WHAT ARE SYSTEM SECTORS? Sectors are not files but simply small areas on your disk that your /hardware reads in single chunks. Under DOS, sectors are most commonly 512 bytes in length. These sectors are invisible to normal programs but are vital for correct operation of your PC. You can not see system sectors on your diskette in the same way that you can see files. They are simply a special reserved location at the beginning of your diskette. DOS Boot Sectors The very first sector on disk or diskette that DOS is aware of is the boot sector. From a DOS perspective, this is the first sector on the disk. Every floppy and hard disk has a boot sector containing executable code. This sector contains an executable program whether the disk is bootable or not. Since this program 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 the 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. Partition Sector (AKA Master Boot Record or MBR) On hard (fixed) disk drives, the very first sector is the partition sector (also known as the master boot record or partition table). Each physical hard disk drive has one of these sectors. A single physical disk can be partitioned into one or more logical disks. For example, you may have a physical drive partitioned into C: and D: logical disks so that your single physical disk appears (to DOS) to be two logical disks. The single partition sector contains the information that describes both logical disks. If the partition sector is damaged, then DOS may not even recognize that your disk exists. The partition sector also contains a program which is executed every time you power up or boot your PC. This program executes and reads the DOS boot sector (or other operating system boot sector) which also contains a program. Numerous viruses plant their code in the partition sector. Copyright © 1995 Stiller Research. Document Last Modified 6/07/95.