UNIT 1 JAGANNATH UNIVERSITY UNIT 2. Define Operating system and its functions. Explain different types of Operating System

Transcription

1 JAGANNATH UNIVERSITY BCAII OPERATING SYSTEM MODEL TEST PAPER (SOLVED) UNIT 1 Q1 Q2 Q3 Q4 Q5 Define Operating system and its functions Explain different types of Operating System Describe different types of services provided by operating system Explain different properties of Operating System Define System Calls and Explain various types of System calls UNIT 2 Q1 Q2 Q3 Q4 Define process, and explain various states of a process with the help of a process state transition diagram. What is a scheduler,explain various types of Schedulers Explain different types of Scheduling Algorithms Discuss various operations on a process and define co-operating process Q5 Explain: a) PCB b) Thread, Types of Thread

2 SOLUTIONS UNIT 1 ANS 1: An operating System (OS) is an intermediary between users and computer hardware. It provides users an environment in which a user can execute programs conveniently and efficiently. In technical terms, it is software which manages hardware. An operating System controls the allocation of resources and services such as memory, processors, devices and information. 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. An operating system is a program that manages the computer hardware. It provided a basis for application programs and acts as an intermediary between the computer user and the computer hardware. An operating system (OS) is a set of computer program that manages the hardware and software resources of a computer. At the foundation of all system software, the OS performs basic tasks such as controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating networking, and managing files. It also may provide a graphical user interface for higher level functions Following are some of important functions of an operating System. Memory Management Processor Management Device Management File Management Security Control over system performance Job accounting Error detecting aids Coordination between other software and users

3 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 access directly by the CPU. So for a program to be executed, it must in the main memory. Operating System does the following activities for memory management. Keeps tracks of primary memory i.e. what part of it are in use by whom, what part are not in use. In multiprogramming, OS decides which process will get memory when and how much. Allocates the memory when the process requests it to do so. De-allocates the memory when the process no longer needs it or has been terminated. Processor Management In multiprogramming environment, OS decides which process gets the processor when and how much time. This function is called process scheduling. Operating System does the following activities for processor management. Keeps tracks of processor and status of process. Program responsible for this task is known as traffic controller. Allocates the processor (CPU) to a process. De-allocates processor when processor is no longer required. Device Management OS manages device communication via their respective drivers. Operating System does the following activities for device management. 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. Operating System does the following activities for file management. Keeps track of information, location, uses, status etc. The collective facilities are often known as file system. Decides who gets the resources.

4 Allocates the resources. De-allocates the resources. Other Important Activities Following are some of the important activities that Operating System does. Security -- By means of password and similar other techniques, preventing unauthorized access to programs and data. Control over system performance -- Recording delays between request for a service and response from the system. Job accounting -- Keeping track of time and resources used by various jobs and users. Error detecting aids -- Production of dumps, traces, error messages and other debugging and error detecting aids. Coordination between other software and users -- Coordination and assignment of compilers, interpreters, assemblers and other software to the various users of the computer systems. A ANS2 Operating systems are there from the very first computer generation. Operating systems keep evolving over the period of time. Following are few of the important types of operating system which are most commonly used. 1. Batch operating system The users of 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. Thus, the programmers left their programs with the operator. The operator then sorts programs into batches with similar requirements. The problems with Batch Systems are following. Lack of interaction between the user and job. CPU is often idle, because the speeds of the mechanical I/O devices is slower than CPU. Difficult to provide the desired priority.

5 2. Multiprogramming System When two or more programs are in memory at the same time, sharing the processor is referred to the multiprogramming operating system. Multiprogramming assumes a single processor that is being shared. It increases CPU utilization by organizing jobs so that the CPU always has one to execute. The operating system keeps several jobs in memory at a time. This set of jobs is a subset of the jobs kept in the job pool. The operating system picks and begins to execute one of the job in the memory. Multiprogrammed system provide an environment in which the various system resources are utilized effectively, but they do not provide for user interaction with the computer system. Jobs entering into the system are kept into the memory. Operating system picks the job and begins to execute one of the job in the memory. Having several programs in memory at the same time requires some form of memory management. Multiprogramming operating system monitors the state of all active programs and system resources. This ensures that the CPU is never idle unless there are no jobs. Advantages 1. High CPU utilization. 2. It appears that many programs are allotted CPU almost simultaneously. Disadvantages 1. CPU scheduling is requires. 2. To accommodate many jobs in memory, memory management is required. 3. 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 of multiprogramming. Processor's time which is shared among multiple users simultaneously is termed as time-sharing. The main difference between Multiprogrammed Batch Systems and Time-Sharing Systems is that in case of Multiprogrammed batch systems, objective is to maximize processor use, whereas in Time-Sharing Systems 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 receives an immediate response. For example, in a transaction processing, processor execute each user program in a short burst or quantum of computation. That is if n users are present, each user can get time quantum. When the user submits the command, the response time is in few seconds at most.

6 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 timesharing systems. Advantages of Timesharing operating systems are following Provide advantage of quick response. Avoids duplication of software. Reduces CPU idle time. Disadvantages of Timesharing operating systems are following. Problem of reliability. Question of security and integrity of user programs and data. Problem of data communication. 4. Distributed operating System Distributed systems use multiple central processors to serve multiple real time application and multiple users. Data processing jobs are distributed among the processors accordingly to which one can perform each job most efficiently. 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 are following. With resource sharing facility 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. If one site fails in a distributed system, the remaining sites can potentially continue operating. Better service to the customers. Reduction of the load on the host computer.

7 Reduction of delays in data processing. 5. Network operating System Network Operating System runs on a server and and provides server the capability to manage data, users, groups, security, applications, and other networking functions. The primary purpose of the network operating system is to allow shared file and printer access among multiple computers in a network, typically a local area network (LAN), a private network or to other networks. Examples of network operating systems are Microsoft Windows Server 2003, Microsoft Windows Server 2008, UNIX, Linux, Mac OS X, Novell NetWare, and BSD. The advantages of network operating systems are following. Centralized servers are highly stable. Security is server managed. Upgrades to new technologies and hardwares can be easily integrated into the system. Remote access to servers is possible from different locations and types of systems. The disadvantages of network operating systems are following. High cost of buying and running a server. Dependency on a central location for most operations. Regular maintenance and updates are required. 6. Real Time operating System Real time system is defines as a data processing system in which the time interval required to process and respond to inputs is so small that it controls the environment. Real time processing is always on line whereas on line system need not be real time. The time taken by the system to respond to an input and display of required updated information is termed as response time. So in this method response time is very less as compared to the online processing. Real-time systems are used when there are rigid time requirements on the operation of a processor or the flow of data and real-time systems can be used as a control device in a dedicated application. Real-time operating system has well-defined, fixed time constraints otherwise system will fail. For example

8 Scientific experiments, medical imaging systems, industrial control systems, weapon systems, robots, and home-applicance controllers, Air traffic control system etc. There are two types of real-time operating systems. HARD REAL-TIME SYSTEMS Hard real-time systems guarantee that critical tasks complete on time. In hard real-time systems secondary storage is limited or missing with data stored in ROM. In these systems virtual memory is almost never found. SOFT REAL-TIME SYSTEMS Soft real time systems are less restrictive. Critical real-time task gets priority over other tasks and retains the priority until it completes. Soft real-time systems have limited utility than hard real-time systems. For example, Multimedia, virtual reality, Advanced Scientific Projects like undersea exploration and planetary rovers etc. ANS3 An Operating System provides services to both the users and to the programs. 1. It provides programs, an environment to execute. 2. It provides users, services to execute the programs in a convenient manner. Following are few common services provided by operating systems. Program execution I/O operations File System manipulation Communication Error Detection Resource Allocation Protection

9 Program execution Operating system handles many kinds of activities from user programs to system programs like printer spooler, name servers, file server etc. Each of these activities is encapsulated as a process. A process includes the complete execution context (code to execute, data to manipulate, registers, OS resources in use). Following are the major activities of an operating system with respect to program management. Loads a program into memory. Executes the program. Handles program's execution. Provides a mechanism for process synchronization. Provides a mechanism for process communication. Provides a mechanism for deadlock handling. I/O Operation I/O subsystem comprised of I/O devices and their corresponding driver software. Drivers hides the peculiarities of specific hardware devices from the user as the device driver knows the peculiarities of the specific device. Operating System manages the communication between user and device drivers. Following are the major activities of an operating system with respect to I/O Operation. 1. I/O operation means read or write operation with any file or any specific I/O device. 2. Program may require any I/O device while running. 3. Operating system provides the access to the required I/O device when required. File system manipulation A file represents a collection of related information. Computer can store files on the disk (secondary storage), for long term storage purpose. Few examples of storage media are magnetic tape, magnetic disk and optical disk drives like CD, DVD. Each of these media has its own properties like speed, capacity, data transfer rate and data access methods. A file system is normally organized into directories for easy navigation and usage. These directories may contain files and other directions. Following are the major activities of an operating system with respect to file management. Program needs to read a file or write a file.

10 The operating system gives the permission to the program for operation on file. Permission varies from read-only, read-write, denied and so on. Operating System provides an interface to the user to create/delete files. Operating System provides an interface to the user to create/delete directories. Operating System provides an interface to create the backup of file system. Communication In case of distributed systems which are a collection of processors that do not share memory, peripheral devices, or a clock, operating system manages communications between processes. Multiple processes with one another through communication lines in the network. OS handles routing and connection strategies, and the problems of contention and security. Following are the major activities of an operating system with respect to communication. Two processes often require data to be transferred between them. The both processes can be on the one computer or on different computer but are connected through computer network. Communication may be implemented by two methods either by Shared Memory or by Message Passing. Error handling Error can occur anytime and anywhere. Error may occur in CPU, in I/O devices or in the memory hardware. Following are the major activities of an operating system with respect to error handling. OS constantly remains aware of possible errors. OS takes the appropriate action to ensure correct and consistent computing. Resource Management In case of multi-user or multi-tasking environment, resources such as main memory, CPU cycles and files storage are to be allocated to each user or job. Following are the major activities of an operating system with respect to resource management. OS manages all kind of resources using schedulers. CPU scheduling algorithms are used for better utilization of CPU.

11 Protection Considering computer systems having multiple users the concurrent execution of multiple processes, then the various processes must be protected from each another's activities. Protection refers to mechanism or a way to control the access of programs, processes, or users to the resources defined by computer systems. Following are the major activities of an operating system with respect to protection. OS ensures that all access to system resources is controlled. OS ensures that external I/O devices are protected from invalid access attempts. OS provides authentication feature for each user by means of a password. Ans 4 Following are few of very important tasks that Operating System handles and may be considered as the properties of OS. 1. Batch processing Batch processing is a technique in which Operating System collects one programs and data together in a batch before processing starts. Operating system does the following activities related to batch processing. o OS defines a job which has predefined sequence of commands, programs and data as a single unit. o o o OS keeps a number a jobs in memory and executes them without any manual information. Jobs are processed in the order of submission i.e. first come first served fashion. When job completes its execution, its memory is released and the output for the job gets copied into an output spool for later printing or processing.

12 Advantages Batch processing takes much of the work of the operator to the computer. Increased performance as a new job gets started as soon as the previous job finished without any manual intervention. Disadvantages Difficult to debug program. A job could enter an infinite loop. Due to lack of protection scheme, one batch job can affect pending jobs.

13 3. Multitasking Multitasking refers to term where multiple jobs are executed by the CPU simultaneously by switching between them. Switches occur so frequently that the users may interact with each program while it is running. Operating system does the following activities related to multitasking. The user gives instructions to the operating system or to a program directly, and receives an immediate response. Operating System handles multitasking in the way that it can handle multiple operations / executes multiple programs at a time. Multitasking Operating Systems are also known as Time-sharing systems. These Operating Systems were developed to provide interactive use of a computer system at a reasonable cost. A time-shared operating system uses concept of CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared CPU. Each user has at least one separate program in memory.

14 A program that is loaded into memory and is executing is commonly referred to as a process. When a process executes, it typically executes for only a very short time before it either finishes or needs to perform I/O. Since interactive I/O typically runs at people speeds, it may take a long time to complete. During this time a CPU can be utilized by another process. Operating system allows the users to share the computer simultaneously. Since each action or command in a time-shared system tends to be short, only a little CPU time is needed for each user. As the system switches CPU rapidly from one user/program to the next, each user is given the impression that he/she has his/her own CPU, whereas actually one CPU is being shared among many users.

15 4. Multiprogramming When two or more programs are residing in memory at the same time, then sharing the processor is referred to the multiprogramming. Multiprogramming assumes a single shared processor. Multiprogramming increases CPU utilization by organizing jobs so that the CPU always has one to execute. Following figure shows the memory layout for a multiprogramming system. Operating system does the following activities related to multiprogramming. The operating system keeps several jobs in memory at a time. This set of jobs is a subset of the jobs kept in the job pool. The operating system picks and begins to execute one of the job in the memory. Multiprogramming operating system monitors the state of all active

16 programs and system resources using memory management programs to ensures that the CPU is never idle unless there are no jobs Advantages High and efficient CPU utilization. User feels that many programs are allotted CPU almost simultaneously. Disadvantages CPU scheduling is required. To accommodate many jobs in memory, memory management is required. 5. Interactivity Interactivity refers that a User is capable to interact with computer system. Operating system does the following activities related to interactivity. o o o o OS provides user an interface to interact with system. OS managers input devices to take inputs from the user. For example, keyboard. OS manages output devices to show outputs to the user. For example, Monitor. OS Response time needs to be short since the user submits and waits for the result. 6. Real Time System Real time systems represents are usually dedicated embedded systems.

17 Operating system does the following activities related to real time system activity. o In such systems, Operating Systems typically read from and react to sensor data. o The Operating system must guarantee response to events within fixed periods of time to ensure correct performance. 7. Distributed Environment Distributed environment refers to multiple independent CPUs or processors in a computer system. Operating system does the following activities related to distributed environment. OS Distributes computation logics among several physical processors. The processors do not share memory or a clock. Instead, each processor has its own local memory. OS manages the communications between the processors. They communicate with each other through various communication lines. 8. Spooling Spooling is an acronym for simultaneous peripheral operations on line. Spooling refers to putting data of various I/O jobs in a buffer. This buffer is a special area in memory or hard disk which is accessible to I/O devices. Operating system does the following activities related to distributed environment. OS handles I/O device data spooling as devices have different data access rates.

18 OS maintains the spooling buffer which provides a waiting station where data can rest while the slower device catches up. OS maintains parallel computation because of spooling process as a computer can perform I/O in parallel fashion. It becomes possible to have the computer read data from a tape, write data to disk and to write out to a tape printer while it is doing its computing task. Advantages The spooling operation uses a disk as a very large buffer. Spooling is capable of overlapping I/O operation for one job with processor operations for another job.

19 ANS 5 System Calls System calls provide an interface to the services made available by an operating system. An example to illustrate how system calls are used: Writing a simple program to read data from one file and copy them to another file- a) First input required is names of two files input file and output file. Names can be specified in many ways- One approach is for the program to ask the user for the names of two files. In an interactive system, this approach will require a sequence of system calls, to write a prompting message on screen and then read from the keyboard the characters that define the two files. On mouse based and icon based systems, a menu of file names is displayed in a window where the user can use the mouse to select the source names and a window can be opened for the destination name to be specified. b) Once the two file names are obtained, program must open the input file and create the output file. Each of these operations requires another system call.when the program tries to open input file, no file of that name may exist or file is protected against access. Program prints a message on console and terminates abnormally. If input file exists, we must create a new output file. If the output file with the same name

20 Pictorial representation of system callsexists, the situation caused the program to abort or delete the existing file and create a new one. Another option is to ask the user(via a sequence of system calls) whether to replace the existing file or to abort the program. When both files are set up, a loop reads from the input file and writes to the output file (system calls respectively). Each read and write must return status information regarding various possible error conditions. After entire file is copied, program closes both files, write a message to the console or window and finally terminate normally.application developers design programs according to application programming interface (API). API specifies set of functions that are available to an application programmer.three of the most common API s available to applicat ion programmers are the Win32API for Windows Systems; POSIX API for POSIX based systems (which include all versions of UNIX, Linux and Mac OS X) and Java API for designing programs that run on Java virtual machine. The functions that make up the API typically invoke the actual system calls on behalf of the application programmer.

21 UNIT 2 ANS 1 A process is sequential program in execution. A process defines the fundamental unit of computation for the computer. Components of process are : 1. Object Program 2. Data 3. Resources 4. Status of the process execution. Object program i.e. code to be executed. Data is used for executing the program. While executing the program, it may require some resources. Last component is used for verifying the status of the process execution. A process can run to completion only when all requested resources have been allocated to the process. Two or more processes could be executing the same program, each using their own data and resources. Processes Process is a dynamic entity that is a program in execution. A process is a sequence of information executions. Process exists in a limited span of time. Two or more processes could be executing the same program, each using their own data and resources. Process State When process executes, it changes state. Process state is defined as the current activity of the process. the general form of the process state transition diagram. Process state contains five states. Each process is in one of the states. The states are listed below.

22 1. New 2. Ready 3. Running 4. Waiting 5. Terminated (exist) 1. New: A process that just been created. 2. Ready: Ready processes are waiting to have the processor allocated to them by the operating system so that they can run. 3 Running: The process that is currently being executed. A running process possesses all the resources needed for its execution, including the processor. 4. Waiting: A process that can not execute until some event occurs such as the completion of an I/O operation. The running process may become suspended by invoking an I/O module. 5. Terminated: A process that has been released from the pool of executable processes by the operating system. Diagram for Process State

23 Whenever processes changes state, the operating system reacts by placing the process PCB in the list that corresponds to its new state. Only one process can be running on any processor at any instant and many processes may be ready and waiting state. Suspended Processes Characteristics of suspend process 1. Suspended process is not immediately available for execution. 2. The process may or may not be waiting on an event. 3. For preventing the execution, process is suspend by OS, parent process, process itself and an agent. 4. Process may not be removed from the suspended state until the agent orders the removal. Swapping is used to move all of a process from main memory to disk. When all the process by putting it in the suspended state and transferring it to disk. Reasons for process suspension 1. Swapping 2. Timing 3. Interactive user request 4. Parent process request Two State Process Model Two state process model refers to running and non-running states which are described below. Running When new process is created by Operating System that process enters into the system as in the running state.

24 Non-Running Processes that are not running are kept in queue, waiting for their turn to execute. Each entry in the queue is a pointer to a particular process. Queue is implemented by using linked list. Use of dispatcher is as follows. When a process is interrupted, that process is transferred in the waiting queue. If the process has completed or aborted, the process is discarded. In either case, the dispatcher then selects a process from the queue to execute. ANS 2 Schedulers Schedulers are special system software which handles process scheduling in various ways. Their main task is to select the jobs to be submitted into the system and to decide which process to run. Schedulers are of three types Long Term Scheduler Short Term Scheduler Medium Term Scheduler Long Term Scheduler It is also called job scheduler. Long term scheduler determines which programs are admitted to the system for processing. Job scheduler selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling. The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound. It also controls the degree of multiprogramming. If the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system.

25 On some systems, the long term scheduler may not be available or minimal. Time-sharing operating systems have no long term scheduler. When process changes the state from new to ready, then there is use of long term scheduler. Short Term Scheduler It is also called CPU scheduler. Main objective is increasing system performance in accordance with the chosen set of criteria. It is the change of ready state to running state of the process. CPU scheduler selects process among the processes that are ready to execute and allocates CPU to one of them. Short term scheduler also known as dispatcher, execute most frequently and makes the fine grained decision of which process to execute next. Short term scheduler is faster than long term scheduler. Medium Term Scheduler Medium term scheduling is part of the swapping. It removes the processes from the memory. It reduces the degree of multiprogramming. The medium term scheduler is incharge of handling the swapped out-processes. Running process may become suspended if it makes an I/O request. Suspended processes cannot make any progress towards completion. In this condition, to remove the process from memory and make space for other process, the suspended process is moved to the secondary storage. This process is called swapping, and the process is said to be swapped out or rolled out. Swapping may be necessary to improve the process mix.

26 S.N Comparison between Scheduler Long Term Scheduler Short Term Scheduler Medium Term Scheduler 1 It is a job scheduler It is a CPU scheduler It is a process swapping scheduler. 2 Speed is lesser than short Speed is fastest among Speed is in between both term scheduler other two short and long term scheduler. It controls the degree of It provides lesser control It reduces the degree of

27 3 over degree of multiprogramming multiprogramming multiprogramming. 4 It is almost absent or It is also minimal in It is a part of Time minimal time sharing in time sharing system sharing system systems. It selects those It selects processes from processes 5 pool and loads them into which are ready to It can re-introduce the process into memory and execution memory for execution Execute can be continued. ANS 3 Four major scheduling algorithms here which are following First Come First Serve (FCFS) Scheduling Shortest-Job-First (SJF) Scheduling Priority Scheduling

28 Round Robin(RR) Scheduling Multilevel Queue Scheduling 1. First Come First Serve (FCFS) Jobs are executed on first come, first serve basis. Easy to understand and implement. Poor in performance as average wait time is high. Wait time of each process is following Process Wait Time : Service Time - Arrival Time P0 0-0 = 0 P1 5-1 = 4 P2 8-2 = 6 P = 13 Average Wait Time: ( ) / 4 = 5.55

29 2. Shortest Job First (SJF) Best approach to minimize waiting time. Impossible to implement Processer should know in advance how much time process will take. Wait time of each process is following Process Wait Time : Service Time - Arrival Time P0 3-0 = 3 P1 0-0 = 0 P = 14 P3 8-3 = 5 Average Wait Time: ( ) / 4 = 5.50

30 3. Priority Based Scheduling Each process is assigned a priority. Process with highest priority is to be executed first and so on. Processes with same priority are executed on first come first serve basis. Priority can be decided based on memory requirements, time requirements or any other resource requirement. Wait time of each process is following Process Wait Time : Service Time - Arrival Time P0 0-0 = 0 P1 3-1 = 2 P2 8-2 = 6 P = 13 Average Wait Time: ( ) / 4 = 5.25

31 4.Round Robin Scheduling Each process is provided a fix time to execute called quantum. Once a process is executed for given time period. Process is preempted and other process executes for given time period. Context switching is used to save states of preempted processes. Wait time of each process is following Process Wait Time : Service Time - Arrival Time P0 (0-0) + (12-3) = 9 P1 (3-1) = 2 P2 6-2) + (15-9) = 10 P3 (9-3) + (18-12) = 12 Average Wait Time: ( ) / 4 = Multi Queue Scheduling

32 Multiple queues are maintained for processes. Each queue can have its own scheduling algorithms. Priorities are assigned to each queue.

33 Ans 4 Operation on Processes Several operations are possible on the process. Process must be created and deleted dynamically. Operating system must provide the environment for the process operation. We discuss the two main operations on processes. 1. Create a process 2. Terminate a process Create Process Operating system creates a new process with the specified or default attributes and identifier. A process may create several new subprocesses. Syntax for creating new process is : CREATE ( processed, attributes ) Two names are used in the process they are parent process and child process. Parent process is a creating process. Child process is created by the parent process. Child process may create another subprocess. So it forms a tree of processes. When operating system issues a CREATE system call, it obtains a new process control block from the pool of free memory, fills the fields with provided and default parameters, and insert the PCB into the ready list. Thus it makes the specified process eligible to run the process. When a process is created, it requires some parameters. These are priority, level of privilege, requirement of memory, access right, memory protection information etc. Process will need certain resources, such as CPU time, memory, files and I/O devices to complete the operation. When process creates a subprocess, that sub process may obtain its resources directly from the operating system. Otherwise it uses the resources of parent process. When a process creates a new process, two possibilities exist in terms of execution. 1. The parent continues to execute concurrently with its children. 2. The parent waits until some or all of its children have terminated. For address space, two possibilities occur: 1. The child process is a duplicate of the parent process.

34 2. The child process has a program loaded into it. Terminate a Process DELETE system call is used for terminating a process. A process may delete itself or by another process. A process can cause the termination of another process via an appropriate system call. The operating system reacts by reclaiming all resources allocated to the specified process, closing files opened by or for the process. PCB is also removed from its place of residence in the list and is returned to the free pool. The DELETE service is normally invoked as a part of orderly program termination. Following are the resources for terminating the child process by parent process. 1. The task given to the child is no longer required. 2. Child has exceeded its usage of some of the resources that it has been allocated. 3. Operating system does not allow a child to continue if its parent terminates. Co-operating Processes Co-operating process is a process that can affect or be affected by the other processes while executing. If suppose any process is sharing data with other processes, then it is called co-operating process. Benefit of the co-operating processes are : 1. Sharing of information 2. Increases computation speed 3. Modularity 4. Convenience Co-operating processes share the information : Such as a file, memory etc. System must provide an environment to allow concurrent access to these types of resources. Computation speed will increase if the computer has multiple processing elements are connected together. System is constructed in a modular fashion. System function is divided into number of modules. Behavior of co-operating processes is nondeterministic i.e. it depends on relative execution sequence and cannot be predicted a priori. Co-operating processes are also Reproducible. ANS 5 A) Process Control Block, PCB Each process is represented in the operating system by a process control block (PCB) also called a task control

35 block. PCB is the data structure used by the operating system. Operating system groups all information that needs about particular process. PCB contains many pieces of information associated with a specific process which is described below. Pointer Pointer points to another process control block. Pointer is used for maintaining the scheduling list. Process State Process state may be new, ready, running, waiting and so on. Program Counter Program Counter indicates the address of the next instruction to be executed for this process. CPU registers CPU registers include general purpose register, stack pointerindex registers and accumulators etc. number of register and type of register totally depends upon the computer architecture. Memory management information This information may include the value of base and limit registers, the page tables, or the segment tables depending on the memory system used by the operating system. This information is useful for deallocating the memory when the process terminates. Accounting information This information includes the amount of CPU and real time used, time limits, job or process numbers, account numbers etc.

36 PCB Process control block includes CPU scheduling, I/O resource management, file management information etc. The PCB serves as the repository for any information which can vary from process to process. Loader/linker sets flags and registers when a process is created. If that process gets suspended, the contents of the registers are saved on a stack and the pointer to the particular stack frame is stored in the PCB. By this technique, the hardware state can be restored so that the process can be scheduled to run again. B) What is Thread? A thread is a flow of execution through the process code, with its own program counter,system registers and stack. A thread is also called a light weight process. Threads provide a way to improve application performance through parallelism. Threads represent a software approach to improving performance of operating system by reducing the overhead thread is equivalent to a classical process. Each thread belongs to exactly one process and no thread can exist outside a process. Each thread represents a separate flow of control. Threads have been successfully used in implementing network servers and web server. They also provide a suitable foundation for parallel execution of applications on shared memory multiprocessors. Following figure shows the working of the single and multithreaded processes.

37 Difference between Process and Thread S.N Process Thread Process heavy weight Resour Thread is light weight taking lesser is or ce resources 1 intensive. than a process. intera Process switching needs Threa switching does not c interaction with d need to t 1 operating system. with operating system. 1 In multiple processing environments each process executes the same code but has its own memory and file resources. All threads can share same set of open files, child processes.

38 1 If one process is blocked then no other process can execute until the first process is unblocked. While one thread is blocked and waiting, second thread in the same task can run. 1 Multiple processes without using threads use more resources. Multi p l ethreaded processes usefewer resources. 1 In mult iple processes each process operates independently of the others. One thread can read, write or change another thread's data. Advantages of Thread Thread minimizes context switching time. Use of threads provides concurrency within a process. Efficient communication.

39 Economy- It is more economical to create and context switch threads. Utilization of multiprocessor architectures to a greater scale and efficiency. Types of Thread Threads are implemented in following two ways User Level Threads -- User managed threads Kernel Level Threads -- Operating System managed threads acting on kernel, an operating system core. User Level Threads In this case, application manages thread management kernel is not aware of the existence of threads. The thread library contains code for creating and destroying threads, for passing message and data between threads, for scheduling thread execution and for saving and restoring thread contexts. The application begins with a single thread and begins running in that thread. Advantages Thread switching does not require Kernel mode privileges.

40 User level thread can run on any operating system. Scheduling can be application specific in the user level thread. User level threads are fast to create and manage. Disadvantages In a typical operating system, most system calls are blocking. Multithreaded application cannot take advantage of multiprocessing. Kernel Level Threads In this case, thread management done by the Kernel. There is no thread management code in the application area. Kernel threads are supported directly by the operating system. Any application can be programmed to be multithreaded. All of the threads within an application are supported within a single process. The Kernel maintains context information for the process as a whole and for individuals threads within the process. Scheduling by the Kernel is done on a thread basis. The Kernel performs thread creation, scheduling and management in Kernel space. Kernel threads are generally slower to create and manage than the user threads. Advantages Kernel can simultaneously schedule multiple threads from the same process on multiple processes. If one thread in a process is blocked, the Kernel can schedule another thread of the same process. Kernel routines themselves can multithreaded. Disadvantages Kernel threads are generally slower to create and manage than the user threads. Transfer of control from one thread to another within same process requires a mode switch to the Kernel.