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Operating System Two mark Questions & Answers

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Please find the two mark questions and answers below for object oriented programming (OOPS) subject related to computer science engineering (CSE) branch. Please do cross refer it before using it. These is for the anna university syllabus - regulation 2008. Please feel free to share you feedback and comments so that i can improve this blog.


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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.

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

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
- Increased throughput
- Economy of scale
- 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, 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 are ten processors & if one fails 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 to indicate the current mode. Mode bit is ‘0’ for

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
- Issuing illegal I/O operations
- By accessing memory locations within the OS itself
- 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 operating Systems
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
- File management
- I/O-system management
- Secondary-storage management
- Networking
- Protection system
- Command-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:
- New
- Running
- Waiting
- Ready
- Terminated

18. What is process control block?
Each process is represented in the operating system by a process
It contains the following information:
- Process state
- Program counter
- CPU registers
- CPU-scheduling information
- Memory-management information
- Accounting information
- I/O status information

19. What are 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 a thread?
A thread otherwise called a lightweight process (LWP) is a basic unit of CPU
utilization, it comprises of a thread id, a program counter, a register set and a stack.
It shares with other threads belonging to the same process its code section, data
section, and operating system resources such as open files and signals.

22. What are the benefits of multithreaded programming?
The benefits of multithreaded programming can be broken down into four
major categories:
- Responsiveness
- Resource sharing
- Economy
- Utilization of multiprocessor architectures

23. Compare user threads and kernel threads.
User threads
User threads are supported above the kernel and are implemented by a thread library at the user level
Thread creation & scheduling are done in the user space, without kernel Kernel threads
Kernel threads are supported directly by the operating system Thread creation, scheduling and
management are done by the operating intervention. Therefore they are fast to create and manage Blocking system call will cause the entire process to block If the thread performs a blocking
system call, the kernel can schedule another thread in the application for execution

24. What is the use of fork and exec system calls?
Fork is a system call by which a new process is created. Exec 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.

25. Define thread cancellation & target thread.
The thread cancellation is the task of terminating a thread before it has
completed. A thread that is to be cancelled is often referred to as the target thread.
For example, if multiple threads are concurrently searching through a database and
one thread returns the result, the remaining threads might be cancelled.

26. What are the different ways in which a thread can be cancelled?
Cancellation of a target thread may occur in two different scenarios:
- Asynchronous cancellation: One thread immediately terminates the target thread is called asynchronous cancellation.
- Deferred cancellation: The target thread can periodically check if it should
terminate, allowing the target thread an opportunity to terminate itself in an orderly fashion.

27. Define CPU scheduling.
CPU scheduling is the process of switching the CPU among various
processes. CPU scheduling is the basis of multiprogrammed operating systems. By
switching the CPU among processes, the operating system can make the computer
more productive.

28. What is preemptive and nonpreemptive scheduling?
Under nonpreemptive scheduling once the CPU has been allocated to a
process, the process keeps the CPU until it releases the CPU either by terminating
or switching to the waiting state.
Preemptive scheduling can preempt a process which is utilizing the CPU in between
its execution and give the CPU to another process.

29. What is a Dispatcher?
The dispatcher is the module that gives control of the CPU to the process
selected by the short-term scheduler. This function involves:
- Switching context
- Switching to user mode
- Jumping to the proper location in the user program to restart that program.

30. What is dispatch latency?
The time taken by the dispatcher to stop one process and start another running is known as dispatch latency. The various scheduling criteria are
- CPU utilization
- Throughput
- Turnaround time
- Waiting time
- Response time 

32. Define throughput?
Throughput in CPU scheduling is the number of processes that are completed
per unit time. For long processes, this rate may be one process per hour; for short
transactions, throughput might be 10 processes per second.

33. What is turnaround time?
Turnaround time is the interval from the time of submission to the time of
completion of a process. It is the sum of the periods spent waiting to get into
memory, waiting in the ready queue, executing on the CPU, and doing I/O.

34. Define race condition.
When several process access and manipulate same data concurrently, then
the outcome of the execution depends on particular order in which the access takes
place is called race condition. To avoid race condition, only one process at a time
can manipulate the shared variable.

35. What is critical section problem?
Consider a system consists of ‘n‘ processes. Each process has segment of
code called a critical section, i n which the process may be changing common
variables, updating a table, writing a file. When one process is executing in its critical
section, no other process can allowed to execute in its critical section.

36. What are the requirements that a solution to the critical section problem must satisfy?
The three requirements are
- Mutual exclusion
- Progress
- Bounded waiting

37. Define entry section and exit section.
The critical section problem is to design a protocol that the processes can use
to cooperate. Each process must request permission to enter its critical section. The
section of the code implementing this request is the entry section. The critical section
is followed by an exit section. The remaining code is the remainder section.

38. Give two hardware instructions and their definitions which can be used for implementing mutual exclusion.
- TestAndSet
- Swap
target = true;
return rv;

void Swap (boolean &a, boolean &b)
boolean temp = a;
a = b;
b = temp;

39. What is semaphores?
A semaphore ‘S’ is a synchronization tool which is an integer value that, apart
from initialization, is accessed only through two standard atomic operations; wait and
signal. Semaphores can be used to deal with the n-process critical section problem.
It can be also used to solve various synchronization problems.
The classic definition of ‘wait’
wait (S)
while (S<=0)
The classic definition of ‘signal’
signal (S)

40. Define busy waiting and spinlock.
When a process is in its critical section, any other process that tries to enter its critical section must loop continuously in the entry code. This is called as busy waiting and this type of semaphore is also called a spinlock, because the process while waiting for the lock.

41. Define deadlock.
A process requests resources; if the resources are not available at that time, the process enters a wait state. Waiting processes may never again change state, because the resources they have requested are held by other waiting processes. This situation is called a deadlock.

42. What is the sequence in which resources may be utilized?
Under normal mode of operation, a process may utilize a resource in the following sequence:
- Use: The process can operate on the resource.
- Release: The process releases the resource.

43. What are conditions under which a deadlock situation may arise?
A deadlock situation can arise if the following four conditions hold
simultaneously in a system:
a. Mutual exclusion
b. Hold and wait
c. No pre-emption
d. Circular wait

44. What is a resource-allocation graph?
Deadlocks can be described more precisely in terms of a directed graph called a system resource allocation graph. This graph consists of a set of vertices V and a set of edges E. The set of vertices V is partitioned into two different types of nodes; P the set consisting of all active processes in the
system and R the set consisting of all resource types in the system.

45. Define request edge and assignment edge.

46. What are the methods for handling deadlocks?
The deadlock problem can be dealt with in one of the three ways:
a. Use a protocol to prevent or avoid deadlocks, ensuring that the system will never enter a deadlock state.
b. Allow the system to enter the deadlock state, detect it and then recover. c. Ignore the problem all together, and pretend that deadlocks never occur in the system.

47. Define deadlock prevention.
Deadlock prevention is a set of methods for ensuring that at least one of the four necessary conditions like mutual exclusion, hold and wait, no pre- emption and circular wait cannot hold. By ensuring that that at least one of these conditions cannot hold, the occurrence of a deadlock can be prevented.

48. Define deadlock avoidance.
An alternative method for avoiding deadlocks is to require additional information about how resources are to be requested. Each request requires the system consider the resources currently available, the resources currently allocated to each process, and the future requests and releases of each possible future deadlock.

49. What are a safe state and an unsafe state?
A state is safe if the system can allocate resources to each process in some order and still avoid a deadlock. 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 resource that Pi
can still request can be satisfied by the current available resource plus the resource held by all the Pj, with j

50. What is banker’s algorithm?
Banker’s algorithm is a deadlock avoidance algorithm that is applicable
to a resource-allocation system with multiple instances of each resource type.
The two algorithms used for its implementation are:
a. Safety algorithm: The algorithm for finding out whether or not a system is in a safe state.
b. Resource-request algorithm: if the resulting resource-allocation is safe, the transaction is completed and process Pi is allocated its resources. If the new state is unsafe Pi must wait and the old resource-allocation state is restored.

51. Define logical address and physical address.
An address generated by the CPU is referred as logical address. An address seen by the memory unit that is the one loaded into the memory address register of the memory is commonly referred to as physical address.

52. What is logical address space and physical address space?
The set of all logical addresses generated by a program is called a logical address space; the set of all physical addresses corresponding to these logical addresses is a physical address space.

53. What is the main function of the memory-management unit?
The runtime mapping from virtual to physical addresses is done by a hardware device called a memory management unit (MMU).

54. Define dynamic loading.
To obtain better memory-space utilization dynamic loading is used. With dynamic loading, a routine is not loaded until it is called. All routines are kept on disk in a relocatable load format. The main program is loaded into memory and executed. If the routine needs another routine, the calling routine
checks whether the routine has been loaded. If not, the relocatable linking loader is called to load the desired program into memory.

55. Define dynamic linking.
Dynamic linking is similar to dynamic loading, rather that loading being postponed until execution time, linking is postponed. This feature is usually library routine, or how to load the library if the routine is not already present.

56. What are overlays?
To enable a process to be larger than the amount of memory allocated to it, overlays are used. The idea of overlays is to keep in memory only those instructions and data that are needed at a given time. When other instructions are needed, they are loaded into space occupied previously by instructions
that are no longer needed.

57. Define swapping.
A process needs to be in memory to be executed. However a process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued execution. This process is called swapping.

58. What are the common strategies to select a free hole from a set of available holes?
The most common strategies are
a. First fit
b. Best fit
c. Worst fit

59. What do you mean by best fit?
Best fit allocates the smallest hole that is big enough. The entire list has to be searched, unless it is sorted by size. This strategy produces the smallest leftover hole.

60. What do you mean by first fit?
First fit allocates the first hole that is big enough. Searching can either start at the beginning of the set of holes or where the previous first-fit search ended. Searching can be stopped as soon as a free hole that is big enough is found.

61. What is virtual memory?
Virtual memory is a technique that allows the execution of processes that may not be completely in memory. It is the separation of user logical memory from physical memory. This separation provides an extremely large virtual memory, when only a smaller physical memory is available.

62. What is Demand paging?
Virtual memory is commonly implemented by demand paging. In demand paging, the pager brings only those necessary pages into memory instead of swapping in a whole process. Thus it avoids reading into memory pages that will not be used anyway, decreasing the swap time and the
amount of physical memory needed.

63. Define lazy swapper.
swapper is used. A lazy swapper never swaps a page into memory unless that page will be needed.

64. What is a pure demand paging?
When starting execution of a process with no pages in memory, the operating system sets the instruction pointer to the first instruction of the process, which is on a non-memory resident page, the process immediately faults for the page. After this page is brought into memory, the process continues to execute, faulting as necessary until every page that it needs is in memory. At that point, it can execute with no more faults. This schema is pure demand paging.

65. Define effective access time.
Let p be the probability of a page fault (0?p?1). The value of p is
expected to be close to 0; that is, there will be only a few page faults. The
effective access time is
Effective access time = (1-p) * ma + p * page fault time.
ma : memory-access time

66. Define secondary memory.
This memory holds those pages that are not present in main memory.
The secondary memory is usually a high speed disk. It is known as the swap device, and the section of the disk used for this purpose is known as swap space.

67. What is the basic approach of page replacement?
If no frame is free is available, find one that is not currently being used and free it. A frame can be freed by writing its contents to swap space, and changing the page table to indicate that the page is no longer in memory. Now the freed frame can be used to hold the page for which the process faulted.

68. What are the various page replacement algorithms used for page replacement?
- FIFO page replacement
- Optimal page replacement
- LRU page replacement
- LRU approximation page replacement
- Counting based page replacement
- Page buffering algorithm.

69. What are the major problems to implement demand paging?
The two major problems to implement demand paging is developing
a. Frame allocation algorithm
b. Page replacement algorithm

70. What is a reference string?
reference is called a reference string.

71. What is a file?
A file is a named collection of related information that is recorded on
secondary storage. A file contains either programs or data. A file has certain “structure” based on its type.
- File attributes: Name, identifier, type, size, location, protection, time, date
- File operations: creation, reading, writing, repositioning, deleting, truncating, appending, renaming
- File types: executable, object, library, source code etc.

72. List the various file attributes.
A file has certain other attributes, which vary from one operating system to another, but typically consist of these: Name, identifier, type, location, size, protection, time, date and user identification

73. What are the various file operations?
The six basic file operations are
- Creating a file
- Writing a file
- Reading a file
- Repositioning within a file
- Deleting a file
- Truncating a file

74. What are the information associated with an open file?
Several pieces of information are associated with an open file which may be:
- File pointer
- File open count
- Disk location of the file
- Access rights

75. What are the different accessing methods of a file?
The different types of accessing a file are:
- Sequential access: Information in the file is accessed sequentially
- Direct access: Information in the file can be accessed without any particular order.
- Other access methods: Creating index for the file, indexed sequential access method (ISAM) etc.

76. What is Directory?
The device directory or simply known as directory records information- such as name, location, size, and type for all files on that particular partition. The directory can be viewed as a symbol table that translates file names into their directory entries.
- Search for a file
- Create a file
- Delete a file
- Rename a file
- List directory
- Traverse the file system

78. What are the most common schemes for defining the logical structure of a directory?
The most common schemes for defining the logical structure of a directory
- Single-Level Directory
- Two-level Directory
- Tree-Structured Directories
- Acyclic-Graph Directories
- General Graph Directory

79. Define UFD and MFD.
In the two-level directory structure, each user has her own user file directory (UFD). Each UFD has a similar structure, but lists only the files of a single user. When a job starts the system’s master file directory (MFD) is searched. The MFD is indexed by the user name or account number, and each entry points to the UFD for that user.

80. What is a path name?
A pathname is the path from the root through all subdirectories to a specified file. In a two-level directory structure a user name and a file name define a path name.

81. What are the various layers of a file system?
The file system is composed of many different levels. Each level in the design uses the feature of the lower levels to create new features for use by higher levels.
- Application programs
- Logical file system
- File-organization module
- Basic file system
- I/O control
- Devices

82. What are the structures used in file-system implementation?
Several on-disk and in-memory structures are used to implement a file system
a. On-disk structure include
-  Boot control block
-  Partition block
b. In-memory structure include
-  In-memory partition table
-  In-memory directory structure
-  System-wide open file table
-  Per-process open table 

83. What are the functions of virtual file system (VFS)?
It has two functions
a. It separates file-system-generic operations from their implementation defining a clean VFS interface. It allows transparent access to different types of file systems mounted locally.
b. VFS is based on a file representation structure, called a vnode. It contains a numerical value for a network-wide unique file .The kernel maintains one vnode structure for each active file or directory. 

84. Define seek time and latency time.
The time taken by the head to move to the appropriate cylinder or track is called seek time. Once the head is at right track, it must wait until the desired block rotates under the read- write head. This delay is latency time. 

85. What are the allocation methods of a disk space?
Three major methods of allocating disk space which are widely in use are
a. Contiguous allocation
b. Linked allocation
c. Indexed allocation 

86. What are the advantages of Contiguous allocation?
The advantages are
a. Supports direct access
b. Supports sequential access
c. Number of disk seeks is minimal. 

87. What are the drawbacks of contiguous allocation of disk space?
The disadvantages are
a. Suffers from external fragmentation
b. Suffers from internal fragmentation
c. Difficulty in finding space for a new file
d. File cannot be extended
e. Size of the file is to be declared in advance 

88. What are the advantages of Linked allocation?
The advantages are
a. No external fragmentation
b. Size of the file does not need to be declared 

89. What are the disadvantages of linked allocation?
The disadvantages are
a. Used only for sequential access of files. 

90. What are the advantages of Indexed allocation?
The advantages are
a. No external-fragmentation problem
b. Solves the size-declaration problems.
c. Supports direct access 

91. How can the index blocks be implemented in the indexed allocation scheme?
The index block can be implemented as follows
a. Linked scheme
b. Multilevel scheme
c. Combined scheme 

92. Define rotational latency and disk bandwidth.
Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. The disk bandwidth is the total number of bytes transferred, divided by the time between the first request for service and the completion of the last transfer. 

93. How free-space is managed using bit vector implementation?
The free-space list is implemented as a bit map or bit vector. Each block is represented by 1 bit. If the block is free, the bit is 1; if the block is allocated, the bit is 0. 

94. Define buffering.
A buffer is a memory area that stores data while they are transferred between two devices or between a device and an application. Buffering is done for three reasons
a. To cope with a speed mismatch between the producer and consumer of a data stream
b. To adapt between devices that have different data-transfer sizes
c. To support copy semantics for application I/O 

95. Define caching.
A cache is a region of fast memory that holds copies of data. Access to the cached copy is more efficient than access to the original. Caching and buffering are distinct functions, but sometimes a region of memory can be used for both purposes. 

96. Define spooling.
A spool is a buffer that holds output for a device, such as printer, that cannot accept interleaved data streams. When an application finishes printing, the spooling system queues the corresponding spool file for output to the printer. The spooling system copies the queued spool files to the printer one at a time. 

97. What are the various disk-scheduling algorithms?
c. SCAN Scheduling
d. C-SCAN Scheduling
e. LOOK scheduling 

98. What is low-level formatting?
Before a disk can store data, it must be divided into sectors that the disk controller can read and write. This process is called low-level formatting or physical formatting. Low-level formatting fills the disk with a special data structure for each sector. The data structure for a sector consists of a header,
a data area, and a trailer. 

99. What is the use of boot block
For a computer to start running when powered up or rebooted it needs to have an initial program to run. This bootstrap program tends to be simple. It finds the operating system on the disk loads that kernel into memory and jumps to an initial address to begin the operating system execution. The full bootstrap program is stored in a partition called the boot blocks, at fixed location on the disk. A disk that has boot partition is called boot disk or system disk. 

100.What is sector sparing?
Low-level formatting also sets aside spare sectors not visible to the operating system. The controller can be told to replace each bad sector logically with one of the spare sectors. This scheme is known as sector sparing or forwarding.

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