Secondary Memory/Auxiliary Memory

All Computers contain both semiconducter memory as well as magnetic memory. The semiconductor memory is primary memory and the magnetic memory is used as secondary memory. The secondary memory is employed for bulk storage of programs, data and other information. The magnetic memory retains the information once stored in it till it is not deleted. They are cost effective. Some common secondary storage devices are hard disk, floppy disk, optical disk, magnetic disk, magnetic tape etc.

Hard Disk :- Hard disk is a part of memory unit, often called a "disk drive" , "hard drive" , "hard disk drive" , that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today's computers typically come with a hard disk that contains several billion bytes (gigabytes) of storage. It is a device used for mass storage needed for direct access.

A hard disk unit comes with a set of rotation speed varying from 4500 to 7200 rpm. Disk access time is measured in milliseconds. Although the physical location can be identified with cylinder, track and sector location, these are actually mapped to a Logical Block Address (LBA) that works with the larger address range on today's hard disks.

Floppy Disk :- The recording medium on floppies are a myler or vinyl plastic material with magnetic coating on one or both sides. These are called floppy because of their flexible physical property without any fear of loss of data. These plastic disks coated with magnetic material are permanently sealed in a square plastic jacket to protect them from dust and scratches.

Optical Disk :- This disk is made up of a resine, such as polycarbonate. It is coated with a material which changes its reflecting property when a high intensity laser beam is focused on it. The coating material is highly reflective, usually aluminium. The high intensity laser beam forms a tiny pit along a trace to represent '1' and the surface without a pit known as 'land' which represents '0' (Zero). For reading the data a laser beam of less intensity is employed.

Magnetic Disk :- A magnetic disk is a circular plate constructed of metal or plastic coated with magnetized material. Often both sides of the disk are used and several disks may be stacked on one spindle with read/write heads available on each surface. All disks rotate together at high speed and are not stopped or started for access purpose. Bits are stored in the magnetized surface in spot along concentric circles called tracks. The tracks are commonly divided into sections called sector. In most systems, the minimum quantity of information which can be transferred is a sector.

Some units use a single read/write head for each disk surface. In this type of unit, the track addeess bits are used by a mechanical assembly to move the head into the specified track position before reading or writing. In other disk system, separate read/write heads are provided for each track in each surface. The address bits can then select a particular track electronically through a decoder circuit. This type of unit is more expensive and is found only in very large computer system.

Magnetic Tape :- A magnetic tape transport consists of the electrical, mechanical and electronic components to provide the parts and control mechanism for a magnetic tape unit. The tape itself is a strip of plastic coated with a magnetic recording medium. Bits are recorded as magnetic spots on the tape along several tracks. Usually, seven or nine bits are recorded simultaneously to form a character together with a parity bit. Read/write heads are mounted one in each tracks so that data can be recorded and read as a sequence of characters.

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