Boot Sequence of Linux

Linux Boot Sequence:
Reboot/Poweron --> SMPS(Switched mode powersupply) --> Processor --> BIOS --> MBR(MasterBootRecord) --> GRUB/LILO ---> /etc/inittab --> /etc/motd
Reboot/Poweron --> Switch on or reboot the system
SMPS(Switched mode power supply)--> It is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power.An SMPS is usually employed to efficiently provide a regulated output voltage, typically at a level different from the input voltage.
Processor --> the unit that reads and executes program instructions.
BIOS (Basic Input Output System) --> The BIOS software is built into the PC, and is the first code run by a PC when powered on. The primary function of the BIOS is to load and start an operating system.It Will initialize POST and Bootable Media devices
POST- Power On self test --> It will check whether all the peripheral devices are properly connected.
Bootable Media Devices --> It will check the bootable devices like cdrom etc are connected properly.
MBR(Master Boot Record)--> It is first sector of first partition which will initiate GRUB/LILO.It is 512 bytes (446byte for Executable codes and logmessages,64 bytes for Partition table information,2 bytes for Magic number).On 446 bytes loads the Kernel and initrd,64 bytes is divided into 4 Partition by defaultm in which first 3 are primary partition and 1 for Extendable partition.MBR uses virtual filesystem called NASH file systems from MBR to GRUB
/boot is an active partition or primary partition.
MBR will load the secondary boot loader which loads GRUB/LILO
LILO --> Linux loader , boot loader which has several limitation while booting so it is not used instead GRUB is used
GRUB --> Grand Unified Bootloader, which holds the address of kernel and initrd (path: /boot/grub/grub.conf)
/etc/initrd - contains required module to boot
/etc/inittab --> to indicate in which run level it should initialize
Runlevel -->
0 - halt
1 - Single user mode
2 - Multiuser mode without NFS
3 - Multiuser mode with NFS
4 - Not used
5 - GUI
6- Reboot
(All Run level specification module are defined in /etc/rc/)
/etc/rc.local --> It will be used in run day jobs or scripts after booting
/etc/motd - To set message of the day.