EFI system partition

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The EFI system partition (ESP) is a small section on your hard drive or SSD that UEFI-based computers use to start your operating system and run early boot tools. It mainly stores boot loaders, kernel images, device drivers used at boot, and some system utilities.

The ESP uses a FAT-based file system, as specified by UEFI. FAT32 is common on fixed drives, while removable drives (like USB sticks) can use FAT12, FAT16, or FAT32. The ESP’s files follow a standard layout so the firmware can find and start operating systems.

Both GPT and MBR partition schemes can include an ESP, because UEFI supports booting from either. Some CD/DVD boot formats (El Torito) are also supported. To stay compatible with older BIOS systems, the ESP reserves the first block for compatibility code. A legacy BIOS can load this code to start, or the firmware can switch to BIOS-compatibility mode (CSM) on some disks, which may stop EFI boot from the ESP on those disks.

UEFI can boot from removable storage like USB drives, too. Those drives are usually FAT-formatted, and a boot loader on the ESP follows the standard layout so the firmware can start the OS from the drive.

Linux and other systems use various boot managers. Common Linux options include GRUB 2, elilo, and systemd-boot. They can load kernel images from many places, not just the ESP. Where you mount the ESP in Linux can vary by boot loader: /boot/efi is a common location for older boot loaders, while systemd-boot often uses /efi or /boot.

EFI Boot Stub is a feature that lets a Linux kernel run directly as a UEFI application, without a separate bootloader. The kernel must be built with EFI stub support (CONFIG_EFI_STUB). This option merged into Linux in 2012 (kernel 3.3). Systemd-boot is a simple UEFI boot manager that reads its configuration and boot files from the ESP and doesn’t access other partitions.

Apple and Windows specifics
- On Intel-based Macs, the ESP starts out unused for macOS, but it’s used for firmware updates and for Windows Boot Camp. iBoot controls boot selection on Apple systems.
- Older Apple Macs that used pre-UEFI sometimes formatted the ESP as HFS+. Apple Silicon Macs (M1/M2 and later) do not use EFI/UEFI in the same way, so they don’t have an ESP.
- Windows places its boot manager in the ESP at the path \EFI\Microsoft\Boot\. Windows boot support began with Windows Vista SP1. Access to the ESP on some Windows versions can be done with the mountvol command.

In short, the ESP is the key EFI boot area on the disk, holding the files needed to start the OS and manage booting, with support for various partitioning schemes, boot methods, and bootloaders.