Boot Process

Bios -> POST -> Sector 0 of bootable device -> Boot loader -> Kernel & Initrd (initramfs) -> Kernel -> Init

• Bios
  • Basic input output
  • Is a frimware stored on R/O chip
  • First program that runs when sys powers on
  • Performs POST
  • Locate and runs boot loader
  • Provides an interface to configure hardware
    • Active onboard HW
    • Boot order, etc.
  • non-volatile

• CMOS
  • Is a special memory chip
  • Hold basic informations (256 Byte)
    • Current date and time
    • HW configuration needed by BIOS
      • Boot sequence
    • volatile (Battery)
    • Complementary metal oxide semiconductor

• Post
  • Power on self-test
  • Does moderboard works?
  • Is GPU installed?
  • Is memory working correctly?

• MBR
  • Master Boot Record (512 Byte)
  • The first sector of the computer hard drive [sector 0]
  • Magic Code [2 Byte] (0xAA55) (0b1010101001010101)
    • BIOS will try booting the system
    • Garbled or 0x0000 [error/tries booting the next disk]
  • How the hard drive is partitioned [Partition table] (64 Byte)
  • How to load the operating system [Boot Code / Bootloader] (446 Byte)

MBR layout

Bootloaders [N05]

• Lilo
  • Does not understand or parse filesystem
  • /etc/lilo.conf
  • information about location of kernel and ram disks, the configuration file is written with bootloader code into MBR

• Grub legacy
  • Logic to read common file systems
  • 3 Stages

• Grub2
  • automatically generated
  • chain-loading
  • full scripting language: variables, conditionals, and loops are available
  • available for several other types of system
  • can read files directly from LVM and RAID
  • Changes from legacy

• Syslinux
  • Specializes in booting full Linux installations from FAT filesystems
  • Live USBs
  • Linux live CDs and bootable install CDs

Initrd or Initramfs [N01.1]

Init

• Process N1
  • Runlevels
    • Daemons (services)
    • Desktop login manager (if any)
    • All other programs

• init systems
  • BSD-Style
  • SysV
  • Upstart
  • openrc
  • systemd
  • shepherd

How knows where these stuff are? [N20]

References

https://en.wikipedia.org/wiki/Linux_startup_process
http://www.wisegeek.org/what-is-cmos.htm
http://www.computerhope.com/issues/ch001360.htm
https://www.howtogeek.com/180798/where-is-the-bios-stored/
https://en.wikipedia.org/wiki/Nonvolatile_BIOS_memory

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Lecture notes

• All lecture notes are available at github in ipynb format: https://github.com/ravexina/linux-notes.

• There are html exports of project available at: https://ravexina.github.io/linux-notes.

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License

Linux Notes by Milad As (Ravexina) is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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