Overview of the boot process

How Linux Kernel Boots? Commemorating the beginnings of an industry

Back in my college years, I stumbled upon a captivating article on "The Art of Linux Booting". The author used vivid and engaging language to walk me through the entire process, from pressing the power button to logging in. It was fascinating to learn about the hardware, software, protocols, commands, and parameters involved. As I followed the guide, I savored each moment of achievement, feeling joyful, accomplished, and proud. Finally, when I started up Linux for the first time according to the guide, it felt like I was facing a vast unknown sea as an adventurer. A rainbow of excitement soared in my heart, and I almost leaped out of my chair.

It was a remarkable experience, as I saw countless software kingdoms waiting for me to explore and conquer. Since then, I've delved into the world of Linux, mastering every skill and each unique aspect. This blog post is a tribute to that exhilarating first encounter and an attempt to share my excitement with you.

Linux Kernel Boots Process

Overview

[BIOS] - First Sector Interrupt with a 446-byte boot image

-> [GRUB Stage 1.0: 446-byte boot image (boot.img)] -> Locates and loads Stage 1.5 code

-> [GRUB Stage 1.5: (Located between the boot record and the first partition on the device): Code image core.img file with larger storage space to accommodate some generic filesystem drivers] -> First partition

-> [GRUB Stage 2: /boot/grub2] -> Loads some kernel runtime modules

-> [Kernel: Device kernel boot process systemd process -> systemd mounts /etc/fstab (e.g., init 3) runlevel] -> Executes /bin/login program -> Provides a user login interface -> After login, Shell controls the host

Details

In the boot process of a computer system, the BIOS (Basic Input/Output System) is responsible for initializing hardware and starting the operating system. During startup, the BIOS is loaded into the first sector of the memory, and it checks if there is a boot code in that sector.

After the first sector interrupt, control is transferred to the boot image (boot.img) located in that sector. This boot image, also known as Stage 1.0, is typically a 446-byte block of code. Its main function is to locate and load the Stage 1.5 code.

The Stage 1.5 code is usually loaded by GRUB (GRand Unified Bootloader), a commonly used boot loader program known for its flexibility and powerful features. In this stage, GRUB loads the Stage 2 file located in the /boot/grub2 directory.

The GRUB code in Stage 2 is responsible for locating and loading the operating system's kernel. The kernel is typically located in the first partition, which is a partition containing the operating system files.

Once the kernel is loaded, the system enters the systemd process. systemd is a system management daemon responsible for initializing the system and managing processes. During the startup process, systemd mounts file systems according to the configuration specified in the /etc/fstab file.

The runlevel (e.g., init 3) specifies the services and processes to run after the system starts. In the specified runlevel, systemd executes the corresponding configuration and starts the appropriate services and processes.

After executing the /bin/login program, the system provides a user login interface, prompting the user to enter a valid username and password.

Once the user successfully logs in, the system provides a Shell interface, allowing the user to interact with the host through a command-line interface. Users can run commands, access the file system, and start/stop processes within the Shell.

In summary, the entire process can be summarized as follows:

BIOS loads the boot image (boot.img) from the first sector.

The boot image locates and loads the Stage 1.5 code.

Stage 1.5 code is loaded by GRUB, which then loads Stage 2 code.

Stage 2 code in GRUB locates and loads the operating system's kernel.

Once the kernel is loaded, the system enters the systemd process, which mounts file systems.

Based on the specified runlevel (e.g., init 3), systemd executes the corresponding configuration and starts services and processes.

The /bin/login program is executed, providing a user login interface.

After successful login, the system provides a Shell interface for controlling the host through a command-line interface.