Note for Operating Systems - OS By Dr. Manish Saraswat

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Chapter -1 INTRODUCTION TO OPERATING SYSTEMS OBJECTIVES: After ending of This chapter you will be able to Answer: ❖ Concept of O.S ❖ Interaction of OS ❖ Components of OS ❖ Kernel system and Services ❖ Various types of OS 1.0 INTRODUCTION Operating systems are an essential part of any computer system. An operating system is a program that manages the computer hardware. It also provides a basis for application programs and acts as an intermediary between a user of a computer and the computer hardware. An amazing aspect of operating systems is how varied they are in accomplishing these tasks. Mainframe operating systems are designed primarily to optimize utilization of hardware. Personal computer (PC) operating systems support complex games, business applications, and everything in between. Handheld computer operating systems are designed to provide an environment in which a user can easily interface with the computer to execute programs. Thus, some operating systems are designed to be convenient, others to be eficient, and others some combination of the two. 1.1 What is Operating System? Operating system consists of a set of programs (software) that manages the hardware and provides an environment to run multiple application programs. The OS shares the hardware (CPU, Memory, I/O controllers and I/O devices) among multiple application programs. OR An operating system is software, consisting of programs and data, which runs on computers, manages computer hardware resources, and provides common services for execution of various application software. Operating system is the most important type of system software in a computer system. Without an operating system, a user cannot run an application program on their computer, unless the application program is self booting. For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between application programs and the computer hardware, although the application code is usually executed directly by the

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hardware and will frequently call the OS or be interrupted by it. Operating systems are found on almost any device that contains a computer—from cellular phones and video game consoles to supercomputers and web servers. Examples of popular modern operating systems are: Linux OpenSuSE, Mandriva, Arch Linux, Debian, Linux mint etc.), Mac OS X,Microsoft Windows and UNIX. 1.2. Components of an Operating System Operating system (OS) is not a single program; it is a set of programs. However the real core of an operating system is called Kernel. The second important program of an OS is User Interface (UI) program. Old operating systems used to have only Command Line user Interface (CLI) program called Shell. But all modern operating systems are coming with Graphical User Interfaces (GUI) called Windows. All the other programs that come with the OS are utility application programs. Finally there are lots of applications, which we separately buy (which is legal) or get pirated copy of application (illegal) or download freeware/shareware applications from the Internet. This is illustrated in the following figure. Fig:1.0 Component and Interaction of OS with User

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The set of programs that comes with OS, and which we call OS is shown inside the dashed lines. The most important part of the OS, is the Kernel. But just kernel alone is not enough. So OS comes with two types of user interface programs. One is graphics based and other is command (or text) based. Besides these user interface programs, OS also includes many utility programs, to create and list file, directories etc.. 1.3 Components of an Operating System Operating system (OS) is not a single program; it is a set of programs. However the real core of an operating system is called Kernel. The second important program of an OS is User Interface (UI) program. Old operating systems used to have only Command Line user Interface (CLI) program called Shell. But all modern operating systems are coming with Graphical User Interfaces (GUI) called Windows. All the other programs that come with the OS are utility application programs. Finally there are lots of applications, which we separately buy (which is legal) or get pirated copy of application (illegal) or download freeware/shareware applications from the Internet. This is illustrated in the above Figure The set of programs that comes with OS, and which we call OS is shown inside the dashed lines. The most important part of the OS, is the Kernel. But just kernel alone is not enough. So OS comes with two types of user interface programs. One is graphics based and other is command (or text) based. Besides these user interface programs, OS also includes many utility programs, to create and list file, directories etc.. Even though GUI, CLI and utility programs come with OS, still they are considered as applications. One major difference between applications and kernel is that, Kernel runs in kernel mode or Supervisor mode of the CPU, where as all applications (that comes with OS also) will run in user mode of the CPU. It is very difficult to distinguish between applications you separately buy and applications that come along with OS. For example windows bundles lot of applications as part of the OS. Examples are Internet explorer (Browser), Outlook express (E-mail client), Media player etc.. Finally people use OS for different purposes. Ordinary users will use it to run the applications. Administrators are responsible for maintaining the OS and other applications. And programmers are responsible for the development of new programs. We may also classify the programmers into System Programmers and Applications programmers. System programmers directly deal with the services (system calls) provided by the OS. Where as application programmers use the APIs (Application Programming Interface functions) provided by the application environments such as Database, Web, Graphics, and distributed application environments. These application environments in turn run on top of the OS.

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What normal user sees in an operating system is its user interface that could be either CLI based or GUI based, and application programs that comes along with the OS. For normal users, OS provides an interface to run their favorite applications. Not just one, but multiple applications at a time. But what system programmer sees in an operating system are its kernel, the system calls provided by the kernel, and libraries that comes with the operating system. The programmer wants to develop some quality useful applications for this operating system. So she/he should know the services (system calls) provided by the kernel. So that programs can use these kernel services. You are reading this because you want to become a system programmer. So as a system programmer we will concentrate on the kernel. For you OS means it is kernel. Its user interface and application programs are not important for you, but they are very important for all normal users. Following are the typical services provided by the Kernel to the system applications: · File Services · I/O Services · Multi-processing services · Multi-threading services · Memory allocation and mapping services · Signal/Event services · Inter Process Communication and Synchronization services · Time services · Network communication services All most all applications need to use all or some of the above services provided by the operating system. Review Questions What is Operating System? What are the various programs that belong to the operating system? What are the two types of user interfaces provided by the OS, and what are the differences between these two types of interfaces? Is it possible to differentiate the applications that are part of OS, and applications, which are not part of OS? What is the main difference between Kernel and Application programs? How do you classify the people using the operating system? What are the differences between Application programs and System programs? What are the types of services provided by the kernel? What are the components present in the computer hardware?