Two Main Categories

The two main types of computer drives used for storage are Magnetic Drives and Optical Drives. Under the topic of magnetic storage the two main types of drives that will be discussed are Hard Disk Drives and Floppy Disk Drives. Under the topic of optical storage the two types of drives that will be discussed are CD-Rom Drives and CD-R Drives. For both topics, other types of drives will be discussed briefly.

Magnetic Drives

A disk that uses magnetic storage stores data on a physical disk by magnetizing some particles, but not others. In a blank magnetic disk the particles which have an oxide- based surface coating are in a random order with no inherent meaning. The particles are aligned by a read-write head, which magnetizes the particles.

A read-write head of a magnetic disk drive has a positive and negative pole, as well as the particles. When the head passes over a particle, the positive pole of the read-write head attracts the negative pole of the particle, which causes the particle to line up with the read-write head. If the read-write head reverses its positive and negative ends, then it will line up particles in the opposite direction.

The distance between the disk surface and the read-write head is less then 0.1 microns. To put this into persepective, a single human hair is 50 microns, which is 500 times larger then the gap between the disk and the read-write head.

Magnetic storage is much more fragile than optical storage. Magnetic disks can lose data if they come into contact with most devices possessing a magnetic field. This can disrupt the particles. As well, magnetic storage will gradually degrade over time as the particles use their magnetic charge.

Hard Disk Drives

A hard drive is the main storage device in almost all computers. This drive consists of a series of platters, which are frisbee-like disks on which the data is stored. These disks are stacked one on top of the other, forming a cylinder held together by a center shaft, called a spindle, on which they sit.

Each platter has two surfaces, and each surface has a read-write head. Each platter is split into a series of circles, called tracks, and each track is split into a series of sections known as sectors. Each sector is a group of data blocks. The reason for tracks and sectors on a platter is for addressing and accessing information on a disk so that you know where the data is located.

When you look at the specifications of a hard drive, they always have several seek times which are used to define the speed of the different operations of the drive. The "track-to-track latency" is the time it takes for the read-write heads to move to a specific track on the disk. The "rotational latency" is the time it takes for the data blocks sought on the disk to rotate under the read-write head. The speed at which the platter rotates is referred to as rpm (revolutions per minute). The faster the disk rotates, the quicker the read-write head can retrieve the information for which you are looking.

Data on disk is stored in groups called "clusters". A cluster is a group of logically related data. When a file is stored on disk, it is stored in a series of clusters. The file can use any available clusters on the disk. They do not have to be side-by-side. All the file system has to do is keep track of which clusters belong to a specific file. One of type of these file systems is "The Fat File System".

The applet following demonstrates a Fat File System receiving a file of a given size, then determining which clusters, and how many, can be used to store the file. Fat maintains the list of clusters that belong to a file and their physical location on disk. This applet uses as an example a hard drive with a 16-bit fat, and clusters of size 4K.

Instructions and The Fat File System

Click here for instructions!

Floppy Disk Drives

These drives are very similar to computer hard drives, except that each floppy disk only consists of one platter, and the floppy disk is portable storage since it can be removed from the floppy disk drive and replaced with another disk. Floppy disks today come in the basic standard 3.5 inch diskette, which is a high-density disk that holds 1.44 MB of data, which is written on both surfaces of the disk.

The parts of a floppy disk are the diskette which is the data disk housed inside a hard plastic cover, which prevents it from being bent. There is a movable cover on the edge of the plastic cover that, when the disk is inserted into a drive, pushes to the side, allowing the read-write head to have access to the diskette.

The part of the floppy disk drive is a motor which turns the diskette inside of its plastic case. There is also another motor, called a stepper motor, which moves the read-write head back and forth accross the tracks of the diskette.

Floppy drives also store data on them using clusters. The difference is that they use a 12-bit Fat system and smaller cluster sizes.

Optical Drives

Optical storage is data that is permanently burned into a disk with a laser light. A blank disk consists of a reflective surface. The burns put on the disk cause that spot to become a dark non-reflective spot. It is these small pits in the disks surface that represent the data. The lands are the reflective surfaces on the disk.

The reason why pits and lands are important is that laser light is used to read data off of the disk. When the laser stikes a pit, it has less distance to travel than when it strikes a land. The reflected laser light is picked up by a photo-electric cell. The different light picked up by the photo-electric cell produces a varying current. This allows it to distinquish between a pit and a land, which allows it to read the disk.

The laser moves accross the surface of the disk while a motor spins the disk. The laser reflected from the disk is seen by a light sensor, which then transfers the signal into a current.

Optical storage disks are much more durable than magnetic storage disks: coming into contact with magnetic devices does not disrupt them, if they get dirty they can be cleaned, and they do not degrade over time, since there is no magnetic charge for them to lose.

CD-ROM Disk Drives

The term CD-ROM stands for "Compact Disk Read Only Memory". Read only means that the pits on the disk, which represent the data stored on the disk, are permanent and cannot be changed. Therefore, a CD-ROM cannot have new data saved on it. The data that was on it when you purchased it is permanent.

CD-ROM disks are fairly cheap to manufacture and can store up to 680 megabytes of data. A CD-ROM disk has several layers. The bottom layer of the disk contains a plastic substance which has pits and lands on it. Bellow this layer is a reflective coating, which is usually aluminum, that coats the pits and lands. On top of the reflective coating is a protective lacquer coating.

What year and from what companys was the very first CD-ROM player released. Answer

CD-R Disk Drives

CD-R stands for "Compact Disk-Recordable". This is a compact disk on which you can write data. Similar to a regular CD-ROM, the data on the disk cannot be changed. Once the data has been stored on a CD-R, it can no longer be changed. A CD-R, once recorded, can be used in a regular CD-ROM drive.

The CD-R is made up of a layer of gold covered by a layer of dye, which replaces the aluminum layer that is on the CD-ROM. To write to the CD-R, the laser beam burns a pit into the dye layer on the disk. This reduces how reflective the spot on the disk is. During reading, the laser light does not reflect back to the photoelectric cell when it passes over one of these burnt spots.

Other Types

The CD-RW stands for "Compact Disk-Rewriteable". This type of disk allows you to not only write data to a cd, but to also rewrite the data on the CD-RW many times. CD-RW's work on the principle that the reflectivity of spots on the disk can be changed. This is done by heating the material on the disk. Heating a particular spot at exactly the right temperature can return the spot to being reflective, which allows it to be rewritten.

Hard Drives Information on all types of drives http://www.harddrives.org/

References