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Note for Operating Systems - OS By Ashish Kachhi

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Downloaded from be.rgpvnotes.in UNIT-I Operating System An Operating System (OS) is an interface between computer user and computer hardware. An operating system is software which performs all the basic tasks like file management, memory management, process management, handling input and output, and controlling peripheral devices such as disk drives and printers. Examples-Some Popular Operating Systems include Linux Operating System, Windows Operating System, VMS, OS/400, AIX, z/OS, etc. Definition An operating system is a program that acts as an interface between the user and the computer hardware and controls the execution of all kinds of programs. Following are some of important functions of an operating System.       Page no: 1 Memory Management Processor Management Device Management File Management Security Control over system performance Follow us on facebook to get real-time updates from RGPV

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Downloaded from be.rgpvnotes.in  Job accounting  Error detecting aids  Coordination between other software and users Memory Management Memory management refers to management of Primary Memory or Main Memory. Main memory is a large array of words or bytes where each word or byte has its own address. Main memory provides a fast storage that can be accessed directly by the CPU. For a program to be executed, it must in the main memory. An Operating System does the following activities for memory manage e t −  Keeps tracks of primary memory, i.e., what part of it are in use by whom, what part are not in use.  In multiprogramming, the OS decides which process will get memory when and how much.  Allocates the memory when a process requests it to do so.  De-allocates the memory when a process no longer needs it or has been terminated. Processor Management In multiprogramming environment, the OS decides which process gets the processor when and for how much time. This function is called process scheduling. An Operating System does the follo i g a ti ities fo p o esso a age e t −  Keeps tracks of processor and status of process. The program responsible for this task is known as traffic controller.  Allocates the processor (CPU) to a process.  De-allocates processor when a process is no longer required. Device Management An Operating System manages device communication via their respective drivers. It does the follo i g a ti ities fo de i e a age e t −  Keeps tracks of all devices. Program responsible for this task is known as the I/O controller.  Decides which process gets the device when and for how much time.  Allocates the device in the efficient way.  De-allocates devices. File Management A file system is normally organized into directories for easy navigation and usage. These directories may contain files and other directions. A Ope ati g S ste Page no: 2 does the follo i g a ti ities fo file a age e t − Follow us on facebook to get real-time updates from RGPV

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Downloaded from be.rgpvnotes.in  Keeps track of information, location, uses, status etc. The collective facilities are often known as file system.  Decides who gets the resources.  Allocates the resources.  De-allocates the resources. Other Important Activities Follo i g a e so e of the i po ta t a ti ities that a Ope ati g S ste pe fo s−  Security− B ea s of pass o d a d si ila othe techniques, it prevents unauthorized access to programs and data.  Control over system performance− Re o di g dela s et ee e uest fo a se i e a d response from the system.  Job accounting− Keepi g t a k of ti e a d esou es used a ious jo s a d use s.  Error detecting aids− P odu tio of du ps, t a es, e o essages, a d othe de uggi g a d error detecting aids.  Coordination between other software’s and users− Coo di atio a d assig e t of compilers, interpreters, assemblers and other software to the various users of the computer systems. Types of Operating System Operating systems are there from the very first computer generation and they keep evolving with time. In this chapter, we will discuss some of the important types of operating systems which are most commonly used. Batch operating system The users of a batch operating system do not interact with the computer directly. Each user prepares his job on an off-line device like punch cards and submits it to the computer operator. To speed up processing, jobs with similar needs are batched together and run as a group. The programmers leave their programs with the operator and the operator then sorts the programs with similar requirements into batches. The problems with Batch Systems are as follo s −  Lack of interaction between the user and the job.  CPU is often idle, because the speed of the mechanical I/O devices is slower than the CPU.  Difficult to provide the desired priority. Time-sharing operating systems Time-sharing is a technique which enables many people, located at various terminals, to use a particular computer system at the same time. Time-sharing or multitasking is a logical extension Page no: 3 Follow us on facebook to get real-time updates from RGPV

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Downloaded from be.rgpvnotes.in of multiprogramming. Processor's time which is shared among multiple users simultaneously is termed as time-sharing. The main difference between Multi-programmed Batch Systems and Time-Sharing Systems is that in case of Multi-programmed batch systems, the objective is to maximize processor use, whereas in Time-Sharing Systems, the objective is to minimize response time. Multiple jobs are executed by the CPU by switching between them, but the switches occur so frequently. Thus, the user can receive an immediate response. For example, in a transaction processing, the processor executes each user program in a short burst or quantum of computation. That is, if n users are present, then each user can get a time quantum. When the user submits the command, the response time is in few seconds at most. The operating system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time. Computer systems that were designed primarily as batch systems have been modified to time-sharing systems. Advantages of Timesharing operating systems  Provides the advantage of quick response.  Avoids duplication of software.  Reduces CPU idle time. Disadvantages of Time-sharing operating systems  Problem of reliability.  Question of security and integrity of user programs and data.  Problem of data communication. Distributed operating System Distributed systems use multiple central processors to serve multiple real-time applications and multiple users. Data processing jobs are distributed among the processors accordingly. The processors communicate with one another through various communication lines (such as high-speed buses or telephone lines). These are referred as loosely coupled systems or distributed systems. Processors in a distributed system may vary in size and function. These processors are referred as sites, nodes, computers, and so on. The advantages of distributed systems  With resource sharing facility, a user at one site may be able to use the resources available at another.  Speedup the exchange of data with one another via electronic mail. Page no: 4 Follow us on facebook to get real-time updates from RGPV

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