entries bear the block count for blocks of 512 bytes
 whereas Apple Block0 announces blocks of 2048 bytes.)

At byte 0x1800 = 6144, there is Apple partition map entry 3:

   50  4d  00  00  00  00  00  03  00  00  01  51  00  00  08  c0
   45  46  49  00  00  00  00  00  00  00  00  00  00  00  00  00
    E   F   I
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   41  70  70  6c  65  5f  48  46  53  00  00  00  00  00  00  00
    A   p   p   l   e   _   H   F   S
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  08  c0  00  00  00  33  00  00  00  00

3 partitions, start block is 0x0151 = 337 (LBA of /isolinux/macboot.img),
block count = 0x08c0 = 2240 (should be 560), name is "EFI",
type is "Apple_HFS", logical block start = 0, lb_count = 2240 (should be 560),
flags = 0x33 : valid, allocated, readable, writable.
(Potential isohybrid.c bug #2:
 Apple partition map entries bear the block count for blocks of 512 bytes
 whereas Apple Block0 announces blocks of 2048 bytes.)

At byte 0x2000 = 8192 begins the GPT partition array.
It ends at byte 0x4000 = 16384.

   a2  a0  d0  eb  e5  b9  33  44  87  c0  68  b6  b7  26  99  c7
   a1  87  a1  ba  4d  2c  27  45  ae  05  cf  ab  a6  fa  87  c1
   00  00  00  00  00  00  00  00  28  49  14  00  00  00  00  00
   00  00  00  00  00  00  00  00  49  53  4f  48  79  62  72  69
                                    I   S   O   H   y   b   r   i
   64  20  49  53  4f  00  49  53  4f  48  79  62  72  69  64  00
    d       I   S   O       I   S   O   H   y   b   r   i   d   
   41  70  70  6c  00  00  00  00  00  00  00  00  00  00  00  00
    A   p   p   l
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00

Byte Range | Value      | Meaning (numbers are stored little endian)
---------- | ---------- | ----------------------------------------------------
   0 -  15 | type_guid  | Partition type GUID = Basic data partition
           |            | Wikipedia: "EBD0A0A2-B9E5-4433-87C0-68B6B72699C7"
           |            | (Note: The first three words are shown byte swapped)
  16 -  31 | part_guid  | Unique partition GUID
           |            | Random, produced by uuid_generate()
           |            | 32,16,16 byte swapped
  32 -  39 | start_lba  | First LBA = 0
  40 -  47 | end_lba    | Last LBA (inclusive) = 0x144828 = 1329448
           |            | This is the ISO image size in blocks of 512.
           |            | (Potential isohybrid.c bug #3:
           |            |  The end_lba in the first GPT entry should be 1 less
           |            |  than the count of 512 byte blocks of the ISO image.)
  48 -  55 | flags      | Attribute flags = 0
  56 - 127 | name       | Wikipedia says UTF-16LE.
           |            | (Potential isohybrid.c bug #4:
           |            |  The name in Fedora-LiveCD.iso is 8 bit and result
           |            |  of faulty memory operation on a text constant.)
---------- | ---------- | ----------------------------------------------------

Next entry is at 0x2800 = 10240:

   a2  a0  d0  eb  e5  b9  33  44  87  c0  68  b6  b7  26  99  c7
   c8  de  c8  1f  fb  f0  51  40  8c  8a  d2  f6  b1  46  16  dc
   a4  00  00  00  00  00  00  00  13  05  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  49  53  4f  48  79  62  72  69
                                    I   S   O   H   y   b   r   i
   64  00  41  70  70  6c  65  00  41  70  70  6c  00  00  00  00
    d       A   p   p   l   e       A   p   p   l
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00

Partition type GUID : "EBD0A0A2-B9E5-4433-87C0-68B6B72699C7" = Basic data
Start at block 0xa4 = 164 * 512 = 41 * 2048. The VFAT image file.
Last block is 0x0513 = 1299 = 164 + 1135. This end is correct.
(Potential isohybrid.c bug #4:
 Wrong character set and incidential bytes in GPT partition name.)
(Potential isohybrid.c bug #5:
 The EFI System Partition should have type GUID :
 "C12A7328-F81F-11D2-BA4B-00A0C93EC93B")

Next entry at byte 0x02100 = 8448:

   00  53  46  48  00  00  aa  11  aa  11  00  30  65  43  ec  ac
   c8  de  c8  1f  fb  f0  51  40  8c  8a  d2  f6  b1  46  16  dc
   44  05  00  00  00  00  00  00  03  0e  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  49  53  4f  48  79  62  72  69
                                    I   S   O   H   y   b   r   i
   64  00  41  70  70  6c  65  00  41  70  70  6c  00  00  00  00
    d       A   p   p   l   e       A   p   p   l
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
   00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00

Partition type GUID : "48465300-0000-11AA-AA11-00306543ECAC" = HFS+
Start block is 0x0544 = 1348 * 512 = 337 * 2048. The HFS+ image file.
Last block is  0x0e03 = 3587 = 1348 + 2239. Correct.
(Potential isohybrid.c bug #4:
 Wrong character set and incidential bytes in GPT partition name.)

The rest of the System Area is 0 up to the Primary Volume Descriptor
at block 16.


The ISO image file gets padded up to full MiB with sufficient room for the GPT
backup which is stored near the very end of the image file. There is need for
at least 16.5 KiB, which effectively occupy 18 KiB.

The backup partition array is stored 17 KiB before the end of MBR partition 1,
which is also the end of the image file.
(Potential isohybrid.c bug #1:
 Wikipedia suggests "LBA -33" counted from end. This would be 16.5 KiB before
 end.)

The GPT header is stored 1 KiB before the end of MBR partition 1.
(Potential isohybrid.c bug #1:
 Wikipedia suggests "LBA -1" counted from end. This would be 512 bytes.)

   45  46  49  20  50  41  52  54  00  00  01  00  5c  00  00  00
    E   F   I       P   A   R   T
   f6  61  10  1c  00  00  00  00  fe  4f  14  00  00  00  00  00
   01  00  00  00  00  00  00  00  30  00  00  00  00  00  00  00
   de  4f  14  00  00  00  00  00  73  23  c8  79  19  e6  97  4d
   95  17  69  30  c5  38  e2  99  de  4f  14  00  00  00  00  00
   80  00  00  00  80  00  00  00  5b  6b  8a  65 

It differs by its own CRC and by some of its parameters:
Own address is 0x144ffe. The backup lba is 0x01 = Primary GPT header.
Partition entries start is 0x144fde
(Potential isohybrid.c bug #1:
 This overlaps with the last usable LBA. The backup GPT must move up by
 512 bytes.)

------------------------------------------------------------------------------


                          GRUB2 grub-mkrescue MBR

Sources:
    Mailing list conversations with Vladimir Serbinenko.


The MBR file that is used with older versions of GRUB2 script grub-mkrescue
needs only a partition table entry which describes the image size.
Newer versions get patched by the block address of the content of the first
El Torito boot image. See grub-mkrescue use of xorrisofs option --grub2-mbr.

Byte Range | Value      | Meaning
---------- | ---------- | ----------------------------------------------------
   0 - 431 | = opaque = | GRUB2 machine code provided by MBR template
           |            |
 432 - 439 | bootimg_adr| With newer versions of grub-mkrescue:
           |            | Block address of the start of the boot image file
           |            | content plus 4. Block size is 512.
           |            | 64 bit Little-endian.
           |            |
 440 - 445 | = opaque = | provided by MBR template
           |            |
 446 - 509 | ========== | Partition table
           |            |
 446 - 461 | part_entry | Partition table entry 1 describing ISO image size
           |            | Peculiar is the start offset of 1 block.
           |            | This prevents mounting of the partition.
           |            | See above for partition table entry format.
           |            |
 462 - 509 |          0 | Unused partition entries 2 to 4
 510 - 511 |     0xaa55 | MBR signature
---------- | ---------- | ----------------------------------------------------

Vladimir Serbinenko about the partition table entry:
"Currently we use first and not last entry. You need to:
 1) Zero-fill 446-510
 2) Put 0x55, 0xAA into 510-512
 3) Put 0x80 (for bootable partition), 0, 2, 0 (C/H/S of the start), 0xcd
   (partition type), [3 bytes of C/H/S end], 0x01, 0x00, 0x00, 0x00 (LBA
   start in little endian), [LBA end in little endian] at 446-462
"

------------------------------------------------------------------------------

>>> Mac and/or PowerPC bootable GRUB2 image with HFS+/FAT, APM,
    EFI GPT partition, PreP MBR partition, mountable FAT partition

>>> ? With PC-BIOS MBR x86 Code ?

This storage layout was mainly defined by Vladimir Serbinenko. It relies much
on the embedded HFS+/FAT filesystem for which he provided the code to libisofs.

Start Blocks (2 KiB):
     0  System Area
    16  Volume Descriptors
  TREE  ISO-RR tree, Joliet tree, other trees and meta data, except HFS+/FAT
   EFI  EFI boot image partition (optional)
  PREP  Prep image partition (optional)
  HFAT  HFS+/FAT metadata (optional)
  DATA  Data file content (including El Torito boot images)
  HFSB  HFS superblock backup (if HFS+/FAT metadata)
  TAIL  Further tails and paddings (optional)
  GPTB  GPT backup (if GPT in System Area) 
   END  End of ISO image
 
System Area may contain simultaneously:
   MBR (x86 boot code must leave room for 8 bytes mock-up of APM Block0)
   APM
   GPT

MBR Partitions:
   0xee from    0 to PREP-1, protective partition, announcing presence of GPT
   0x41 from PREP to HFAT-1, PreP partition
   0x0c from HFAT to  END-1, FAT partition, bootable bit on
   0x00                      Empty partition

GPT Partitions:
   The primary GPT itself covers the System Area.
   Basic Data  from   16 to  EFI-1, protects first part of ISO image
   EFI System  from  EFI to PREP-1, offers EFI image for booting
   Basic Data  from PREP to HFAT-1, protects PreP partition
   HFS+        from HFAT to TAIL-1, offers HFS+ for mounting
   Basic Data  from TAIL to GPTB-1, protects rest of ISO image (if there is)

APM Partitions:
   The range from end of APM to end of System Area stays unprotected.
   (The primary GPT might sit there.) 
   Apple_partition_map from    1 to 3 or 4, covers the APM itself
   ISO9660_data        from   16 to HFAT-1, covers first part of ISO image
   Apple_HFS           from HFAT to GPTB-1, offers HFS+ for boot and mount
   ISO9660_data        from GPTB to  END-1, covers rest of ISO image
                                            (might be omitted if empty)

El Torito:
   Boot image for 80x86 PC-BIOS 
   Boot image for EFI (usually the same file as the partition EFI to PREP-1).

If optional components are not present, then their addresses coincide with
the address of the next component that is present. E.g. HFAT == DATA if no
HFS+/FAT filesystem is present.
If no FAT filesystem is present within HFS+/FAT, then the type of the last
partition is 0xcd.
If neither EFI, nor PreP, nor FAT within HFS+/FAT, are present, then there
is only one partition. It has type 0xee, if GPT is present in the System Area.
It has type 0xcd and offset 1*512, if no GPT is present.

Involved -as mkisofs options:
  -hfsplus
  -fat
  -efi-boot-part DISKFILE
  -prep-boot-part DISKFILE


>>> What boots by what ?



------------------------------------------------------------------------------