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  Write a note on real time OS. Ans. In real time operating system , job is to be completed within the rigid time constraints otherwise job loses its meaning. A real time system functions correctly only if it return the correct result within its time constraints Thus, in a real tome system , the correctness of the computation not only depends upon the logical correctness of the but also upon the time at which the result is produced. A real time system is often used as a central device in a dedicated applications like fuel-injection system, robotics, air- traffic control, medical imaging systems, systems that control scientific  experiment, industrial control system and weapon systems .  

  List of five service provided by an operating system. Explain how  each provides convenience to users?     The main purpose of operating system is to provide environment for the execution of programs. Thus an operating system provides certain services to program and the users of those programs. However , different operating system can provide different set of service. Some of the basis services provided by operating system are: 1.         Program execution ·          Operating system provides a convenient environment where users can run their programs ·          The operating system performs memory allocation to programs, load them into appropriate location so that they can execute. The users need not to worry about all these tasks.   2.         I/O Operation ·          in order to execute a program,it usually requires an I/O operations. For example , it may need a file and print the output. ·          When all these I/O operation are performed users cannot I/O dev

  S ecurity Thus securing a system involves following measures :                   1.        Preventing unauthorized access i.e. hiding the information from unauthorisation users.   2.        Protection the system from malicious damage or misuse. Such intential misuse can be of the following kinds :   ·          Theft of information ·          Unauthorized modification of data ·          Unauthorized destruction of data   3.        Protection the system against accidental misuse.   4.        The term security is often associated with protection with however, there is a difference between them. Protection is defined as guarding the user information against user within the system whereas security is defined as guarding the user information against unauthorized user outside the system

  Protection and security Protection in computer system   ·           In the system with contiguous memory allocation,    protection is usually accomplished by the use of limit registers. Each program    is assigned a specific limit in term of address space . whenever any reference is made to an instruction of the limit register is checked to ensure that reference is within the bound of program otherwise the access to memory is denied. ·           In paging system, protection is achieved by the use of a page table. This page table list all the pages that a program can access. ·           In addition the table store access rights such as read, write or execute for each individual page. ·           In a system that uses segmentation, protection is accomplished by the use of segment table . each process has its segment table   ·           In case of files stored on secondary storage devices suchj as disks, access rights are associated with each file system. Any user who tries to access a

 C omparison between real time and time sharing operating system P rotection and s ecurity  • Protection refers to a mechanism for controlling the access of program s processes, or users to the resources defined by computer system. • The concept of protection came with the advent of multiprogramming where several processes compete for the use of CPU. • the purpose was to confine each users program to its assigned areaof memory so that the programs cannot interface and harm each other. • Protection in main memory is particularly important because of address translation. The purpose of protection is to allow concurrently running process to share the common physical address space. • Protection also ensure that only process that have gained proper authorization from the operating system can operate on memory segment , the CPU, files and other resources.

  Multi threading  operating system ·           Multi threading is a technique in which a process, executing an application is divided into threads that can run concurrently ·           A thread is a dispatch able unit of work. It includes    a processor context and its own data area for sack . ·           A thread execute sequentially and is interruptible so that the processor can turn to another thread. ·           Thus, a thread represents a light weight process and is the smallest unit of CPU utilization it is like a mini process. ·           A process, on other hand is a collection of one or more threads and associated system resources ·           A thread is not a process by itself. Cannot run on its own. It always run within a process. ·           Thus, a multithreaded process may have multiple execution flows, different once belonging to difficult threads.   M ulti Threading Operating  System A thread is not a process by itself .it cannot run on its own. It always run within a

  R eal-time system ·           In a real time operating system, a job is to be completed within the    rigid time constraints otherwise job loses its meaning. ·           A real time system function correctly only if it returns the correct result within its time constraints ·           Thus, in a    real-time system, the correctness of the    computation not only depends upon the logical correctness of the computation but also upon the time at which the result is produced. R eal-time system In a real time system various functions like process scheduling, memory management, I/O management and file  management are performed as follows : 1.         Process scheduling  . the real time system uses priority based pre-emptive scheduling. each process is assigned a certain level of priority and the process with the highest priority is allocated the processor among those which are in the ready state. 2.         Memory managemen t . In real time system, the processes remain in primary memory fo

  Distributed system ·           A distributed system is a collection of processor located in geographical dispersal physical location. ·           In this system, the workhold is distributed or divided between two or more computers that are linked together by communication network . that is the different processors communicate using communication links, such as telephone lines and buses3 ·           The various processors do not share memory, clock ,or peripheral devices instead each processors has its local memory. ·           The purpose of distributed system is to provide an efficient and convenient environment for sharing of resources. ·           Some example of distributed operating system are amoeba, angle, chorus, much, alpha kernel. ·           The processors in a distributed system vary in size and function. They may include small microprocessors, workstations microcomputers, mainframe computers and large general purpose computers. M odel of distributed operating system The

  Time sharing  system ·           Time sharing refers to the allocation of computer resources in a time dependent fashion to several program simultaneously ·           A time sharing system has many user terminals that are connected to same computer simultaneously. Using these terminal, different users can work on a system at the same time ·           Thus, it uses multi programming with a special CPU scheduling among all the last one, and then again beginning from the first one ·           In time sharing system, the CPU time is divided among all the users on schedule basis. ·           It release the CPU under any of the following three conditions: 1.         When the allotted time slice expires. 2.         When the program needs to perform I/O operation. 3.         When the execution of the program is over during the time slice ·           Thus the time sharing, when the CPU is allocated to user program, the user uses the CPU for the period of time slot.                single user