Ch 08 Linux Boot Process

After power ON

power on  ---------------->





POST (power on self test)

(First it detect disply)

Post bring first boot device in kernel.


A.C power connector with 4 pin.

+12 V (for motors)

-12 V (ports Like monitor, serial extra )

+5  V (I.C)

-5  V (Refresh the RAM)



Bios : software and firmware manufacturer of M/B puts

BIOS in Rom chip Rom get deposited in RAM

Now,Ram is play ground BIOS start playing , bios

Comes from ROM CPU sends BIOS to RAM, BIOS wants to

Load OS but Doesn’t know which OS to load then BIOS Learns.

CMOS: It comes with M/B and has program in it we can make Changes in  CMOS Data, which is inside it press DEL Key to change the settings of CMOS.

Bios load the Cmos in Ram ,Ram consist of Cmos+BIOS

Cmos as why have you called me.

Bios : to load OS.

Cmos : who asked.

Bios : CPU & I want to load OS.

CMOS : which OS

BIOS : Any

CMOS : OK, Go to 1st boot Device

In First boot device it look for MBR ( Master boot Record )

MBR is located at cylinder 0 ,head 0 ,sector 1

MBR = 512 bytes

1)boot sector     = 446 bytes

2)partition Table = 64  bytes

3)Magic NO        = 2   bytes

in boot sector existing files ntdlr , CHS value , grub binary.

Disk geometry exist with Partition Table



Ground unify boot loader

After MBR comes in RAM it will execute boot loader binary from boot


If magic NO is ok it will load OS

If there are multi OS

Grub send 13 H interbred when ntdlr start  executing and task

Control to him self In the system BIOS checks the system and

Launches the first stage boot loader.


The first stage boot loader load itself into the memory and

Launches the second stage boot loader from the first boot



The fist stage boot loader is small machine code binary on the MBR

It’s sole job is to locate the second stage boot loader and load

The first into the memory.


Once the second stage boot loader is in the RAM it present the

User with Graphical screen showing different OS and kernel it

Has been configured to boot the graphical screen comes because






After selecting OS (LINUX) when the kernel is located it

Immediately initialize and configure the computer memory and

Various hardware attach to PC including all processor input output

Sub system and storage devices it looks for compress init RAM_FS

Image in pre determine location in memory it uncompress it

Directory to /sysroot to load all necessary drivers.

After this init co-ordinate the rest of the boot process and

Configure enviourment of users.

When the init program start it become parent and grand parent

Of all other process and all process start automatically.

 #ps –a |less

 (to know the all process and ID then it first Runs also)


 which state the enviourment path, swap checks the file system

execute all there steps requirement for system initialization.


Then the init command start /etc/inittab which describe how

System should be setup in each Run level.

 (from any file put #for particular line To comment the line)


NOTE: Linux system can has 11 terminals configured.

 The init start all the background process by looking appropriate

RC directory for the run level specified as a default in

/etc/inittab file.

 When booting to run level 5 the init program looks in


 last script is /etc/rc.local mingetty lords drivers for

Terminals from  /etc/issue

 Login prompt is because of /bin/login , then login name

checked in /etc/passwd and the password check in authorization

 in /etc/shadow .

 Then login file goes in sleep state until you go in different

Terminals when login goes sleep state mingetty comes into the

Action and then /etc/motd send mal for user read.


# vi /etc/motd  (If we want to add some banner after login. we can

add text in this file.


 #stty –echo

(TO do not appear typed command )

#stty echo

(To enable in previous display command)