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The FAT filesystem By Andries Brouwer, 2002-09-20. The traditional DOS filesystem types are FAT12 and FAT16. Here FAT stands for File Allocation Table: the disk is divided into clusters, the unit used by the file allocation, and the FAT describes which clusters are used by which files. Let us describe the FAT filesystem in some detail. The FAT12/16 type is important, not only because of the traditional use, but also because it is useful for data exchange between different operating systems, and because it is the filesystem type used by all kinds of devices, like digital cameras. We'll see: the 512 bytes of the boot sector of a DRDOS boot floppy ( here the FAT12 version, that is also the common part of the FAT12, FAT16 and FAT32 boot sectors ); the File Allocation Table of FAT12, FAT16, FAT32; the directory entry; FAT under Linux ( Linux has support for FAT12 and FAT16 filesystems since 0.97. Linux has support for FAT32 since 2.0.34 ). To see the content of your disk, please use a debugger, like the DOS/Windows debug.exe. Remember: since the master boot code, in the MBR ( Master Boot Record ) is the first program executed when you turn on your PC, this is a favorite place for virus writers to target. There is always code in any disk ( hard or floppy ) that is formatted, whether or not the system files are present on the disk. Since the Volume Boot Code ( in the Volume Boot Sector ) is always executed when the system attempts to boot from a device, a virus can be present on a floppy disk even if you don't format it with the system files on it using "FORMAT /S" or the "SYS" command. As soon as you see the "Non-system disk..." message, the virus could already be in your system memory ( source: Reference Guide - Hard Disk Drives: http://www.storagereview.com/guide/index.html ).
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