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
Kernel:
·
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.
Init:
·
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/
nicely explained.. good job..
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