What Files Are in the EFI BOOT Directory?

The EFI/BOOT directory on an EFI System Partition (ESP) serves as the standardized fallback location for system firmware to locate bootloaders when no specific NVRAM boot entries exist. This article outlines the typical executable binaries, architecture-specific fallback files, and auxiliary configuration components commonly stored in this directory across different operating systems.

Standard Architecture Executables

The UEFI specification defines a standardized naming convention for default bootloaders. When a system boots from removable media (like a USB drive) or a fixed drive lacking a configured NVRAM boot entry, the firmware searches EFI/BOOT for a binary matched to the processor architecture:

  • BOOTX64.EFI: The default fallback bootloader for standard 64-bit x86 systems (x86_64/AMD64). This is the most common file found on modern PCs.
  • BOOTIA32.EFI: Used for 32-bit x86 UEFI implementations (often found on older Intel Atom tablets).
  • BOOTAA64.EFI: The fallback bootloader for 64-bit ARM architectures (AArch64).
  • BOOTARM.EFI: The bootloader for 32-bit ARM architectures.
  • BOOTRISCV64.EFI: Used for 64-bit RISC-V platforms.

Linux and GRUB Ecosystem Files

On Linux distributions utilizing Secure Boot and the GRUB bootloader, the EFI/BOOT directory typically contains components designed to initialize the kernel securely:

  • BOOTX64.EFI (Shim): On Secure Boot-enabled systems, this file is usually a renamed version of the Microsoft-signed "shim" pre-loader (shimx64.efi). Its role is to verify the secondary bootloader's signature before executing it.
  • grubx64.efi: The core GRUB binary executed by the shim once cryptographic verification succeeds.
  • mmx64.efi: The Machine Owner Key (MOK) Manager. If Secure Boot validation fails on a custom driver or kernel, the shim launches this utility to allow the user to enroll custom keys manually.
  • fbx64.efi: The Fallback application. It automatically detects, reconstructs, and re-adds missing NVRAM boot entries pointing back to the distribution's primary directory (such as EFI/ubuntu or EFI/fedora).
  • grub.cfg: A small stub configuration file that directs GRUB to find the root partition or the primary configuration file located in the main distribution directory.

Windows Boot Manager Files

When Windows writes to the default boot directory, it populates it with a standard set of recovery and loader files:

  • bootx64.efi: A copy of the Windows Boot Manager (bootmgfw.efi), allowing the Windows installation media or system drive to boot if BIOS/UEFI settings are reset.
  • boot.sdi: A System Deployment Image file used during recovery and installation routines to host a virtual RAM disk.
  • BCD: The Boot Configuration Data registry hive, which stores parameters and menu options for starting the Windows operating system.

Diagnostic and Shell Utilities

On administrative media, flash drives, or custom recovery environments, other auxiliary tools may reside in the directory:

  • Shell.efi: An interactive UEFI Shell binary allowing direct manipulation of drives, memory maps, and EFI variables from a command line prior to OS initialization.
  • Driver Files (.efi): Pre-boot UEFI drivers required to initialize specific storage controllers, file systems (such as ext4_x64.efi or ntfs_x64.efi), or network adapters.