Booting Process

Booting may be defined as the process of loading the Operating System onto the RAM

Once the computer system is turned on,the following are the steps that a typical boot sequence involves.

• The BIOS performs Power on Self Test (POST) that checks to see whether the devices in the system are in perfect order. 

• After the BIOS is done with the pre boot activities, it searches for the Master Boot Record in the first physical sector of the bootable disk. When it finds one, it implies that it is the bootable partition, and, the Operating System loader, also called the boot strap loader is loaded from that partition onto the memory. A boot strap loader is a special program that is present in the boot sector of the bootable drive.

• MS DOS Operating system comprises of the following files

1. IO.Sys
2. MSDOS.Sys
3. Command.Com
4. Config.Sys
5. Autoexec.bat

Note: The first three files are mandatory while the rest two are optional. Further, the first three files should be present in the bootable drive of the disk

• The boot strap loader first loads the IO.SYS file. The IO.Sys file as the name suggests, is responsible for Input Output in the DOS environment. 

• The next file that is loaded is the MSDOS.SYS which is the core of the DOS operating system. The MSDOS.Sys file is  responsible for Memory management and Processor Management in the DOS environment. 

• The MSDOS.SYS file now searches to find the name of the command interpreter in the CONFIG.SYS file and when it finds one, it loads the same onto the memory. 

• If no command interpreter is specified in the CONFIG.SYS file, the COMMAND.COM file is loaded as it is the default command interpreter of DOS Operating system.

• The last file to be loaded and executed is the AUTOEXEC.BAT file that contains a sequence of DOS commands.

• Now, the prompt is displayed and you can see the drive letter of the bootable drive displayed on your screen indicative of the fact that the Operating System has been loaded successfully from that drive.