Transcript Operating Systems and System Software

System Software
Grade 12
Computer Studies HG
Computer Overview
Computer
Hardware
Software
System
Software
Operating
Systems
Application
Software
System
Utilities
Computer Software
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Computer software can be divided into
two categories:
1.
2.
Application Software – this is software
that aids the user in performing a specific
task e.g. word-processors or media
players.
System Software – this is software that
controls, manages, supports and
maintains the hardware of the computer.
Plan of Action
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We’ll first discuss operating systems and
all the wonderful things that go with
them.
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And then we’ll enter the wonderful world
of system utilities.
Definition of an Operating System
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“A operating system is a suite of software
which manages the hardware and software of
the computer.”
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This means that the OS decides which applications
run and when they run as well as which hardware is
used by these applications.
It also implies that the OS is responsible for
ensuring that applications all run effectively and
efficiently and that they have the resources they
require to execute.
Why do we need Operating Systems?
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In the beginning…
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There was only hardware.
If you wanted to use a computer you would need to
have intricate knowledge of how the electronics of
that hardware worked.
Then IBM created machine language
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Computers understand machine language and a
programmer can give the computer instructions in
machine language.
However this was also nearly impossible for the
average person to comprehend
Still haven’t answered the question!
Operating Systems were created in order
to “abstract” the user from the complexities of hardware and programming.
 The idea is that the user needs to know
nothing about the hardware or programming in order to use the computer.
 So… average users can click use
applications and the operating system
will ensure that the program runs!
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Software Interaction Diagram
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Software can be seen as having different
layers. You don’t need to know how a lower
layer works in order to use a higher one!
Application Software
System Utilities
Operating System
Computer Hardware
Reality Check
The modern computer is not usually
doing one job! It often is doing hundreds
of things that keep your computer
running. These tasks appear to the user
as if they are happening all at once.
 In addition users take it for granted that
things happen relatively safely and
without errors. This is also untrue!
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Development of Operating Systems
It is important to understand how the
operating system developed.
 If we know how the operating system
developed we can better understand the
needs of previous computers and thus
understand why they decided to
incorporate certain features which are
still in today’s operating systems
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Back in the day…
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This is how people used to run a program on a
computer:
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The programmer coded her/his program on coding
sheets.
These coding sheets were then typed out onto
punch cards.
The punch cards were fed into the machine and the
job was processed.
The printed output was then taken back to the
programmer.
If something was wrong with the output the
programmer would have to recode his program
from scratch.
Picture Time
A program on punch cards
The OS Dream
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The dream that developers had from the very
beginning was to create an computer system which
could:
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Be used without expert knowledge
Perform fundamental tasks in the background that could keep
the computer operational
And at the “same time” execute programs on behalf of the
user
As well as provide a means that the user could input data and
get meaningful output from the computer
Many would say we all use such a computer system
everyday. Has the dream already come true? Or are
we still dreaming?
Development #1:
The Batch System
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At first computers could only process one job
and then another job had to be programmed
into the computer.
 The first major development was the batch
system.
 The batch system allowed a number of jobs to
be stored on a magnetic tape. The computer
would complete the jobs in the order that they
were stored on the tape.
 Once a job was completed it would move on to
the next job on the tape.
Batch Systems Today
Batch systems are stilled used today.
 The term is used to refer to any set
instructions that can be processed
without user intervention.
 “Batch” means “many” or “a whole lot”
 So if you were to execute a batch of jobs
you would be instructing the computer to
perform a bunch of operations.
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Development #2:
Multiprogramming
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There were often huge delays while data was
read in from magnetic tapes.
The ideal situation was to keep the CPU busy
at all times.
The technique that was developed was called
multiprogramming.
Multiprogramming is a variety of techniques
that allow the computer to share its hardware
resources between several independent jobs .
This meant that the computer could give it’s
“attention” to whatever user/job needed it.
Development #3:
User Interface
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Once all these nifty features had been added
programmers got bored and decided to do something
fun.
So they started developing different user interfaces.
These interfaces started out as text-based systems
where users had to type commands they wanted to
computer to execute.
Interfaces evolved into what we have today. GUI
(Graphical User Interfaces).
GUI’s consists of graphical objects that can be utilised
and manipulated in order to perform a task.
GUI’s are necessarily easy to use! A lot of programs
don’t behave in a logical intuitive manner (MS Word!).
Features of Modern Operating Systems
What should modern operating systems
be able to do?
 Users have come to expect certain basic
functionalities of a computers operating
system. We’ll have a look at what these
features are in no particular order.
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Coincidentally Windows usually never includes any of these pertinent
features yet everyone uses it.
Feature #1:
Error Handling
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When an error occurs with the execution of a
program or a piece of hardware the operating
system should react accordingly.
The following are suitable error handling
procedures:
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The OS should attempt to prevent the error before it
occurs.
Failing that the OS should attempt to correct the error
when it occurs without user intervention.
If this is not possible the user should be notified of the
error.
The error notification should include the cause of the error,
RELEVANT information concerning the error and a
procedure to follow to correct the error.
Feature #2:
Input / Output
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The operating system needs to be able to handle input
and output in an efficient manner.
For instance a slow output device like a printer should
not need to be monitored by the computer while it is
printing a document. The print data should be made
available to the printer and the CPU should be freed to
process other jobs.
Techniques such as buffering and spooling are used to
manage input and output.
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Spooling – Placing data in a queue for later processing by a
device or program.
Buffering – placing data from an input device in memory
space until all the data is there and ready for processing.
Feature #3:
User Interface
We’ve already discussed user interfaces
a few slides ago.
 A user interface should be easy and
logical to use.
 The user should not need any expert
knowledge of hardware or programming
in order to use the computers interface.
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Feature #4:
Resource Control and Scheduling
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This means that when a program requires the use of
hardware resources the OS is responsible for ensuring
that the correct resources are allocated to the
program.
The OS should also ensure that resources are equally
shared among programs and that each has “a turn” to
the resource.
However the OS must be careful not to allow one
program to access the data another program is
currently working on! Why is this important?
The MOST important aspect of the OS in my humble
opinion!
Feature #5:
Backing Storage Management
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Most data and instructions are stored on
secondary storage like hard-disks.
 The operating system is responsible for
ensuring that this data is organised in a logical
manner and is readily available when needed.
 The OS is responsible for retrieving and
loading data from secondary memory into
primary memory when needed.
 Most OS’s use a technique called virtual
memory.
Different kinds of OS’s
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Operating Systems come in many shapes,
sizes, colours and flavours and all have their
own unique way of carrying out the tasks and
features we’ve just looked at.
 In the next few slides we’ll consider the
different ways in which an OS can execute
programs on behalf of the user.
 (Remember that computers can “seemingly”
do many jobs at once)
Method #1:
Time-Slicing
The CPU’s processing time is divided up
and each process is allocated a time slot
in which it can utilise the CPU.
 If the process does not finish in the
allocated time it must return to the end of
the queue and wait for the next “round”.
 Time can be allocated to processes
according to a set priority.
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Method #2:
Pre-emptive and Co-operative Multitasking
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Two main ways of running numerous
processes at the “same time”
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Pre-emptive – each process is allocated the
process for a set amount of time. When the time is
up the next process uses the CPU. The change
over is FORCED by the OS.
Co-operative – process co-operate in the use of the
processor. A process uses the CPU until is it
satisfied that it no longer needs it and then allows
another process to utilise it. Is this good or bad?
What happens when a program crashes?!
Method #3:
Time-Sharing
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Although the concept of time-sharing is very similar to
that of time-slicing they are not to be confused!
Time-sharing allocates a time slice to USERS that are
using the same computer. Therefore each user has a
small time slot in which to use the resources of the
computer.
These time periods are so small that it appears to
each user that they have all the resources of the PC to
themselves.
So… time-slicing allocates time to separate processes
while time-sharing allocated time to separate users.
Method #4:
Threads
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Threads are little sub-tasks of a bigger process.
These threads perform operations that relate to the
bigger process but both the threads and the process
are working towards the same goal.
Unlike separate process that operate in difference
memory spaces, threads operate in the same memory
space as the process they belong to.
E.g. If you are typing a document called “bob.doc” the
main process is you typing the document but various
threads are being carried out at the same time. For
example there is a thread that is spell-checking as you
type and another thread that is auto-saving and yet
another that is doing auto-formatting.
Method #5:
Multiprocessing
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This involves actually having two physical
CPU’s.
 The operating system must be support
multiple processors before this can be useful.
 The OS runs different processes on different
CPU’s or different threads of the same process
on each CPU.
 These systems are the only systems that can
truly carry out more than one process at a
time.
Parts of the OS
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Nucleus or Kernel
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This term is used interchangeably and refers to the
unchangeable aspects of the OS. Such as how the
OS handles interrupts and resource scheduling.
The kernel also contains information about driver
software that is installed.
Shell
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This is the part of the operating system that the
user sees and interacts with. For example the shell
in Windows consists of the task bar, the desktop,
icons, buttons and windows. The user uses these to
interact with the kernel.
Abstraction abstraction abstraction
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It is important to understand that users are
abstracted from the operating system to a
large degree. Most users have no desire to
know how files are stored or how programs
are run. They just want the damn computer to
do the work. ICONs are a perfect example the
user double clicks on a icon and something
happens. So most users don’t distinguish
between opening a web-page, opening an
image and running a program!
House-keeping
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The operating system needs to do a lot to keep the
computer operational.
In order to achieve this it needs to keep track of
certain aspects of the computer.
Windows does this by using something called the
registry.
The registry stores all the information, settings and
preferences about the hardware, software and users
on a computer.
This information in Windows is stored in USER.DAT
(user info and settings) and SYSTEM.DAT (hardware
and software info and settings)
File Allocation Table
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Another important aspect that the OS need to
keep track of is the file system.
 Files stored on your hard-disk are categorised
meticulously so that they can be retrieved.
Very much like a filing system.
 Windows uses FAT to stored data relating to
files and folders.
 FAT is a table of all the files and folders on
your hard-disk. It stores info on the file name,
size and location on the physical disk.
Onwards!
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Now that we’ve completed out look at
operating systems we can take a closer
look at system utilities.
What are System Utilities?
Definition: “System utilities aid the
operating system in maintaining and
running applications. They also allow the
user to construct her/his own software.”
 In other words system utilities are the
OS’s little helpers.
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Examples
BIOS
 Compilers and Interpreters
 Utility Programs
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Disk Utilities (like scan disk)
 Compression Programs
 Antivirus Utilities
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Compilers
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Compilers compile directly to machine code.
The compiled code will only work on machines
of the same type that is was compiled on.
So a Visual Basic program compiled on a
Windows machine will only run on another
Windows machine.
A c++ program compiled under linux will only
run on another a linux machine.
We call this compiled code “platform specific”
Interpreters
Compilers take the whole piece of code
and compile it in one go and then run it.
 Interpreters take a different approach
and run each line of code one after the
other without first compiling it.
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The Java Approach
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Java compiles it code to run on a “pretend”
machine called the java virtual machine.
The java code can only run on the JVM.
Each platform (mac, unix and windows) needs
the JVM in order to run java programs.
The allows java to be compiled once and run
everywhere.
Why is this a super nifty idea?
Utility Programs
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Compression Programs
These compress files so that they take up
less space on secondary storage devices.
 E.g. winzip, arj, winrar
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Antivirus Utilities
Detects and clean viruses
 E.g. MacAfee, Norton, AVG, Trend PCCillin
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Disk Utilities
Disk-cleanup – cleans unwanted, temporary
and unused files in order to save space.
 Scan-disk – fixes a variety or errors on disk
storage mediums:
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Bad or damaged sectors
Lost file fragments
Cross-linked files
Disk-defragmenter – finds scattered file
segments of the same file/program and
replaces them in a consecutive order in order
to increase efficiency.