Booting in Linux: Legacy BIOS versus UEFI

Modern Linux distributions prefer the UEFI booting process over legacy BIOS booting. Let's find out why.

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Every time we press the power button on a computer, a complex chain of events begins—one that ends with our Linux desktop or server environment becoming fully functional. This process, called booting, involves multiple layers of interaction between firmware, storage devices, bootloaders, and the kernel.

For decades, computers relied on the BIOS (Basic Input/Output System) firmware, which operated in a simple but rigid way. However, as hardware evolved, BIOS showed its limitations: inability to handle disks larger than 2TB, restricted partitioning, and limited extensibility.

UEFI (Unified Extensible Firmware Interface) was introduced to overcome these challenges. It modernised the boot process with modular phases, support for larger disks, and features like Secure Boot.

Understanding the boot process Before diving into BIOS and UEFI, it’s important to outline the general flow of a boot sequence.

1 Power-on and firmware initialisation: The firmware (BIOS or UEFI) initialises hardware components and determines the boot device.

2 Bootloader loading: A small program (bootloader) is loaded from the disk into the memory of the computer.

3 Kernel loading: The bootloader loads the Linux kernel and initial RAM filesystem, which is called initramfs.

4 Kernel initialisation: The kernel initialises device drivers, mounts the root filesystem, and launches systemd or init, which brings up user space.

Legacy BIOS booting Let’s first explore the legacy BIOS booting process.