Linux Boot Process

BIOS (Basic Input/output System):
·         When we power on BIOS performs Power On Self Test (POST) for all different hardware components in the system to make sure that everything is working properly.
·         Also it checks, computer is being started from off position (cold boot) or restart(warm boot).
·         Retrieves information from CMOS (Complementary Metal-Oxide Semiconductor) a battery operated memory chip on the motherboard that stores time, date, and critical system information.
·         Once BIOS sees everything is fine, it will start searching for boot loader.
·         It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.
·         Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.
·         So, in simple terms BIOS loads and executes the MBR boot loader.

MBR (Master Boot Record):
·         MBR stands for Master Boot Record.
·         It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda
·         MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.
·         It contains information about GRUB (or LILO in old systems).
·         So, in simple terms MBR loads and executes the GRUB boot loader.

Boot Loader:
·         A boot loader, also called a boot manager, is a small program that places the operating system (OS) of a computer into memory.
·         GRUB stands for Grand Unified Boot loader:
o   GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.
o   GRUB has the knowledge of the file system (the older Linux loader LILO didn’t understand filessystem).
o   Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this).
·         LILO(Linux Loader):
o   LILO is a linux boot loader which is too big to fit into single sector of 512-bytes.
o   So it is divided into two parts: an installer and a runtime module.
o   The installer module places the runtime module on MBR.The runtime module has the info about all operating systems installed.
o   When the runtime module is executed it selects the operating system to load and transfers the control to kernel.
o   LILO does not understand filesystems and boot images to be loaded and treats them as raw disk offsets
·         Mounts Root file system.
·         Initializes devices and loads initrd module.
·         initrd stands for Initial RAM Disk.
·         initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.
·         init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1.

·         First process which is started in linux is init process.
·         Looks at the /etc/inittab file to decide the Linux run level.
·         Following are the available run levels
o   0 – halt
o   1 – Single user mode
o   2 – Multiuser, without NFS
o   3 – Full multiuser mode
o   4 – unused
o   5 – X11
o   6 – reboot
·         Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.
Run Levels:
·         When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.
·         Depending on your default init level setting, the system will execute the programs from one of the following directories.
o   Run level 0 – /etc/rc.d/rc0.d/
o   Run level 1 – /etc/rc.d/rc1.d/
o   Run level 2 – /etc/rc.d/rc2.d/
o   Run level 3 – /etc/rc.d/rc3.d/
o   Run level 4 – /etc/rc.d/rc4.d/
o   Run level 5 – /etc/rc.d/rc5.d/
o   Run level 6 – /etc/rc.d/rc6.d/


  1. nicely explained.. good job..

  2. Hello There,

    A spot on observation on what probably is “the” underlying details of the Programming Tutorial .Too many people don’t even think about wherever there will be actual demand and more importantly what happens if this demand comes later (or maybe a lot later) than they expect

    I want to ask for the newbie of linux, which platform of linux should I start with?
    But nice Article Mate! Great Information! Keep up the good work!

    Many Thanks,