Booting Process in DOS Operating System

What is Booting?

Booting is a process of starting a computer or computer like device such as a smartphone, etc. Therefore, booting of a computing device start when we press the power button of the device, it makes the computer or the device ready for use.

Actually, when a computer or any computing device is switched off, its operating system remains in the secondary memory like hard disk of the system. But for execution of a software, it must be in the main memory of the system. Hence, booting may also be defined in other words as, the process of loading the operating system from secondary memory into main memory of the device is called booting.

When we press power button of the device, the BIOS of the system is activated which locates and loads the operating system into the main memory, this process of loading the operating system into main memory is called booting.

Types of Booting

Based on the current state of the computer or computing device, the booting is classified into the following two types −

• Cold Booting − When computer is in the powerless state, and is started by the user by pressing the power button of the system unit. In this case, the operating system is loaded from the disk to the main memory, this type of booting is referred to as cold booting.

• Warm Booting − Sometimes situations arises when the system has stopped working and needs to be restarted. In other words, when system is already running and required to be restarted so that it will boot again, this type of booting is called warm booting.

Now let's discuss the booting process in the DOS operating system.

Booting Process in DOS Operating System

In DOS (Disc Operating System), the booting process begins when we press the power button of system unit and continues until the DOS command prompt is appeared on the computer screen. In DOS operating system, booting process primarily deals with the loading of the following three DOS system files into the main memory of the system −

• IO.SYS

• MSDOS.SYS

• COMMAND.COM

We can understand the booting process in DOS operating system in the following steps −

Step 1

When the computer system is started by pressing the power button, the BIOS (Basic Input-Output System) performs several important functionality tests on the programs in ROM (Read Only Memory). This is called Power-On Self Test (POST). POST checks the system for correct peripheral configurations, i.e. whether the peripherals are connected in perfect order or not.

Step 2

Once the BIOS is done with the basic functionality test, it finds the master boot record (MBR) in the first physical sector of the bootable disk as per the boot device sequence specified.

For example, if the boot device sequence is −

• Hard Disk Drive

• Floppy Disk Drive

• CD-ROM

Step 3

Hence, the BIOS will search for the master boot record first in the hard disk drive. If it is not found, then search in the floppy disk drive. If not found in the floppy disk drive as well, the it will search in the CD-ROM. In case, when the system is unable to find the master boot record in all these resources, it shows a message “No Boot Device Found” on the screen and the system will stop.

If master boot record is found in any of the bootable disk drivers, the operating system loader (also called Bootstrap Loader) is loaded from the bootable drive into the main memory.

Step 4

The bootstrap loader first loads the DOS system IO.SYS, and after this it loads the MSDOS.SYS file that is the core file of the disk operating system.

Step 5

The MSDOS.SYS file finds the command interpreter in another system file named CONFIG.SYS. When it is found, it is to be loaded into the main memory. In case when no command interpreter is specified in the CONFIG.SYS, then the COMMAND.COM file is loaded into the main memory as the default command interpreter of the disk operating system.

Step 6

At last, the AUTOEXEC.BAT file is loaded and executed. This file contains the sequence of DOS commands. Once the AUTOEXEC.BAT is loaded, the DOS command prompt is displayed on the computer screen. We can see the letter of the bootable drive in the prompt screen. This indicates that the operating system is successfully loaded from disk into the main memory. At this stage, the system is ready to use.

This is how the booting process takes place in the disk operating system.