Code-Memo

The Boot Process

The boot process is the sequence of events that occurs when a computer is powered on and the operating system is loaded into memory. Here’s an overview of the stages in the boot process:

1. BIOS/UEFI Initialization:

   • The Basic Input/Output System (BIOS) or Unified Extensible Firmware Interface (UEFI) initializes hardware components and performs a Power-On Self Test (POST) to ensure everything is functioning correctly.
   • BIOS/UEFI locates and executes the bootloader from the bootable device (e.g., HDD, SSD).

2. Bootloader Execution:

   • The bootloader is a small program that loads the operating system kernel into memory.
   • Common bootloaders include GRUB (GRand Unified Bootloader) and LILO (Linux Loader).

3. Kernel Loading:

   • The bootloader loads the kernel and initial RAM disk (initrd or initramfs) into memory.
   • The kernel initializes hardware components, mounts the root file system, and starts the initial process (init or systemd).

4. System Initialization:

   • The init process or systemd initializes user-space services and sets up the user environment.
   • The system reaches a state where users can log in and interact with the OS.

The Bootloader

What is a Bootloader?

A bootloader is a program that loads the operating system kernel into memory and starts its execution. It also allows users to select from multiple operating systems installed on the system.

Common Bootloaders:

• GRUB (GRand Unified Bootloader): The most widely used bootloader for Linux systems, supporting a wide range of file systems and configurations.
• LILO (Linux Loader): An older bootloader that has largely been replaced by GRUB.
• Syslinux/EXTlinux: Lightweight bootloaders often used for live USBs and embedded systems.

Bootloader Stages:

1. Stage 1: Located in the Master Boot Record (MBR) or the GUID Partition Table (GPT). It loads the Stage 1.5 or Stage 2 bootloader.
2. Stage 1.5: Optional intermediate stage that understands file systems, allowing the Stage 2 bootloader to be located and loaded.
3. Stage 2: The main bootloader that presents the user with a menu to select the operating system and loads the selected OS kernel.