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Types of computer
Computers can be categorised into four types
¨ Microcomputers
desktop, laptop, notebook and palmtop personal computers (PCs)
used in businesses, schools/colleges, hospitals, booking systems and homes range in price from a few
hundred pounds to a few thousand
¨ Minicomputers
often used as multi-user systems, with hundreds of workstations or terminals attached to a central minicomputer, e.g. EPOS
cost from £10,000 to about £150,000.
¨ Mainframe computers
used by large organisations may have thousands of terminals, often remote cost ££ hundreds of thousands
¨ Supercomputers
largest category of computer used mostly by scientific and industrial research departments, NASA, the Weather Centre, stock
exchanges cost ££ millions
CPU
controls the transmission of data from input devices to memory
processes the data held in main memory
controls the transmission of information from main memory to output devices
Main memory
Instructions and data are held in main memory, which is divided into millions of individually-addressable storage units called
bytes.
One byte can hold one character, or it can be used to hold a code representing, for example, a tiny part of
a picture, a sound, or part of a computer program instruction.
The total number of bytes in main memory is referred to as the computer’s memory size. Computer
memory sizes are measured as follows:
1 Kilobyte (Kb) = 1000 bytes (to be exact, 1024 bytes)
1 Megabyte (Mb) = 1,000,000 (1 million) bytes
1 Gigabyte (Gb) = 1,000,000,000 (1 billion) bytes
1 Terabyte (Tb) = 1,000,000,000,000 (1 trillion) bytes
Random Access Memory (RAM)
‘ordinary’ memory
used for storing programs which are currently running and data which is being processed.
This type of memory is volatile - it loses all its contents as soon as the machine is switched off.
Read Only Memory (ROM)
non-volatile, with contents permanently etched
used for example to hold the bootstrap loader, the program which runs as soon as the computer is
switched on and instructs it to load the operating system from disk into memory.
Cache memory
very fast memory used to improve the speed of a computer, doubling it in some cases
acts as an intermediate store between the CPU and main memory
stores the most frequently or recently used instructions and data for rapid retrieval
generally between 1Kb and 512Kb
Disk storage
Hard disks
all standalone PCs have in-built hard disk
typical capacity for Pentium PC is 1–5Gb
used for storing software including the operating system, other systems software, application
programs and data
Floppy disks
thin sheet of mylar plastic in hard 3½” casing
two standard types
double density with 720Kb
high density with 1.44Mb
Hard disks for microcomputers
The hard disk used with PCs consist of one or more disk platters permanently sealed inside a
casing
Hard disks typically have a capacity of between 500Mb and 5Gb (1Gb = 1,000Mb)
External hard drives which can be plugged into a microcomputer are available as extra storage
Hard disks for minis and mainframes
For large-scale applications storing huge amounts of data, several hard disk units will be required.
The disks may be either fixed or removable
Fixed disks are faster, more reliable, and have a greater storage capacity.
¨ Data is stored on concentric tracks, with tracks being divided into sectors.
All the tracks that are accessible from one position of the read-write heads form a cylinder
¨ Data is recorded cylinder by cylinder
to minimise movement of the read-write heads, thereby minimising access time.
Magnetic tape
¨ Data is recorded in ‘frames’ across the tape
with one frame representing one byte.
¨ The frames form tracks along the tape
with 9 tracks being common, giving 8 data tracks and one parity track
1 0 1 Track 1
1 1 1
0 1 1
0 0 0
1 1 0
1 1 0
0 0 1
0 0 1
0 0 1 Parity track
Magnetic tape is a serial medium
Uses of magnetic tape
A cheap and convenient medium for backup
Also used for archiving past transactions
or other data that may be needed again
such as weather records collected over years.
Cartridge tape drives
¨ in common use for backing up the hard disk of personal computers
much more convenient than dozens of floppy disks
¨ Cartridge tapes storing up to 7Gb can be bought for under £10, or about £50 for a 20Gb tape
CD-ROM
¨ Store around 680Mb of data
equivalent to hundreds of floppy disks.
Data may be in the form of text, graphics, photographic images, video clips or sound files.
Do not transfer data as fast as a hard disk drive
but speed is increasing every year and is acceptable for most applications.
¨ Read-only
Laser beam burns tiny holes in the surface of the disk, which has a single spiral track divided into sectors.
To read data from the disk, a laser beam is reflected off the surface, detecting the presence or absence of
pits which represent binary digits.
WORM disks
Write Once, Read Many optical laser disks look similar to CD-ROM disks, but they are often gold rather than silver in
colour
An end-user company can use these disks to write their own material, typically for archiving or storing
say, graphic or photographic images which will not be changed.
Widely used for pirated software
Magneto-optical disks
Integrate optical and laser technology
Read and write storage
A 5½” disk can store up to 1000Mb.
May in future replace current magnetic disks, but currently too expensive, slow and unreliable
Microfiche
¨ COM (Computer Output on Microfilm) devices
used to prepare microfiche, a 4” by 6” hard-copy film that is sometimes seen in libraries and bookshops
being used for reference purposes.
¨ A microfiche sheet has up to 270 frames
each containing a page of information which can be read using a special viewer.
¨ COM is also extensively used for archiving material such as old cheques or income tax returns.
Printers
Come in all shapes and sizes. Type of printer chosen will depend on:
volume of output
quality of print required
location of printer
requirement for multiple copies
requirements for colour
Types of printer
Dot matrix
¨impact printer, print head with 9 - 24 pins strikes paper through ribbon
‘near letter quality’ (NLQ) by printing line twice, but takes twice as long to print. Many are ‘bidirectional’.
versatile, most can print in condensed, standard and enlarged mode, in ‘bold’ or normal print,
some graphics
noisy
Ink jet
non-impact printer, fires ink droplets at the page
compact, quiet, cheap and high resolution but slow
large coloured areas can get wet, buckle, and smear
colour ink cartridges expensive
Laser
toner (powdered ink) fused to page by heat and pressure
increasingly popular, price falling
output of very high quality at up to ten pages per minute
virtually silent in operation
toner cartridge expensive but lasts 5,000 copies
Plotters
¨produce high quality line drawings such as building plans or electronic circuits. Pen (vector plotters) or penless (raster plotters)
Visual display unit (VDU)
Three basic attributes: size, colour and resolution.
It has its own fixed amount of RAM associated with it to store the image being displayed on the
screen: the amount of RAM will determine the resolution and the maximum number of colours that
can be displayed. Note that:
The resolution is determined by the number of pixels (addressable picture elements) used to
represent a full-screen image;
The number of colours that can be displayed is determined by how many bits are used to represent
each pixel.
Installing hardware devices
When you add a new device such as a printer, scanner, mouse, soundcard or videodisk player to a computer system, you have
to install a piece of software called a device driver to control the device.
Example
When you attach a new printer to a computer, you have to do the following before you can print anything:
Install the printer-driver file for the printer
Select the port you want to assign the printer to
A printer-driver file is a piece of software that specifies information about the printer, including details about
printer features
descriptions of fonts (sizes and styles)
control sequences that the printer uses to produce various formats, accents etc.
When you give an instruction to print, the driver translates the information about fonts, formatting,
highlighting etc. into a form that the printer can understand.
© LEV
