GPT is short for GUID Partition Table. It's a more modern alternative to MBR (master boot record) partition tables. Although it's defined in the UEFI specification, it often gets used on non-UEFI systems too. There are a couple big advantages of using GPT over MBR:

  • It has a relatively clear and precise standard, unlike MBR where implementations often just try to match what other implementations do.
  • It supports very large disks and very large numbers of partitions.

A GPT disk contains a primary header near the beginning of the disk, followed by a partition entry array. The header and partition entry array have a secondary copy at the end of the disk for redundency. The partition entry arrays contain structures that describe each partition, including a GUID to identify the individual partition, a partition type GUID to indicate the purpose of the partition, and start/end block addresses. In between the entry arrays is the actual partition data.

System partition

The system partition is UEFI's version of a bootable partition. The system partition is sometimes called the ESP, or EFI System Partition. It is identified by a partition type of c12a7328-f81f-11d2-ba4b-00a0c93ec93b. The system partition always contains a FAT file system. There are various standardized paths that can exist within the file system, and of particular importance are the boot files. These are the files that UEFI will try to boot from by default (in the absence of a different boot configuration set through special UEFI variables).

Boot files are under \EFI\BOOT, and are named BOOT<ARCH>.efi, where <ARCH> is a short architecture name.

ArchitectureFile name
Intel 32-bitBOOTIA32.EFI