Stage 1: BIOS / UEFI (Firmware Stage)

• BIOS (Basic Input/Output System) or UEFI runs first when the computer starts.

• Performs POST (Power-On Self Test) — checks CPU, RAM, keyboard, etc.

• Looks for a bootable disk (based on boot order).

• Hands control to the bootloader stored in the MBR (Master Boot Record) or EFI partition.

File involved: Firmware chip on motherboard
Output: Bootloader code in memory

 Stage 2: MBR / GPT

• MBR (Master Boot Record) or GPT (GUID Partition Table) contains partition info and the bootloader’s location.

• The MBR is located in the first 512 bytes of the disk.

• Points to the bootloader program (like GRUB).

Files: /dev/sda, partition tables
Output: Bootloader execution

Stage 3: Bootloader (GRUB, LILO, systemd-boot)

• The bootloader is responsible for loading the Linux kernel into memory.

• Common Linux bootloader: GRUB (GRand Unified Bootloader).

• Displays a boot menu if multiple OSes are installed.

Main Tasks:

1. Locate and load the kernel image (/boot/vmlinuz).

2. Load the initial RAM disk (/boot/initrd or /boot/initramfs).

3. Pass control to the kernel.

Files:

• /boot/grub/grub.cfg (GRUB configuration)

• /boot/vmlinuz-* (kernel)

• /boot/initrd.img-* (initial RAM disk)

Output: Linux kernel loaded in memory

Stage 4: Kernel Initialization

• Kernel is now running — it’s the core of Linux.

• Initializes CPU, memory, device drivers, and mounts the root file system.

• Switches from real mode to protected mode (full CPU control).

• Loads necessary modules (network, file system, etc.).

• Mounts the root filesystem / (read-only initially).

Files: /boot/vmlinuz-*, /lib/modules/
Output: Root filesystem mounted

Stage 5: initramfs / initrd (Initial RAM Disk)

• A temporary root file system loaded into RAM before the real root filesystem mounts.

• Used to load drivers needed to access the actual root partition (like RAID, LVM, or encrypted disks).

• Once real root mounts, initramfs is removed from memory.

Files: /boot/initramfs-* or /boot/initrd-*
Output: Real root file system mounted

Stage 6: Init Process / systemd

• After mounting the root filesystem, kernel executes the init process — traditionally /sbin/init, but now systemd in most Linux distros.

Systemd Responsibilities:

1. Mount remaining file systems

2. Start essential background services (network, logging, SSH, etc.)

3. Manage targets (runlevels)

Old Runlevels (SysVinit) → systemd Targets

📍 Files: /etc/systemd/, /lib/systemd/system/
📍 Output: All services started

 Stage 7: Login / GUI

• After all system services start, the system presents:

  • CLI Login Prompt (tty)

  • or Graphical Login Screen (GDM, LightDM, etc.)

📍 Files: /etc/passwd, /etc/profile, ~/.bashrc
📍 Output: User shell or desktop environment