As you read in previous lesson that computer have Primary memory or internal memory. Now, let us see the secondary memory.Secondary memory is also called External memory of a computer. We use secondary memory devices to store data and information permanently. There are many types of secondary devices available. Some of secondary memory devices are given below:
The information contained in the tape can be accessed only serially. You can read the information any number of times. You can read and write in this tape. It is a plastic tape coated with iron oxide. There are several tracks on a tape length, normally 9 tracks on a tape assumed to be ideal. Totally length of a tape varies from 1200 feet to 3600 feet. depending on the storage capacity of the tape. In normal cases, the width of the tape is 1/2 inch.
Floppy disk is one of the important auxiliary (External) memory devices. It is flexible plastic sheet, rounded in orientation. Plastic sheet is coated with magnetic material either zinc oxide or iron oxide. This rounded sheet is kept in a plastic jacket, which is of square geometry.This external plastic jacket protects the floppy from environmental effects such as noise, dust, sunlight etc. Normally , floppy disk are available in two sizes, i.e. 5.2″ and 3.5″ Inches. Data storing capacity of a 5.25″ inch floppy is 1.2 MB whereas 3.5″ inch floppy can store 1.44 MB data. 3.5″ inch floppy is becoming more and more popular due to its size / capacity ratio. The information can be stored on one or both sides of a disk. Some double sided disks are used for storing data in both sides. These disks are called double sided while those which use one side are called single are sided disk.It can be inserted and removed when needed.
Hard disk is the most important secondary storage device. It is permanently fixed in the system unit. It provides high storage capacity and also provides fast access to information. There are one or more circular disks with storing capacity in both surfaces .It spins continuously .These disks are coated with magnetic material. Information is recorded on surface by magnetic heads as small magnetic spots. Information is recorded in bands on different disks. Each band of information is called track. Each track is divided into eight sections these sections are called sectors.
CD-ROM (Compact Disc Read Only Memory)
Optical disk or CD-ROM is very popular now a day.These disk do not work on magnetic mechanism.You must have read about CD , The compact disk . This is similar to the record player we used to listen music. When the needle passes through these portions, we get the music. If we use it frequently, it will get worn out. To get better quality music, CD players have been developed.
It is a circular plate made of polycarbonate and is about 120 mm diameter and 1.22 mm thick with a hole in the center. CD-ROM have a spiral track moving outward from the center to the periphery where data is digitized and stored.
CD-ROM are useful for distributing large data bases, specially those combining text , image and sounds.
Despite of importance ,there are limitations also. We can write it only one and it can not be erased.