Chapter Computer systems

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7 nov. 2013 (il y a 7 années et 10 mois)

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Chapter 1 Computer systems
Computers are information processing machines. They process data to produce
information. The most common mistake made by people when they talk about
computers is to believe they are intelligent ‘thinking machines’. This could not be
further from the truth. Every computer must be told exactly what to do and how to
do it by a human. The instructions humans give computers are called programs or
software. Without software to tell them what to do, computers would be useless.
Fig 1.1 A designer working on a personal computer
Some of the reasons for using computers are:
• computers can work much faster than humans
• computers never get tired or need a rest
• computers can do jobs that it would be dangerous for a human to do
• computers can store large amounts of information in a very small space
• computers can find information very quickly
• computers never lose or misplace information.
Chapter 1
Input, processing and output
Whenever a computer is used, it must work its way through three basic stages
before any task can be completed: input, processing and output.
A computer works through these stages by ‘running’ a program. A program is a
set of step-by-step instructions which tells the computer exactly what to do with
input in order to produce the required output.
The input stage of computing is concerned with getting the data needed by the
program into the computer. Input devices are used to do this. The most
commonly used input devices are the mouse and the keyboard.
The program contains instructions about what to do with the input. During the
processing stage the computer follows these instructions using the data that has
been input. What the computer produces at the end of this stage, the output,
will only be as good as the input. In other words, if garbage has been input,
garbage will be output. This is known as GIGO, or ‘garbage in, garbage out’.
The output stage of computing is concerned with producing processed data as
information in a form useful to the user. Output devices are used to do this.
The most commonly used output devices are the screen, which is also called a
monitor or visual display unit (VDU), and the printer.
Computer systems
Data and information
Data is any collection of numbers, characters or other symbols that has been
coded into a format which can be input to a computer and processed. Data on its
own has no meaning or context. A computer processes data and turns it into
useful information. For example, the string of numbers 13568180320003600 has
no meaning. It is the correct processing of this data by the computer that
transforms it into the information that “Employee number 13568 worked 36 hours
in the week ending March 18th 2000”. Exactly the same numbers processed in a
different way could produce the information that “Student number 135 scored
68,18,3,20,0,36 and 0 in the last 7 tests”.
In the examination you could be asked to explain what is meant by the terms data
and information. Make sure you can:
• explain what each of these terms means – a key point to include in your answer is
that data has no meaning whereas information does have a meaning or context;
• give some examples of data and information.
Data types
There are many types of data. The main types of data that can be input into a
computer and processed are numeric, text, dates, graphics and sound. Data can
be input to a computer by the user in many different ways. Every type of data
ends up being stored as a series of numbers inside a computer.
Numeric data types are split into two different sorts. The first is integer, a whole
number which has no decimal point in it. The second is real. A real number is one
with a decimal point in it. In both cases the symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 are
used to represent the numbers.
Text data includes any character on the keyboard. Text data types are also called string.
Diagrams, pictures and scanned images can be stored on a computer in special
graphics files.
Dates are often treated as a kind of data on their own and stored inside the
computer in a special way that makes them easier to process.
Any sound can be recorded and stored in digital form on a computer.
Chapter 1
The parts of a computer
Hardware is the name given to any part of a computer you can actually touch.
An individual piece of hardware is called a device. The basic hardware of any
computer consists of a central processing unit (CPU) and main memory together
with input, output and backing storage devices. Any part of a computer other
than the CPU and main memory can also be referred to as a peripheral device.
Figure 1.2 The parts of a computer
The Central Processing Unit (CPU)
The Central Processing Unit (CPU) is the part of a computer where the searching and
sorting of data, calculating and decision-making goes on. The CPU is sometimes
described as the ‘brain’ of the computer but this isn’t really true because computers
aren’t able to think for themselves. The CPU contains the Main Memory, the Control
Unit and the Arithmetic and Logic Unit (ALU). The Main Memory holds the program
instructions and data. It contains two types of memory chip, called ROMand RAM
(the meaning of these terms will be explained later). The Control Unit fetches
instructions from the main memory, decodes them and causes them to be executed
by the ALU. The ALU performs calculations and makes decisions using these
Figure 1.3 The movement of data and instructions inside the CPU
Central Processing Unit
data flow
control flow
Types of computer
There are many different types of computer available today. These range from giant
supercomputers to small hand-held electronic personal organisers. Some of the
main types of computers in use today are described below.
Mainframe computers
Mainframe computers cost millions of pounds to buy and install. They can process
extremely quickly massive amounts of data, which is stored on hundreds of disk
drives. A mainframe can have hundreds of terminals (and users) connected to it at
the same time. The most powerful mainframes are called supercomputers.
Examples of organisations that use mainframes include the DVLA who use one
to store data about all of the drivers and cars in the country, and the television
licensing authority who use one to store data about all the households with a
TV licence and everyone who buys a new television. ATM machines all over the
country are linked to bank and building society mainframes, which store data
about every customer and their account transactions and balances.
Figure 1.4 A Cray mainframe computer
Computer systems
Chapter 1
Minicomputers cost tens of thousands of pounds to buy and install. They are
much more powerful than microcomputers but not as powerful as mainframes.
Medium-sized businesses use minicomputers to control their company network
and act as a central store for all the company’s data.
The microcomputer is the most common type of computer. Microcomputers are
used in the workplace, schools and homes. Microcomputers are usually called
desktop personal computers or desktop PCs.
A typical desktop PC system consists of:
• a base unit, or tower unit, that contains a CPU and backing storage devices
(normally a floppy disk drive, a hard disk drive and CD-ROM or DVD-ROM
• input devices, normally a mouse and keyboard;
• output devices, typically a screen, printer and speakers.
Figure 1.5 The parts of a personal computer
Figure 1.6 A laptop computer connected to a PDA
Computer systems
Portable computers
Portable computers are smaller than desktop PCs and can be easily carried
around. The most common type of portable computer is the laptop which, as the
name suggests, will fit comfortably on a user’s lap. Other types of portable
computer include palmtops and Personal Digital Assistants (PDAs); these are
much smaller than laptops and will fit in a shirt or jacket pocket. PDAs typically
offer users facilities such as a calendar, diary, address book and, in many cases,
access to e-mail and the Internet. These facilities allow users to manage their
schedules and messages without the need to carry their laptop computer around.
Both palmtops and PDAs can be linked to a user’s laptop allowing data to be
synchronised so that it is the same on both devices.
Chapter 1
1. Describe what happens during each of the stages of computing listed below.
(a) Input (2)
(b) Processing (2)
(c) Output (2)
2.Draw and label a diagram to show the parts of a personal computer.(6)
3.(a) Explain what is meant by the term data.(2)
(b) Explain what is meant by the term information.
(c) Tick one box next to each item in the table below to show
whether it is data or information.
(i) 04022002
(ii) 4th February 2002
(iii) it is a sunny day
(iv) day a it sunny is
AQA 2002 Paper 2 Tier H
4.Give one suitable data type for each item listed below.
(a) Date of birth (1)
(b) Height (1)
(c) Town of birth (1)
(d) Shoe size (1)
(e) Gender (1)
5.Every computer contains a CPU and main memory.
(a) What do the letters CPU stand for?(1)
(b) Why does a computer need a CPU?(2)
(c) Give two types of memory chip found in main memory.(2)
(d) Why does a computer need main memory?(2)
Web tasks
Computer systems
1.Visit the Free On-Line Dictionary of Computing (FOLDOC) at:
(a) Look up definitions of the keywords highlighted in this chapter.
(b) Prepare a summary list of keywords and definitions to use
later for revision.
2. Visit
(a) Read the article “Why people think computers can’t”.
(b) Prepare a summary of the article.
3. Visit the computer history museum at
(a) Research the history of computers.
(b) Construct a timeline to summarise what you find out.
4. Take a tour of Carl Friend’s minicomputer museum at:
5.In 1997 the IBM supercomputer Deep Blue played a chess match with the
world chess champion Gary Kasparov and defeated him.
Read about this match at