Formatting GUIDs in GPT header fields

October 12, 2020

“GPT” stands for “GUID Partition Table”, and “GUID” stands for “Globally-Unique IDentifier”. A GUID is a number generated according to the GUID specification, which is a method for generating numbers such that it is very unlikely that any two thus generated numbers will be the same.

GPT uses GUIDs to identify disks, partitions, and partition types.

One normally writes a GUID as several dash-separated hexidecimal numbers. An example GUID is C12A7328-F81F-11D2-BA4B-00A0C93EC93B. This particular GUID is special, in that it indicates an “EFI System Partition” partition type. This is the partition type that my tool will create, as it’s aimed at generating UEFI-bootable disk images.

Looking at an example UEFI-bootable disk, the numeric value for its partition type is 0x3bc93ec9a0004bba11d2f81fc12a7328. Looking closely at this number and the EFI GUID C12A7328-F81F-11D2-BA4B-00A0C93EC93B, every byte (pair of hex digits) is present. The order of fields is reversed, and the order of bytes within some fields is reversed. I need to do some more digging to find out the general rule for encoding GUIDs in GPT headers, though I suspect the pattern observable from this one example will suffice.