BOOT- UP PROCESS

When you switch ON your system within a few seconds a login window appears. Ever wandered how this window appears? What all steps are performed before this window
appears?
The whole process starting from the switching ON the power supply till the time your
computer is ready for you to work on is called bootup process.
It is 6 stage process that includes steps:

step1: BIOS(Basic Input Output System)
it performs some system integrity tests.
It searches for the boot loader in floppy, cd-rom,or hard drive depending on the
priority set by you,.and after finding it loads it and executes it.
This boot loader is called MBR(Master Boot Record) which is in the 1st sector of the
booting device.
So basically BIOS loads and executes MBR boot loader.
Step 2: MBR(Master Boot Record)
It is located in the first sector of booting device in less than 512 bytes. It has 3
components 1. primary boot loader info in 1st 446 bytes 2nd partition table info in
next 64 bytes 3rd MBR validation check in last 2 bytes.
It contain the info of the secondary boot loader i.e. The GRUB or LILO.
So in short MBR loads and executes the GRUB boot loader.
Step 3: GRUB(Grand Unified Bootloader)
it loads the kernel image and initrd images
it is the GRUB that displays the screen asking you to choose the kernel image to load
among the various kernel images you have on your system.
GRUB configuration file is in /boot/grub/grub.conf which contains info about the
kernel and initrd image to load.
Step 4: Kernel
it mounts the root file system as specified in grub.conf.
Kernel then executes the /sbin/init program which is the first program to beexecuted by the kernel so it has a pid of 1.
initrd stands for initial RAM disk.
It is used by kernel as the 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 in access of hard drives partitions and other hardware.
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Step 5: Init program
this program checks the /etc/inittab file to decide the linux runlevel.
The run levels availabe are
0-halt
1-single user mode
2-multiuser without NFS
3-full multiuser mode
4-unused
5-X11
6-reboot
Step 6:Runlevel Programs
Depending upon the runlevel set the system wilRun level 0- /etc/rc.d/rc0.d/l execute
the programs from one of the directories.Run level 0 – /etc/rc.d/rc0.d/
Run level 1 – /etc/rc.d/rc1.d/
Run level 2 – /etc/rc.d/rc2.d/
Run level 3 – /etc/rc.d/rc3.d/
Run level 4 – /etc/rc.d/rc4.d/
Run level 5 – /etc/rc.d/rc5.d/
Run level 6 – /etc/rc.d/rc6.d/