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Member "syslinux-6.03/doc/gpt.txt" (6 Oct 2014, 2786 Bytes) of package /linux/misc/syslinux-6.03.tar.gz:


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    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.