Operating System Interview Online Test
Operating system technical interview questions and answers help students and job seekers understand core computing concepts that are essential for technical interviews. The operating system acts as a bridge between hardware and software, making it one of the fundamental subjects tested in campus placement interviews. Recruiters frequently ask questions about process management, memory allocation, deadlocks, scheduling algorithms, threads, file systems, and OS architecture. These OS interview questions appear repeatedly in companies like TCS, Wipro, Infosys, Cognizant, Capgemini, and Accenture. This guide explains the most commonly asked OS interview questions with easy-to-understand answers, practical examples, and simple definitions. Whether you are preparing for your first job, a technical round, or a written placement test, understanding OS concepts will help you perform confidently. You can also practice mock questions or download PDFs to strengthen your preparation.
1. Explain the concept of Reentrancy.
Answer: It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instanc
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2. Explain Belady's Anomaly.
Answer: Also called FIFO anomaly. Usually, on increasing the number of frames allocated
to a process' virtual memory, the process execution is faster, because fewer page faults
occur. Sometimes, the reverse happens, i.e., the execution time increases even when more
frames are allocated to the process. This is Belady's Anomaly. This is true for certain
page reference patterns.
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3. What is a binary semaphore? What is its use?
Answer: A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.
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4. What is thrashing?
Answer: It is a phenomenon in virtual memory schemes when the processor spends most of its time swapping pages, rather than executing instructions. This is due to an inordinate number of page faults.
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5. List the Coffman's conditions that lead to a deadlock.
Answer: Mutual Exclusion: Only one process may use a critical resource at a time.
Hold & Wait: A process may be allocated some resources while waiting for others.
No Pre-emption: No resource can be forcible removed from a process holding it.
Circular Wait: A closed chain of processes exist such that each process holds at least
one resource needed by another process in the chain.
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6. What are short-, long- and medium-term scheduling?
Answer: Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.
Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on memorymanagement criteria.
Short term scheduler, also know as a dispatcher executes most frequ
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7. What are turnaround time and response time?
Answer: Turnaround time is the interval between the submission of a job and its completion. Response time is the interval between submission of a request, and the first response to that request.
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8. What are the typical elements of a process image?
Answer: User data: Modifiable part of user space. May include program data, user stack area,
and programs that may be modified.
User program: The instructions to be executed.
System Stack: Each process has one or more LIFO stacks associated with it. Used to
store parameters and calling addresses for procedure and system calls.
Process control Block (PCB): Info needed by the OS to control processes.
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9. What is the Translation Lookaside Buffer (TLB)?
Answer: In a cached system, the base addresses of the last few referenced pages is maintained in registers called the TLB that aids in faster lookup. TLB contains those page-table entries that have been most recently used. Normally, each virtual memory reference causes 2 physical memory accesses-- one to fetch appropriate page-table entry,and one to fetch the desired data. Using TLB in-between, this is reduced to just one physical memory access in cases of TLB-hit.
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10. What is the resident set and working set of a process?
Answer: Resident set is that portion of the process image that is actually in real-memory at
a particular instant. Working set is that subset of resident set that is actually needed for
execution. (Relate this to the variable-window size method for swapping techniques.)
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11. When is a system in safe state?
Answer: The set of dispatchable processes is in a safe state if there exists at least one temporal order in which all processes can be run to completion without resulting in a deadlock.
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12. What is cycle stealing?
Answer: We encounter cycle stealing in the context of Direct Memory Access (DMA).Either the DMA controller can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific break points in an instruction cycle.
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13. What is meant by arm-stickiness?
Answer: If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track. This generally happens with most common device scheduling algorithms (LIFO, SSTF, CSCAN, etc). High-density multisurface disks are more likely to be affected by this than low density ones.
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14. What are the stipulations of C2 level security?
Answer: C2 level security provides for:
Ø Discretionary Access Control
Ø Identification and Authentication
Ø Auditing
Ø Resource reuse
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15. What is busy waiting?
Answer: The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The CPU is not engaged in any real productive activity during this period, and the process does not progress toward completion.
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16. Explain the popular multiprocessor thread-scheduling strategies.
Answer: Load Sharing: Processes are not assigned to a particular processor. A global queue of threads is maintained. Each processor, when idle, selects a thread from this queue.Note that load balancing refers to a scheme where work is allocated to processors on a more permanent basis.
Ø Gang Scheduling: A set of related threads is scheduled to run on a set of processors at the same time, on a 1-to-1 basis. Closely related threads / processes may be scheduled this way to reduce synchronization blocking,
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17. When does the condition 'rendezvous' arise?
Answer: In message passing, it is the condition in which, both, the sender and receiver are blocked until the message is delivered.
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18. What is a trap and trapdoor?
Answer: Trapdoor is a secret undocumented entry point into a program used to grant access without normal methods of access authentication. A trap is a software interrupt, usually the result of an error condition.
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19. What are local and global page replacements?
Answer: Local replacement means that an incoming page is brought in only to the relevant process' address space. Global replacement policy allows any page frame from any process to be replaced. The latter is applicable to variable partitions model only.
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20. Define latency, transfer and seek time with respect to disk I/O.
Answer: Seek time is the time required to move the disk arm to the required track. Rotational delay or latency is the time it takes for the beginning of the required sector to reach the head. Sum of seek time (if any) and latency is the access time. Time taken to actually transfer a span of data is transfer time.
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21. Describe the Buddy system of memory allocation.
Answer: Free memory is maintained in linked lists, each of equal sized blocks. Any such block is of size 2^k. When some memory is required by a process, the block size of next higher order is chosen, and broken into two. Note that the two such pieces differ in address only in their kth bit. Such pieces are called buddies. When any used block is freed, the OS checks to see if its buddy is also free. If so, it is rejoined, and put into the original free-block linked-list.
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22. What is time-stamping?
Answer: It is a technique proposed by Lamport, used to order events in a distributed
system without the use of clocks. This scheme is intended to order events consisting of
the transmission of messages. Each system 'i' in the network maintains a counter Ci.
Every time a system transmits a message, it increments its counter by 1 and attaches the
time-stamp Ti to the message. When a message is received, the receiving system 'j' sets
its counter Cj to 1 more than the maximum of its current value and t
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23. How are the wait/signal operations for monitor different from those for semaphores?
Answer: If a process in a monitor signal and no task is waiting on the condition variable,
the signal is lost. So this allows easier program design. Whereas in semaphores, every
operation affects the value of the semaphore, so the wait and signal operations should be
perfectly balanced in the program.
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24. In the context of memory management, what are placement and replacement algorithms?
Answer: Placement algorithms determine where in available real-memory to load a
program. Common methods are first-fit, next-fit, best-fit. Replacement algorithms are
used when memory is full, and one process (or part of a process) needs to be swapped out
to accommodate a new program. The replacement algorithm determines which are the partitions to be swapped out.
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25. In loading programs into memory, what is the difference between load-time dynamic linking and run-time dynamic linking?
Answer: For load-time dynamic linking: Load module to be loaded is read into memory.Any reference to a target external module causes that module to be loaded and the references are updated to a relative address from the start base address of the application module.
With run-time dynamic loading: Some of the linking is postponed until actual reference during execution. Then the correct module is loaded and linked.
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26. What are demand- and pre-paging?
Answer: With demand paging, a page is brought into memory only when a location on that
page is actually referenced during execution. With pre-paging, pages other than the one
demanded by a page fault are brought in. The selection of such pages is done based on
common access patterns, especially for secondary memory devices.
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27. Paging a memory management function, while multiprogramming a processor management function, are the two interdependent?
Answer: Yes.
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28. What is page cannibalizing?
Answer: Page swapping or page replacements are called page cannibalizing.
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29. What has triggered the need for multitasking in PCs?
Answer: Ø Increased speed and memory capacity of microprocessors together with the support
fir virtual memory and
Ø Growth of client server computing
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30. What are the four layers that Windows NT have in order to achieve independence?
Answer: Ø Hardware abstraction layer
Ø Kernel
Ø Subsystems
Ø System Services.
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31. What is SMP?
Answer: To achieve maximum efficiency and reliability a mode of operation known as symmetric multiprocessing is used. In essence, with SMP any process or threads can be assigned to any processor.
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32. What are the key object oriented concepts used by Windows NT?
Answer: Ø Encapsulation
Ø Object class and instance
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33. Is Windows NT a full blown object oriented operating system? Give reasons.
Answer: No Windows NT is not so, because its not implemented in object oriented language and the data structures reside within one executive component and are not represented as objects and it does not support object oriented capabilities .
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34. What is a drawback of MVT?
Answer: It does not have the features like
Ø ability to support multiple processors
Ø virtual storage
Ø source level debugging
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35. What is process spawning?
Answer: When the OS at the explicit request of another process creates a process, this action is called process spawning.
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37. List out some reasons for process termination.
Answer: Ø Normal completion
Ø Time limit exceeded
Ø Memory unavailable
Ø Bounds violation
Ø Protection error
Ø Arithmetic error
Ø Time overrun
Ø I/O failure
Ø Invalid instruction
Ø Privileged instruction
Ø Data misuse
Ø Operator or OS intervention
Ø Parent termination.
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38. What are the reasons for process suspension?
Answer: Ø swapping
Ø interactive user request
Ø timing
Ø parent process request
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39. What is process migration?
Answer: It is the transfer of sufficient amount of the state of process from one machine to the target machine
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40. What is mutant?
Answer: In Windows NT a mutant provides kernel mode or user mode mutual exclusion with the notion of ownership.
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41. What is an idle thread?
Answer: The special thread a dispatcher will execute when no ready thread is found.
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43. What are the possible threads a thread can have?
Answer: Ø Ready
Ø Standby
Ø Running
Ø Waiting
Ø Transition
Ø Terminated.
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44. What are rings in Windows NT?
Answer: Windows NT uses protection mechanism called rings provides by the process to implement separation between the user mode and kernel mode.
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45. What is Executive in Windows NT?
Answer: In Windows NT, executive refers to the operating system code that runs in kernel mode.
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46. What are the sub-components of I/O manager in Windows NT?
Answer: Ø Network redirector/ Server
Ø Cache manager.
Ø File systems
Ø Network driver
Ø Device driver
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47. What are DDks? Name an operating system that includes this feature.
Answer: DDks are device driver kits, which are equivalent to SDKs for writing device drivers. Windows NT includes DDks.
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49. What is an operating system?
Answer: An operating system is a program that acts as an intermediary between the user and the computer hardware. The purpose of an OS is to provide a convenient environment in which user can execute programs in a convenient and efficient manner.It is a resource allocator responsible for allocating system resources and a control program which controls the operation of the computer h/w.
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50. What are the various components of a computer system?
Answer: 1. The hardware
2. The operating system
3. The application programs
4. The users.
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51. What is purpose of different operating systems?
Answer: The machine Purpose Workstation individual usability &Resources utilization Mainframe Optimize utilization of hardware PC Support complex games, business application Hand held PCs Easy interface & min. power consumption
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52. What are the different operating systems?
Answer: 1. Batched operating systems
2. Multi-programmed operating systems
3. timesharing operating systems
4. Distributed operating systems
5. Real-time operating systems
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53. Explain the concept of the batched operating systems?
Answer: In batched operating system the users gives their jobs to the operator who sorts the programs according to their requirements and executes them. This is time consuming but makes the CPU busy all the time.
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54. Explain the concept of the multi-programmed operating systems?
Answer: A multi-programmed operating systems can execute a number of programs concurrently. The operating system fetches a group of programs from the job-pool in the secondary storage which contains all the programs to be executed, and places them in the main memory. This process is called job scheduling. Then it chooses a
program from the ready queue and gives them to CPU to execute. When a executing program needs some I/O operation then the operating system fetches another program and hands it to the
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55. Explain the concept of the timesharing operating systems?
Answer: It is a logical extension of the multi-programmed OS where user can interact with the program. The CPU executes multiple jobs by switching among them, but the switches occur so frequently that the user feels as if the operating system is running only his program
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56. Explain the concept of the multi-processor systems or parallel systems?
Answer: They contain a no. of processors to increase the speed of execution, and reliability, and economy. They are of two types:
1. Symmetric multiprocessing
2. Asymmetric multiprocessing
In Symmetric multi processing each processor run an identical copy of the OS, and these copies communicate with each other as and when needed.But in Asymmetric multiprocessing each processor is assigned a specific task.
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57. Explain the concept of the Distributed systems?
Answer: Distributed systems work in a network. They can share the network resources,communicate with each other
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58. Explain the concept of Real-time operating systems?
Answer: A real time operating system is used when rigid time requirement have been placed on the operation of a processor or the flow of the data; thus, it is often used as a control device in a dedicated application. Here the sensors bring data to the computer. The computer must analyze the data and possibly adjust controls to
modify the sensor input.
They are of two types:
1. Hard real time OS
2. Soft real time OS
Hard-real-time OS has well-defined fixed time constraints. But soft real time opera
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59. Define MULTICS?
Answer: MULTICS (Multiplexed information and computing services) operating system was developed from 1965-1970 at Massachusetts institute of technology as a computing utility. Many of the ideas used in MULTICS were subsequently used in UNIX.
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62. What is cache-coherency?
Answer: In a multiprocessor system there exist several caches each may containing a copy of same variable A. Then a change in one cache should immediately be reflected in all other caches this process of maintaining the same value of a data in all the caches s called cache-coherency.
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63. What are residence monitors?
Answer: Early operating systems were called residence monitors.
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64. What is dual-mode operation?
Answer: In order to protect the operating systems and the system programs from the malfunctioning programs the two mode operations were evolved:
1. System mode.
2. User mode.
Here the user programs cannot directly interact with the system resources, instead they request the operating system which checks the request and does the required task for the user programs-DOS was written for / intel 8088 and has no dual-mode. Pentium provides dual-mode operation.
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65. What are the operating system components?
Answer: 1. Process management
2. Main memory management
3. File management
4. I/O system management
5. Secondary storage management
6. Networking
7. Protection system
8. Command interpreter system
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66. What are operating system services?
Answer: 1. Program execution
2. I/O operations
3. File system manipulation
4. Communication
5. Error detection
6. Resource allocation
7. Accounting
8. Protection
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67. What are system calls?
Answer: System calls provide the interface between a process and the operating system. System calls for modern Microsoft windows platforms are part of the win32 API, which is available for all the compilers written for Microsoft windows.
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68. What is a layered approach and what is its advantage?
Answer: Layered approach is a step towards modularizing of the system, in which the operating system is broken up into a number of layers (or levels), each built on top of lower layer. The bottom layer is the hard ware and the top most is the user interface.The main advantage of the layered approach is modularity. The layers are
selected such that each uses the functions (operations) and services of only lower layer. This approach simplifies the debugging and system verification.
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69. What is micro kernel approach and site its advantages?
Answer: Micro kernel approach is a step towards modularizing the operating system where all nonessential components from the kernel are removed and implemented as system and user level program, making the kernel smaller.The benefits of the micro kernel approach include the ease of extending the operating system. All new services are added to the user space and consequently do not require modification of the kernel. And as kernel is smaller it is easier to upgrade it. Also this approach provides more sec
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70. What are a virtual machines and site their advantages?
Answer: It is the concept by which an operating system can create an illusion that a process has its own processor with its own (virtual) memory. The operating system implements virtual machine concept by using CPU scheduling and virtual memory.
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71. What are a virtual machines and site their advantages?
Answer: It is the concept by which an operating system can create an illusion that a process has its own processor with its own (virtual) memory. The operating system implements virtual machine concept by using CPU scheduling and virtual memory.
1. The basic advantage is it provides robust level of security as each virtual machine is isolated from all other VM. Hence the system resources are completely protected.
2. Another advantage is that system development can be done without disrupting normal o
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72. What is a process?
Answer: A program in execution is called a process. Or it may also be called a unit of work. A process needs some system resources as CPU time, memory, files, and i/o devices to accomplish the task. Each process is represented in the operating system by a process control block or task control block (PCB).Processes are of two types:
1. Operating system processes
2. User processes
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73. What are the states of a process?
Answer: 1. New
2. Running
3. Waiting
4. Ready
5. Terminated
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74. What are various scheduling queues?
Answer: 1. Job queue
2. Ready queue
3. Device queue
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75. What is a job queue?
Answer: When a process enters the system it is placed in the job queue.
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76. What is a ready queue?
Answer: The processes that are residing in the main memory and are ready and waiting to execute are kept on a list called the ready queue.
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77. What is a device queue?
Answer: A list of processes waiting for a particular I/O device is called device queue.
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78. What is a long term scheduler & short term schedulers?
Answer: Long term schedulers are the job schedulers that select processes from the job queue and load them into memory for execution. The short term schedulers are the CPU schedulers that select a process form the ready queue and allocate the CPU to one of them.
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79. What is context switching?
Answer: Transferring the control from one process to other process requires saving the state of the old process and loading the saved state for new process. This task is known as context switching.
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80. What are the disadvantages of context switching?
Answer: Time taken for switching from one process to other is pure over head. Because the system does no useful work while switching. So one of the solutions is to go for threading when ever possible.
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81. What are co-operating processes?
Answer: The processes which share system resources as data among each other. Also the processes can communicate with each other via interprocess communication facility generally used in distributed systems. The best example is chat program used on the www.
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82. What is a thread?
Answer: A thread is a program line under execution. Thread sometimes called a light-weight process, is a basic unit of CPU utilization; it comprises a thread id, a program counter, a register set, and a stack.
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83. What are the benefits of multithreaded programming?
Answer: 1. Responsiveness (neednt to wait for a lengthy process)
2. Resources sharing
3. Economy (Context switching between threads is easy)
4. Utilization of multiprocessor architectures (perfect utilization of the multiple processors).
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84. What are types of threads?
Answer: 1. User thread
2. Kernel thread
User threads are easy to create and use but the disadvantage is that if they perform a blocking system calls the kernel is engaged completely to the single user thread blocking other processes. They are created in user space.Kernel threads are supported directly by the operating system. They are slower to create and manage. Most of the OS like Windows NT, Windows 2000, Solaris2, BeOS, and Tru64 Unix support kernel threading.
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85. Which category the java thread do fall in?
Answer: Java threads are created and managed by the java virtual machine, they do not easily fall under the category of either user or kernel thread……
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86. What are multithreading models?
Answer: Many OS provide both kernel threading and user threading. They are called multithreading models. They are of three types:
1. Many-to-one model (many user level thread and one kernel thread).
2. One-to-one model
3. Many-to many
In the first model only one user can access the kernel thread by not allowing multi-processing. Example: Green threads of Solaris.The second model allows multiple threads to run on parallel processing systems. Creating user thread needs to create corresponding kernel
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87. What is a P-thread?
Answer: P-thread refers to the POSIX standard (IEEE 1003.1c) defining an API for thread creation and synchronization. This is a specification for thread behavior, not an implementation. The windows OS have generally not supported the P-threads.
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88. What are java threads?
Answer: Java is one of the small number of languages that support at the language level for the creation and management of threads. However, because threads are managed by the java virtual machine (JVM), not by a user-level library or kernel, it is difficult to classify Java threads as either user- or kernel-level.
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89. What is process synchronization?
Answer: A situation, where several processes access and manipulate the same data concurrently and the outcome of the execution depends on the particular order in which the access takes place, is called race condition. To guard against the race condition we need to ensure that only one process at a time can be manipulating
the same data. The technique we use for this is called process synchronization.
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90. What is critical section problem?
Answer: Critical section is the code segment of a process in which the process may be changing common variables, updating tables, writing a file and so on. Only one process is allowed to go into critical section at any given time (mutually exclusive).The critical section problem is to design a protocol that the processes can use to
co-operate. The three basic requirements of critical section are:
1. Mutual exclusion
2. Progress
3. bounded waiting
Bakery algorithm is one of the solutions to CS probl
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91. What is a semaphore?
Answer: It is a synchronization tool used to solve complex critical section problems. A semaphore is an integer variable that, apart from initialization, is accessed only through two standard atomic operations: Wait and Signal.
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92. What is bounded-buffer problem?
Answer: Here we assume that a pool consists of n buffers, each capable of holding one item. The semaphore provides mutual exclusion for accesses to the buffer pool and is initialized to the value 1.The empty and full semaphores count the number of empty and full buffers, respectively. Empty is initialized to n, and full is initialized to 0.
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93. What is readers-writers problem?
Answer: Here we divide the processes into two types:
1. Readers (Who want to retrieve the data only)
2. Writers (Who want to retrieve as well as manipulate)
We can provide permission to a number of readers to read same data at same time.But a writer must be exclusively allowed to access. There are two solutions to this problem:
1. No reader will be kept waiting unless a writer has already obtained permission to use the shared object. In other words, no reader should wait for other readers to complet
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94. What is dining philosophers problem?
Answer:
Ans: Consider 5 philosophers who spend their lives thinking and eating. The philosophers share a common circular table surrounded by 5 chairs, each belonging to one philosopher. In the center of the table is a bowl of rice, and the table is laid with five single chop sticks. When a philosopher thinks, she doesnt interact with her colleagues.
From time to time, a philosopher gets hungry and tries to pick up two chop sticks that are closest to her .A philosopher may pick up only one chop stick
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95. What is a deadlock?
Answer: Suppose a process request resources; if the resources are not available at that time the process enters into a wait state. A waiting process may never again change state, because the resources they have requested are held by some other waiting processes. This situation is called deadlock.
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96. What are necessary conditions for dead lock?
Answer: 1. Mutual exclusion (where at least one resource is non-sharable)
2. Hold and wait (where a process hold one resource and waits for other resource)
3. No preemption (where the resources cant be preempted)
4. circular wait (where p[i] is waiting for p[j] to release a resource. i= 1,2,…n
j=if (i!=n) then i+1
else 1 )
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97. What is resource allocation graph?
Answer: This is the graphical description of deadlocks. This graph consists of a set of edges E and a set of vertices V. The set of vertices V is partitioned into two different types of nodes P={p1,p2,…,pn}, the set consisting of all the resources in the system, R={r1,r2,…rn}.A directed edge Pi?Rj is called a request edge; a directed edge Rj?
Pi is called an assignment edge. Pictorially we represent a process Pi as a circle, and each resource type Rj as square.Since resource type Rj may have more than
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98. What are deadlock prevention techniques?
Answer: 1. Mutual exclusion : Some resources such as read only files shouldnt be mutually
exclusive. They should be sharable. But some resources such as printers must be
mutually exclusive.
2. Hold and wait : To avoid this condition we have to ensure that if a process is
requesting for a resource it should not hold any resources.
3. No preemption : If a process is holding some resources and requests another
resource that cannot be immediately allocated to it (that is the process must wait),
then
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99. What is a safe state and a safe sequence?
Answer: A system is in safe state only if there exists a safe sequence. A sequence of processes is a safe sequence for the current allocation state if, for each Pi, the resources that the Pi can still request can be satisfied by the currently available resources plus the resources held by all the Pj, with j
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100. What are the deadlock avoidance algorithms?
Answer: A dead lock avoidance algorithm dynamically examines the resource-allocation state to ensure that a circular wait condition can never exist. The resource allocation state is defined by the number of available and allocated resources, and the maximum demand of the process.There are two algorithms:
1. Resource allocation graph algorithm
2. Bankers algorithm
a. Safety algorithm
b. Resource request algorithm
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101. What are the basic functions of an operating system?
Answer: Operating system controls and coordinates the use of the hardware among the various applications programs for various uses. Operating system acts as resource allocator and manager. Since there are many possibly conflicting requests for resources the operating system must decide which requests are allocated resources to operating the computer system efficiently and fairly. Also operating system is control program which controls the user programs to prevent errors and improper use of the computer.
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102. Explain briefly about, processor, assembler, compiler, loader, linker and the functions executed by them.
Answer: Processor:A processor is the part a computer system that executes instructions .It is also called a CPU
Assembler: — An assembler is a program that takes basic computer instructions and converts them into a pattern of bits that the computers processor can use to perform its basic operations. Some people call these instructions assembler language and others use the term assembly language.
Compiler: — A compiler is a special program that processes statements written in a particular programming
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103. What is a Real-Time System?
Answer: A real time process is a process that must respond to the events within a certain time period. A real time operating system is an operating system that can run real time processes successfully
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104. What is the difference between Hard and Soft real-time systems?
Answer: A hard real-time system guarantees that critical tasks complete on time. This goal requires that all delays in the system be bounded from the retrieval of the stored data to the time that it takes the operating system to finish any request made of it. A soft real time system where a critical real-time task gets priority over other tasks and retains that priority until it completes. As in hard real time systems kernel delays need to be bounded
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105. What is virtual memory?
Answer: A virtual memory is hardware technique where the system appears to have more memory that it actually does. This is done by time-sharing, the physical memory and storage parts of the memory one disk when they are not actively being used
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106. What is cache memory?
Answer: Cache memory is random access memory (RAM) that a computer microprocessor can access more quickly than it can access regular RAM. As the microprocessor processes data, it looks first in the cache memory and if it finds the data there (from a previous reading of data), it does not have to do the more time-consuming reading of data
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107. Differentiate between Complier and Interpreter?
Answer: An interpreter reads one instruction at a time and carries out the actions implied by that instruction. It does not perform any translation. But a compiler translates the entire instructions.
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108. What are different tasks of Lexical Analysis?
Answer: The purpose of the lexical analyzer is to partition the input text, delivering a sequence of comments and basic symbols. Comments are character sequences to be ignored, while basic symbols are character sequences that correspond to terminal symbols of the grammar defining the phrase structure of the input
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109. Why paging is used?
Answer: Paging is solution to external fragmentation problem which is to permit the logical address space of a process to be noncontiguous, thus allowing a process to be allocating physical memory wherever the latter is available.
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110. What is Context Switch?
Answer: 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 a context switch.Context-switch time is pure overhead, because the system does no useful work while switching. Its speed varies from machine to machine, depending on the memory speed, the number of registers which must be copied, the existed of special instructions(such as a single instruction to load or store all registers).
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111. Distributed Systems?
Answer: Distribute the computation among several physical processors.
Loosely coupled system each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines
Advantages of distributed systems:
->Resources Sharing
->Computation speed up load sharing
->Reliability
->Communications
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112. Difference between Primary storage and secondary storage?
Answer: Main memory: only large storage media that the CPU can access directly.
Secondary storage: extension of main memory that provides large nonvolatile storage capacity.
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113. What is CPU Scheduler?
Answer: ->Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.
->CPU scheduling decisions may take place when a process:
1.Switches from running to waiting state.
2.Switches from running to ready state.
3
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114. What do you mean by deadlock?
Answer: Deadlock is a situation where a group of processes are all blocked and none of them can become unblocked until one of the other becomes unblocked.The simplest deadlock is two processes each of which is waiting for a message from the other
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115. What is Dispatcher?
Answer: ->Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:
Switching context
Switching to user mode
Jumping to the proper location in the user program to restart that program
Dispatch latency t
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116. What is Throughput, Turnaround time, waiting time and Response time?
Answer: Throughput number of processes that complete their execution per time unit
Turnaround time amount of time to execute a particular process
Waiting time amount of time a process has been waiting in the ready queue
Response time amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
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117. Explain the difference between microkernel and macro kernel?
Answer: Micro-Kernel: A micro-kernel is a minimal operating system that performs only the essential functions of an operating system. All other operating system functions are performed by system processes.
Monolithic: A monolithic operating system is one where all operating system code is in a single executable image and all operating system code runs in system mode.
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118. What is the cause of thrashing? How does the system detect thrashing? Once it detects thrashing, what can the system do to eliminate this problem?
Answer: Thrashing is caused by under allocation of the minimum number of pages required by a process, forcing it to continuously page fault. The system can detect thrashing by evaluating the level of CPU utilization as compared to the level of multiprogramming. It can be eliminated by reducing the level of multiprogramming.
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119. Under what circumstances do page faults occur? Describe the actions taken by the operating system when a page fault occurs?
Answer: A page fault occurs when an access to a page that has not been brought into main memory takes place. The operating system verifies the memory access, aborting the program if it is invalid. If it is valid, a free frame is located and I/O is requested to read the needed page into the free frame. Upon completion of I/O, the process table and page table are updated and the instruction is restarted
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120. Explain Segmentation with paging?
Answer: Segments can be of different lengths, so it is harder to find a place for a segment in memory than a page. With segmented virtual memory, we get the benefits of virtual memory but we still have to do dynamic storage allocation of physical memory. In order to avoid this, it is possible to combine segmentation and paging into a two-level virtual memory system. Each segment descriptor points to page table for that segment.This give some of the advantages of paging (easy placement) with some of the
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121. Define Demand Paging, Page fault interrupt, and Trashing?
Answer: Demand Paging: Demand paging is the paging policy that a page is not read into memory until it is requested, that is, until there is a page fault on the page.
Page fault interrupt: A page fault interrupt occurs when a memory reference is made to a page that is not in memory.The present bit in the page table entry will be found to be off by the virtual memory hardware and it will signal an interrupt.
Trashing: The problem of many page faults occurring in a short time, called “page thrashing,”
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122. What is fragmentation? Different types of fragmentation?
Answer: Fragmentation occurs in a dynamic memory allocation system when many of the free blocks are too small to satisfy any request.
External Fragmentation: External Fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much external fragmentation occurs, the amount of usable memory is drastically reduced.Total memory space exists to satisfy a request, but it is not contiguous
Internal Fragmentatio
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123. What are Dynamic Loading, Dynamic Linking and Overlays?
Answer: Dynamic Loading:
->Routine is not loaded until it is called
->Better memory-space utilization; unused routine is never loaded.
->Useful when large amounts of code are needed to handle infrequently occurring cases.
->No special support from the operating system is required implemented through program design.
Dynamic Linking:
->Linking postponed until execution time.
->Small piece of code, stub, used to locate the appropriate memory-resident library routine.
->Stub replaces itself with the
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124. What is Memory-Management Unit (MMU)?
Answer: Hardware device that maps virtual to physical address.
In MMU scheme, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory.
->The user program deals with logical addresses; it never sees the real physical addresses
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125. Binding of Instructions and Data to Memory?
Answer: Address binding of instructions and data to memory addresses can happen at three different stages
Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes.
Load time: Must generate relocatable code if memory location is not known at compile time.
Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps (e.g.,
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126. Difference between Logical and Physical Address Space?
Answer: ->The concept of a logical address space that is bound to a separate physical address space is central to proper memory management.
Logical address generated by the CPU; also referred to as virtual address.
Physical address address seen by the m
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127. Recovery from Deadlock?
Answer: Process Termination:
->Abort all deadlocked processes.
->Abort one process at a time until the deadlock cycle is eliminated.
->In which order should we choose to abort?
Priority of the process.
How long process has computed, and how much longer to completion.
Resources the process has used.
Resources process needs to complete.
How many processes will need to be terminated?
Is process interactive or batch?
Resource Preemption:
->Selecting a victim minimize cost.
->Rollback return
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128. What is a Safe State and its use in deadlock avoidance?
Answer: When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state
->System is in safe state if there exists a safe sequence of all processes.
->Sequence is safe if for each Pi, the resources that Pi can still request can be satisfied by
currently available resources + resources held by all the Pj, with j
If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished.
When Pj is finished, Pi can obtain
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129. What are the Methods for Handling Deadlocks?
Answer: ->Ensure that the system will never enter a deadlock state.
->Allow the system to enter a deadlock state and then recover.
->Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including
UNIX.
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130. Different types of Real-Time Scheduling?
Answer: Hard real-time systems required to complete a critical task within a guaranteed amount of time.
Soft real-time computing requires that critical processes receive priority over less fortunate ones.
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131. What is starvation and aging?
Answer: Starvation: Starvation is a resource management problem where a process does not get the resources it needs for a long time because the resources are being allocated to other processes.
Aging: Aging is a technique to avoid starvation in a scheduling system. It works by adding an aging factor to the priority of each request. The aging factor must increase the requests priority as time passes and must ensure that a request will eventually be the highest priority request (after it has waited long
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132. Give a non-computer example of preemptive and non-preemptive scheduling?
Answer: Consider any system where people use some kind of resources and compete for them. The non-computer examples for preemptive scheduling the traffic on the single lane road if there is emergency or there is an ambulance on the road the other vehicles give path to the vehicles that are in need. The example for preemptive scheduling is people standing in queue for tickets.
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133. What is multi tasking, multi programming, multi threading?
Answer: Multi programming: Multiprogramming is the technique of running several programs at a time using timesharing.It allows a computer to do several things at the same time. Multiprogramming creates logical parallelism.
The concept of multiprogramming is that the operating system keeps several jobs in memory simultaneously. The operating system selects a job from the job pool and starts executing a job, when that job needs to wait for any i/o operations the CPU is switched to another job. So the mai
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134. What do you mean by deadlock?
Answer:
Ans : Deadlock is a situation where a group of processes are all blocked and none of them can become unblocked until one of the other becomes unblocked.The simplest deadlock is two processes each of which is waiting for a message from the other
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135. What is Dispatcher?
Answer: ->Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:
Switching context
Switching to user mode
Jumping to the proper location in the user program to restart that program
Dispatch latency t
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136. What is Throughput, Turnaround time, waiting time and Response time?
Answer: Throughput number of processes that complete their execution per time unit
Turnaround time amount of time to execute a particular process
Waiting time amount of time a process has been waiting in the ready queue
Response time amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
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137. Distributed Systems?
Answer: Distribute the computation among several physical processors.
Loosely coupled system each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines
Advantages of distributed systems:
->Resources Sharing
->Computation speed up load sharing
->Reliability
->Communications
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