Linux Boot Process

Linux Boot Process

Basic Structural block diagram:

BIOS: 

• BIOS stands for Basic Input Output System.
• It is a small program that checks system integrity.
• It conducts first hardware checks.
• In personal computer it resides inside ROM chip populated on mother board.
• It checks for secondary memory availability.
• Finally it loads and execute MBR (Master Boot Record).

MBR:

• MBR stands for master Boot Record.
• It is located in first sector of  HDD or flash drive. "/dev/hda" or "/dev/sda".
• MBR is pretty small in size, less than 512 Bytes. It has following three components:
• Primary bootloader information in first   : 446 Bytes.
• partition table information                       :   64 Bytes.
• MBR validation check                              :     2 Bytes.
• It contains information about bootloader (here we consider GRUB), it simply load and executes GRUB.

GRUB:

• GRUB stands for Grand Unified Bootloader.
• It displays the list of all available kernel images in system. I has option to choose any one of them.
• GRUB has knowledge of Filesystem.
• Present in /etc/grub.conf or /boot/grub/grub.conf
• It loads kernel and initrd images.

Kernel:

• It is a core part of Operating System.
• It executes /sbin/init program file.
• It initiates initrd image.
• initrd stands for initial ramdisk.
• It is a temporary file system mounted until kernel is booted.
• initrd has basic device drivers for HDD, other hardware.
• Mounts root filesystem.

init: 

• It is very first program which executed by kernel after booting.
• It has PID =1.
• It has following runlevels:
• 0 :  halt
• 1 : Single user
• 2 : multi user
• 3 :
• 4 : unused
• 5 : multi user with full GUI.
• 6 : reboot