Grade 8: Lesson 5 -Primary and Secondary Storage

Primary Storage

This is a group of silicon chips that resides on the computer’s motherboard (main circuit board). Primary storage consists of two types of memory chips: RAM and ROM chips. A memory chip is an integrated circuit (IC) made up of millions of transistors and capacitors.

RAM

This chip is used to hold data and instructions (programs) temporarily while processing is taking place using that data and program. RAM is volatile i.e. it is temporary and changeable. If the power is turned off or the computer is rebooted (started up again) all the information held in RAM will be lost.

 

ROM

This chip holds data and instructions necessary for starting up the computer when it is switched on. It is used to store system-level programs such as the BIOS (Basic Input Output System) program.

 

Secondary Storage Media

Secondary Storage allows the storing of information of information permanently. Secondary storage devices are chosen for a particular use based on their:

• Storage capacity (how much data the device can store)
• Access speed ( the time needed to locate the data and transmit it to the CPU)
• Portability ( ability to be easily removed and used on another system)
• Cost
• Size (necessary for storage on shelves or portability)

Magnetic tape

A magnetic tape looks like an audiocassette tape. Tapes may come in different sizes. It is used mainly to backup hard disks because it can store large amounts of data at a low cost. Accessing data on a tape is very slow since data is stored sequentially.

Magnetic disks (Floppy disk, Hard disk)

All magnetic disks provide direct access to stored data. This means that you can go directly to specific piece of data without having to access any other either before or after the data you want.

Floppy disk

This is also called a diskette. It is a removable, flexible plastic disk, coated with a magnetisable material. The disk is held in a plastic case, usually 3.5 inches by 3.5 inches in dimension. The plastic case protects the disk from dust and grease. The floppy disk is used to store and transfer small amounts of data between computers. A 3.5 inch floppy disk holds up to 1.44 MB of data.

Hard disk

A hard disk is a thin but rigid, inflexible disk made of highly polished metal. The surface of each side of the disk (also called platters) is covered with a substance that can be magnetized.

 

Optical Disks

Optical disks are disks that are read by laser lights. They are made from plastic. The data is stored on layers inside the plastic. CDs and DVDs are the two main types of optical disks.

Compact Disc

A Compact Disc (CD) is an optical disc used to store digital data. The physical dimension of a CD is 12 cm or occasionally 8 cm in diameter. CDs can hold up to 750 MB. They are three types of CDs: CD-ROM, CD-R and CD-RW.

CD-ROM (read only)-Storing multimedia (text, graphics, sound and videos). Storing software packages for sale or distribution e.g. application software packages such as encyclopedias, word processors, training programs, games and graphics packages.

CD-R (recordable)-Storing large volumes of data that does not need to change e.g music CDs.

CD-RW (re-writeable) – Storing large volumes of data that change frequently e.g backups copies

DVD

“Digital Versatile Disc” commonly called “DVD” is an optical disc storage media that can be used for storing movies with high video and sound quality. DVDs look like compact discs, their physical dimensions are the same (12 cm or occasionally 8 cm in diameter). DVDs are encoded in a different format to CDs and they have a much higher density (more data is stored per unit area). Some DVDs are double sided i.e. they can hold data on both sides. DVDs can hold between 4.7 GB and 17 GB of data. Three types of DVDs are the DVD-ROM (read only), DVD-R (recordable) and DVD-RW (rewritable).

USB Flash drive

USB Flash drives are storage devices which consist of a small circuit board encased in a plastic or metal casing built with a USB interface. They are typically small, lightweight, removable and rewritable.  Flash drives are more compact, generally faster, hold more data and may be more reliable (due to their lack of moving parts) than floppy disks.

Flash Memory Cards

These come in the form of a card shaped like a stick of chewing gum, with dimensions 21.5 x 50 x 2.8 mm and storage capacities up to 128 GB.  They are used in PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras, mobile phones and video game consoles.