Boot Process Steps to Know for Operating Systems

The boot process is crucial for starting up a computer and loading the operating system. It involves several steps, from hardware checks to user login, ensuring everything is ready for a smooth user experience. Understanding these steps helps grasp how operating systems function.

1. Power-on self-test (POST)

   • Conducts hardware checks to ensure all components (CPU, RAM, etc.) are functioning properly.
   • Identifies and initializes hardware devices connected to the system.
   • Reports errors through beeps or error codes if any hardware issues are detected.

2. BIOS/UEFI initialization

   • BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface) firmware is loaded.
   • Configures system settings and prepares the hardware for booting.
   • Provides an interface for users to configure hardware settings and boot order.

3. Boot device selection

   • Determines the order of devices (HDD, SSD, USB, etc.) to search for bootable media.
   • Checks the configured boot sequence in BIOS/UEFI settings.
   • Initiates the boot process from the first detected bootable device.

4. Master Boot Record (MBR) or GUID Partition Table (GPT) loading

   • MBR contains the partition table and bootloader for legacy systems, limited to 2TB drives.
   • GPT supports larger drives and more partitions, providing redundancy and improved data integrity.
   • The system reads the partition table to locate the active partition for booting.

5. Bootloader execution

   • The bootloader (e.g., GRUB, LILO) is loaded from the active partition.
   • It prepares the system to load the operating system kernel.
   • Provides options for selecting different operating systems or kernel versions if multiple are installed.

6. Kernel loading

   • The operating system kernel is loaded into memory from the bootloader.
   • Initializes core components necessary for the OS to function.
   • Sets up memory management, process scheduling, and hardware abstraction.

7. Kernel initialization

   • The kernel initializes device drivers to communicate with hardware components.
   • Sets up system calls and prepares the environment for user processes.
   • Establishes the root filesystem and mounts it for access.

8. Init process or systemd startup

   • The init process (or systemd) is the first user-space program executed by the kernel.
   • Manages system initialization and service management.
   • Launches essential services and processes required for the operating system to function.

9. User space initialization

   • Initializes user-space applications and services based on configuration files.
   • Sets up user environments and prepares for user interactions.
   • Ensures that background services and daemons are running as needed.

10. Login prompt or GUI display

    • Presents a login interface for user authentication (command line or graphical).
    • Allows users to enter credentials to access the system.
    • Initiates the user session and loads the user-specific environment upon successful login.