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Describe initial process sequence while the system boots up.1) BIOS
2) Master Boot Record (MBR)
3) LILO or GRUB
4) Kernel
5) init
6) Run Levels
1) BIOS: Loads the boot sector from the storage media. The order of booting can be changed using BIOS.
2) (DOS) Master Boot Record (MBR): DOS option includes MS-DOS, Windows. The first 512 bytes off the disk i.e., /dev/had will be executed by BIOS. The standard MBR looks for the primary partition i.e., /dev.hda1-4 which is marked bootable.
3) LILO: The kernel image is to be loaded as it does not understand the file system code. The kernel image is persisted as raw disk offsets using BIOS routines to load the sequence. Typically the menu code is available in /boot/boot.b and prompts for the partition or kernel for image (Linux) or other (DOS) to load first 512 bytes of the partition.
4) Kernel: Initializes devices, loads ‘initrd’ optionally, mounts root file system that is specified by lilo or loadin with root=parameter. Runs /sbin/init which has the process number 1(PID = 1) and prints the version number.
5) /sbin/init: Reads the /etc/inittab. Executes the boot scripts debian in turn runs /etc/rcS.d/S* scripts, runs the programs available in /etc/inittab.
6) Run Levels: The default level is defined in /etc/inittab. Ex: id: 3: initdefault: The other levels are:
- 0 halt
- 1 single user
- 2 Full Multi-User modes (default)
- 3-5 Same as 2
- 6 Reboot
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