| 1 | GPT boot protocol |
|---|
| 2 | |
|---|
| 3 | There are two ways to boot a GPT-formatted disk on a BIOS system. |
|---|
| 4 | Hybrid booting, and the new GPT-only booting protocol originally |
|---|
| 5 | proposed by the author, and later adopted by the T13 committee in |
|---|
| 6 | slightly modified form. |
|---|
| 7 | |
|---|
| 8 | |
|---|
| 9 | *** Hybrid booting *** |
|---|
| 10 | |
|---|
| 11 | Hybrid booting uses a standard MBR, and has bootable ("active") |
|---|
| 12 | partitions present, as partitions, in the GPT PMBR sector. This means |
|---|
| 13 | the PMBR, instead of containing only one "protective" partition (type |
|---|
| 14 | EE), may contain up to three partitions: a protective partition (EE) |
|---|
| 15 | *before* the active partition, the active partition, and a protective |
|---|
| 16 | partition (EE) *after* the active partition. The active partition is |
|---|
| 17 | limited to the first 2^32 sectors (2 TB) of the disk. |
|---|
| 18 | |
|---|
| 19 | All partitions, including the active partition, should have GPT |
|---|
| 20 | partition entries. Thus, changing which partition is active does NOT |
|---|
| 21 | change the GPT partition table. |
|---|
| 22 | |
|---|
| 23 | This is the only known way to boot Microsoft operating systems from a |
|---|
| 24 | GPT disk with BIOS firmware. |
|---|
| 25 | |
|---|
| 26 | |
|---|
| 27 | *** New protocol *** |
|---|
| 28 | |
|---|
| 29 | This defines the T13-approved protocol for GPT partitions with BIOS |
|---|
| 30 | firmware. It maintains backwards compatibility to the extent |
|---|
| 31 | possible. It is implemented by the file mbr/gptmbr.bin. |
|---|
| 32 | |
|---|
| 33 | The (P)MBR format is the normal PMBR specified in the UEFI |
|---|
| 34 | documentation, with the first 440 bytes used for the boot code. The |
|---|
| 35 | partition to be booted is marked by setting bit 2 in the GPT Partition |
|---|
| 36 | Entry Attributes field (offset 48); this bit is reserved by the UEFI |
|---|
| 37 | Forum for "Legacy BIOS Bootable". |
|---|
| 38 | |
|---|
| 39 | |
|---|
| 40 | -> The handover protocol |
|---|
| 41 | |
|---|
| 42 | The PMBR boot code loads the first sector of the bootable partition, |
|---|
| 43 | and passes in DL=<disk number>, ES:DI=<pointer to $PnP>, sets EAX to |
|---|
| 44 | 0x54504721 ("!GPT") and points DS:SI to a structure of the following |
|---|
| 45 | form: |
|---|
| 46 | |
|---|
| 47 | Offset Size Contents |
|---|
| 48 | --------------------------------------------------------- |
|---|
| 49 | 0 1 0x80 (this is a bootable partition) |
|---|
| 50 | 1 3 CHS of partition (using INT 13h geometry) |
|---|
| 51 | 4 1 0xED (partition type: synthetic) |
|---|
| 52 | 5 3 CHS of partition end |
|---|
| 53 | 8 4 Partition start LBA |
|---|
| 54 | 12 4 Partition length in sectors |
|---|
| 55 | 16 4 Length of the GPT entry |
|---|
| 56 | 20 varies GPT partition entry |
|---|
| 57 | |
|---|
| 58 | The CHS information is optional; gptmbr.bin currently does *NOT* |
|---|
| 59 | calculate them, and just leaves them as zero. |
|---|
| 60 | |
|---|
| 61 | Bytes 0-15 matches the standard MBR handover (DS:SI points to the |
|---|
| 62 | partition entry), except that the information is provided |
|---|
| 63 | synthetically. The MBR-compatible fields are directly usable if they |
|---|
| 64 | are < 2 TB, otherwise these fields should contain 0xFFFFFFFF and the |
|---|
| 65 | OS will need to understand the GPT partition entry which follows the |
|---|
| 66 | MBR one. The "!GPT" magic number in EAX and the 0xED partition type |
|---|
| 67 | also informs the OS that the GPT partition information is present. |
|---|
| 68 | |
|---|
| 69 | Syslinux 4.00 and later fully implements this protocol. |
|---|
