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CS101 - Introduction to Computing - Lecture Handout 11

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Operating Systems

Focus of the last lecture: computer SW
We found out about the role SW plays in a computing environment
We learned to distinguish between SW belonging to the system & application categories
Also discussed the different types of SW licenses:
Open source

Learning Goals for Today

The role of the operating system in a computing environment
The various functions that an operating system performs
The main components of an operating system
Various types of operating systems

Why Have OSes?

User/programmer convenience
Greater resource utilization

The Role of An OS

The 1st program that runs when a typical computer is turned ON, and the last one to finish running when the computer is turned OFF.
It manages the HW and SW resources of the computer system, often invisibly. These include the processor, memory, disk drives, etc.
It provides a simple, consistent way for applications to interact with the HW without having to know all the details of the HW

Advantage for App. Developers

Application developers do not need to know much about the HW while they are developing their app
They just develop with a particular OS in mind. If the OS runs on many types of computers having different HW configurations, so will the app – without making any HW-specific modifications in the app SW. The OS hides the HW differences from the app

Are OS’es Essential?

No. If a computer has been designed for limited functionality (e.g. it runs just a single program all the time as in a automatic clothes washing machine), it does not require a traditional OS
In limited-functionality computers, an OS just adds to the overhead unnecessarily, which impedes the computer’s performance
In these situations, the required parts of the OS are integrated into the the only program that is going to run

In the beginning …

A single user ran a single program ran on a single computer – there was no need for an OS
Then came computer operators who ran multiple programs for multiple users one after the other – still, no need for an OS
Later computers became powerful, & became able to run multiple programs, simultaneously. That’s when the need for OS’es arose for:
Managing the resources of the computers efficiently
Making use of computers convenient for users/programmers

Core Tasks of an OS

Processor management
Memory management
Device management
Storage management

Application Interface
User Interface

Processor Management
Memory Management

Straight forward for a single-user, single tasking
Each app must have enough private memory in which to execute
App can neither run into the private memory space of another app, nor be run into by another app
Different types of memory (e.g. main, cache) in the system must be used properly, so that each app can run most effectively

Storage Management

The OS manages storage through one of its sub-modules, the File Manager
A file system is a collection of directories, subdirectories, and files organized in a logical order.
File manager maintains an index of the filenames & where they are located on the disk.
File manager make it easy to find the required file in a logical and timely fashion.

Device Management

Applications talk to devices through the OS and OS talks to and manages devices through Device Drivers
Example: When we print to a laser printer, we do not need to know its details. All we do is to tell the printer device driver about what needs to be printed and it takes care of the details

Application Interface

App developers do not need to know much about the HW, especially the uP, while they are developing their app
The OS provides all apps with a straight-forward and consistent interface to the HW

Example: An app uses the OS to store data on the disk drive. For that, the app does not need to know about the exact physical characteristics of that drive; it just tells the OS to do that through the app interface, and the OS takes cares of all the details of the task

User Interface

Users communicate with the computer using a consistent user interface provided by the OS
This UI can be a command-line interface in which a user types in the commands.

copy a:/file1.html c:/file1.html

Or, it can be a graphical UI, where Windows, Icons, Menus, and a Pointing device (such as a mouse) is used to receive and display information. Example:

With the help of the mouse, drag file1.html from drive a to drive c

OS Components


OS Components


The heart of the OS
Responsible for all the essential operations like basic house keeping, task scheduling, etc.
Also contains low-level HW interfaces
Size important, as it is memory-resident

Types of OS’es

Classification w.r.t. the type of computers they run on and the type of applications they support
Real-Time Operating System (RTOS)
Single-User, Single Task
Single-User, Multi-Tasking

RTOS (1)

Used to run computers embedded in machinery, robots, scientific instruments and industrial systems
Typically, it has little user interaction capability, and no end-user utilities, since the system will be a "sealed box" when delivered for use
Examples: Wind River, QNX, Real-time Linux, Real-time Windows NT

RTOS (2)

An important part of an RTOS is managing the resources of the computer so that a particular operation executes in precisely the same amount of time every time it occurs In a complex machine, having a part move more quickly just because system resources are available may be just as catastrophic as having it not move at all because the system was busy

Single-User, Single Task

OS’es designed to manage the computer so that one user can effectively do one thing at a time
The Palm OS used in many palmtop computers (PDA’s) is an example of a single-user, single-task OS

Single-User, Multi-Tasking

Most popular OS
Used by most all PC’s and Laptops
Examples: Windows, Mac OS, Linux
Lets a single user interact with several programs, simultaneously


A multi-user OS allows many users to take advantage of the computer's resources, simultaneously
The OS must make sure that the requirements of the various users are balanced, and that the programs they are using each have sufficient and separate resources so that a problem with one user doesn't affect any of the other users
Examples: Linux, Unix, VMS and mainframe OS’es, such as MVS

Another Way of Classifying

Uni-processor OS’es
Designed to schedule tasks on a single uP only
Example: DOS
Multi-processor OS’es
Can control computers having multiple uPs, at times 1000’s of them
Example: Current versions of Windows, Mac OS, Linux, Solaris

How many different OS’es are there?

OS’es from the Windows family dominate the desktops and run on millions of PC’s
OS’es from the Unix family (Unix, Linux, etc) are quite popular on servers

There are hundreds more. Some designed for mainframes only. Some for embedded applications only.

Comparing Popular OS’es

Comparing Popular OS’es

What have we learnt today?

The role of the OS in a computing environment
The various functions that an OS performs
The main components of an OS
Various types of OS’es

Next Lecture: Application SW

We’ll learn about application SW, i.e. programs that interact directly with the user for the performance of a certain type of work
We’ll try to become familiar with various SW used in the following application areas: