Once system powered on, first thing will be POST check i.e

power on self test.

Then BIOS loads the MBR from the disk into RAM. Then, the

BIOS executes the MBR code. now boot loader list the number

of OS installed in to the system. select the OS wants to be

boot once OS selected initial RAM disk (temporary root file

system) are loaded into memory. When the images are loaded,

the boot loader passes control to the kernel image and the

kernel is decompressed and initialized.At this stage, the

boot loader checks the system hardware, enumerates the

attached hardware devices, mounts the root device, and then

loads the necessary kernel modules. When complete, the first

user-space program (init /sbin/init) starts, which will

looks file /etc/inittab to find the which runlevel kernel

will be boot then it will be check /etc/rc?.d (? means

runlevel got from inittab file)and check every script which

starts with name S will start in to that level once

completed, initrd(temporary root file system) will be

unmounted and mount the physical filesystem and loads the

main kernel then login prompt will be available.