linux-boot-process

Date: 2026-01-23

Intro

When you turn on a computer or restart it, it must load an operating system. This process is called the boot process.

BIOS

In the past before transitioning into boot loaders, BIOS (Basic Input and Output Service), was being used. This was a code that was stored in a non-volatile memory such as ROM, EEPROM or flash memory. When a PC is turned on, this code is executed and performs a power on self test (POST) to check the machine. This code also determines the boot drive from the available removable/fixed storage and loads the first sector from the Master Boot Record (MBR) on that drive eg hard drive/ ssd or usb stick or DVD/CD

1. BIOS/UEFI Initialization

• The computer’s firmware (BIOS or UEFI) performs a Power-On Self Test (POST) to check hardware (RAM, CPU etc)
• BIOS/UEFI locates the boot loader from the disk defined in the boot order

2. Bootloader (GRUB) Execution

• This is located in the MBR (Master Boot Record) for BIOS or EFI System Partition for UEFI.
• The common boot loader is GRUB
• A boot menu is then displayed if configured allowing the user to select different kernels or OS
• A Linux kernel is loaded into memory if selected
• The kernel parameters are passed from its configuration file

3. Linux kernel Initialization

• After the kernel which is a compressed image is loaded into memory, It extracts itself and initializes hardware drivers like CPU,memory
• Then the root filesystem is mounted from the partition specified by the bootloader
• The init process (PID 1) starts from the initial RAM disk

4. initrd or initramfs (Initial RAM Disk)

• Temporary filesystem is loaded into RAM
• Once the root filesystem is accessible, the real root is mounted and the temporary one is discarded

References