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5 System software
In this chapter, you will learn about
★ why computers need an operating system
★ key management tasks, such as memory management, file
management, security management, hardware management and
process management
★ the need for utility software, including disk formatters, virus
checkers, defragmentation software, disk content analyse and repair
software, file compression and back-up software
★ program libraries, software under development using program library
software and the benefits to software developers, including the use of
dynamic link library (DLL) files
★ the need for these language translators: assemblers, compilers and
interpreters
★ the benefits and drawbacks of using compilers or interpreters
★ an awareness that high level language programs may be partially
compiled and partially interpreted (such as Java
tm
)
★ the features of a typical integrated development environment (IDE) for
– coding (using context-sensitive prompts)
– initial error detection (including dynamic syntax checks)
– presentation (including pretty print, expand and collapse code
blocks)
– debugging (for example, single stepping, use of breakpoints,
variables/expressions report windows).
WHAT YOU SHOULD ALREADY KNOW
Try these five questions before you read the first
part of this chapter.
1 Microprocessors are commonly used to
control microwave ovens, washing machines
and many other household items.
Explain why it is not necessary for these
devices to have an operating system.
2 a) Name three of the most common
operating systems used in computers and
other devices, such as mobile phones and
tablets.
b) A manufacturer makes laptop computers,
mobile phones and tablets.
Explain why it is necessary for the
manufacturer to develop different versions
of its operating system for use on its
computers, mobile phones and tablets.
3 Most operating systems offer a graphic user
interface (GUI) as well as a command line
interface (CLI).
a) What are the main differences between the
two types of interface?
5.1 Operating systems
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Key terms
CMOS – complementary metal-oxide semiconductor.
Operating system – software that provides an
environment in which applications can run and provides
an interface between hardware and human operators.
HCI – human–computer interface.
GUI – graphical user interface.
CLI – command line interface.
Icon – small picture or symbol used to represent, for
example, an application on a screen.
WIMP – windows, icons, menu and pointing device.
Post-WIMP – interfaces that go beyond WIMP and use
touch screen technology rather than a pointing device.
Pinching and rotating – actions by fingers on a touch
screen to carry out tasks such as move, enlarge,
reduce, and so on.
Memory management – part of the operating system
that controls the main memory.
Memory optimisation – function of memory
management that determines how memory is allocated
and deallocated.
Memory organisation – function of memory
management that determines how much memory is
allocated to an application.
Security management – part of the operating system
that ensures the integrity, confidentiality and availability
of data.
Contiguous – items next to each other.
Virtual memory systems – memory management (part
of OS) that makes use of hardware and software to
enable a computer to compensate for shortage of actual
physical memory.
Memory protection – function of memory management
that ensures two competing applications cannot use
same memory locations at the same time.
Process management – part of the operating system
that involves allocation of resources and permits the
sharing and exchange of data.
Hardware management – part of the operating system
that controls all input/output devices connected to
a computer (made up of sub-management systems
such as printer management, secondary storage
management, and so on).
Device driver – software that communicates with the
operating system and translates data into a format
understood by the device.
Utility program – parts of the operating system which
carry out certain functions, such as virus checking,
defragmentation or hard disk formatting.
Disk formatter – utility that prepares a disk to allow
data/files to be stored and retrieved.
Bad sector – a faulty sector on an HDD which can be
soft or hard.
Antivirus software – software that quarantines and
deletes files or programs infected by a virus (or other
malware). It can be run in the background or initiated by
the user.
Heuristic checking – checking of software for
behaviour that could indicate a possible virus.
Quarantine – file or program identified as being
infected by a virus which has been isolated by
antivirus software before it is deleted at a later
stage.
False positive – a file or program identified by a virus
checker as being infected but the user knows this
cannot be correct.
Disk defragmenter – utility that reorganises the
sectors on a hard disk so that files can be stored in
contiguous data blocks.
Disk content analysis software – utility that checks
disk drives for empty space and disk usage by reviewing
files and folders.
Disk compression – software that compresses data
before storage on an HDD.
Back-up utility – software that makes copies of files on
another portable storage device.
b) What are the pros and cons of both types
of interface?
c) Who would use each type of interface?
4 Before the advent of the operating system,
computers relied on considerable human
intervention.
Find out the methods used to start up early
computers to prepare them for the day’s
tasks.
5 Describe the role of buffers and interrupts
when a printing job is being sent to an inkjet
printer.
Consider the different operational speeds of a
processor and a printer, together with size of
printing job and interrupt priorities.
Describe potential error scenarios – such as
paper jam, out of paper or out of ink – and
how these could affect the printing job.
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5.1.1 The need for an operating system
Early computers had no operating system at all. Control software had to be
loaded each time the computer was started – this was done using either paper
tape or punched cards.
In the 1970s, the home computer was becoming increasingly popular. Early
examples, such as the Acorn BBC B, used an internal ROM chip to store part
of the operating system. A cassette tape machine was also used to load the
remainder of the operational software (see Figure 5.1). This was necessary to
‘get the computer started’ and used a welcome cassette tape which had to be
used each time the computer was turned on.
▲ Figure 5.1 An Acorn BBC B (left) and its cassette tape machine (right)
As the hard disk drive (HDD) was developed, operating systems were stored on
the hard disk, and start-up of the motherboard was handled by the basic input/
output system (BIOS). Initially, the BIOS was stored on a ROM chip but, in
modern computers, the BIOS contents are stored on a flash memory chip. The
BIOS configuration is stored in CMOS memory (complementary metal-oxide
semiconductor) which means it can be altered or deleted as required.
The required part of the operating system is copied into RAM – since operating
systems are now so large, it would seriously affect a computer’s performance
if it was all loaded into RAM at once. An operating system provides both the
environment in which applications can be run, and a useable interface between
humans and computer. An operating system also disguises the complexity
of computer hardware. Common examples include Microsoft Windows
®
, Apple
MacOS, Google Android and IOS (Apple mobile phones and tablets).
The human–computer interface (HCI) is usually achieved through a graphical
user interface (GUI), although it is possible to use a command line interface
(CLI) if the user wishes to directly communicate with the computer.
A CLI requires a user to type instructions to choose options from menus, open
software, and so on. There are often a number of commands that need to be
typed; for example, to save or load a file. The user, therefore, has to learn a
number of commands (which must be typed exactly with no errors) just to carry
out basic operations. Furthermore, it takes time to key in commands every time
an operation has to be carried out.
Program library – a library on a computer where
programs and routines are stored which can be freely
accessed by other software developers for use in their
own programs.
Library program – a program stored in a library for
future use by other programmers.
Library routine – a tested and ready-to-use routine
available in the development system of a programming
language that can be incorporated into a program.
Dynamic link file (DLL) – a library routine that can
be linked to another program only at the run time
stage.
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The advantage of CLI is that the user is in direct communication with the
computer and is not restricted to a number of pre-determined options.
For example, the following section of CLI imports data from table A into table B.
It shows how complex it is just to carry out a straightforward operation.
1. SQLPrepare(hStmt,
2. ? (SQLCHAR *) "INSERT INTO tableB SELECT * FROM tableA",
3. ? SQL_NTS):
4. ? SQLExecute(hStmt);
A GUI allows the user to interact with a computer (or MP3 player, gaming
device, mobile phone, and so on) using pictures or symbols (icons). For
example, the whole of the above CLI code could have been replaced by a single
icon, like the one on the left.
Selecting this icon would execute all of the steps shown in the CLI without the
need to type them.
GUIs use various technologies and devices to provide the user interface.
One of the first commonly used GUI environments was known as windows,
icons, menu and pointing device (WIMP), which was developed for use on
personal computers (PCs). Here, a mouse is used to control a cursor and icons
are selected to open and run windows. Each window contains an application.
Modern computer systems allow several windows to be open at the same time.
An example is shown in Figure 5.2.
▲ Figure 5.2 An example of WIMP
A windows manager looks after the interaction between windows, the
applications and windowing system (which handles the pointing devices and
the cursor’s position).
However, smart phones, tablets and many computers now use a post-WIMP
interaction where fingers are in contact with the screen, allowing actions such
as pinching and rotating which are difficult using a single pointer and device
such as a mouse. Also, simply tapping the icon with a finger (or stylus) will
launch the application. Developments in touch screen technology mean these
flexible HCIs are now readily available.
Table update
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5.1.2 Operating system tasks
operating
system
hardware management
file management
security management
memory management
process management
▲ Figure 5.3 Operating system tasks
Memory management
Memory management, as the name suggests, is the management of a
computer’s main memory. This can be broken down into three parts: memory
optimisation, memory organisation and memory protection.
Memory optimisation
Memory optimisation is used to determine how computer memory is allocated
and deallocated when a number of applications are running simultaneously.
It also determines where they are stored in memory. It must, therefore,
keep track of all allocated memory and free memory available for use by
applications. To maintain optimisation of memory, it will also swap data to
and from the HDD or SSD.
Memory organisation
Memory organisation determines how much memory is allocated to an
application, and how the memory can be split up in the most appropriate or
efficient manner.
This can be done with the use of
» a single (contiguous) allocation, where all of the memory is made available
to a single application. This is used by MS-DOS and by embedded systems
» partitioned allocation, where the memory is split up into contiguous
partitions (or blocks) and memory management then allocates a partition
(which can vary in size) to an application
» paged memory, which is similar to partitioned allocation, but each partition
is of a fixed size. This is used by virtual memory systems
» segmented memory, which is different because memory blocks are not
contiguous – each segment of memory will be a logical grouping of data
(such as the data which may make up an array).
Memory protection
Memory protection ensures that two competing applications cannot use the
same memory locations at the same time. If this was not done, data could
be lost, applications could produce incorrect results, there could be security
issues, or the computer may crash.
Memory protection and memory organisation are different aspects of an
operating system. An operating system may use a typical type of memory
organisation (for example, it may use paging or segmentation) but it is always
important that no two applications can occupy the same part of memory.
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Therefore, memory protection must always be a part of any type of memory
organisation used.
Figure 5.4 shows how different applications can be kept separate from each other.
These boundaries mark the
upper limits of the
addresses available to each
application (addresses start
at 0 and end at Z); the
boundaries (A + 1, B + 1,
C + 1) are often referred to
as a FENCE
A FENCE defines the
boundary between the
operating system and the
applications; it is not
possible for an application
to access a memory
location which is lower than
the FENCE address
Address
Boundary location is at address (A+1)
Boundary location is at address (B+1)
Boundary location is at address (C+1)
Upper limit to address value (Z)
Memory
operating system
memory allocated
to application 1
memory allocated
to application 2
memory allocated
to application 3
A + 1
B
0
A
B + 1
C
C + 1
Z
▲ Figure 5.4 Memory protection
Security management
Security management is another part of a typical operating system. The
function of security management is to ensure the integrity, confidentiality and
availability of data.
This can be achieved by
» carrying out operating system updates as and when they become available
» ensuring that antivirus software (and other security software) is always up-
to-date
» communicating with, for example, a firewall to check all traffic to and from
the computer
» making use of privileges to prevent users entering ‘private areas’ on a
computer which permits multi-user activity (this is done by setting up user
accounts and making use of passwords and user IDs). This helps to ensure
the privacy of data
» maintaining access rights for all users
» offering the ability for the recovery of data (and system restore) when it has
been lost or corrupted
» helping to prevent illegal intrusion to the computer system (also ensuring
the privacy of data).
Note: many of these features are covered in more depth elsewhere in this
chapter or in other chapters.
EXTENSION ACTIVITY 5A
While working through the remainder of Chapters 5 and 6, find out all of
the methods available to ensure the security, privacy and integrity of data
and how these link into the operating system security management. It is
important to distinguish between what constitutes security, privacy and
integrity of data.
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Process management
A process is a program which is being run on a computer. Process management
involves the allocation of resources and permits the sharing and exchange of
data, thus allowing all processes to be fully synchronised (for example, by the
scheduling of resources, resolution of software conflicts, use of queues and so
on). This is covered in more depth in Chapter 16.
Hardware management
Hardware management involves all input and output peripheral devices.
The functions of hardware management include
» communicating with all input and output devices using device drivers
» translating data from a file (defined by the operating system) into a format
that the input/output device can understand using device drivers
» ensuring each hardware resource has a priority so that it can be used and
released as required.
The management of input/output devices is essentially the control and
management of queues and buffers. For example, when printing out a
document, the printer management
» locates and loads the printer driver into memory
» sends data to a printer buffer ready for printing
» sends data to a printer queue (if the printer is busy or the print job has a
low priority) before sending to the printer buffer
» sends various control commands to the printer throughout the printing
process
» receives and handles error messages and interrupts from the printer.
File management
The main tasks of file management include
» defining the file naming conventions which can be used (filename.docx,
where the extension can be .bat, .htm, .dbf, .txt, .xls, and so on)
» performing specific tasks, such as create, open, close, delete, rename, copy,
move
» maintaining the directory structures
» ensuring access control mechanisms are maintained, such as access rights to
files, password protection, making files available for editing, locking files,
and so on
» specifying the logical file storage format (such as FAT or NTFS if Windows
is being used), depending on which type of disk formatter is used
(see Section 5.1.3)
» ensuring memory allocation for a file by reading it from the HDD/SSD and
loading it into memory.
EXTENSION ACTIVITY 5B
Write down the tasks carried out by a keyboard manager when a user types
text using a word processor. Consider the use of buffers and queues in your
answer.
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5.1.3 Utility software
Computer users are provided with a number of utility programs that are part of
the operating system. However, users can also install their own utility software
in addition. This software is usually initiated by the user, but some, such as
virus checkers, can be set up to constantly run in the background. Utility
software offered by most operating systems includes
» hard disk formatter
» virus checker
» defragmentation software
» disk contents analysis/repair software
» file compression
» back-up software.
Hard disk formatter
A new hard disk drive needs to be initialised ready for formatting. The
operating system needs to know how to store files and where the files will
be stored on the hard disks. A disk formatter will organise storage space by
assigning it to data blocks (partitions). A disk surface may have a number of
partitions (see Chapter 3 for more details regarding the organisation of data on
hard disks). Note that partitions are contiguous blocks of data.
Once the partitions have been created, they must be formatted. This is usually
done by writing files which will hold directory data and tables of contents
(TOC) at the beginning of each partition. This allows the operating system
to recognise a file and know where to find it on the disk surface. Different
operating systems will use different filing systems; Windows, for example, uses
new technology filing system (NTFS).
When carrying out full formatting using NTFS, all disk sectors are filled with
zeros; these zeros are read back, thus testing the sector, but any data already
stored there will be lost. So, it is important to remember that reformatting
an HDD which has already been used will result in loss of data during the
formatting procedure.
Disk formatters also have checking tools, which are non-destructive tests that
can be carried out on each sector. If any bad sector errors are discovered, the
sectors will be flagged as ‘bad’ and the file tracking records will be reorganised–
this is done by replacing the bad sectors with new unused sectors, effectively
repairing the faulty disk. A damaged file will now contain an ‘empty’ sector,
which allows the file to be read but it will be corrupted since the bad sector will
have contained important (and now lost) data. It would, therefore, be prudent to
delete the damaged file leaving the rest of the HDD effectively repaired.
Bad sectors can be categorised as hard or soft. There are a number of ways that
they can be produced, as shown in Table 5.1.
Hard bad sectors (difficult to repair) Soft bad sectors
n caused by manufacturing errors
n damage to disk surface caused by allowing the read-
write head to touch the disk surface (for example, by
moving HDD without first parking the read-write head)
n system crash which could lead to damage to the disk
surface(s)
n sudden loss of power leading to data corruption in
some of the sectors
n effect of static electricity leading to corruption of
data in some of the sectors on the hard disk surfaces
▲ Table 5.1 Hard and soft bad sectors
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Virus checkers
Any computer (including mobile phones and tablets) can be subject to a virus
attack (see Chapter 6).
There are many ways to help prevent viruses, such as being careful when
downloading material from the internet, not opening files or links in emails
from unknown senders, and by only using certified software. However, virus
checkers – which are offered by operating systems – still provide the best
defence against malware, as long as they are kept up to date and constantly run
in the background.
Running antivirus software in the background on a computer will constantly
check for virus attacks. Although various types of antivirus software work in
different ways, they have some common features. They
» check software or files before they are run or loaded on a computer
» compare possible viruses against a database of known viruses
» carry out heuristic checking – this is the checking of software for types of
behaviour that could indicate a possible virus, which is useful if software is
infected by a virus not yet on the database
» put files or programs which may be infected into quarantine, to
– automatically delete the virus, or
– allow the user to decide whether to delete the file (it is possible that the
user knows that the file or program is not infected by a virus – this is known
as a false positive and is one of the drawbacks of antivirus software).
Antivirus software needs to be kept up to date since new viruses are constantly
being discovered. Full system checks need to be carried out once a week, for
example, since some viruses lie dormant and would only be picked up by this
full system scan.
Defragmentation software
As an HDD becomes full, blocks used for files will become scattered all over
the disk surface (in potentially different sectors and tracks as well as different
surfaces). This happens as files are deleted, partially-deleted, extended and
so on. The consequence is slower data access time: the HDD read-write head
requires several movements just to find and retrieve the data making up the
required file. It would be advantageous if files could be stored in contiguous
sectors, considerably reducing HDD head movements.
Note that, due to their different operation when accessing data, this is less of
a problem with SSDs.
Consider the following example using a disk with 12 sectors per surface.
We have three files (1, 2 and 3) stored on track 8 of the disk surface.
sectors:
track 8:
0 1 2 3 4 5 6 7 8 9 10 11
File 1 File 2 File 3
▲ Figure 5.5
File 2 is deleted by the user and file 1 has data added to it. However, the file 2
sectors which become vacant are not filled up straight away by new file 1 data
since this would require ‘too much effort’ for the HDD resources.
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We get the following.
track 8: File 1File 1 File 3
▲ Figure 5.6
File 1 has been extended to write data in sectors 10 and 11.
Now, suppose file 3 is extended with the equivalent of 3.25 blocks of data. This
requires filling up sector 9 and then moving to some empty sectors to write the
remainder of the data – the next free sectors are on track 11.
track 8: File 1File 1 File 3
track 11: File 3
▲ Figure 5.7
If this continues, the files just become more and more scattered throughout
the disk surfaces. It is possible for sectors 4, 5 and 6 (on track 8) to eventually
become used if the disk starts to fill up and it has to use up whatever space
is available. A disk defragmenter will rearrange the blocks of data to store
files in contiguous sectors wherever possible; however, if the disk drive
is almost full, defragmentation may not work. Assuming we can carry out
defragmentation, then track 8 now becomes:
track 8: File 1 File 3
▲ Figure 5.8
This allows for much faster data access and retrieval since the HDD now requires
fewer read-write head movements to access and read files 1 and 3. Some
defragmenters also carry out clean up operations. Data blocks can become
damaged after several read/write operations (this is different to bad sectors). If
this happens, they are flagged as ‘unusable’ and any subsequent write operation
will avoid writing data to data blocks which have become affected.
Disk content analysis/repair software
The concept of disk repair software was discussed in the above section. Disk
content analysis software is used to check disk drives for empty space and
disk usage by reviewing files and file folders. This can lead to optimal use
of disk space by the removal of unwanted files and downloads (such as the
deletion of auto saving files, cookies, download files, and so on).
Disk compression and file compression
File compression is essential to save storage space and make it quicker to
download/upload files and quicker to send files via email. It was discussed in
Chapter 1.
Disk compression is much less common these days due to the vast size of HDDs
(often more than 2 TB). The disk compression utility compresses data before
writing it to hard disk (and decompresses it again when reading this data). It is
a high priority utility and will essentially override all other operating system
routines – this is essential because all applications need to have access to the
HDD. It is important not to uninstall disk compression software since this would
render any previously saved data to be unreadable.
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Back-up software
While it is sensible to take manual back-ups using, for example, a memory stick
or portable HDD, it is also good practice to use the operating system back-up
utility. This utility will
» allow a schedule for backing up files to be made
» only carry out a back-up procedure if there have been any changes made to a file.
For total security, there should be three versions of a file:
1 The current (working) version stored on the internal HDD.
2 A locally backed up copy of the file (stored on a portable SSD, for example).
3 A remote back-up version stored well away from the computer (using cloud
storage, for example).
Windows environment offers the following facilities using the back-up utility:
» The ability to restore data, files or the computer from the back-up (useful
if there has been a problem and files have been lost and need to be
recovered).
» The ability to create a restore point (this restores a computer to its state at
some point in the past; this can be very useful if a very important file has
been deleted and cannot be recovered by any of the other utilities).
» Options of where to save back-up files; this can be set up from the utility to
ensure files are automatically backed up to a chosen device.
Windows uses File History, which takes snapshots of files and stores them on an
external HDD at regular intervals. Over a period of time, File History builds up a
vast library of past versions of files – this allows a user to choose which version
of the file they want to use. File History defaults to backing up every hour and
retains past versions of files forever unless the user changes the settings.
Mac OS offers the Time Machine back-up utility. This erases the contents of a
selected drive and replaces them with the contents from the back-up. To use
this facility it is necessary to have an external HDD or SSD (connected via USB
port) and ensure that the Time Machine utility is installed and activated on the
selected computer. Time machine will automatically
» back up every hour
» keep daily back-ups for the past month, and
» keep weekly back-ups for all the previous months.
Note that once the back-up HDD or SSD is almost full, the oldest back-ups are
deleted and replaced with the newest back-up data. Figure 5.9 shows the
Time Machine message:
▲ Figure 5.9 Screen shot of Time Machine message
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5.1.4 Program libraries
Program libraries are used
» when software is under development and the programmer can utilise
pre-written subroutines in their own programs, thus saving considerable
development time
» to help a software developer who wishes to use dynamic link library (DLL)
subroutines in their own program, so these subroutines must be available at
run time.
When software routines are written (such as a sort routine), they are frequently
saved in a program library for future use by other programmers. A program
stored in a program library is known as a library program. We also have the
term library routines to describe subroutines which could be used in another
piece of software under development.
Suppose we are writing a game for children with animated graphics (of a
friendly panda) using music routines and some scoreboard graphics.
▲ Figure 5.10
This game could be developed using existing routines from a library.
new game under development
friendly panda
animation routines
children’s music routines
final scoring graphics
▲ Figure 5.11
Well done
Freddie!!
You got 6 right
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Developing software in this way
» removes the need to rewrite the many routines every single time (thus
saving considerable time and cost)
» leads to modular programming, which means several programmers can be
working on the same piece of software at the same time
» allows continuity with other games that may form part of a whole range
(in education, where there may be a whole suite of programs, for example)
» allows the maintenance of a ‘corporate image’ in all the software being
developed by a particular company
» saves considerable development time having to test each routine, since the
routines are all fully tested in other software and should be error-free.
All operating systems have two program libraries containing library programs
and library routines: static and dynamic.
In static libraries, software being developed is linked to executable code in the
library at the time of compilation. So the library routines would be embedded
directly into the new program code.
In dynamic libraries, software being developed is not linked to the library
routines until actual run time (these are known as dynamic link library files
or DLL). These library routines would be stand-alone files only being accessed
as required by the new program – the routines will be available to several
applications at the same time.
When using DLL, since the library routines are not loaded into RAM until
required, memory is saved, and software runs faster. For example, suppose
we are writing new software which allows access to a printer as part of its
specification. The main program will be developed and compiled. Once the
object code is run, it will only access (and load up) the printer routine from DLL
when required by the user of the program. The main program will only contain a
link to the printer library routine and will not contain any of the actual printer
routine coding in the main body. Table 5.2 summarises the pros and cons of
using DLL files.
Pros of using DLL files Cons of using DLL files
the executable code of the main program is much smaller
since DLL files are only loaded into memory at run time
the executable code is not self-contained, therefore all
DLL files need to be available at run time otherwise error
messages (such as missing .dll error) will be generated and
the software may even crash
it is possible to make changes to DLL files independently
of the main program, consequently if any changes are
made to the DLL files it will not be necessary to recompile
the main program
any DLL linking software in the main program needs to be
available at run time to allow links with DLL files to be
made
DLL files can be made available to a number of
applications at the same time
if any of the DLL files have been changed (either
intentionally or through corruption) this could lead to the
main program giving unexpected results or even crashing
all of the above save memory and also save execution time malicious changes to DLL files could be due to the result
of malware, thus presenting a risk to the main program
following the linking process
▲ Table 5.2 Pros and cons of using DLL files
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5.2 Language translators
WHAT YOU SHOULD ALREADY KNOW
Try these two questions before you read the
second part of this chapter:
1 a) Name two types of language translator.
b) Identify a method, other than using a
translator, of executing a program written
in a high-level language.
2 Most modern language translators offer an
Integrated Development Environment (IDE)
for program development.
a) Which IDE are you using?
b) Describe five features offered by the IDE
you use.
c) Which feature do you find most useful?
Why is it useful to you?
ACTIVITY 5A
1 a) i) Explain why a computer needs an
operating system.
ii) Name two management tasks carried out
by the operating system.
b) A new program is to be written in a high
level language. The developer has decided
to use DLL files in the design of the new
program.
i) Explain what is meant by a DLL file.
How does this differ from a static library
routine?
ii) Describe two potential drawbacks of
using DLL files in the new program.
2 A company produces glossy geography
magazines. Each magazine is produced using
a network of computers where thousands of
photographs and drawings need to be stored.
The computers also have an external link to the
internet.
Name, and describe the function of, three utility
programs the company would use on all its
computers.
3 A computer user has a number of important
issues, listed below.
For each issue, name a utility which could help
solve it. Give a reason for each choice.
a) The user wants to send a number of
very large attachments by email, but the
recipient cannot accept attachments greater
than 20 MB.
b) The user has accidentally deleted files in the
past. It is essential that this cannot happen
in the future.
c) The user has had their computer for a
number a years. The time to access and
retrieve data from the hard disk drive is
increasing.
d) Last week, the user clicked on a link in an
email from a friend, since then the user’s
computer is running slowly, files are being
lost, and they are receiving odd messages.
e) Some of the files on the user’s HDD have
corrupted and will not open and this is
affecting the performance of the HDD.
Key terms
Translator – the systems software used to translate
a source program written in any language other than
machine code.
Compiler – a computer program that translates a
source program written in a high-level language to
machine code or p-code, object code.
Interpreter – a computer program that analyses and
executes a program written in a high-level language line
by line.
Prettyprinting – the practice of displaying or printing
well set out and formatted source code, making it
easier to read and understand.
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Integrated development environment (IDE) – a suite of
programs used to write and test a computer program
written in a high-level programming language.
Syntax error – an error in the grammar of a source
program.
Logic error – an error in the logic of a program.
Debugging – the process of finding logic errors in a
computer program by running or tracing the program.
Single stepping – the practice of running a program
one line/instruction at a time.
Breakpoint – a deliberate pause in the execution
of a program during testing so that the contents of
variables, registers, and so on can be inspected to aid
debugging.
Report window – a separate window in the run-time
environment of the IDE that shows the contents of
variables during the execution of a program.
5.2.1 Translation and execution of programs
Instructions in a program can only be executed when written in machine code
and loaded into the main memory of a computer. Programming instructions
written in any programming language other than machine code must be
translated before they can be used. The systems software used to translate
a source program written in any language other than machine code are
translators. There are three types of translator available, each translator
performs a different role.
Assemblers
Programs written in assembly language are translated into machine code by
an assembler program. Assemblers either store the program directly in main
memory, ready for execution, as it is translated, or they store the translated
program on a storage medium to be used later. If stored for later use, then
a loader program is also needed to load the stored translated program into
main memory before it can be executed. The stored translated program can be
executed many times without being re-translated.
Every different type of computer/chip has its own machine code and assembly
language. For example, MASM is an assembler that is used for the X86 family
of chips, while PIC and GENIE are used for microcontrollers. Assembly language
programs are machine dependent; they are not portable from one type of
computer/chip to another.
Here is a short sample PIC assembly program:
movlw B’00000000’
tris PORTB
movlw B’00000011’
movwf PORTB
stop: goto stop
Assembly language programs are often written for tasks that need to be
speedily executed, for example, parts of an operating system, central heating
system or controlling a robot.
EXTENSION
ACTIVITY 5C
Find out what
task this very
short sample PIC
assembly program
is performing.
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Compilers and interpreters
Programs written in a high-level language can be either translated into
machine code by a compiler program, or directly executed line-by-line using an
interpreter program.
Compilers usually store the translated program (object program) on a storage
medium ready to be executed later. A loader program is needed to load the
stored translated program into main memory before it can be executed.
The stored translated program can be executed many times without being
retranslated. The program will only need to be retranslated when changes are
made to the source code.
With an interpreter, no translated program is generated in main memory or
stored for later use. Every line in a program is interpreted then executed each
time the program is run.
High-level language programs are machine independent, portable and can be
run on any type of computer/chip, provided there is a compiler or interpreter
available. For example, Java, Python and Visual Basic® (VB) are high-level
languages often used for teaching programming.
The similarities and differences between assemblers, compilers and interpreters
are shown in Table 5.3.
Assembler Compiler Interpreter
Source program written in assembly language high-level language high-level language
Machine dependent yes no no
Object program generated yes, stored on disk or in
main memory
yes, stored on disk or in
main memory
no, instructions are
executed under the control
of the interpreter
Each line of the source
program generates
one machine code
instruction, one to one
translation
many machine code
instructions, instruction
explosion
many machine code
instructions, instruction
explosion
▲ Table 5.3 Similarities and differences between assemblers, compilers and interpreters
5.2.2 Pros and cons of compiling or interpreting a program
Both compilers and interpreters are used for programs written in high-level
languages. Some integrated development environments (IDEs) have both
available for programmers, since interpreters are most useful in the early stages
of development and compilers produce a stand-alone program that can be
executed many times without needing the compiler.
Table 5.4 shows the pros (in the blue cells) and cons (in the white cells) of
compilers and interpreters.
Compiler Interpreter
The end user only needs the executable code, therefore, the
end user benefits as there is no need to purchase a compiler
to translate the program before it is used.
The end user will need to purchase a compiler or an
interpreter to translate the source code before it is used.
The developer keeps hold of the source code, so it cannot
be altered or extended by the end user, therefore, the
developer benefits as they can charge for upgrades and
alterations.
The developer relinquishes control of the source code,
making it more difficult to charge for upgrades and
alterations. Since end users can view the source code, they
could potentially use the developer’s intellectual property.
EXTENSION
ACTIVITY 5D
Find out about
three more high-
level programming
languages that are
being used today.
➔
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Compiler Interpreter
Compiled programs take a shorter time to execute as
translation has already been completed and the machine
code generated may have been optimised by the compiler.
An interpreted program can take longer to execute than
the same program when compiled, since each line of the
source code needs to be translated before it is executed
every time the program is run.
Compiled programs have no syntax or semantic errors. Interpreted programs may still contain syntax or semantic
errors if any part of the program has not been fully tested,
these errors will need to be debugged.
The source program can be translated on one type of
computer then executed on another type of computer.
Interpreted programs cannot be interpreted on one type
of computer and run on another type of computer.
A compiler finds all errors in a program. One error detected
can mean that the compiler finds other dependent errors
later on in the program that will not be errors when the
first error is corrected. Therefore, the number of errors
found may be more than the actual number of errors.
It is easier to develop and debug a program using an
interpreter as errors can be corrected on each line and
the program restarted from that place, enabling the
programmer to easily learn from any errors.
Untested programs with errors may cause the computer to
crash.
Untested programs should not be able to cause the
computer to crash.
The developer needs to write special routines in order to
view partial results during development, making it more
difficult to assess the quality of particular sections of
code.
Partial results can be viewed during development,
enabling the developer to make informed decisions about
a section of code, for example whether to continue,
modify, or scrap and start again.
End users do not have access to the source code and
the run-time libraries, meaning they are unable to
make modifications and are reliant on the developer for
updates and alterations.
If an interpreted program is purchased, end users have all the
source code and the run-time libraries, enabling the program
to be modified as required without further purchase.
▲ Table 5.4 Pros (blue cells) and cons (white cells) of compilers and interpreters.
5.2.3 Partial compiling and interpreting
In order to achieve shorter execution times, many high-level languages
programs use a system that is partially compilation and partially interpretation.
The source code is checked and translated by a compiler into object code.
The compiled object code is a low-level machine independent code, called
intermediate code, p-code or bytecode. To execute the program, the object
code can be interpreted by an interpreter or compiled using a compiler.
For example, Java and Python programs can be translated by a compiler into a
set of instructions for a virtual machine. These instructions, called bytecode,
are then interpreted by an interpreter.
Below are examples of Java and Python intermediate code (bytecode):
Source code:
public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello World");
}
}
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5.2.4 Integrated development environment (IDE)
An integrated development environment (IDE) is used by programmers to
aid the writing and development of programs. There are many different IDEs
available; some just support one programming language, others can be used for
several different programming languages. NetBeans®, PyCharm®, Visual Studio®
and SharpDevelop are all IDEs currently in use.
EXTENSION ACTIVITY 5F
In small groups investigate different IDEs. See how many different features
are available for your group’s IDE and identify which programming
language(s) are supported. Compare the features of the IDE investigated by
your group with the IDEs investigated by other groups in the class.
Bytecode:
Compiled from "HelloWorld.java"
public class HelloWorld extends java.lang.Object{
p u b l i c H e l l o W o r l d();
Code:
0: aload _ 0
1: invokespecial #1; //Method java/lang/
O b j e c t.”< i n i t > ”:()V
4: retu r n
public static void main(java.lang.String[]);
Code:
0: getstatic #2; //Field java/lang/System.
out:Ljava/io/PrintStream;
3: ldc #3; //String Hello World
5: invokevirtual #4; //Method java/io/PrintStream.
println:(Ljava/lang/String;)V
8: retur n
EXTENSION ACTIVITY 5E
Visual Basic also has an interpreter for bytecode. Find an example of
bytecode for Visual Basic. See if you can find the bytecode for displaying
‘Hello World’ on the screen as in the Python example above.
Source code:
print ("Hello World")
Bytecode:
1 0 LOAD _ NAME 0 (print)
2 LOAD _ CONST 0 ('Hello World')
4 CALL _ FUNCTION 1 (1 positional, 0 keyword pair)
6 RETURN _ VALUE
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IDEs usually have
» a source code editor
» a compiler, an interpreter, or both
» a run-time environment with a debugger
» an auto-documenter.
Source code editor
A source code editor allows a program to be written and edited without the
need to use a separate text editor. The use of an integrated source code editor
speeds up the development process, as editing can be done without changing
to a different piece of software each time the program needs correcting or
adding to. Most source code editors colour code the words in the program and
layout the program in a meaningful way (prettyprinting). Some source code
editors also offer context sensitive prompts with text completion for variable
names and reserved words, and provide dynamic syntax checking. Figures 5.12
and 5.13 show these features in the PyCharm source code editor.
colour coded words
context sensitive
prompt offering
text completion
▲ Figure 5.12 PyCharm IDE showing source code editor
Here, string values are shown coloured green and integer values are shown
coloured blue.
▲ Figure 5.13 PyCharm IDE showing dynamic syntax checking
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5.2 Language translators
5
Dynamic syntax checking finds possible syntax errors as the program code
is being typed in to the source code editor and alerts the programmer at the
time, before the source code is interpreted. Many errors can therefore be found
and corrected during program writing and editing before the program is run.
Logic errors can only be found when the program is run.
For larger programs that have more than one code block, some code blocks can
be collapsed to a single line in the editor allowing the programmer to just see
the code blocks that are currently being developed.
Compilers and interpreters
Most IDEs usually provide a compiler and/or an interpreter to run the program.
The interpreter is often used for developing the program and the compiler to
produce the final version of the object code.
source program
run-time environment
▲ Figure 5.14 PyCharm IDE showing both program code and program run
With PyCharm there can be more than one interpreter available for different
versions of the Python language. The program results are shown using the
run-time environment provided.
A run-time environment with a debugger
A debugger is a program that runs the program under development and aids
the process of debugging. It allows the programmer to single step through
the program a line at a time (single stepping) or to set a breakpoint to stop
the execution of the program at a certain point in the source code. A report
window then shows the contents of the variables and expressions evaluated at
that point in the program. This allows the programmer to see if there are any
logic errors in the program and check that the program works as intended.
single step
type and contents of the variables
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single step
type and contents of the variables
▲ Figure 5.15 PyCharm IDE showing the report window after line 2 (page 155) and after
line 4 (above)
Each variable used is shown in the report window together with the type and
the contents of the variable at that point in the program. The top variable
shown is the last one that was used.
Answers to calculations and other expressions can also be shown.
▲ Figure 5.16 PyCharm IDE showing the report window with the answer to an expression
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Auto-documenter
Most IDEs usually provide an auto-documenter to explain the function and
purpose of programming code.
▲ Figure 5.17 PyCharm IDE showing the quick documentation window for print
1 A programmer is writing a program that includes code from a program library.
a) Describe two benefits to the programmer of using one or more library
routines. [4]
b) The programmer decides to use a Dynamic Link Library (DLL) file.
i) Describe two benefits of using DLL files. [4]
ii) State one drawback of using DLL files. [2]
Cambridge International AS & A Level Computer Science 9608
Paper 12 Q8 November 2016
2 a) The operating system contains code for performing various management
tasks. The appropriate code is run when the user performs actions.
Copy the diagram below and connect each OS management task to the
appropriate user action. [3]
ACTIVITY 5B
1 a) i) Describe the difference between a compiler and an assembler.
ii) Describe the difference between a compiler and an interpreter.
b) State two benefits and two drawbacks of using an interpreter.
2 A new program is to be written in a high-level language. The programmer
has decided to use an IDE to develop the new program.
a) Explain what is meant by an IDE.
b) Describe three features of an IDE.
End of chapter
questions
➔
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OS management task action
main memory management
The user moves the mouse on the
desktop
input/output management
The user closes the spreadsheet
program
secondary storage
management
The user selects the SAVE command to
save their spreadsheet
human–computer interface
management
The user selects the PRINT command
to output their spreadsheet
b) A user has the following issues with the use of his PC.
State the utility software which should provide a solution.
i) The hard disk stores a large number of video files. The computer
frequently runs out of storage space. [1]
ii) The user is unable to find an important document. He thinks it
was deleted in error some weeks ago. This must not happen again. [1]
iii) The operating system reports ‘bad sector’ errors. [1]
iv) There have been some unexplained images and advertisements
appearing on the screen. The user suspects it is malware. [1]
Cambridge International AS & A Level Computer Science 9608
Paper 11 Q6 June 2017
3 File History and Time Machine are examples of back-up utilities offered as part of
two different operating systems.
a) Explain why it is important to back up files on a computer. [2]
b) One of the features offered by both utilities is the possibility of
‘turning back the internal computer clock’.
Explain why this is an important feature and give two occasions when a
user may wish to use this feature. [4]
c) By using diagrams and written explanation, describe how defragmentation
software works. [4]
4 Assemblers, compilers and interpreters are all used to translate programs.
Discuss the different roles played by each translator. [6]
5 State four features of an IDE that are helpful when coding a program. [4]
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