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//proc/self/root/usr/share/doc/syslinux-4.02/gpt.txt
GPT boot protocol There are two ways to boot a GPT-formatted disk on a BIOS system. Hybrid booting, and the new GPT-only booting protocol originally proposed by the author, and later adopted by the T13 committee in slightly modified form. *** Hybrid booting *** Hybrid booting uses a standard MBR, and has bootable ("active") partitions present, as partitions, in the GPT PMBR sector. This means the PMBR, instead of containing only one "protective" partition (type EE), may contain up to three partitions: a protective partition (EE) *before* the active partition, the active partition, and a protective partition (EE) *after* the active partition. The active partition is limited to the first 2^32 sectors (2 TB) of the disk. All partitions, including the active partition, should have GPT partition entries. Thus, changing which partition is active does NOT change the GPT partition table. This is the only known way to boot Microsoft operating systems from a GPT disk with BIOS firmware. *** New protocol *** This defines the T13-approved protocol for GPT partitions with BIOS firmware. It maintains backwards compatibility to the extent possible. It is implemented by the file mbr/gptmbr.bin. The (P)MBR format is the normal PMBR specified in the UEFI documentation, with the first 440 bytes used for the boot code. The partition to be booted is marked by setting bit 2 in the GPT Partition Entry Attributes field (offset 48); this bit is reserved by the UEFI Forum for "Legacy BIOS Bootable". -> The handover protocol The PMBR boot code loads the first sector of the bootable partition, and passes in DL=<disk number>, ES:DI=<pointer to $PnP>, sets EAX to 0x54504721 ("!GPT") and points DS:SI to a structure of the following form: Offset Size Contents --------------------------------------------------------- 0 1 0x80 (this is a bootable partition) 1 3 CHS of partition (using INT 13h geometry) 4 1 0xED (partition type: synthetic) 5 3 CHS of partition end 8 4 Partition start LBA 12 4 Partition end LBA 16 4 Length of the GPT entry 20 varies GPT partition entry The CHS information is optional; gptmbr.bin currently does *NOT* calculate them, and just leaves them as zero. Bytes 0-15 matches the standard MBR handover (DS:SI points to the partition entry), except that the information is provided synthetically. The MBR-compatible fields are directly usable if they are < 2 TB, otherwise these fields should contain 0xFFFFFFFF and the OS will need to understand the GPT partition entry which follows the MBR one. The "!GPT" magic number in EAX and the 0xED partition type also informs the OS that the GPT partition information is present. Syslinux 4.00 and later fully implements this protocol.