Linux

Boot Process

flowchart TB
  0["Power on"]
  1["CPU executes BIOS/EUFI from ROM"]
  2["BIOS/UEFI runs POST"]
  3["BIOS/UEFI loads partition table into memory"]
  4["First partition starts bootloader"]
  5["Bootloader loads kernel into memory"]
  6["kernel initiates kernel modules, loads device drivers, starts system services"]
  7["init system starts background services (network, logging, daemons), user login"]

  0 --> 1
  1 --> 2
  2 -->|POST fails| halt["boot stops"]
  2 -->|POST succeeds| 3
  3 --> 4
  4 --> 5
  5 --> 6
  6 --> 7

1. BIOS/UEFI
   • BIOS
     • older
     • Master Boot Record (MBR) for disk partitioning
       • uses 32 bits for block addressing, limiting disk sizes to ~2TB ($2^{32}\times 512\text{-byte sized sectors}$)
   • UEFI
     • GUID Partition Table (GPT) for disk partitioning
       • uses 64 bits for block addressing
     • faster, more security features (secure boot)
2. Power-on self-test (POST)
   • set initial device state from firmware
   • detect any hardware component errors
   • any failure ends the boot process
3. Bootloader (GRUB2)
   • locate operating system kernel, load kernel into memory
4. kernel
   • kernel
   • initiate background processes, device drivers, kernel modules
   • start init system
5. init system (systemd)
   • mount file systems
   • start background services, daemons, drivers
     • networking, sound, power management
     • targets for dependency ordering
   • user login