it linux boot process
6 Stages of Linux Boot Process (Startup Sequence)
Press the power button on your system, and after few moments you see the Linux login prompt.
Have you ever wondered what happens behind the scenes from the time you press the power button until the Linux login prompt appears?
The following are the 6 high level stages of a typical Linux boot process.
1. BIOS
BIOS stands for Basic Input/Output System
Performs some system integrity checks
Searches, loads, and executes the boot loader program.
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.
2. MBR
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.
3. GRUB
GRUB stands for Grand Unified Bootloader.
If you have multiple kernel images installed on your system, you can choose which one to be executed.
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.
GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).
Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.
#boot=/dev/sda
default=0 timeout=5 splashimage=(hd0,0)/boot/grub/splash.xpm.gz hiddenmenu title CentOS (2.6.18-194.el5PAE)
Press the power button on your system, and after few moments you see the Linux login prompt.
Have you ever wondered what happens behind the scenes from the time you press the power button until the Linux login prompt appears?
The following are the 6 high level stages of a typical Linux boot process.
1. BIOS
BIOS stands for Basic Input/Output System
Performs some system integrity checks
Searches, loads, and executes the boot loader program.
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.
2. MBR
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.
3. GRUB
GRUB stands for Grand Unified Bootloader.
If you have multiple kernel images installed on your system, you can choose which one to be executed.
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.
GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).
Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.
#boot=/dev/sda
default=0 timeout=5 splashimage=(hd0,0)/boot/grub/splash.xpm.gz hiddenmenu title CentOS (2.6.18-194.el5PAE)