GPT
GPT stands for GUID Partition Table. It is made to replace MBR partitioning.
Contents |
Layout
A GPT partitioned media looks like this:
| LBA 0 | Protective Master Boot Record (PMBR). Holds a partition pointing to GPT to avoid accidental overwrite by old programs. |
| 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 entries |
| ...data on disk... | |
| LBA -33..-2 | mirror of partition table |
| LBA -1 | mirror of partition header on last addressable sector |
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:
| 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.
LBA 1: Partition Table Header
Right after the first sector, there's a special sector to identify GPT. This sector is repeated in the last sector of the media (pointed by the field at 0x20).
| Offset | Length (bytes) | Description |
| 0x0 | 8 | Signature, can be identified by 8 bytes magic "EFI PART" (45h 46h 49h 20h 50h 41h 52h 54h) |
| 0x8 | 4 | GPT Revision |
| 0xC | 4 | Header size |
| 0x10 | 4 | CRC32 checksum of the GPT header (0x0 to 0x5c) |
| 0x14 | 4 | Reserved |
| 0x18 | 8 | The LBA containing this header |
| 0x20 | 8 | The LBA of the alternate GPT header |
| 0x28 | 8 | The first usable block that can be contained in a GPT entry |
| 0x30 | 8 | The last usable block that can be contained in a GPT entry |
| 0x38 | 16 | GUID of the disk |
| 0x48 | 8 | Starting LBA of the GUID Partition Entry array |
| 0x50 | 4 | Number of Partition Entries |
| 0x54 | 4 | Size (in bytes) of each entry in the Partition Entry array - must be a value of 128×2ⁿ where n ≥ 0 (in the past, multiples of 8 were acceptable) |
| 0x58 | 4 | CRC32 of the Partition Entry array. |
| 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.
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
Usually after the EFI header sector, comes the table itself, however this could be defined in a GPT header field at 0x48 otherwise. For compatibility it is recommended to use LBA 2. The table is repeated at the end of the media before the alternate GPT header sector.
Each entry in the table looks like this:
| Offset | Length (bytes) | Description |
| 0x0 | 16 | Partition Type GUID (zero means unused entry) |
| 0x10 | 16 | Unique Partition GUID |
| 0x20 | 8 | Starting LBA |
| 0x28 | 8 | Ending LBA |
| 0x30 | 8 | Attributes |
| 0x38 | 72 | Partition Name |
Partition type has no central registry, but with 16 bytes it is unlikely that there'll ever be a collision. You can find types for OSes on their corresponding corporation's website (for MacOSX see Apple; for Windows see Microsoft etc.)
Attributes is a bit set. Bit 0 (attrib & 1) means that the partition is required by the firmware and should not be touched. EFI System Partition would be a good example, however that's always explicitly assumed to have this bit set. Bit 2 (attrib & 2) means that the partition contains necessary files to boot an operating system (defined as EFI_PART_USED_BY_OS). This is somewhat similar to 0x80 boot flag for MBR partitions, but not entirely.
Name is UNICODE16-LE encoded, meaning it can only store the UNICODE Bilingual Plane (code points 32 to 65535), and each character consumes 2 bytes. However the EFI spec defines name length as 72 bytes (36 characters), you should never hard-code this into your application. Always use (Partition Entry Size in header at 0x54) - 0x38 instead.
Utilities
The following utilities can handle GPT:
- parted, gparted, gdisk on Linux
- diskutil on MacOSX
- diskpart on Windows (Vista and upwards)
- gpt on MacOSX and BSDs.
Boot loaders
EFI firmware is capable of booting from a specific GPT partition, EFI System Partition which is basically a FAT32 partition. It should contain a slightly modified PE-executable, like ELILO. On old machines with legacy BIOS only you can use GRUB, or you can write a custom boot code.
Limine fully supports GPT both when using UEFI and when using BIOS. Furthermore, unlike GRUB, when using BIOS, it supports embedding itself into the GPT structures as to make an additional reserved partition unnecessary (but an can be optionally configured to use one anyways).
BOOTBOOT by default boots your kernel from EFI System Partition. On UEFI machines it does that natively, and on legacy BIOS systems and other platforms (ARM for example) it interprets GPT and ESP on its own.
