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DOS Boot Sequence

The boot sequence begins with CMOS indicating which drive to scan first for a boot record. If the boot device is the C: drive, the Master Boot Record (MBR) is located and this tells which partition to boot to. If the drive is the A: drive, DOS Boot Record loads and control is passed to that.

The DBR loads the IO.SYS and MSDOS.SYS, then passes control on to it. IO.SYS loads the CONFIG.SYS, which has the settings for the environment and loads device drivers. Next, the COMMAND.COM is loaded which executes the AUTOEXEC.BAT, the displays the user interface. Thus, the DOS boot sequence is IO.SYS, MSDOS.SYS, CONFIG.SYS, COMMAND.COM, then AUTOEXEC.BAT.

The DOS boot sequence ends when the AUTOEXEC.BAT finishes and the user interface begins.

DOS Boot Errors

There will be several questions on the boot process and errors that may occur.

Non-system disk or disk error, replace and press any key - Most often a floppy disk is left in the drive to cause this error. Remove the disk to fix. If you receive this error with no floppy disks or CD-ROMs in the drives, it could mean a corrupt disk, try rebooting several times. If all else fails, recover any data you can and rebuild the machine.

Missing Operating System - The Master Boot Record couldn't find an operating system on the bootable partition. Could mean the O/S hasn't been installed, the disk isn't formatted, the boot partition is corrupt, or you could have a virus. Boot with a floppy, format the disk, or run a virus checker.

Bad or Missing COMMAND.COM - The COMMAND.COM is missing from the root directory. Copy it there from a floppy or reinstall DOS.

Error in CONFIG.SYS line # - Something is wrong with the CONFIG.SYS. Use a text editor (like EDIT) to edit the CONFIG.SYS and determine the problem.