Computer Boot Up Sequence

A computer without a program running is just an inert hunk of electronics. The first thing a computer has to do when it is turned on is start up a special program called an operating system. The operating system’s job is to help other computer programs to work by handling the messy details of controlling the computer’s hardware. For starting the OS computer follow some set of sequence is called Boot UP Sequence.


Basic Input/Output System, the BIOS, ROM BIOS, or System BIOS is a chip located on all motherboards that contain instructions and setup for how your system should boot and how it operates. The BIOS includes instructions on how to load basic computer hardware and includes a test referred to as a POST (Power On Self Test) that helps verify the computer meets requirements to boot up properly. If the computer does not pass the POST, you will receive a combination of beeps indicating what is malfunctioning within the computer.

The four main functions of a PC BIOS :


Short for power-on self-test, the POST is a test the computer must complete to verify all hardware is working properly before starting the remainder of the boot process. If the computer passes the POST the computer may return a single beep and if unsuccessful generate a beep code to indicate the error and not boot.


Bootstrapping, bootloader, or boot program, a bootstrap loader is a program that resides in the computers EPROM, ROM, or other non-volatile memory that automatically executed by the processor when turning on the computer. The bootstrap loader reads the hard drives boot sector to continue the process of loading the computers operating system. The term boostrap comes from the old phrase “Pull yourself up by your bootstraps”.

The boot loader has been replaced in computers that have an Extensible Firmware Interface (EFI). The boot loader is now part of the EFI BIOS.

BIOS drivers:

Low level drivers that give the computer basic operational control over your computer’s hardware.

BIOS or CMOS Setup:

Configuration program that allows you to configure hardware settings including system settings such as computer passwords, time, and date.

Different between Bios and CMOS:

The BIOS is a computer chip on the motherboard that resembles the picture to the right. This chip contains a special program that helps the computer processor interact and control the other components in the computer. These other components include disc drives, video cards, sound cards, network cards, floppy drives, USB ports, hard drives, and others. Without the BIOS, the processor would not know how to interact or interface with the computer components, and the computer would not be able to function.

The CMOS is also a computer chip on the motherboard, but more specifically, it is a RAM chip. This is a type of memory chip which stores information about the computer components, as well as settings for those components. However, normal RAM chips lose the information stored in them when power is no longer supplied to them. In order to retain the information in the CMOS chip, a CMOS battery on the motherboard supplies constant power to that CMOS chip. If the battery is removed from the motherboard or runs out of juice (e.g. a dead CMOS battery), the CMOS would lose the information stored in it. Any settings you made in the CMOS setup would be lost, and you would need to make those settings changes again after a new CMOS battery was put on the motherboard. For example, with a dead CMOS battery the time and date will reset back to the manufactured date if it has been off for a long period of time.

The BIOS program on the BIOS chip reads information from the CMOS chip when the computer is starting up, during the boot up process. You may notice on the initial start up screen, called the POST screen, an option is available to enter the BIOS or CMOS setup. When you enter this setup area, you are entering the CMOS setup, not the BIOS setup. The BIOS chip and program cannot be updated directly by a user. The only way to update the BIOS is using a BIOS flash program called a BIOS update, which updates the BIOS to a different version. These updates usually are provided by either the motherboard manufacturer or the computer manufacturer.

The CMOS setup lets you change the time and date and settings for how devices are loaded at start up, like hard drives, disc drives, and floppy drives. The CMOS setup lets you enable and disable various hardware devices, including USB ports, the onboard video card and sound card (if present), parallel and serial ports, and other devices.


Master Boot Record, MBR is also sometimes referred to as the master boot block, master partition boot sector, and sector 0. The MBR is the first sector of the computer hard drive that tells the computer how to load the operating system, how the hard drive is partitioned.It contains information about GRUB.


GRUB stands for Grand Unified Boot loader. If you have multiple kernel images installed on your system, you can choose which one to be executed. In simple terms GRUB just loads and executes Kernel 4.Mounts the root file system as specified in the “root=” in grub.conf.

Grup config file:

The configuration file (/boot/grub/grub.conf), which is used to create the list of operating systems to boot in GRUB’s menu interface, essentially allows the user to select a pre-set group of commands to execute





# section to load Linux

title Red Hat Enterprise Linux (2.4.21-1.ent)

root (hd0,0)

kernel /vmlinuz-2.4.21-1 ro root=/dev/sda2

initrd /initrd-2.4.21-1.img

# section to load Windows

title Windows

rootnoverify (hd0,0)

chainloader +1

Kernel executes the /sbin/init program

INIT – RunLevels:

Looks at the /etc/init tab file to decide the Linux run level.Following are the available run levels:

0 – halt   (Shuts down the system)

1 – Single user mode (Mode for administrative tasks)

2 – Multiuser (Does not configure network interfaces and does not export networks services)

3 – Multiuser mode with N/W (Starts the system normally)

4 – unused (For special purposes)

5 – X11 (As runlevel 3 + display manager (starts Login Screen))

6 – reboot (Reboots the system.)

INIT – RunLevels

In standard practice, when a computer enters runlevel zero, it halts, and when it enters runlevel six, it reboots.The intermediate runlevels (1-5) differ in terms of which drives are mounted, and which network services are started.

Lower run levels are useful for maintenance or emergency repairs, since they usually don’t offer any network services at all. The particular details of run level configuration differ widely among operating systems, and also among system administrators.

PDF Download Link Here:Boot Up sequence

Follow Innovation Begins Here on Twitter, innovationBeginsHere, Facebook , Google+ & Linkdin.

Copyright © 2015[InnovationBeginsHere].All Rights Reserved.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s