The computer system consists of the primary memory, which is referred to as the semiconductor memory, which is high speed, high cost and low capacity. Secondary memory is low speed, low cost and high speed. The parameters of memory are however changing, to meet consumer and application demands.
Secondary memory provides a means of storing data over a long period of time. This type of memory can be further classified into direct access and sequential access. The floppy is a type of direct access and a magnetic tape a sequential.
Secondary memory types
Hard disk
Floppy drive
CD-ROM
CD-RW
Magnetic Tape, cartridge and cassette
Zip drive
Jaz Drive
Compact Tape
Magnetic disk Is a non volatile mass storage device, which allows fast access to its data, modify and pass it back to the processor of the computer. Disks achieve fast access because they use a identifier or key to identify a block or record, of data.
By using ferromagnetic materials, data can be stored onto the these materials to represent a logic 1 or a logic 0. In very simple terms a current passing through a magnetic material, generates a magnetic field, the direction of the current determines the direction of the field and hence the logic level.
Hard disk
The hard disk was developed in 1956 by IBM who introduced the Rampac 350 which stored 5MB of data in a 50 , 24 inch disks platters. A typical layout of a disk pack is shown in fig 1. The disks are made of aluminium, and coated with ferric oxide. Disk packs are usually used in large mainframe computers.
The hard disk is one of the commonest secondary storage mediums. The communication, i.e. read/write operations, between the hard disk and the computer is performed by the hard disk controller .
Data is stored on the surface of the disk in a circular pattern, known as a track. The collection of tracks that can be accessed without moving the heads is a cylinder.
Data is stored onto the disk by a mechanism which magnetises the surface of the disk. Typical rotation speed of a disk is 3600 rpm to 10000rpm.
Floppy Disk
Floppy disks were originally designed by IBM in the mid 1960s, and were fully introduced in 1970. The success of this system, led to the eventual replacement of punched cards. Unlike the fixed disk system, in the read/write head makes contact with the disk. The capacity, of an extra high density disk, is typical 2.8MB.
originally the disk came as a 5.25 inch disk, it comes in general as 3.5 inch.
CD-ROM
CD technology was developed in 1976, the result of a joint effort between PHILIPS
and Sony Corporation. The CD-ROM, is a high capacity, low cost storage medium, for read only, and appeared in 1984. The CD-ROM measures 120mm, has a 15mm hole for the spindle and a thickness of 1.2mm. Very simply the operation of reading from a CD-ROM, is essentially a laser shinning light onto land, or pit. The reflective light is received by a light sensitive diode. When light hits a pit, half the light falls on the pit and half onto the land. When this happens the resolution of light received is essentially zero. However when the light hits a land the all the light is received.
In order to over come imperfections in manufacture or other imperfections e.g. scratches on the surface, two error detecting systems maybe used:
CIRC ( Cross Interleased Read Soloman Code )
EFM Eight to Fourteen modulation
