1. First we power on
machine, then BIOS (Basic Input Output System) takes over. BIOS runs POST(Power
on Self Test), i.e. BIOS is responsible to check the RAM, Key Board, Storage
devices etc.
2.
BIOS find a first
bootable device (hard disk). It searches for the Master Boot Record (MBR), and
the control transfers to the master boot record. This stage is called Stage
1 GRUB. After Stage 1 GRUB, Stage 1.5 GRUB is
consulted, which it is located in the first 30KB hard disk immediately followed
byMBR, and it loads the Stage 2 GRUB
3.
Stage
2 GRUB receives the control,
and displays the GRUB boot menu for the user, and GRUB load an user selected(or
default) kernel image & required kernel modules into
the memory and control passes to the kernel phase.
4.
During the kernel phase,
it initiates the /sbin/init program, and control passes to
init phase
5.
Finally the init process
reads the /etc/inittab and starts the /etc/rc.d/rc.sysinit scripts
accordingly, and it will reach to appropriate runlevel
NOTE:
1.
The 1st &
2nd boot loaders combined are called Linux Loader (LILO)
or Grand Unified Boot loader (GRUB) in the x 86 PC environments as LILO
has some disadvantages that were corrected in GRUB. TheLILO is two
stage boot loader and the GRUB is three stage boot loader
(i.e. 1 (MBR), 1.5, 2)
2.
To check the MBR content,
need to run below commands
# dd if=/dev/hda of=mbr.bin
bs=512 count=1
# od -xa mbr.bin
Here dd command
reads the first 512 bytes from /dev/hda, writes them into mbr.bin file,
and odcommand prints the binary file in HEXA and ASCII formats
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