Booting Process
1) POST (Power On Self Test):- BIOS is group of program and it is written by
system programmer to do the POST so it checks the hardware and put the list on
memory chip.
2) CMOS (Complimentary Metal Oxide
Semiconductor) :- CMOS
comes with motherboard and has program in it, we can make changes in. CMOS
battery powers the CMOS data which is inside it and to access CMOS data we have
to press DEL key or other appropriate key for it.
BIOS
reads CMOS and the CMOS in the RAM :-
RAM=BOIS+CMOS
CMOS:-
Why you called me?
BIOS:-To
load OS.
CMOS:-
Who asked?
BIOS:- CPU
CMOS:-
Which OS?
BIOS:-
Any (then BIOs checks Boot Priority)
CMOS:- Ok go to the first boot device.
In the
first boot device it looks for MBR, it is located at CHS(Cylinder-0, Head-0,
Sector-1)
MBR(Master
Boot Recorder), MBR is divided into three part:-
1. Boot
Partition (446 Bytes)
2.
Partition Table (64 Bytes)
3. Magic
No. (2 Bytes)
In the
system BIOS check the system and launches first stage boot loader on the
MBR of primary hard disk. The first stage boot loader load itself into memory
and launches the second boot stage boot loader from first boot partition.
The first
stage boot loader it’s a small machine code on the MBR and its job is to locate
the second boot stage loader and load first part into memory once the second
boot stage loader is in the memory its resent the user with graphical screen
showing different OS and kernel, it has been configured to boot the graphical
screen comes because of /etc/grub.conf script which is a link file and actual
stored in /boot/grub/grub.conf.
Over here
user can use arrow keys to choose which OS or kernel they which to boot and
press enter, if no keys are pressed the
boot loader loads the default selected OS after the configuration period has
passed.
Now once
the second stage boot loader has determine which kernel to boot it locates the
corresponding kernel binary ( /boot directory) the kernel binary named used the
following format which is located in /boot directory
vmlinuz
After
selecting OS the kernel is loaded and CPU immediately initialized and configure
computer, memory and various hardware attached to the system including all process,
input & output system and a storage devices. Then it look for a compressed init.ramfs
image in predetermined location in memory. It compresses it directly to
/sysroot then it loads all necessary drivers.
Now the
init program starts and coordinate the rest of booting process and configure
the environment for user. Init is father of all other process and its PID is 1.
When
init program start it become parental and grant parent for all other process
and all the processes start automatically on that system. Then it first run
/etc/rd.d.rc.sysinit script which states the environment path starts the swap
memory checks the file system execute all other steps required for system
initialization. Then the init command runs /etc/inittab script which describes
how the system should be set up in init each runlevel, run levels are more
defined by services list.
RUNLEVELS
are
0 – Halt
1 –
single user mode
2 -
Multiuser without NFS
3 – Full
multiuser
4 -
Unused
5 – X11
6 –
Rebbot
The init
command sets the source function library /etc/init.d/functions script. First
system to start, kill and determine the PID of program. The init start all the processes by looking appropriate rc directory
from the runlevels specified as default in file /etc/inittab.
For
example when booting to runlevel 5 the init looks in /etc/rc.d/rc5.d script to
determine which process to start and stop.
All the
files in the /etc/rc.d/rc5.d are symbolic link tp script located in
/etc/rc.d/init.d. the name of each symbolic link begain witj k or s where k
links are process that kill on runlevel while those beginning with s are
started.
After
the init command has processed to rc directory for runlevel in /etc/inittab
files it directly point to /sbin/mingettys process for each virtual console.
The
/sbin/mingettys process opens communication path way to tty devices. Set there
mode print a login prompt except the username and password and initialize the
login screen.
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