Jones: The EFI System Partition and the Default Boot Behavior

On his blog, Peter Jones writes about the boot process for UEFI, looking at the requirements for EFI System Partitions (ESPs), how the BootOrder variable is used, falling back to removable media, and more. It may be more than you wanted to know about UEFI booting. "There’s nothing truly special about an ESP. It isn’t an ESP because of the GPT GUID and label, nor because of the file system type. Those are how the firmware identifies a partition, and the file system it contains, as candidates to treat as the ESP, when it really needs to find one. The only factor in determining if a partition is the ESP is this: is the firmware attempting to use it as the ESP? At the same time, the requirements for the ESP give us latitude; we know that we can use UEFI’s APIs to find correctly constructed FAT file systems, but there’s no need for those to be the ESP. In fact, even when we create multiple partitions with the ESP’s GUID and label, there’s no requirement that the firmware looks at more than one of them if it needs to find the ESP, and there’s no guarantee as to which one it will pick, either."

Comments (14 posted)