OPERATING SYSTEM CS2252 CS2411 IMPORTANT 2 MARKS AND 16 MARKS ANNA UNIVERSITY QUESTIONS WITH ANSWERS FOR EEE AND CSE STUDENTS


OPERATING SYSTEM CS2252 CS2411 IMPORTANT 2 MARKS AND 16 MARKS ANNA UNIVERSITY QUESTIONS WITH ANSWERS FOR EEE AND CSE STUDENTS

OPERATING SYSTEM CS1252
SEMESTER - iV

OPERATING SYSTEM CS 1252
PART – A(TWO MARKS)
UNIT I
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
• 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 usersfrom 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' formonitor 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
• 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 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 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 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.
UNIT II
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 intervention. Therefore they are fast to create and manage blocking system call
will cause the entire process to block
Kernel threads
Kernel threads are supported directly by the operating system .Thread creation,
scheduling and management are done by the operating system. Therefore they are
slower to create & manage compared to user threads. 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.
31.What are the various scheduling criteria for CPU scheduling?
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, in 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
boolean TestAndSet (boolean &target)
{
boolean rv = target;
target = true;
return rv;
}
• Swap
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)
;
S--;
}
The classic definition of 'signal'
signal (S)
{
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.
UNIT III
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:
• Request: If the request cannot be granted immediately,
then the requesting process must wait until it can acquire the
resource.
• 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
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.
A directed edge from process Pi to resource type Rj is denoted by PiàRj; it signifies that
process Pi requested an instance of resource type Rj and is currently waiting for that
resource. A directed edge from resource type Rj to process Pi is denoted by RjàPi, it
signifies that an instance of resource type has been allocated to a process Pi. A directed
edge PiàRj is called a request edge. A directed edge RjàPi is called an 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 preemption 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 process, to decide whether the could be satisfied
or must wait to avoid a 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
sequence exists, then the system state is said to be unsafe.
50.What is banker's algorithm?
Banker's algorithm is a deadlock avoidance algorithm that is applicable to a resourceallocation
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 resourceallocationis 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 used with system libraries,
such as language subroutine libraries. A stub is included in the image for each libraryroutine
reference. The stub is a small piece of code that indicates how to locate the
appropriate memory-resident 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 tore 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. he entire list has to be searched,
unless it is sorted by size. his strategy produces the smallest leftover hole.
60.What do you mean by first fit?
First fit allocates the first hole that is big enough. earching can either start at the
beginning of the set of holes orwhere the previous first-fit search ended. Searching can be
stopped as soon as a free hole that is big enough is found.
UNIT IV
61.What is virtual memory?
Virtual memory is a technique that allows the execution of rocesses that may not be
completely in memory. It is the eparation of user logical memory from physical memory.
This separation providesan 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 demandpaging, the
pager brings only those necessary pages intomemory instead of swapping in a whole
process. Thus it avoidsreading into memory pages that will not be used
anyway,decreasing the swap time and the amount of physical memoryneeded.
63.Define lazy swapper.
Rather than swapping the entire process into main memory, alazy swapper is used. A lazy
swapper never swaps a page intomemory unless that page will be needed.
64.What is a pure demand paging?
When starting execution of a process with no pages inmemory, the operating system sets
the instruction pointer to thefirst instruction of the process, which is on a non-memory
resident page, the process immediately faults for the page. Afterthis page is brought into
memory, the process continues toexecute, faulting as necessary until every page that it
needs isin 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 valueof p is expected to be close to
0; that is, there will be only afew page faults. The effective access time isEffective 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 mainmemory. The secondary
memory is usually a high speed disk. It isknown as the swap device, and the section ofthe
disk used forthis 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 notcurrently being used and free it. A frame can be freed by writingits contents
to swap space, and changing the page table toindicate 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?
An algorithm is evaluated by running it on a particularstring of memory references and
computing the number of pagefaults. The string of memory reference is called a reference
string.
71.What is a file?
A file is a named collection of related information that isrecorded on secondary storage.
A file contains either programs ordata. 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 oneoperating system to another,
buttypically 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.
77.What are the operations that can be performed on a directory?
The operations that can be performed on a directory are
• 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.
UNIT V
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
· Directory structure used to organize the files
· File control block (FCB)
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.
b. Direct access is not supported
c. Memory space required for the pointers.
d. Reliability is compromised if the pointers are lost or damaged
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 datatransfer
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?
The various disk-scheduling algorithms are
a. First Come First Served Scheduling
b. Shortest Seek Time First Scheduling
c. SCAN Scheduling
d. C-SCAN Scheduling
f. 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.
UNIT IV
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.
Rather than swapping the entire process into main memory, a
lazy 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?
An algorithm is evaluated by running it on a particular
string of memory references and computing the number of page
faults. The string of memory 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.
77.What are the operations that can be performed on a directory?
The operations that can be performed on a directory are
• 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.
UNIT V
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
· Directory structure used to organize the files
· File control block (FCB)
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
d.
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.
b. Direct access is not supported
c. Memory space required for the pointers.
d. Reliability is compromised if the pointers are lost or damaged
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 datatransfer
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?
The various disk-scheduling algorithms are
a. First Come First Served Scheduling
b. Shortest Seek Time First Scheduling
c. SCAN Scheduling
d. C-SCAN Scheduling
f. 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.
PART – B
SIXTEEN MARK QUESTIONS WITH HINTS
1. Explain the various types of computer systems.
Mainframe systems
Desktop systems
Multiprocessor systems
Distributed systems
Clustered systems
Real-time systems
Handheld systems
2. Explain how protection is provided for the hardware resources by the operating
system.
Dual mode operation
I/O protection with diagram
Memory protection with diagram
CPU protection
3. What are the system components of an operating system and explain them?
Process management
Main-memory management
File management
I/O management
Secondary storage management
Networking
Protection system
Command-interpreter system
4. Write about the various system calls.
Process control
File management
Device management
Information maintenance
Communication
5. What are the various process scheduling concepts
Scheduling queues with diagram
Queueing diagram
Schedulers
Context switch with diagram
6. Explain about interprocess communication.
Message-passing system
Naming
Direct communication
Indirect communication
Synchronization
Buffering
7. Give an overview about threads.
Thread definition
Motivation
Diagram
Benefits
User and kernel threads
8. Explain in detail about the threading issues.
The fork and exec system calls
Cancellation
Signal handling
Threads pools
Thread-specific data
9. Write about the various CPU scheduling algorithms.
First-come, first-served scheduling
Shortest-job-first scheduling
Priority Scheduling
Round-robin scheduling
Multilevel queue scheduling
Multilevel feedback queue scheduling
10.Write notes about multiple-processor scheduling and real-time scheduling.
Homogeneous systems
Load sharing
Self-scheduling
Resource reservation
Priority inversion
Priority inheritance protocol
Dispatch latency with diagram
11.What is critical section problem and explain two process solutions and
multiple process solutions?
Critical section problem definition
Two process solutions
Algorithm 1, 2 & 3
Multiple-process solution with algorithm
12.Explain what semaphores are, their usage, implementation given to avoid
busy waiting and binary semaphores.
Semaphore definition
Usage for mutual exclusion and process synchronization
Implementation to avoid spinlock using block and wakeup
Binary semaphores
13.Explain the classic problems of synchronization.
The bounded-buffer problem with structure
The readers-writers problem with structure
The dining-philosophers problem with structure
14.Write about critical regions and monitors.
Critical region definition
Implementation of the conditional-region construct
Monitor definition
Syntax of monitor
Schematic view of monitors
Monitor with condition variables
Monitor solution to dining-philosopher problem
15.Give a detailed description about deadlocks and its characterization
Deadlock definition
Deadlock conditions
Mutual exclusion
Hold and wait
No pre-emption
Circular wait
Resource allocation graph
16.Explain about the methods used to prevent deadlocks
Ensure that at least one of the following does not hold
Mutual exclusion
Hold and wait
No pre-emption
Circular wait
17.Write in detail about deadlock avoidance.
Safe state and safe sequence
Diagram for safe, unsafe & deadlock states
Resource-allocation graph algorithm
18.Explain the Banker's algorithm for deadlock avoidance.
Deadlock avoidance definition
Data structures used
Safety algorithm
Resource request algorithm
19.Give an account about deadlock detection.
Single instance of each resource type
Wait-for graph
Several instances of a resource type
Detection-algorithm usage
20.What are the methods involved in recovery from deadlocks?
Process termination
Resource pre-emption
21.Explain about contiguous memory allocation.
Contiguous allocation
Memory protection with diagram
Memory allocation
First fit
Best fit
Worst fit
Fragmentation
22.Give the basic concepts about paging.
Paging definition
Basic method-page, frame, page table, page number & page offset
Paging hardware diagram
TLB with diagram
Protection-protection bits & valid-invalid bits
23.Write about the techniques for structuring the page table.
Hierarchical paging-two-level & multi-level with diagram
Hashed page table with diagram
Inverted page table with diagram
24.Explain the basic concepts of segmentation.
User view of program
Segmentation definition
Hardware used with diagram-segment table, base, limit & offset
Protection and sharing with diagram
Fragmentation
25.What is demand paging and what is its use?
Demand paging definition
Virtual memory implementation
Lazy swapper, page fault, pure demand paging, valid-invalid bit
Diagrams
26.Explain the various page replacement strategies.
Page replacement-basic scheme with diagram
FIFO page replacement
Optimal page replacement
LRU page replacement
LRU approximation page replacement
Counting-based page replacement
Page buffering algorithm
27.What is thrashing and explain the methods to avoid thrashing?
Thrashing definition
Cause of thrashing
Working set model
Page-fault frequency
28.What are files and explain the access methods for files?
File definition
Attributes, operations and types
Sequential access with diagram
Direct access
Other access methods-index with diagram
29.Explain the schemes for defining the logical structure of a directory.
Single level directory with diagram
Two level directory with diagram
Tree structured directory with diagram
Acyclic-graph directory with diagram
General graph directory with diagram
30.Write notes about the protection strategies provided for files.
Types of access
Access control list (ACL)
Three classifications-owner, group & universe
Other protection approaches-passwords
31.Explain the allocation methods for disk space.
Contiguous allocation advantage, disadvantage & diagram
Linked allocation advantage, disadvantage & diagram
Indexed allocation advantage, disadvantage & diagram
Performance
32.What are the various methods for free space management?
Bit vector with example
Linked list with diagram
Grouping
Counting
33.Write about the kernel I/O subsystem.
I/O scheduling
Buffering
Caching
Spooling & device reservation
Error handling
Kernel data structures
34.Explain the various disk scheduling techniques
FCFS scheduling
SSTF scheduling
SCAN scheduling
C-SCAN scheduling
LOOK scheduling
35.Write notes about disk management and swap-space management.
Disk formatting-low level formatting
Boot block-bootstrap loader, boot block, boot disk & system disk
Bad blocks-sector sparing, sector slipping
Swap-space use
Swap-space location
Swap-space management
OPERATING SYSTEM CS2252 CS2411 IMPORTANT 2 MARKS AND 16 MARKS ANNA UNIVERSITY QUESTIONS WITH ANSWERS FOR EEE AND CSE STUDENTS Reviewed by Rejin Paul on 7:03 PM Rating: 5

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