Updated description of boot sectors by sparse info about PReP, CHRP.

Described the layout for GRUB2 based hybrid images.

doc/boot_sectors.txt

@@ -17,9 +17,9 @@ EL Torito CD booting, for PC-BIOS x86, PowerPC, (old) Mac, EFI.
 
 Master Boot Record (MBR), for PC-BIOS x86 from (pseudo-) hard disk
 
-Apple Partition Map (APM) 
+Apple Partition Map (APM), for more modern Mac
 
-GUID Partition Table (GPT)
+GUID Partition Table (GPT), for EFI from (pseudo-) hard disk
 
 MIPS Volume Header, for MIPS Big Endian, e.g. SGI Indigo2.
 
@@ -27,10 +27,16 @@ DEC Boot Block, for MIPS Little Endian , e.g. DECstation.
 
 SUN Disk Label and boot images, for SUN SPARC
 
+PowerPC Reference Platform (PReP), for IBM PowerPC
+
+Common Hardware Reference Platform (CHRP), for IBM PowerPC
+
 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
 
 ------------------------------------------------------------------------------
 
@@ -50,7 +56,13 @@ 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.
 
-The Boot Record is a ECMA-119 Volume Descriptor which is eventually located
+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 eventually located
 at 2 kB block number 17 (decimal). 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
@@ -373,8 +385,8 @@ HFS+ filesystem.
 Block0 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 with no negative side effects.
+and is also harmless x86 machine code with no negative side effects.
-So it is possible to combine APM with specially prepared MBR.
+So it is possible to combine APM with an especially prepared MBR.
 
 The layout of a Block0 of an APM is:
 
@@ -424,6 +436,7 @@ Byte Range | Value      | Meaning (all numbers are stored big endian)
            |            | bit1= entry is allocated
            |            | bit4= partition is readable
            |            | bit5= partition is writable
+           |            | 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
@@ -457,11 +470,11 @@ start_block = start_of_hfs
 block_count = end_of_hfs - start_of_hfs
 name        = "HFSPLUS_Hybrid"
 type        = "Apple_HFS"
-flags       = 0x13
+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 forth APM entry will be written.
+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"
@@ -479,7 +492,7 @@ flags       = 0x13
 
                         GUID Partition Table (GPT)
                     for alternative mountability paths
-        and for booting of some Apple Macs from (pseudo-) hard disk
+      and for EFI booting of some Apple Macs from (pseudo-) hard disk
 
 Sources:
   http://mjg59.dreamwidth.org/11285.html
@@ -536,6 +549,7 @@ A globally registered class of GUID are the partition type GUIDs.
 This example uses two of them
 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-endia interpretation.
 
@@ -581,6 +595,11 @@ Byte Range | Value      | Meaning (little endian numbers, LBA unit is 512 byte)
            |            | 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.
+
+
 ------------------------------------------------------------------------------
 
 
@@ -866,6 +885,36 @@ Byte Range | Value      | Meaning
 ---------- | ---------- | ----------------------------------------------------
 
 
+------------------------------------------------------------------------------
+
+
+                      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.
+
+
+CHRP is marked by an MBR partition entry of type 0x96 spanning the whole
+ISO 9660 image.
+
+PReP boot may be preferable. At least it can co-exist with other partitions
+in the ISO image.
+
+
 ------------------------------------------------------------------------------
 
 >>> ??? HP-PA
@@ -1299,9 +1348,73 @@ Vladimir Serbinenko about the partition table entry:
 
 ------------------------------------------------------------------------------
 
->>> Mac bootable GRUB2 image with HFS+ and APM
+>>> Mac and/or PowerPC bootable GRUB2 image with HFS+/FAT, APM,
->>> ? With EFI partition ?
+    EFI GPT partition, PreP MBR partition, mountable FAT partition
->>> ? With PC-BIOS MBR ?
+
+>>> ? 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
 
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