EFI System Partition
The EFI System Partition is a dedicated partition on GPT. Usually a small one (128-256MiB), located at the beginning of the disk and its partition record at the beginning of the GPT.
Identify
Unlike normal bootable partitions, which have the attribute flag bit 2 set, ESP is identified by it's GUID, which is
C12A7328-F81F-11D2-BA4B-00A0C93EC93B
Format
In theory ESP can be formatted to any FAT file system, FAT12/16/32. In reality most firmware only support FAT32, and some platforms with limited resources may support FAT16 too. FAT12 is very rarely implemented, and it's safe to assume it won't work on a real hardware.
There's one notable exception, Apple machines (like iMac and MacBooks) use a special firmware which only accepts HFS+ file system on the EFI System Partition.
Access
In the EFI Shell, the ESP is the very first file system, denoting as FS0:. GRUB and ELILO recognize ESP, and they install their files there by default. Other boot loaders, like rEFInd can only work if ESP exists.
Important Files on ESP
- FS0:\STARTUP.NSH - an EFI Shell script, similar to MS-DOS autoexec.bat
- FS0:\BOOTMGR.EFI - the EFI boot manager
- FS0:\BOOT\BOOTX86.EFI - the default x86_32 boot loader
- FS0:\BOOT\BOOTX64.EFI - the default x86_64 boot loader
To avoid filename clashes, you should not create files in the root directory of ESP. The specification mandates that root directory is for vendor directories, like FS0:\APPLE, FS0:\MICROSOFT etc. Some applications (like GRUB) just creates a one level directory without vendor parent directory. Feel free to create a directory for your own OS, and put your files there.