Talk:GUID Partition Table

New version created by Scorpiuss

I've created a new version of the GPT page. The fundamentals are all there. Could use a chart of the things represented at each bit position. Microsoft's got a nice one on the external link indicated (which, by the way, is a great source).

-Scorpiuss, Aug 26 2005, 13:25 GMT

EFI System partition GUID contradiction

The GUID for an EFISys partition under the Partition Entries section is different than the one in the list of GUIDs. On the MS page linked at the bottom, the one given in the Partition Entries section is given.

-Scorpiuss

Error in GPT disk layout figure

I have been trying to understand the GPT disk layout. From reading the UEFI spec and other pages, my impression is that the secondary (alternate) GPT header is located in the last addressable block on the disk. However, the figure (GUID Partition Table Scheme) is not consistent. The caption says that -1 is the last addressable block, but following the same graphical logic as used at the top of the figure the figure indicates that the secondary GPT header starts at -2 and ends at -1, i.e., it indicates that that the secondary GPT header is located at the second addressable block from the end, which should be wrong.

Am I right in this? Who has made the figure and how can we modify it?

this is indeed misleading. the backup GPT header is in the very last LBA. nothing follows it. The figure must be fixed. I'll see if I can fix it. --Crackwitz (talk) 20:10, 21 January 2019 (UTC)

I meditated on this briefly. The alignment of the text relative to the dashed lines can be misleading. It was probably intended to be relative to blocks. "LBA-1" is beside the last block. It's not meant to label the dashed line. --Crackwitz (talk) 20:12, 21 January 2019 (UTC)

I made some edits: https://gist.github.com/crackwitz/e492e43cbf28d4a3877fe6108f99e6fa (vertically aligned the labels, added/removed some labels, removed some dividers) If anyone knows how to replace/overwrite a file in here, feel free to do it.--Crackwitz (talk) 20:29, 21 January 2019 (UTC)

Nevermind, had to go to Wikimedia Commons to find the feature. It's done.--Crackwitz (talk) 20:34, 21 January 2019 (UTC)

Endianness

This is a mess. Someone needs to go through all those GUIDs and check them. I can only check a few right now. To lay it out:

• UUIDs, according to the RFC, are always written as hex integer ASCII strings (i.e. MSB first, the mathematical way)
• the natural layout in memory would be exactly the same
• some systems, such as Microsoft, and Intel who specified GPT, didn't feel like that and instead use mixed endian marshalling
• you can recognize the byte order a GUID was written in by checking its version field (most significant nibble in the third field, i.e. Vxxx) for a valid number. you'll notice that microsoft uses version 4 GUIDs. on-disk structures have that field in little endian, so the top four version bits sit like xxVx

Example: a "MS Reserved" partition (reserved for being turned into an EFI partition), has the bytes 16 E3 C9 E3 5C 0B B8 4D 81 7D F9 2D F0 02 15 AE on disk, but everyone agrees that the GUID is E3C9E316-0B5C-4DB8-817D-F92DF00215AE

So... #Partition_type_GUIDs has a footnote that explains this wrong. These GUIDs are written as specified. There's nothing little-endian about them.

I'll go ahead and fix this. My little explanation here because I know someone will need it.--Crackwitz (talk) 19:42, 21 January 2019 (UTC)