Functions of secondary storage
Secondary storage also called auxiliary storage or backing storage is used to store data and instructions when they are not being processed. Secondary storage is much cheaper than primary storage and is also used to back up for later use.
Storage media are the physical hardware on which a computer keeps data, instructions and information. E.g. a CD, a DVD, a floppy disk, hard disk.
Storage devices record and retrieve data, instructions and information to and from storage media.
Serial access means the records are accessed in the order in which they were stored. To read a serial file a computer has to read each record until it reaches the one required.
Sequential access means reading or writing data consecutively. With sequential access the records are in a particular order and the records are accessed in sequence. Magnetic tape is accessed sequentially.
Direct access or random access it is possible to go directly to a record without having to look at any other records. Magnetic disks, optical disks
The Zip disk is a medium-capacity removable disk storage system that was introduced by Iomega in late 1994. Originally, Zip disks launched with capacities of 100 MB, but later versions increased this to first 250 MB and then 750 MB. It is a small portable disk. Zip drives are used to backup important data.
* Inexpensive (general)
* Can transfer large files from one computer to another
* Cost more than DVd’s or CDR’s
* Not many computers have zip drives any more
Magnetic tape is a ribbon of plastic material coated with a metal oxide film on which data is recorded as magnetized or unmagnified spots. The magnetized spots represent ones and the unmagnetized spots represent zeros. In microcomputers tapes are stored in cartridges. On mainframe computers the tape is stored on reels or cartridges. Magnetic tape cartridges can hold up to 1.6TB
* Easy to transport and store
* Used for backups or archives
* Sequential access
* Limited shelf life- 2 years
* Must be stored properly in a suitable environment away from smoke dust and humidity * Difficult to update files(cannot make a change to a record without saving over the entire tape)
The diagram for representing data on magnetic tape is in the textbook page 10 chapter 3.
A 3.5 inch floppy disk is a removal flexible plastic disk coated with a magnetized material. The disk is contained in a hard plastic case. The term floppy refers back to the 5.24 “diskettes that were used in the 1980’s and were actually soft and flexible. Each disk contains a number of concentric rings known as tracks. The tracks are divided into pizza slices called sectors. Data is stored as magnetized or unmagnetized spots. They are used to transfer data between computers and for backup of files. They are slow to access because of slow rotation speeds. They hold 1.44 MB hold data.
* Direct access
* Suitable for backup
* Erasable, reusable and rewritable
* Small storage capacity
* Limited shelf life
* Slow read/write speeds
Before a disk is used to stored data it must be formatted. Formatting is the process of preparing a disk to receive and store data. When a disk is formatted the following happens: * All data on the disk is from a erased
* The surface of the disk is checked for any physical and magnetic defects.
Care of floppy disks
* Keep your disks away from liquids, dust, dirt
* Do not bend disks or handle them roughly
* Take care when removing or inserting a disk from a computer’s disk drive * Keep disks in a disk box
* Do not leave in the drive
Fixed Hard disk
Thin rigid, inflexible disks...
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