Secondary memory

The computer has secondary memory data and storage for the program which is not currently in use. In addition to screwed card and paper tape, early computers used magnetic tape for secondary storage. The tape is cheap, either in big reels or small cassettes, but its disadvantage is that it should be sequentially read or written from one end to the other.

IBM introduced the first magnetic disk, Ramac in 1955; It had 5 megabytes and was hired for $ 3,200 per month. Magnetic discs are platters coated with iron oxide like tape and drum. A hand with a small wire wire, reading / writing (R / W) head, basically runs on the disk, which is divided into data centered centered from small arcs, or areas of data. The magnetic field of the disk produces small currents in the coil because it passes, allowing it to "read" an area; Similarly, a small flow in the wire will induce local magnetic changes in the disc, which will "write" in one area. The disk rotates rapidly (up to 15,000 rotation per minute), and therefore R / W head can reach the disk faster than any area.

Initial discs were large removable platters. In the 1970s, IBM introduced sealed discs with fixed plaques known as Winchester Disk - probably because there were two earlier 30 megabyte platters, which suggested the Winchester 30-30 rifle. Not only was the sealed disk protected against dirt, R / W head could also "fly" on thin air film, which was very close to the platter. By putting the head closer to the platter, the area of oxide film that represents a bit, can be very small, thus increasing the storage capacity. This basic technique is still used.

computer hard drive

To increase the storage and data transfer rates, with the addition of R / W heads for two surfaces of each platter, many platitors in a disk drive include refining to add 10 or more. Due to even greater advantage the control of the radial speed of the disk arm has improved with the track hand, resulting in the density distribution of the data on the disc. By 2002, such a density had reached more than 8,000 tracks per centimeter (20,000 per track per inch), and the diameter of a plate could gigabytes of data. In 2002, the cost of an 80-gigabyte disc was $ 200- just about one millionth of the cost of 1955 and the decline in the price of main memory represented an annual decline of approximately 30 percent.

Optical storage devices- CD-ROMs (compact discs, read-only memory) and DVD-ROMs (digital video, or versatile discs) - appeared in the mid and late 90s of the 1980s. They both represent bits in the form of small pits in plastics, which are held in a longer spiral, written as a phonograph record and read with laser. A CD-ROM can hold 2 gigabytes of data. But by including the error-correction code (to correct for dust, small flaws and scratches), data can be reduced to 650 megabytes. DVDs are dens, small pits, and can hold up to 17 gigabytes with error correction.

The DVD player uses a laser which is high power and has a consistently better focus point compared to the CD player. This enables it to solve small pits and compressed separation tracks and thus has an account for more storage capacity of DVDs.

Optical storage devices are slower than magnetic disks, but they are well suited for making master copies of software or for multimedia (audio and video) files that are read sequentially. There are also writable and rewritable CD-ROMs (CD-R and CD-RW) and DVD-ROMs (DVD-R and DVD-RW) that can be used like magnetic tapes for inexpensive archiving and sharing of data.

The decreasing cost of memory continues to make new uses possible. A single CD-ROM can store 100 million words, more than twice as many words as are contained in the printed Encyclopædia Britannica. A DVD can hold a feature-length motion picture. Nevertheless, even larger and faster storage systems, such as three-dimensional optical media, are being developed for handling data for computer simulations of nuclear reactions, astronomical data, and medical data, including X-ray images. Such applications typically require many terabytes (1 terabyte = 1,000 gigabytes) of storage, which can lead to further complications in indexing and retrieval.