DOS Boot Sector

When a floppy diskette is formatted (it doesn't matter if it's a system or data disk) the very first portion of the disk is set aside for two main purposes:

• storing information about the disk, and
• storing a short program that either puts a message on the screen saying the disk cannot be used to start the computer if it's a data disk, or a short program that starts to load the operating system if it's a system disk which can start the computer (boot disk).

This special sector is numbered 0,0 and is called the Boot Sector.

A hard disk also has a DOS boot sector, but it's located in a different sector on the hard disk (see note below). Other removable devices have DOS boot sectors that are defined by the format utility for that device. Bootable CD-ROMs, if infected by whomever wrote out the disc, could also be the source of an infection.

Since the DOS boot sector is executed every time you power on or boot your PC, it is very vulnerable to virus attack. Damage to this sector can make your disk appear to be unreadable. This sector is rewritten whenever you do a "SYS" or a "FORMAT /S" to a disk.

Warning: Even a non-bootable floppy can contain a virus in the boot sector. If you leave an infected floppy in your PC when you power on or boot, you will be infected even though the PC won't successfully boot from that floppy.

When a hard disk is formatted (FORMAT command) a boot sector, similar to that on a floppy diskette, is also created. Note: This boot sector should not be confused with the Master Boot Record (MBR) on a hard disk. In order to distinguish between the two, we've adopted the terminology of DOS Boot Sector (DBS) for the boot sector created by FORMAT on a hard disk.