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Operating Systems/Introduction to Operating System Lecture Notes PCP Bhatt/IISc, Bangalore M1/V1/June 04/1
Module 1: Introduction to Operating System
Operating System (or shortly OS) primarily provides services for running applications on a computer system.
Need for an OS:
The primary need for the OS arises from the fact that user needs to be provided with services and OS ought to facilitate the provisioning of these services. The central part of a computer system is a processing engine called CPU. A system should make it possible for a user’s application to use the processing unit. A user application would need to store information. The OS makes memory available to an application when required. Similarly, user applications need use of input facility to communicate with the application. This is often in the form of a key board, or a mouse or even a joy stick (if the application is a game for instance).

The output usually provided by a video monitor or a printer as some times the user may wish to generate an output in the form of a printed document. Output may be available in some other forms. For example it may be a video or an audio file. Let us consider few applications.

• Document Design
• Accounting
• E-mail
• Image processing
Operating Systems/Introduction to Operating System Lecture Notes PCP Bhatt/IISc, Bangalore M1/V1/June 04/2
• Games
We notice that each of the above application requires resources for • Processing information
• Storage of Information
• Mechanism to inputting information
• Provision for outputting information
• These service facilities are provided by an operating system regardless of the nature of application.
The OS offers generic services to support all the above operations. These operations in turn facilitate the applications mentioned earlier. To that extent an OS operation is application neutral and service specific.

User and System View:
From the user point of view the primary consideration is always the convenience. It should be easy to use an application. In launching an application, it helps to have an icon which gives a clue which application it is. We have seen some helpful clues for launching a browser, e-mail or even a document preparation application. In other words, the human computer interface which helps to identify an application and its launch is very useful. This hides a lot of details of the more elementary instructions that help in selecting the application. Similarly, if we examine the programs that help us in using input devices like a key board – all the complex details of character reading program are hidden from the user. The same is true when we write a program. For instance, when we use a programming language like C, a printf command helps to generate the desired form of output. The following figure essentially depicts the basic schema of the use of OS from a user stand point. However, when it comes to the view point of a system, the OS needs to ensure that all the system users and applications get to use the facilities that they need. Operating Systems/Introduction to Operating System Lecture Notes PCP Bhatt/IISc, Bangalore M1/V1/June 04/3

Also, OS needs to ensure that system resources are utilized efficiently. For instance, there may be many service requests on a Web server. Each user request need to be serviced. Similarly, there may be many programs residing in the main memory. The system need to determine which programs are active and which need to await some form of input or output. Those that need to wait can be suspended temporarily from engaging the processor. This strategy alone enhances the processor throughput. In other words, it is important for an operating system to have a control policy and algorithm to allocate the system resources.

The Evolution:
It would be worthwhile to trace some developments that have happened in the last four to five decades. In the 1960s, the common form of computing facility was a mainframe computer system....
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