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Described the CRC algorithm of GPT

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This commit is contained in:
Thomas Schmitt 2012-05-07 20:33:53 +02:00
parent dc2b27ca51
commit 977161cda1
1 changed files with 20 additions and 15 deletions
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35
doc/boot_sectors.txt
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@ -426,15 +426,15 @@ firmware as possible.
1024 - 2047 | unused |
2048 - 4095 | apm_entry1 | APM entry 1 describing ISO image size.
4096 - 6143 | apm_entry2 | APM entry 2 describing the UEFI VFAT boot image.
6144 - 8195 | apm_entry3 | APM entry 2 describing the HFS+ 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 UEFI 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
34815 - 36863 | el_torito | EL Torito boot block pointing to a boot catalog
| | with entries for the MBR boot image (platform
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)
-------------- | ---------- | -------------------------------------------------
@ -518,18 +518,22 @@ Byte Range | Value | Meaning (little endian numbers, LBA unit is 512 byte)
92 - 511 | reserved | Must be 0
---------- | ---------- | ----------------------------------------------------
>>> CRC-32: Generating polynomial 0x104c11db7, mirrored bit order in and out,
>>> seed 0xffffffff, result gets exored by 0xffffffff.
>>> ??? This works as byte shifter with a bitwise mirrored table
>>> ??? but not as simple bit shifter.
>>> ??? Both do match if seed == 0. But not with other seeds.
>>> ??? An algebraic miracle ?
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 0xffffffff.
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 bit mirrored. E.g. bit0 becomes bit31,
bit1 becomes bit30, and so on. Further it gets exored with 0xffffffff.
The ISO image has a size of 332362 blocks of 2K = 1329448 blocks of 512.
Because the block size was announced as 2048, the first Apple partition map
entry is located at byte 0x800.
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
@ -569,7 +573,7 @@ Byte Range | Value | Meaning (all numbers are stored big endian)
136 - 511 | reserved |
---------- | ---------- | ----------------------------------------------------
The next Apple partition map entry is at byte 0x1000:
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
@ -588,7 +592,7 @@ flags = 0x33 : valid, allocated, readable, writable.
This points to file /isolinux/efiboot.img in the ISO image,
>>> but misrepresents its size of 284 blocks of 2048.
At byte 0x1800, there is map entry 3:
At byte 0x1800 = 6144, there is 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
@ -608,7 +612,8 @@ flags = 0x33 : valid, allocated, readable, writable.
>>> GPT partitions marking ISO image, VFAT image within, HFS+ image within
At byte 0x2000 begins the GPT partition array. It ends at byte 0x4000.
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
@ -641,7 +646,7 @@ Byte Range | Value | Meaning (numbers are stored little endian)
| | >>> of faulty memory operation on a text constant.
---------- | ---------- | ----------------------------------------------------
Next entry:
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
@ -658,7 +663,7 @@ Start at block 0xa4 = 164 * 512 = 41 * 2048. The VFAT image file.
Last block is 0x0513 = 1299 = 164 + 1135. This end is correct.
>>> The name is an unintentional patchwork in an 8 bit character set.
Next entry:
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