BOOT PROCESS
In this topic we will discuss in depth of Linux Boot
Sequence.How a Linux system boots? This will help administrators in
troubleshooting some boot-up problem.Before discussing
about I will note down the major component we need to know
who are responsible for the booting process.
1.BIOS(Basic Input/Output System)
2.MBR(Master Boot Record)
3.LILO or GRUB
LILO:-LInux Loader
GRUB:-GRand Unified Bootloader
4.Kernel
5.init
6.Run Levels
1.BIOS:
i.When we power on BIOS
performs a Power-On Self-Test (POST) for all of the
different hardware components in the system to make sure everything is working
properly
ii.Also it checks for
whether the computer is being started from an off position (cold boot) or from
a restart (warm boot) is
stored at this location.
stored at this location.
iii.Retrieves information
from CMOS (Complementary Metal-Oxide Semiconductor) a battery
operated memory chip on the motherboard that stores time, date, and critical
system information.
iv.Once BIOS sees everything
is fine it will begin searching for an operating system Boot Sector on a valid
master boot sector
on all available drives like hard disks,CD-ROM drive etc.
on all available drives like hard disks,CD-ROM drive etc.
v.Once BIOS finds a valid
MBR it will give the instructions to boot and executes the first 512-byte boot
sector that is the first
sector (“Sector 0″) of a partitioned data storage device such as hard disk or CD-ROM etc .
sector (“Sector 0″) of a partitioned data storage device such as hard disk or CD-ROM etc .
2.MBR
i. Normally we use
multi-level boot loader.Here MBR means I am referencing to DOS MBR.
ii.Afer BIOS executes a
valid DOS MBR,the DOS MBR will search for a valid primary partition marked as
bootable on the hard disk.
iii.If MBR finds a valid
bootable primary partition then it executes the first 512-bytes of that
partition which is second level MBR.
iv. In linux we have two types
of the above mentioned second level MBR known as LILO and GRUB
3.LILO
i.LILO is a linux boot
loader which is too big to fit into single sector of 512-bytes.
ii.So it is divided into two
parts :an installer and a runtime module.
iii.The installer module
places the runtime module on MBR.The runtime module has the info about all
operating systems installed.
iv.When the runtime module
is executed it selects the operating system to load and transfers the control
to kernel.
v.LILO does not understand
filesystems and boot images to be loaded and treats them as raw disk offsets
GRUB
i.GRUB MBR consists of 446
bytes of primary bootloader code and 64 bytes of the partition table.
ii.GRUB locates all the
operating systems installed and gives a GUI to select the operating system need
to be loaded.
iii.Once user selects the
operating system GRUB will pass control to the karnel of that operating system.
see below what is the difference between LILO and GRUB
see below what is the difference between LILO and GRUB
4.Kernel
i.Once GRUB or LILO
transfers the control to Kernel,the Kernels does the following tasks
ñ Intitialises
devices and loads initrd module
ñ mounts
root filesystem
5.Init
i.The kernel, once it is
loaded, finds init in sbin(/sbin/init) and executes it.
ii.Hence the first process
which is started in linux is init process.
iii.This init process reads
/etc/inittab file and sets the path, starts swapping, checks the file systems,
and so on.
iv.It runs all the boot
scripts(/etc/rc.d/*,/etc/rc.boot/*)
v.starts the system on
specified run level in the file /etc/inittab
6.Runlevel
i.There are 7 run levels in
which the linux OS runs and different run levels serves for different
purpose.The descriptions are
given below.
given below.
ñ 0
– halt
ñ 1
– Single user mode
ñ 2
– Multiuser, without NFS (The same as 3, if you don’t have networking)
ñ 3
– Full multiuser mode
ñ 4
– unused
ñ 5
– X11
ñ 6 –
Reboot
ii.We can set in which
runlevel we want to run our operating system by defining it on /etc/inittab
file.
Now as per our setting in /etc/inittab the Operating
System the operating system boots up and finishes the bootup process.
Below are given some few important differences about
LILO and GRUB
LILO
|
GRUB
|
LILO has no interactive command interface
|
GRUB has interactive command interface
|
LILO does not support booting from a network
|
GRUB does support booting from a network
|
If you change your LILO config file, you have to
rewrite the LILO stage one boot loader to the MBR
|
GRUB automatically detects any change in config file
and auto loads the OS
|
LILO supports only linux operating system
|
GRUB supports large number of OS
|
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