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Chapter: Operating Systems : Process and Threads

Important Short Questions and Answers: Operating Systems - Process and Threads

Operating Systems - Process and Threads - Important Short Questions and Answers: Operating Systems - Process and Threads


1. What is an Operating System?


An operating system is a program that manages the computer hardware. It also provides a basis for application programs and act as an intermediary between a user of a computer and the computer hardware. It controls and coordinates the use of the hardware among the various application programs for the various users.



2. Why is the Operating System viewed as a resource allocator & control program?


A computer system has many resources hardware & software that may be required to solve a problem, like CPU time, memory space, file-storage space, I/O devices & so on. The OS acts as a manager for these resources so it is viewed as a resource allocator. The OS is viewed as a control program because it manages the execution of user programs to prevent errors & improper use of the computer.


3. What is the Kernel?


A more common definition is that the OS is the one program running at all times on the computer, usually called the kernel, with all else being application programs.


4. What are Batch Systems?


Batch systems are quite appropriate for executing large jobs that need little interaction. The user can submit jobs and return later for the results. It is not necessary to wait while the job is processed. Operators batched together jobs with similar needs and ran them through the computer as a group.


5. What is the advantage of Multiprogramming?


Multiprogramming increases CPU utilization by organizing jobs so that the CPU always has one to execute. Several jobs are placed in the main memory and the processor is switched from job to job as needed to keep several jobs advancing while keeping the peripheral devices in use. Multiprogramming is the first instance where the Operating system must make decisions for the users. Therefore they are fairly sophisticated.


6. What is an Interactive Computer System?


Interactive computer system provides direct communication between the user and the system. The user gives instructions to the operating system or to a program directly, using a keyboard or mouse, and waits for immediate results.




7.What do you mean by Time-Sharing Systems?


Time-sharing or multitasking is a logical extension of multiprogramming. It allows many users to share the computer simultaneously. The CPU executes multiple jobs by switching among them, but the switches occur so frequently that the users can interact with each program while it is running.


8.What are Multiprocessor Systems & give their advantages?


Multiprocessor systems also known as parallel systems or tightly coupled systems are systems that have more than one processor in close communication, sharing the computer bus, the clock and sometimes memory & peripheral devices. Their main advantages are,


v Increased throughput


v Economy of scale


v Increased reliability



9.What are the different types of Multiprocessing?



Symmetric multiprocessing (SMP): In SMP each processor runs an identical copy of the OS


& these  copies communicate with one another as  needed. All  processors are peers.


Examples are Windows NT, Solaris, Digital UNIX, and OS/2 & Linux.


Asymmetric multiprocessing: Each processor is assigned a specific task. A master processor controls the system; the other processors look to the master for instructions or predefined tasks. It defines a master-slave relationship.


Example: SunOS Version 4.


10. What is Graceful Degradation?


In multiprocessor systems, failure of one processor will not halt the system, but only slow it down. If there is ten processors & if any one fails then the remaining nine processors pick up the work of the failed processor. This ability to continue providing service is proportional to the surviving hardware is called graceful degradation.


11. What is Dual- Mode Operation?


The dual mode operation provides us with the means for protecting the operating system from wrong users and wrong users from one another. User mode and monitor mode are the two modes. Monitor mode is also called supervisor mode, system mode or privileged mode. Mode bit is attached to the hardware of the computer in order to indicate the current mode. Mode bit is ‘0’ for monitor mode and ‘1’ for user mode.


12. What are Privileged Instructions?


Some of the machine instructions that may cause harm to a system are designated as privileged instructions. The hardware allows the privileged instructions to be executed only in monitor mode.


13.            How can a user program disrupt the normal operations of a system?


A user program may disrupt the normal operation of a system by,


v Issuing illegal I/O operations


v By accessing memory locations within the OS itself


v Refusing to relinquish the CPU


14.            How is the protection for memory provided?


The protection against illegal memory access is done by using two registers. The base register and the limit register. The base register holds the smallest legal physical address; the limit register contains the size of the range. The base and limit registers can be loaded only by the OS using special privileged instructions


15. What are the various OS Components?


The various system components

are, Process management

Main memory management

ile management


I/O-system management

Secondary storage management


Protection system

Commnd interpreter system


16. What is a Process?


A process is a program in execution. It is the unit of work in a modern operating system. A process is an active entity with a program counter specifying the next instructions to execute and a set of associated resources. It also includes the process stack, containing temporary data and a data section containing global variables.


17. What is a Process State and mention the various States of a Process?


As a process executes, it changes state. The state of a process is defined in part by the current activity of that process.


Each process may be in one of the following states:









18. What is Process Control Block (PCB)?

Each process is represented in the operating system by a process control block also called a task control block. It contains many pieces of information associated with a specific process. It simply acts as a repository for any information that may vary from process to process. It contains the following information:


Process state

Program counters

CPU registers

CPU scheduling information

Memory management information

Accounting information

I/O status information


19. What is the use of Job Queues, Ready Queues & Device Queues?


As a process enters a system, they are put into a job queue. This queue consists of all jobs in the system. The processes that are residing in main memory and are ready & waiting to execute are kept on a list called ready queue. The list of processes waiting for a particular I/O device is kept in the device queue.


20. What is meant by Context Switch?


Switching the CPU to another process requires saving the state of the old process and loading the saved state for the new process. This task is known as context switch. The context of a process is represented in the PCB of a process.


21. What is Spooling?


Spooling means Simultaneous Peripheral Operations On Line. It is a high-speed device like a disk is interposed between a running program and a low speed device involved with the program in input/output. It disassociates a running program from the slow operation of devices like printers.


22. What are System Calls?


System calls provide the interface between a process and the Operating system. System Calls are also called as Monitor call or Operating-system function call. When a system call is executed, it is treated as by the hardware as software interrupt. Control passes through the interrupt vector to a service routine in the operating system, and the mode bit is set to monitor mode.


23. List the services provided by an Operating System?


Program execution

I/O Operation

File System manipulation


Error detection


24. What are the two types of Real Time System?

Hard real time system

Sot real time system


25. What is the difference between Hard Real Time System and Soft Real Time System?


A hard real time system guarantees that critical tasks complete on time. In a soft real time system, a critical real-time task gets priority over the other tasks, and retains that priority until it completes. Soft real time systems have more limited utility than do hard real-time systems.


26. Write the difference between Multiprogramming and Non - Multiprogramming?


The operating system picks and begins to execute one of the jobs in the memory. Eventually, the job may have to wait for some task, such as a tape to be mounted, or an I/O operation to complete. In a non-multiprogrammed system, the CPU would sit idle. In a multiprogramming system, the operating system simply switches to and executes another job.


When that job needs to wait, the CPU is switched to another job, and so on. Eventually, the first job finishes waiting and gets the CPU back. As long as there is always some job to execute, the CPU will never be idle.


27. What are the design goals of an Operating System?


The requirements can be divided into two basic groups: User goals and System goals.Users desire that the system should be convenient and easy to use, easy to learn, reliable, safe and fast. The Operating system should be easy to design, implement, and maintain. Also it should be flexible, reliable, error free and efficient. These are some of the requirements, which are vague and have no general solution.


28. What are the five major categories of System Calls?

Process Control

Ile management

Device management

Information maintainance



29. What is the use of Fork and Execve System Calls?


Fork is a System calls by which a new process is created. Execve is also a System call, which is used after a fork by one of the two processes to replace the process memory space with a new program.


30. Define Elapsed CPU time and Maximum CPU time?


Elapsed CPU Time: Total CPU time used by a process to date.


Maximum CPU Time: Maximum amount of CPU time a process may use.


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