A cylinder is any set of all of tracks of equal diameter in a hard disk drive (HDD). It can be visualized as a single, imaginary, circle that cuts through all of the platters (and both sides of each platter) in the drive.
A platter is a thin, high-precision aluminum or glass disk that is coated on both sides with a high-sensitivity magnetic material and which is used by a HDD to store data. Modern HDDs contain multiple platters, all of which are mounted on a single shaft, in order to maximize the data storage surface in a given volume of space.
The magnetic media on each side of each platter is divided into a series of tracks. A track is any of the concentric circles over which one magnetic head passes while it is stationary but the platter is rotating at high speed. A magnetic head, also commonly referred to as just a head, is a small, high-sensitivity electromagnet that is used for reading and writing data on the magnetic media. Each platter requires two heads, one for each side. All heads in a HDD are mechanically linked and move in unison, and consequently they are always in a common cylinder.
Each track, and thus each cylinder, on a modern HDD has a width of only a few microns (i.e., millionths of a meter). There can be tens of thousands of tracks on each platter, and thus the same number of cylinders in the HDD.
BIOSs (basic input output systems) on older computers were unable to read past the first 1024 cylinders of the primary HDD. A BIOS is a small program that controls a personal computer's hardware from the time that the power is turned on until the operating system is loaded into memory and takes over. This limitation added to the complexity of installing multiple operating systems on a single computer; fortunately, it does not exist in newer BIOSs.
The term cylinder as used with regard to HDDs is a slight misnomer and thus may lead to some confusion for those not yet familiar with the internals of HDDs. This is because the conventional definition of a cylinder is a solid object with a circular cross section, such as a piston in an internal combustion engine, whereas the meaning in a HDD context is only a cylinder's outer curved surface and exclusive of its interior.
Created March 19, 2006.