Thursday, July 12, 2012


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Lecture 1

Topic Functions (Singh, Chapter )

Section A1 � p14

Input Function Output

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0o sin 0.5

x sin sin(x)

x f f(x)

jxgfghfjkghf gklhgklhjglkjb lkjfhlajfhajhfdajghfg ;;ha;jkdgha dgLecture 1

Topic Functions (Singh, Chapter )

Section A1 � p14

Input Function Output

0o sin 0.5

x sin sin(x)

x f f(x)

Example (Singh, Section A, Example 1)

A function f is defined by f(x)=x+1. (We could also say, f is defined by x® x+1). Evaluate f(x) at x=-,-,-1,1,,.


f(-) = (-)+1=+1=10 f(-)=(-)+1=5 f(-1)=(-1)+1=

f() = +1=10 f()=+1=5 f(1)=1+1=

One way of thinking about the properties of functions is to draw schematic pictures of the domain and the range and indicate how numbers in the domain are mapped to numbers in the range. For the function of the previous example

The above picture emphasises that, in this example, f is a many-to-one function; i.e., more than one number in the domain is mapped to each number in the range.

Introduction to Operating Systems

When you use a computer you do not necessarily need to know all about the inner workings of the computer in order to be able to use it. Even if we understand the hardware, we cannot directly access it, the hardware needs software to make it work. This software is the operating system.

Defn An operating system is the collection of software and data that manages the system and performs resource sharing, user dialogue provision and timesharing.

You are probably familiar with lots of different applications software, word processors, spreadsheets and the like. It is important to realise that applications software does not communicate directly with the hardware, it communicates with the operating system. The operating system is a software interface between the hardware and the applications software.

The term system software is sometimes used interchangeably with operating system; there is a slight difference in that system software does include the operating system but it also includes other programs such as programming language interpreters, compilers, assemblers and editors.

The operating system (OS) is a set of programs the most important of which is the supervisor program that manages the OS, this program is resident (remains in memory). The supervisor program controls the entire operating system and loads into memory other operating system programs (called non-resident) from disk storage only as they are needed.

What does the OS do?

Broadly speaking an operating system performs two main functions,

Resource Sharing

· Share computer’s resources (processors, memory, I/O devices) among a number of simultaneous users.

· Aim increase availability of computer to its users and to maximise the utilisation of resources.

· Decline in hardware costs means resource utilisation less important than it was, dedicated machines are now used.

· Large computers still expensive so resource allocation still important.

· Microcomputers used for multiple functions even if only single users so resource allocation still important.

Provision of a virtual machine

· Transformation of raw hardware into a machine which is more easily used.

· Present user with ‘virtual machine’ characteristics of which easier for user than physical machine.

· some differences between real and virtual machine

I/O I/O capabilities of hardware may be extremely complex, virtual machine has I/O which is easier to use and just as powerful

Memory OS provides virtual machine with more memory because uses secondary memory also. OS may provide virtual machine with partitioned main memory so users think memory is smaller.

Filing System OS provides access to stored information using a symbolic name rather than by providing details of physical location. Also imposing structure on the stored information

Protection + Error Handling

Large computer system with multiple users, OS protects users against error or malice in others and prevents them affecting the integrity of the system.

Program Interaction

User programs may interact i.e. output from one program becomes input to another

Program Control

CLI or GUI so user can tell virtual machine what to do e.g. compile programs without having to use machine code instructions.

Precise nature of the virtual machine will vary depending on application to which computer will be used. Operating system design must be strongly influenced by the type of use for which the machine is intended, e.g. airline seat reservation system or desktop computer?

Most of work of an OS is hidden from user, tasks are performed ‘behind the scenes’, who unaware of managing the computer’s resources. Also all I/O invoked by applications software is actually carried out by OS.

Desirable Features of an Operating System

Efficiency No one criterion to judge efficiency; unused CPU time (except when no work to do), response time, resource utilization.

Reliability Ideally the operating system should be completely free of errors and be able to handle all contingencies. In practice this is never the case.

Maintainability It should be possible to maintain an operating system - enhancing it or correcting errors - without employing an army of systems programmers - i.e. modular and well documented.

Small size. Space used to hold the operating system, whether in memory or elsewhere, is wasted so far as productive computing is concerned. Furthermore, a large system is liable to be more prone to error, and will probably take longer to develop than a small one.

OS for mainframes and other large computers are complex, need to keep track of several programs from several users all running in the same time frame. Some PC operating systems can support multiple programs or users, most are concerned with a single user. We will start with the single user scenario.

Operating Systems for Personal Computers.

Software is generally grouped according to the computer, probably IBM (or compatibles) or Macintosh, on which the software can be used. Software is further distinguished according to the application - word processing, spreadsheets, games etc.

Generally an application program can only run on one operating system. Just as you can’t place a Nissan engine in a Ford truck, you cannot take a version of WordPerfect designed to run on an IBM machine and run it on an Apple Macintosh. Why? Because most IBM machines use the Microsoft operating systems (MS-DOS or Microsoft Windows) and Macintoshes use an entirely different operating system - the Macintosh operating system produced by Apple. Most PC’s are limited to one of these two.

Why different OS?

The operating systems are different because the CPU’s are different. Software producers have to decide which OS to write their application for - although some make versions of their software for each operating system. Users do not set out to buy operating systems, they want computers and the applications software to make them useful. However, since the OS determines what software is available for a given computer, many users observe the high volume of software available for MS-DOS machines and make their computer purchases accordingly. Others like user-friendly style of Macs, and choose them instead. Although OS differ, many of their basic functions are similar. The examples here are from MS-DOS


When the computer is turned on, the operating system is loaded from the hard drive into the computer’s memory, thus making it available for use. The process of loading the operating system into memory is called bootstrapping, or booting the system. When the computer is switched on, a small program (in ROM - read only memory) automatically pulls up the basic components of the operating system from the hard disk. From now on we will use DOS for brevity.

Once booted up the prompt C will appear on the screen. C refers to the disk drive and the is a prompt signalling that the system is waiting for you to do something. The user will supply an instruction, possibly just the name of some software application but it could also be one of the commands available from the operating system. The C is actually a signal for direct communication between the user and the operating system.

Although the prompt is the only visible result of booting the system, DOS also provides the basic software that co-ordinates the computers hardware components and a set of programs that lets you perform the many computer system tasks you need to do. To execute a given DOS program a user issues a command, a name that invokes a specific DOS program. There are whole books on DOS commands but some examples are

FORMAT [drive] formatting disks

COPY [filename] [destination] copy files from one disk to another

DEL [filename] erase files from a disk.

DIR [drive] list the files on a disk

Operating Environments

There is another way to interact with the operating system besides the command line (CLI - command line interface). Another layer, the operating environment, is often added to separate the operating system and the user. This layer is often called the shell because it forms a ‘coating’ or ‘barrier’ over the operating system. The operating environment presents a new screen appearance - more palatable to users.

When using an operating environment, you see pictures or menus instead of the C prompt. Instead of having to know the command you make a selection from the choices available on the screen. Apple Macintosh paved the way for simple interfaces between users and operating systems, Microsoft defined the operating environment standard for DOS based computers with Microsoft Windows, X?Windows is the most common interface used with the Linux operating system.

Windows is a colourful graphical interface that allows users to work with icons and menus rather than commands. The interface is a type of GUI (graphical user interface - pronounced ‘goo-ee’). Pull-down and pop-up menus along with icons encourage the use of ‘point and click’ using a mouse, this approach makes the computer both fast and easy to use.

Multi-programming/ Multi-tasking

· Large computers - mainframes - many users etc. OR

· PC & same user, many tasks.

· Computer - one CPU - only do one instruction at a time

· Program may involve disk accesses or printing so wasteful if CPU idle whilst these tasks done

· OS includes multi-programming feature to maximize CPU use - run several programs at same time

· Programs run concurrently, unrelated programs - possibly different users - compete for processor

· They do not run simultaneously, programs take turns using CPU, operating system keeps track.

· From users point of view their program run as if computer resources dedicated to their program (unless time delay)

· In industry jargon, and for all practical purposes, we consider programs to be running at the same time.

· Windows, multi-tasking feature, so Windows achieves

· More than an attractive GUI

Windows8 is different from Windows .x - Windows 5 (onwards) is not a shell, it is a self-contained operating system, it requires no pre-installed DOS.

Network Operating Systems (NOS)

Network operating systems are an extension of operating systems for personal computers, lets computers on network share resources such as hard disks and printers. Similar to standard operating systems like DOS but includes special features for handling network functions. In addition to resource sharing also supports data security, troubleshooting (computers failing to respond to network requests), and administrative control (track of on-line hours and message tracking)

In a client-server relationship, parts of NOS run on server computer (file access and management programs) and other software runs on clients (software for communicating with server and other computers).

Network operating system’s main task is to make resources appear as if they are running from the client’s computer. The whole point of a client/server system is to provide expanded services to individual users at their own networked computers; the network operating system is the software that makes it possible.


1. Why won’t ‘WordProcessorA’, written for a MAC, run on an IBM compatible machine?

. Why do we have ‘systems software’?

. Explain the differences between compilers, interpreters and translators.

4. What factors might make a company who have outgrown their computer decide to stay with the same manufacturer?

5. List the characteristics of a virtual machine.

6. Describe two important functions of an operating system.

7. What is the difference between a CLI and GUI?

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