View
212
Download
0
Category
Preview:
Citation preview
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
1 Computer systems, communications and software
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
Components of a computer system and modes of use1.1
2 Module 1
A “computer system” is made up of hardware and
software. In this chapter we explain the terms relating
to a computer system that will be used throughout the
rest of the coursebook.
Definitions and purposes of hardware devicesThe hardware is the physical electronic and electrical
components that make up the computer system. For
example, these include the motherboard, a keyboard
and a monitor. In this section, we explain terms relating
to hardware devices that we use throughout the rest of
the text.
Peripheral devices are hardware devices that
are outside the central processing unit (CPU). They
are normally connected to the computer by internal
wiring (buses), cables or wireless technology. A
printer is obviously a peripheral as it is separate from
the computer. A hard disk drive is also a peripheral
because it is not part of the CPU. This is less obvious
because it is in the same box as the computer.
Most peripheral devices are one of three types:
● An input device transmits data from the user to
the computer processor. A computer is only of
value if it is possible to give it data and processing
instructions.● An output device conveys the results of processing
from the computer processor to the user.● A storage device can store data outside the
processor. When a computer system is switched off,
data in the processor are lost. Data that are needed
for future use must be saved to a storage device
Figure 1.1.1 shows a typical set of hardware in a
computer system. The arrows show the direction of
T
1.1 a, b
flow of data and information. The storage device is both
an input device and an output device.
Figure 1.1.1 The typical hardware of a computer system.Fi 1 1 1 Th i l h d f
input device output device
main memory(hard disk drive)
processor (CPU)
storage device
In Chapter 1.4, we look at various types of hardware
in more detail.
Definitions and purposes of software typesSoftware is the set of instructions that make the
computer hardware usable. These sets of instructions
are known as “programs”. So, a computer program is a
piece of software. It is important to distinguish between
operating system software and generic applications
software.
Operating system software is the set of instructions
that make the hardware of the computer available for
use. It includes programs:● that allow the user to communicate with the
computer● that control the movement of data around the
computer● that turn instructions that people can understand
into instructions that a computer can recognise.
Applications software is a set of instructions designed
to make the computer do something for the user. For
example, the user might want to write a letter, create a
report or create a slide show presentation.
1.1 a, c
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
1.1 Components of a computer system and modes of use 3
Software that can be used for many tasks is
called generic applications software or general purpose
applications software. Microsoft Office is a suite of
generic applications, including Microsoft Word (a
word processing application), Microsoft Excel
(a spreadsheet application) and Microsoft PowerPoint
(a presentation application).
Other applications software carries out a specific
sequence of tasks. For example, a payroll application
can process payroll data, print payslips and
generate BACS payments. It cannot process electricity
billing data.
In Chapter 1.2, we discuss types of system software
in more detail. You will notice that applications
software is covered throughout much of the rest
of the syllabus, which should give you an idea of
its importance. Chapter 1.7 looks specifically at
applications software.
Summary
● Hardware is the physical components that make up the computer system.
● Software is the set of instructions (programs) that make the computer hardware usable.
● An input device is a physical component that transmits data to the computer.
● A storage device is a physical component that stores data for use by the computer.
● An output device is a physical component displays data transmitted from the computer.
● Operating system software comprises the programs that make the computer hardware available for use, often
unseen by the user.
● Generic/general purpose applications software comprises programs that can be used for many tasks, such as a word
processor, spreadsheet software and database software.
Test yourself1. Explain the difference between hardware and software. [2]
Hint
The question mentions two items (“hardware” and “software”) and two marks are available.
Therefore, you should make two points. Make sure that the second point is not just the opposite
of the first. The word “explain” indicates that the two points need to say how the items differ.
2. Give two reasons why a computer system would need to have some type of external storage device. [2]
Hint
Notice that the question asks for reasons why the system needs external storage. Do not say
what is stored or what sort of device might be used. There are two indicators in the question
that you should make two points: the word in bold and the marks awarded.
3. Describe the difference between system software and application software. [2]
Hint
This question is similar in style to Question 1. Remember that you need to state two distinct
points that are related to each other.
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
System software1.2
4 Module 1
● The OS provides a platform on which the
applications software can run.● The OS hides the complexity of the hardware from
the user.● The OS controls access to the computer system so
that you can put passwords on your files to stop
other people seeing them. It controls the access
rights of different users.
Whether or not a certain feature is classified as
part of the operating system or as a utility program
varies considerably across operating systems. We
consider utility software in more detail at the end of
this chapter.
Types of operating systemBatchWhen computing was still a new science, there
were not enough machines to satisfy the demand
for processor time. There was a “speed mismatch”
between the user sitting at the keyboard, who was very
slow, and the processing by the computer, which was
very fast. One solution to this problem is to buy more
machines. Another solution is to make the machines
work more effectively by taking away the slowest part
of the system – the human being. Nowadays we might
well opt to buy more machines! When computers were
very expensive, an aim for efficiency improvements
gave rise to batch processing.
A batch processing operating system does not allow
for interaction between the user and the processor
during the execution of the program. Lots of programs
(or lots of data to be run through the same program)
are collected together (to form a batch). The batch
operating system then controls their passage through
the computer.
T1.2 b
The purpose of an operating systemAn operating system (OS) is the main item of system
software used by a computer system. The OS is a set
of programs designed to run in the background on a
computer system and provide an environment in which
application software can be executed.
Most operating systems comprise a large set
of programs, only some of which are stored in the
computer’s memory all the time. Many of the routines
available in the OS are stored on the hard drive so that
they can be accessed when required. This saves space
in the computer’s main memory .
When you are using applications software, you
are not communicating directly with the computer
hardware. Your applications software communicates
with OS program modules that communicate with the
computer hardware on its behalf. Without an operating
system, a computer is useless no matter how many
programs you have.
There are many different types of operating system.
Each type makes the computer behave differently
and is appropriate to a given computer system and its
applications. We look at some of the different types of OS
in this chapter and again in other parts of the syllabus.
All operating systems have some common purposes:
● The OS must manage the physical resources of the
computer. Some resources are limited and must be
managed to maximise the use of the computer system:
− A simple system has only one processor.
− Secondary storage is of a fixed size.
− Some input/output devices (e.g. printers) are
shared.● The OS provides a means of communication (the
user interface) between the human user, or the
outside world, and the computer.
TA
1.2 a
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
1.2 System software 5
Nowadays, batch processing is used for applications
where:● there are large amounts of data to be processed● the data are very similar in nature and require
similar processing● the computer system has identifiable times
when it is not being used and so has available
processor time● the application does not require interaction by
a user.
Typical examples of applications which use batch
processing include:● the processing of payroll information● the production of bank statements from customer
files at periodic intervals; e.g. all accounts with
surnames starting A–E could be processed on the
fifth of each month, surnames F–J on the tenth of
each month, and so on● the production of utility (gas, electricity and
telephone) bills from customer records.
Real-timeA real-time operating system can react quickly
enough to affect the next input or process that needs
to be carried out. There is a continuous cycle of input–
processing–output.
Most real-time systems are based on the control of
some process in the real world (where the processing
takes place on a timescale of milliseconds). Some
information processing applications require the
processing to be done within seconds; it still fits the
description of the continuous cycle and is known as a
pseudo real-time operating system.
The following examples of real-time applications
show why immediate response can be vital:● A chemical plant has a reaction vessel in which
the temperature is critical to the result of the
process. The temperature is monitored by a
computer that accepts input from a sensor. The
computer uses the sensor data to make decisions
about adjusting the heating elements in the vessel.
A delay in the decision-making process might
corrupt the reaction.● A robot trolley is controlled by a processor that
takes input from a sensor following a black line
on the floor. The processor makes decisions about
steering to keep the trolley on the black line. The
trolley would very soon lose its direction if it was not
steering quickly enough.● A catalogue shop processes orders. The code for a
product is input and the system compares it with
information in its files. When it finds the correct
code, it can report to the user the quantity of that
product in stock. It is necessary to record a purchase
before the next shopper’s request is processed
otherwise the second person might be sold the
same item.
Single-userA single-user operating system controls a system
which has only one user, and their programs, at any
one time. A perfect example of a single-user system is
the PC or laptop that you may have at home. Only one
person uses the computer system at any one time. Note
that it does not mean a system that only ever has one
user! This means that security measures are important
so that the system only allows access to files that
belong to the appropriate user.
Multi-userAgain, as the name implies, this type of operating
system allows the computer to service more than one
user simultaneously. A multi-user operating system
has a single (normally powerful) computer which is
connected to a number of terminals. These terminals
are not computers, although they may have a very
limited amount of processing power. They rely on the
main computer to carry out any processing.
The computer sends a message to each of the
terminals in turn, asking if it wants any processing
to be carried out. This process is called a time-share
system or a round robin system. Each of the small
amounts of processor time is called a “time slice”. The
length of a time slice varies according to the system but
is typically about a hundredth of a second.
A configuration in which a central computer
(a server) processes data from several terminals
(see Figure 1.2.1 on page 6) is called a “thin-client
network”.
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
6 Module 1
Figure 1.2.1 A multi-user computer system. Since terminals do no processing this is a thin client network.
server
network cable
server processes eachterminal’s application
program
terminals do not haveany processing
capability
server stores:- application programs- users’ data files
terminal 1 terminal 2 terminal 3
There is a limit to the number of terminals that can
be serviced. The system needs to be fast enough that
a user at a terminal does not have to wait for too long
before being given processing time. The system can be
speeded up if the computer ignores terminals that are
not active. Terminals can send a signal to the computer
when they need processor time. Such a system is
only satisfactory if the response time for all users is
acceptable; no user should have to wait for a long time
for a program to respond.
Multi-taskingA multi-tasking operating system allows several
applications to be available simultaneously. On a
single-user system, you are probably used to having
a number of processes loaded in main memory
at the same time. One window may show a spreadsheet
while another shows a word processing application;
you may also be connected to the Internet through an
email client; and you may be playing music while you
work. It appears that more than one task is running
simultaneously but they aren’t, they just appear to be.
The OS can switch between tasks so quickly that it
seems to the user as if they are all being done at once.
The method used to do this is very similar to the multi-
user OS. The tasks are allocated time slices and the
computer goes from one to the other servicing them.
Microsoft Windows is a multi-tasking operating
system. You can view a list of the running processes
in the Task Manager operating system utility
(Figure 1.2.2). In Chapter 3.1, we further consider
multi-tasking in Windows.
NetworkA network comprises a number of computers linked
together for the purposes of communication and the
sharing of resources. Networks are important because
they allow hardware and software to be shared. They
require the computers to run a network operating
system. Often, one of the machines is used to control
the rest of the system; this machine is called the server.
A network OS must carry out tasks such as:● control of access to the network● management of the filing system● management of all applications programs available
from the server● management of all shared peripherals.
A common misunderstanding is to confuse network
systems with multi-user systems. A multi-user system
has many users using one computer at the same time;
a network system has many computers each using
shared hardware and software.
In Chapter 3.8, we consider networking in detail.
Application requirements for operating systemsIn an examination, you will be asked to identify
applications that require batch processing and
applications that require real-time processing.
Applications that require batch processing include:
payroll, the production of bills and the production
of bank statements. Be very careful in choosing any
other application – it must be clear that there are
good reasons for using batch processing. For example,
simply to say that an ATM machine uses batch
processing is not enough. Requests for bank statements
AI
1.2 c
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
1.2 System software 7
are batched for later processing but the PIN for a
customer must be checked in real time. You must be
specific and justify your choice. For example, you could
write “Because statements must be sent out by post,
they can be processed while the computer is not doing
other things. It makes sense to process a large number
of statements at the same time as a batch.”
Control applications, in which the results of a
process are used to inform the next input, use real-
time processing. A good example would be the
control of a robotic machine on a production line.
Information systems also require real-time processing.
It is necessary to update the file of information before
the next enquiry is dealt with. The classic example is
the airline (or theatre) booking system. If a customer
decides to buy a ticket for a flight, the number of tickets
available must be updated before the next person
makes an enquiry, otherwise another person may be
sold the same seat.
In general terms, an examination question will ask
you to decide which of these two types of processing
is most appropriate for a given application. It is not
sensible to try to memorise a list of applications for
each type. You must learn to identify the characteristics
of each of the types of processing for a given
application.
Types of user interfaceThe user interacts with a computer system through its
user interface. The user gives input (e.g. the click of a
mouse) and receives responses from the computer. A
user interface (also called a human–computer interface
(HCI)) consists of both hardware and software.
When a user types instructions into a computer and
the computer responds by displaying details on a screen,
then that is an interface. The keyboard and the screen
are the hardware components; the software components
of the interface allow the computer to understand the
typed instructions. In the early days of computing,
people could use a teleprinter instead of a monitor.
The teleprinter was similar to a typewriter. As the user
typed commands on it, the computer would print the
commands and its response on paper. The hardware
TT
1.2 d
Figure 1.2.2 The Windows Task Manager utility.
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
8 Module 1
and software components of the interface need to be
appropriate to the use. For example, a keyboard and
screen are needed for an interface that enables the user
to make enquiries about theatre tickets; a printer would
enable the user to print a booked ticket.
There are many different types of interface. Their
features vary depending on the application, the
conditions in which it is to be used and the knowledge
and skills of the user. From the many types of HCI, you are
expected to be able to describe the five software interfaces
discussed below and give a typical application for each
of them. In Chapter 1.9, we discuss the hardware that is
necessary to put the software interface into operation.
Form-based interfaceIf the majority of the input to a system is standardised,
a typical interface is an on-screen form to be filled in.
This type of interface is used by an operator inputting
information while talking to a customer over the
telephone.
The interface:● prompts the operator to ask each of the questions
in turn● makes the operator input the information in the
correct order and ensures that nothing is missed out● ensures that the information is input in the
correct format● makes the checking of the information easier.
A form-based interface (see Figure 1.2.3) has a specific
area for each piece of data. For example, there may be:● a drop-down list for restricted input (such as the
person’s title or a date)● boxes for textual information (such as the name or
email address)● check boxes for yes–no information (such as a box
to indicate acceptance of conditions).
The form uses standard widget controls – text boxes,
radio buttons, check boxes and drop-down lists.
When the user enters data, the cursor often moves
automatically to the next box to be filled in. Sometimes a
Figure 1.2.3 Form-based interface.
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
1.2 System software 9
box is highlighted to make it clear to the operator where
the data are to be inserted. Some of the data are more
important than others and the cursor does not move on
until data have been supplied. The interface checks that
the input is sensible for that box before moving on.
When a person orders something from a supplier
on the Internet, a form-based interface is used. The
customer fills in a number of standard personal and
payment details.
Menu-based interfaceA menu-based interface is used in situations where
the user does not know what options are available.
Examples of this are information systems for tourists
and the on-screen menus for digital television. A list
of choices is made available. When the user selects an
option, a further set of choices is displayed, and so on
until the result is obtained.
Imagine an information system at a train station
in a popular holiday location. The first screen may
ask for the general area of interest (accommodation,
trips, shopping or entertainment). If the user selects
“accommodation”, the next screen may offer a choice
of types of accommodation (hotels, guest houses, etc.).
The third screen may offer different price bands. Finally,
the user may see a list of all the available properties
that match the choices of type and price band. Such an
information system often uses a touch screen because
other peripherals are less appropriate. For example, a
mouse connected to an information system in a train
station might be vandalised or damaged by the weather.
Graphical user interfaceA graphical user interface (GUI) uses windows, icons,
menus, and a pointer (giving rise to the acronym
WIMP). The user of a GUI expects to be able to view
different applications or files on the screen at the
same time. This is done by putting each into its own
bordered area known as a window. The user can select
a command by clicking on a small picture (an icon)
that represents it. The user selects further options from
menus. Icons and menu options are selected by using
some sort of pointing device such as a mouse.
In Figure 1.2.4, the main menu is displayed
horizontally as text options (File, Edit, etc.). Selecting
Shape from the menu opens a drop-down menu of
Figure 1.2.4 Graphical user interface.
Cambridge University Press978-0-521-18662-9 – Cambridge International AS and A Level ComputingChris Leadbetter Roger Blackford and Tony PiperExcerptMore information
© in this web service Cambridge University Press www.cambridge.org
10 Module 1
Figure 1.2.5 Command line interface.
selections. Selecting Rotate or Flip gives access to a
final menu from which the user can select the final
command – Rotate Right. The user can save a file in one
step by clicking the “Save” icon (it represents a floppy
disk); in Figure 1.2.4 you can see this button directly
under the “Edit” button.
The whole principle of a GUI is to make using the
system as simple as possible by hiding all the complicated
bits! For example, when the user clicks an icon to bring
a piece of software onto the screen, it involves only one
action. In the background, the computer is executing a
complicated process involving a lot of instructions. When
the icon is chosen, the computer is simply told to run
those instructions so the software will appear. The icon
has hidden the complexity from the user.
Natural language interfaceA natural language interface is sometimes referred to
as a “conversational interface”. The computer displays
or speaks questions that give the user the impression
that the computer is talking to them. However, the
system restricts itself to questions that will provoke very
simple responses which the computer can understand.
If the user does not give one of the expected responses,
a message is produced which makes it clear that the
user must try again.
It is often assumed that a natural language interface
is very close to a spoken language. This may be true but it
is likely to be typed into the machine rather than actually
being spoken. For example, some Internet search engines
or large information systems can process natural language
queries. This is the next step up from a simple keyword
search, where the software picks out the keywords and
then searches for matching documents or files.
Computerised telephone systems may use voice
input to select menu choices. A true spoken interface
might be used by a disabled person to communicate
with a computer.
Command line interfaceIn a command line interface, or “command-based
interface”, the user types a series of commands at
the keyboard and the computer displays appropriate
output (see Figure 1.2.5). The computer does not
Recommended