Floppy disks and disk images
A floppy disk is a thin flexible medium that reacts to magnetic fields. Electromagnetic induction is used to read the disk, and a magnetic field is used to write to the disk.
The surface is divided up into concentric rings, known as cylinders, and each cylinder is split into sectors. On the disks most often used by the Atari ST, there are around 80 cylinders and 9 or 10 sectors per cylinder. Each sector contains exactly 512 bytes of data.
The sector is located by spinning the disk around using the hub in the middle, and the cylinder is located by moving a read/write head back and forth. There are two heads, one for each side of the disk, and they are moved together as a single unit.
A floppy disk on its own is inflexible when it comes to data storage. Sectors are written and read individually, and they each have a fixed size and location. A disk is fine for the storage of a single file of data that will only be read, but for managing several files, different lengths, folders, appending to files and so on, a filesystem is used. On the Atari ST, a very slight modification of the standard MS-DOS 'FAT' filesystem is used.
The filesystem uses a few sectors at the beginning of the disk to store some file management information, and provides a way of storing directories and files that could be located anywhere on the disk. If the files have been repeatedly read and rewritten, then they may end up using little pieces of space from all over the disk, a situation known as fragmentation. A disk defragmentation tool rearranges files on the disk so that they occupy contiguous sectors.
A disk image contains the data from each sector of the disk, without regard for the meaning of that data, including the filesystem information from the beginning of the disk, The sectors are added to the disk image in a particular order; this order is usually side 0, track 0, all sectors in numerical order, then side 1, track 0, all sectors in numerical order, then side 0, track 1, then side 1, track 1, side 0, track 2, and so on.
The data stored in the disk image file may be compressed, and it may also include information about the size of the disk, checksums, occurrences of bad sectors, and other information about odd disk formats.
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