The Boot Process of DOS Operating System by joydipkanjilal

This article discusses the process involved in the boot process of the DOS Operating System.

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

Once the computer system is turned on, the BIOS performs a series of activities called Power on Self Test (POST) that checks to see whether the peripherals in the system are in perfect order.


This Pre Boot Sequence consists of a series of steps that starts with the execution of software stored in the ROM called firmware. These activities are actually known as POST. 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. Note that a floppy disk cannot be partitioned and hence does not contain the MBR. Hence, if the bootable disk is floppy, this process is skipped and the boot strap loader is loaded in the memory.

If the bootable disk is not the floppy, the MBR finds and searches the partition table to load and scan all the extended partitions to find the primary partition that is marked as “active”. 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 that 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 and in the same sequence as shown above for the operating system to function properly. If any of these three files are not found, then the message, “”Non-system disk or disk error – Replace and press any key when ready” is displayed in the console, else the boot process continues.

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 mainly 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. You can load a different command interpreter by specifying the following in the Config.Sys file.

Command =

Here, is a command interpreter that would be used as the command interpreter in lie of the default command interpreter Command.Com.

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.