There are different types of computer of varying size and power, including the following:
Supercomputer (the most powerful type of mainframe)
Mainframe (large, very powerful, multi-user i.e. can be used by many people at the same time, multi-tasking i.e. can run many programs and process different sets of data at the same time)
Minicomputer (smaller than a mainframe, powerful, multi-user, multi-tasking)
Personal computer (PC) (single user)
Desktop computer (suitable size for sitting on an office desk)
Workstation (most powerful type of desktop, used for graphic design, etc.)
Portable (can be carried around, can operate with batteries)
Laptop (large portable, can be rested on user's lap)
Notebook (size of a sheet of notebook paper)
Handheld (can be held in one hand)
Pen-based (main input device is an electronic pen)
PDA (personal digital assistant, has functions such as task lists, diary, address book)
Note that the term PC usually refers to an IBM compatible personal computer i.e. an Apple Mac personal computer is not referred to as a PC. A computer that provides a service on a network e.g. storing files, sharing a printer, is known as a server computer. Server computers usually have a UPS (uninterruptible power supply) attached to them. This is a battery that automatically provides an electricity supply to allow the server to shut itself down properly if the main supply fails.
The processor e.g. Pentium, is the most important part of the computer. It processes the data and controls the computer. Powerful computers used as servers often have more than one processor. There are two main types of memory:
a) RAM (random access memory) holds the program instructions and the data that is being used by the processor,
b) ROM (read only memory) holds the program instructions and settings required to start up the computer.
The combination of the processor and memory is sometimes referred to as the CPU (central processing unit), although sometimes the processor itself is referred to as the CPU. The other parts connected to the CPU are known as peripherals. These can include input devices, output devices, storage devices and communications devices. Input devices include: keyboards, scanners, barcode readers, digital cameras, microphones and video cameras e.g. webcams (small digital video cameras used on the Web). Output devices include: monitors (VDU display screens), printers, plotters, loudspeakers, headphones. Storage devices include: magnetic tape, floppy disks (diskettes), hard disks, CD-ROMs, CD-R disks, CD-RW disks, DVDs and MO disks. A common communications device is a modem (a modulator/demodulator used for converting digital signals to analogue signals and vice versa to allow a computer to be connected to the ordinary telephone system).
A set of connectors used for carrying signals between the different parts of a computer is known as a bus. Data is transferred constantly between the processor and memory along the system bus. Each part of memory has its own memory address and the processor determines where processed data is stored by sending an address signal along an address bus and data along a data bus. This is synchronised by an electronic clock in the CPU that determines the operating speed of the processor. Transferring data between the processor and RAM can slow up the computer; therefore, some very expensive, extremely fast memory is usually used as a cache to hold the most frequently used data.
In a desktop computer, the CPU (central processing unit) and storage devices (pieces of equipment used for reading from and writing to a storage medium) are normally built inside a system unit which consists of a metal chassis enclosed in a flat desktop or a tower shaped case. Other peripherals are attached to the system unit by cables. Each peripheral uses its own driver card or controller (an expansion card that is plugged into special expansion slots in the system unit). Expansion cards contain the electronics required to communicate with and control the device e.g. video or graphics cards are used for monitors, soundcards are used for audio input/output and NICs (network interface cards) are used for connecting to other computers in a network. Extra memory can also be added to the computer using special memory expansion slots inside the computer. A portable computer that does not have enough space inside to fit expansion cards may use an external device called a port replicator to provide connections for peripherals.
Storage devices in the form of a disk or tape are used to store the programs and data that are not being used. Before a program or data can be used, it must be transferred from the storage device to the main RAM memory. Hard disks consist of a set of magnetic coated metal disks that are vacuum-sealed inside a case to keep out the dust. The magnetic surfaces of the disks are formatted using a read/write head to provide magnetic storage areas. These storage areas form concentric circles called tracks and each track is subdivided into sections called sectors. The disks are rotated at high speed and read from or written to by the read/write head that moves across the surface of the disks. In server computers, hard disks can be connected together and made to operate as one unit using RAID (a redundant array of inexpensive disks). This can speed up the system and provide a way of recovering data if the system crashes (fails suddenly and completely, usually referring to the failure of a hard disk). There is a variety of optical storage devices that use laser light to read or write to a disk, including: CD-ROMs (compact disk read only memory), CD-R (recordable compact disk), CD-RW (rewritable compact disk), DVD (digital versatile disk - previously known as digital video disk).
An input device called a barcode reader is a special type of scanner for reading barcodes (a set of printed bars of varying thickness that are used to identify a product e.g. used to price items in supermarkets).
When comparing computers, the power of the computer is important. This is mainly determined by the speed and capacity (size) of each part of the computer.
Speed is measured in hertz (Hz) i.e. cycles per second.
Capacity is measured in bytes (B) where 1 byte = 8 bits (binary digits) = 1 character.