(1) BOOTX64.EFI Therory
We can create a bootable USB by building a BOOTX64.EFI file.
What is an EFI file?
EFI = Extensible Firmware Interface
An .efi file is an executable binary used in systems that support UEFI (Unified Extensible Firmware Interface).
Before the operating system (Windows, Linux, macOS, etc.) starts, the system firmware (UEFI) runs these files.
What is UEFI?
UEFI = Unified Extensible Firmware Interface
UEFI is a firmware interface that connects a PC’s hardware to its operating system.
It was developed to replace the older BIOS system, and is now the standard in almost all modern computers.
UEFI is responsible for booting the operating system, initializing hardware, setting up security features, and loading the bootloader.
UEFI vs BIOS Comparison
| Feature | BIOS (Legacy) | UEFI (Modern) |
| Release Era | 1980s | After 2005 |
| Max Boot Disk Size | 2TB (limited by MBR) | 9.4ZB+ (supports GPT) |
| Partition Scheme | MBR | GPT |
| Boot Speed | Slower | Faster |
| Security Features | Minimal | Secure Boot, encryption, etc. |
| Interface | Text-based UI | GUI with mouse support |
| Executable Format | .bin | .efi (PE format) |
Core Functions of UEFI
Hardware Initialization
UEFI initializes key hardware components such as the CPU, RAM, keyboard, and disk drives during the early boot process.
Boot Manager Role
UEFI acts as a boot manager by locating and executing .efi bootloaders stored in the EFI System Partition (ESP).
Common examples include:
BOOTX64.EFIgrubx64.efibootmgfw.efi
Secure Boot
UEFI can verify the digital signature of the OS boot files.
This helps prevent booting from unauthorized or malicious software (e.g., rootkits).
Passes Control to OS
Once the OS kernel is loaded, UEFI hands over control to the operating system.
How UEFI Relates to the Boot Process
Simplified UEFI Boot Sequence:
Power on the system
UEFI firmware starts
It searches for the EFI System Partition (ESP)
Executes
BOOTX64.EFIorgrubx64.efifrom the ESPLoads and starts the operating system kernel
Bootable USB
We need to create a bootable USB, and for this, the file system must be set to exFAT. NTFS cannot be used if we want to work with .efi files.
NTFS vs exFAT (Summary)
| Feature | NTFS | exFAT |
| Full Name | New Technology File System | Extended File Allocation Table |
| Compatibility | Windows only (limited on UEFI/BIOS) | Cross-platform (Windows, macOS, UEFI) |
| EFI Support | UEFI systems cannot boot from NTFS | UEFI systems can boot from exFAT or FAT32 |
| Max File Size | ~16TB | ~16EB |
| Usage Scenario | Internal drives, system disks | USB drives, SD cards, boot media |
Summary:
NTFS is powerful for Windows internal drives, but not supported by UEFI firmware for booting.
exFAT is a lightweight, fast, and UEFI-compatible file system, ideal for bootable USBs.