BIOS: The Basic Input/Output System is the lowest level interface between the computer and peripherals.
The BIOS performs integrity checks on memory and seeks instructions on the Master Boor Record (MBR) on the floppy drive or hard drive.
The very first sector of the hard disk is reserved for the same purpose and is called the master boot record (MBR).
When booting from a hard disk, the PC system BIOS loads and executes the boot loader code in the MBR. The MBR then needs to know which partitions on the disk have boot loader code specific to their operating systems in their boot sectors and then attempts to boot one of them.
The GRUB MBR boot loader merely searches for a special boot partition and loads a boot loader. This then reads the data in the /boot/grub/grub.conf configuration file. which lists all the available operating systems and their booting parameters. When this is complete splash screen that lists all the configured operating system kernels for our choice
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In some operating systems, such as Debian / Ubuntu, the /boot/grub/grub.conf file may also be referred to by the name /boot/grub/menu.lst
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When Linux begins to boot with its kernel, it first runs the /sbin/init program, which does some system checks, such as verifying the integrity of the file systems, and starts vital programs needed for the operating system to function properly. It then inspects the /etc/inittab file to determine Linux's overall mode of operation or runlevel.
Based on the selected runlevel, the init process then executes startup scripts located in subdirectories of the /etc/rc.d directory. Scripts used for runlevels 0 to 6 are located in subdirectories /etc/rc.d/rc0.d through /etc/rc.d/rc6.d, respectively.
each filename in these directories either starts with an "S" which signifies the script should be run at startup, or a K, which means the script should be run when the system is shutting down
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http://www.ibm.com/developerworks/linux/library/l-linuxboot/
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view of the Linux boot process