Operating Systems Introduction
Author: Brian Brown, 1995-2000. All rights reserved.
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This module introduces you to the subject of operating systems. It is an introductory course that will take you approximately 30-40 hours to complete (including the exercises) depending upon your level of prior knowledge.


What is an operating system?
An operating system (OS) is a collection of system programs that together control the operation of a computer system.


What does an operating system do?
An operating system controls the way in which the computer system functions. In order to do this, the operating system includes programs that


Where are operating systems found?
There are many types of operating systems, the complexity of which varies depending upon what type of functions are provided, and what the system is being used for. Some systems are responsible for managing many users on a network. Other operating systems do not manage user programs at all. These are typically found in hardware devices like petrol pumps, airplanes, video recorders, washing machines and car engines.


What is a general-purpose operating system?

Windows NT Workstation is known as a general-purpose operating system. This is because it provides the ability to run a number of different programs, such as games, word processing, business applications and program development tools.


A simple operating system for a security control system
An operating system for a security control system (such as a home alarm system) would consist of a number of programs. One of these programs would gain control of the computer system when it is powered on, and initialize the system.

The first task of this initialize program would be to reset (and probably test) the hardware sensors and alarms. Once the hardware initialization was complete, the operating system would enter a continual monitoring routine of all the input sensors. If the state of any input sensor changed, it would branch to an alarm generation routine.


What are Input and Output devices?
Input and output devices are components that form part of the computer system. These devices are controlled by the operating system.

Input devices provide input signals such as commands to the operating system. These commands received from input devices instruct the operating system to perform some task or control its behavior. Typical input devices are a keyboard, mouse, temperature sensor, air-flow valve or door switch. In the previous example of our simple security control system, the input devices could be door switches, alarm keypad panel and smoke detector units.

Output devices are instruments that receive commands or information from the operating system. Typical output devices are monitor screens, printers, speakers, alarm bells, fans, pumps, control valves, light bulbs and sirens.


Exercise 1
Classify each of the following devices as an input or output device


Exercise 2
For a simple home security system, make a list of possible input and output devices that you think might be required.

Input Devices Output Devices

Exercise 3

This is a Philips video recorder. Make a list of possible input and output devices that you think the operating system inside this video recorder might be required to manage.
Input Devices Output Devices


What is a single-user operating system?
We are all familiar with the concept of sitting down at a computer system and writing documents or performing some task such as writing a letter. In this instance there is one keyboard and one monitor that you interact with.

Operating systems such as Windows 95, Windows NT Workstation and Windows 2000 professional are essentially single user operating systems. They provide you the capability to perform tasks on the computer system such as writing programs and documents, printing and accessing files.

Consider a typical home computer. There is a single keyboard and mouse that accept input commands, and a single monitor to display information output. There may also be a printer for the printing of documents and images.

In essence, a single-user operating system provides access to the computer system by a single user at a time. If another user needs access to the computer system, they must wait till the current user finishes what they are doing and leaves.

Students in computer labs at colleges or University often experience this. You might also have experienced this at home, where you want to use the computer but someone else is currently using it. You have to wait for them to finish before you can use the computer system.


What is a multi-user operating system?
A multi-user operating system lets more than one user access the computer system at one time. Access to the computer system is normally provided via a network, so that users access the computer remotely using a terminal or other computer.

In the early days of large multi-user computers, multiple terminals (keyboards and associated monitors) were provided. These terminals sent their commands to the main multi-user computer for processing, and the results were then displayed on the associated terminal monitor screen. Terminals were hard-wired directly to the multi-user computer system.

Today, these terminals are generally personal computers and use a network to send and receive information to the multi-user computer system. Examples of multi-user operating systems are UNIX, Linux (a UNIX clone) and mainframes such as the IBM AS400.

The operating system for a large multi-user computer system with many terminals is much more complex than a single-user operating system. It must manage and run all user requests, ensuring they do not interfere with each other. Devices that are serial in nature (devices which can only be used by one user at a time, like printers and disks) must be shared amongst all those requesting them (so that all the output documents are not jumbled up). If each user tried to send their document to the printer at the same time, the end result would be garbage. Instead, documents are sent to a queue, and each document is printed in its entirety before the next document to be printed is retrieved from the queue. When you wait inline at the cafeteria to be served you are in a queue. Imagine that all the people in the queue are documents waiting to be printed and the cashier at the end of the queue is the printer.


Operating system utilities
The operating system consists of hundreds of thousands of lines of program code and stored on hard disk. Portions of the operating system are loaded into computer system memory (RAM) when needed. Utilities are provided for


Operating System Interfaces
In addition, the operating system provides each user with an interface that accepts, interprets and executes user commands or programs. This interface is commonly called a SHELL or command line interpreter (CLI). In some systems this might be a simple text mode line-by-line entry using keywords (such as MSDOS or UNIX), in other systems it might be highly graphical using windows and a pointing device such as a mouse (such as X-Windows).


Advantages and problems of multi-user operating systems
The advantage of having a multi-user operating system is that normally the hardware is very expensive, and it lets a number of users share this expensive resource. This means the cost is divided amongst the users. It also makes better use of the resources. Since the resources are shared, they are more likely to be in use than sitting idle being unproductive.

One problem with multi-user computer systems is that as more users access it, the performance becomes slower and slower. Another disadvantage is the cost of hardware, as a multi-user operating system requires a lot of disk space and memory. In addition, the actual software for multi-user operating systems tend to cost more than single-user operating systems.


What is a multi-tasking operating system?
A multi-tasking operating system provides the ability to run more than one program at once. For example, a user could be running a word processing package, printing a document, copying files to the floppy disk and backing up selected files to a tape unit. Each of these tasks the user is doing appears to be running at the same time.

A multi-tasking operating system has the advantage of letting the user run more than one task at once, so this leads to increased productivity. The disadvantage is that more programs that are run by the user, the more memory that is required.


Revision Exercise 1
List FOUR things an operating system does.


List THREE home appliance devices in your home that probably have an operating system controlling their function.


List FOUR hardware devices on a typical home computer that an operating system has to initialize when it is turned on.


List THREE possible errors that might occur in a typical home computer that an operating system has to handle.


State THREE possible basic routines an operating system might provide for handling a hardware device.


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