• It keeps its data even after the power is turned off.

• New data cannot be entered into ROM.

ROM chips have programs already loaded into them before they are put into the PC. These are programs which the machine uses frequently and needs to have on hand. ROM is typically used for software which translates program languages into the code of the machine. ROM is ideal for programs which have to be used repeatedly without any modification.

Nowadays, floppies are mostly being used for data storage. Like an audio or video cassette, a floppy is used to record data, and, likewise, existing data can also be erased by recording new data on it.

There are three main types of ROM. When a manufacturer preprograms a ROM at the time when a device is manufactured, it is called a masked ROM. This type of ROM cannot be changed by the user. A Programmable ROM (PROM) is supplied without an inbuilt program. However, once a PROM has been programmed, it cannot be changed. The Erasable PROM (EPROM) is a modification of the PROM, and offers the facility of reprogramaming.

Depending on the size of the memory and the processing speed, there are 4 sizes of computers.

- A Micro Computer is small, and is the most common computer.

It is single-user driven.

- A Mini Computer can support 12-30 users at once.

- A Mainframe Computer services several hundreds of users. These are used by large organizations.

- A Super Computer is used for highly complex scientific tasks.

DISK DRIVES

Using tapes are slow. A faster way of saving information is by the use of a machine called a disk drive. A disk drive is a machine, that among other things, reads 0's and l's that are magnetically encoded onto disks. This information then goes to the memory. A disk system provides quick and direct access to specific information located anywhere on a disk. Flexible magnetic disks, called floppy disks, are widely used to provide input to personal computers.

FLOPPY DISKS

Floppy disks are being used today not only for the transport and exchange of data, but also for storing data.

It is made of mylar or polyurethane coated with metal oxide. The inside of the jacket is lined with a feltlike material that cleans and sometimes lubricates the disk as it turns..

The Index hole on the floppy helps the PC to count disk rotations.

The Write-Protect notch is covered with a tape, if data stored in a floppy is only to be read, and not altered.

The access hole allows the drive to read and write data.

The alignment notches fit the disk snugly into the drive.

When the disk drive is working, the red drive light glows. This indicates that the recording head is reading or writing on the disk.

Hard disks

Usually hard disks of 10 Mb are used. This means that they can hold 10 Mb of data, which is equivalent to 3000 pages of single spaced type-written text. Hard disks range from 5 Mb to 70 Mb. Whereas a floppy is organised into 40 tracks and 8 or 9 sectors, the hard disk is organised into 306 tracks and 17 sectors.

Modems are devices that allow computers to communicate with other computers by using telephone lines. A modem translates sounds into tones that represent binary numbers. It can sead the tones over the phone lines to other modems. Other input devices are a joystick and a mouse for moving figures about on a screen and a

tablet consisting of a pad and a special pen for producing illustrations. Voice activators enable computers to understand spoken words.