Disk Physics

A floppy disk is a round sheet of plastic coated with particles of rust (iron-oxide) and enclosed in a protective case. A hard disc is conceptually the same, except it consists of multiple sheets (platters) made of metal.

Each sheet/platter is divided into tracks of data. Unlike a CompactDisc™ (which has one long spiral for a track), the tracks on a floppy or hard disk are a set of concentric rings.

Floppy disks also have an index hole. This allows for the start of track to positioned precisely on disk, but is not required.

For a single-sided floppy disk, the data is recorded on the bottom layer. Naturally a double-sided floppy disk uses the top layer as well; the bottom layer is conventionally called side 1 and the top layer is called side 2. Some software may refer to them as sides 0 and 1 respectively.

A floppy disk rotates at a nominal speed of 300 RPM (revolutions per minute). That's 5 revolutions per second, or 0.2 seconds per revolution. Or said another way, 200 milliseconds per 360 degrees. If a track happened to contain 20 sectors, each sector would have an average span of 360/20 = 18 degrees. Such a sector would 'appear' under the disk head for about 200/20 = 10 milliseconds.

A 1540/1541/1551/1570/1571 disk-drive uses 5¼-inch floppies and can access 40 tracks per disk-side, although the standard CBM format only uses the first 35 tracks per disk-side. The 1571 is the only double-sided drive by CBM that uses 5¼-inch floppies; it uses 35*2 = 70 tracks. Each track has 17 to 21 sectors, depending on zone.

The 1581 disk-drive uses 3½-inch floppies and has 80 tracks per disk-side. All 1581 formats are double-sided and thus consume 160 physical tracks, although the DOS presents these as 80 logical tracks. Each physical track consists of 10 physical sectors which map to 20 logical blocks. Each logical track thus consists of 40 blocks (20 each side).