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|LBA 1||partition header, can be identified by 8 bytes magic "EFI PART" (45h 46h 49h 20h 50h 41h 52h 54h)
|-
|LBA 2..33||partition table entiresentries
|-
| colspan="2" | ...data on disk...
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=== LBA 0: Protective Master Boot Record ===
 
This is kept untouched for backward compatibility. The UEFI specification requires that the PMBR partition table contain one partition record, with the other three partition records set to zero. The partition record should be defined as follows:
This is kept untouched for backward compatibility. There should be a partition covering the entire disk with type 0xEE (unfortunately this is not clear from the spec if this MBR partition should cover the GPT table itself, or all the partitions that the GPT table describes. The phrasing suggest the latter, but since one of the reasons for GPT was to allow larger disks that cannot be described by the 32 bit sector addresses in the MBR table, this makes not much sense.)
 
{| {{wikitable}}
You could also record the [[EFI System Partition]] with the type 0xEF, but this makes not much sense either: GPT-aware firmware and utilities recognize the GPT and use that instead of the MBR table, and the old ones which don't, well, they don't recognize 0xEF partition type either.
|'''Offset'''||'''Length (bytes)'''||'''Description'''||'''Value'''
|-
|0x0||1||Boot Indicator||Set to 0x00 to indicate a non-bootable partition.
|-
|0x1||3||Starting CHS||Set to 0x000200, corresponding to the Starting LBA field.
|-
|0x4||1||OS Type||Set to 0xEE (GPT Protective)
|-
|0x5||3||Ending CHS||Set to the CHS address of the last logical block on the disk. Set to 0xFFFFFF if the value cannot be represented in this field.
|-
|0x8||4||Starting LBA||Set to 0x00000001 (LBA of GPT Partition Header)
|-
|0xC||4||Ending LBA||Set to the size in logical blocks of the disk, minus one. Set to 0xFFFFFFFF if the size of the disk is too large to be represented in this field.
|}
 
The PMBR will always exist if the drive contains a GPT, as it is required in order to maintain compatibility with tools expecting an MBR partition table. It's also worth noting that UEFI firmware implementations are also required to parse legacy MBR partition tables in the event that the drive does not contain a GPT.
The UEFI specification explicitly says (in section 12.3.2 Partition Discovery) that PMBR partitioning table should only be parsed when there's no GPT exists. This means if there's a GPT then the storage's partitions will be specified by the GPT table, and only by the GPT.
 
=== LBA 1: Partition Table Header ===
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|0xC||4||Header size
|-
|0x10||4||CRC32 checksum of the GPT header (0x0 to 0x5c)
|-
|0x14||4||Reserved
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|0x5C||'''blocksize'''-0x5C||Reserved (should be zeroed)
|}
The Partition Entry array can contain unused entries -- that is, the GUID value is set to zero. For the purposes of the GPT header, these should be counted when considering the Number of Partition Entries field, and should also be taken into account when calculating the CRC32 of the entire array. There should be no more entries in the array, unused or otherwise, than are indicated by this field.
Note: it is somewhat vague what the Number of Partition Entries field contain. For many applications that is the number of '''actually used''' entries, while many partitioning tools (most notably fdisk and gdisk) handles that as the number of '''maximum available''' entries, using full zero Partition Type to mark empty entries. Unfortunately both interpretation is suggested by the EFI spec, so this is unclear. One thing is certain, there should be no more entries (empty or not) than this field.
 
For the checksums in the header, the CCITT32 ANSI CRC method is used, the one with the polynomial 0x04c11db7 (same as in gzip, and different to the Castagnoli CRC32 that hardware accelerated CRC instructions calculate). The header checksum field at 0x10 is zeroed during calculation and the reserved data are not included.
 
=== LBA 2: Partition Entries ===
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|0x10||16||Unique Partition GUID
|-
|0x20||8||StartingLBAStarting LBA
|-
|0x28||8||EndingLBAEnding LBA
|-
|0x30||8||Attributes
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The following utilities can handle GPT:
* [http://www.gnu.org/software/parted/manual/parted.html parted], [http://linux.die.net/man/8/gparted gparted], [http://www.rodsbooks.com/gdisk/ gdisk] on Linux
* [httphttps://developerwww.appleunix.com/libraryman-page/macOSX/#documentation8/Darwindiskutil/Reference/ManPages/man8/diskutil.8 diskutil] on MacOSX
* [http://technet.microsoft.com/en-us/library/bb490893.aspx diskpart] on Windows (Vista and upwards)
* [http://www.freebsd.org/cgi/man.cgi?query=gpt&manpath=FreeBSD+7.1-RELEASE gpt] on MacOSX and BSDs.
* [https://gitlab.com/bztsrc/bootboot/tree/master/mkbootimg mkbootimg] an easy to use disk image creator available on Windows, MacOSX and Linux.
 
== Boot loaders ==
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* [http://msdn.microsoft.com/en-us/windows/hardware/gg463524.aspx GPT on MSDN]
* [http://developer.apple.com/library/mac/#technotes/tn2166/_index.html GPT on Apple Developer]
* [http://www.ibm.com/developerworks/linux/library/l-gpt/ GPT on IBM devloperWorksdeveloperWorks]
* [[wikipedia:GUID_Partition_Table|Wikipedia: GUID Partition Table]]
* [[wikipedia:BIOS_Boot_partition|Wikipedia: BIOS Boot partition]]
 
[[Category:X86Disks]]
[[Category:X86-64]]
[[Category:IA-64]]
[[Category:UEFI]]
[[Category:Firmware]]
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