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. 70
