path: root/doc/gpt.txt
diff options
Diffstat (limited to 'doc/gpt.txt')
1 files changed, 17 insertions, 36 deletions
diff --git a/doc/gpt.txt b/doc/gpt.txt
index 14c1ee8e..09099320 100644
--- a/doc/gpt.txt
+++ b/doc/gpt.txt
@@ -1,13 +1,10 @@
GPT boot protocol
-There is no official MBR-to-partition handover protocol defined for
-booting from disks partitioned using GPT partition tables with
-BIOS-style firmware. This is because the GPT partition format comes
-from the EFI spec, which thinks the universe is all going to be EFI.
+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.
-There are thus two alternatives: hybrid booting, and defining a new
*** Hybrid booting ***
@@ -29,31 +26,16 @@ GPT disk with BIOS firmware.
*** New protocol ***
-This defines an alternative (experimental) booting protocol for GPT
-partitions with BIOS firmware. It maintains backwards compatibility
-to the extent possible. It is implemented by the file mbr/gptmbr.bin.
+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 PMBR
+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 PMBR (the first 512-byte sector of the disk) is divided up as
- Offset Size Contents
- ---------------------------------------------------------
- 0 424 PMBR boot code
- 424 16 GUID of the boot partition
- 440 4 MBR-compatible disk ID
- 444 2 Magic number: 1D 9A
- 446 16 PMBR protective entry
- 462 48 PMBR null entries
- 510 2 Boot signature: 55 AA
-To change the bootable partition, verify that the magic number is
-present (to avoid corrupting software not compatible with this
-specification) and enter the GUID of the boot partition at offset
-424. It might be wise to verify that the data already there is a
-valid partition GUID already, or at least warn the user if that is not
-the case.
-> The handover protocol
@@ -66,11 +48,12 @@ form:
0 1 0x80 (this is a bootable partition)
1 3 CHS of partition (using INT 13h geometry)
- 4 1 0xEE (partition type: EFI data partition)
+ 4 1 0xED (partition type: synthetic)
5 3 CHS of partition end
8 4 Partition start LBA
12 4 Partition end LBA
- 16 varies GPT partition entry
+ 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.
@@ -80,9 +63,7 @@ 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 0xEE partition type
+MBR one. The "!GPT" magic number in EAX and the 0xED partition type
also informs the OS that the GPT partition information is present.
-Currently, this is compatible with Syslinux as long as the Syslinux
-partition is < 2 TB; this probably will be improved in a future
+Syslinux 4.00 and later fully implements this protocol.