Booting may be defined as process of loading the operating system into memory. The booting process starts from the moment when we power on computer and continues till moment, computer is ready for use. In the case of DOS, booting process starts when we start computer and continues till DOS prompt is displayed. The booting process of DOS mainly deals with loading three main system files of DOS into memory. These files are IO.SYS, MSDOS.SYS and COMMAND.COM.
In the booting process of DOS, the following steps are performed when we start a computer –
- Once the computer system is turned on, BIOS (Basic Input /Output System) performs a series of activities or functionality test on programs stored in ROM, called on Power-on Self Test (POST) that checks to see whether peripherals in system are in perfect order or not.
- After the BIOS is done with pre-boot activities or functionality test, it read bootable sequence from CMOS (Common Metal Oxide Semiconductor) and looks for master boot record in first physical sector of the bootable disk as per boot device sequence specified in CMOS. For example, if the boot device sequence is –
- Floppy Disk
- Hard Disk
- After this, master boot record will be searched first in a floppy disk drive. If not found, then hard disk drive will be searched for master boot record. But if the master boot record is not even present on hard disk, then CDROM drive will be searched. If the system is not able to read master boot record from any of these sources, ROM displays the message “No Boot device found” and system is halted. On finding master boot record from a particular bootable disk drive, operating system loader, also called Bootstrap loader is loaded from boot sector of that bootable drive· into memory. A bootstrap loader is a special program that is present in boot sector of bootable drive.
- Bootstrap loader first loads the IO.SYS file. After this, MSDOS.SYS file is loaded which is core file of DOS operating system.
- After this, MSDOS.SYS file searches to find Command Interpreter in CONFIG.SYS file and when it finds, it loads into memory. If no Command Interpreter specified in the CONFIG.SYS file, the COMMAND.COM file is loaded as default Command Interpreter of DOS operating system.
- The last file is to be loaded and executed is the AUTOEXEC.BAT file that contains a sequence of DOS commands. After this, the prompt is displayed, and we can see drive letter of bootable drive displayed on the computer system, which indicates that operating system has been successfully on the system from that drive.
- Cold Booting/Switch Booting –
When the user starts computer by pressing power switch on system unit, the operating system is loaded from disk to main memory this type of booting is called Cold Booting. This booting takes more time than Hot or Warm Booting.
- Hot or Warm Booting –
Hot booting is done when computer system comes to no response state/hang state. Computer does not respond to commands supplied by user. There are many reasons for this state, only solution is to reboot computer by using the Reset button on cabinet or by pressing a combination of ALT + CTRL + DEL keys from keyboard.