OS面试题（转载）

转载自：http://placement.freshersworld.com/power-preparation/technical-interview-preparation/os-interview-questions-23351

1. What is an operating system?

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.

2. What are the different operating systems?

1.     Batched operating systems

2.     Multi-programmed operating systems

3.     timesharing operating systems

4.     Distributed operating systems

5.     Real-time operating systems

3. What are the basic functions of an operating system?

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. Also operating system is control program
which controls the user programs to prevent errors and improper use of
the computer. It is especially concerned with the operation and control
of I/O devices.

4. What is kernel? 

Kernel is the core and essential part of computer operating system that provides basic services for all parts of OS.

5. What is difference between micro kernel and macro kernel?

Micro kernel is a
kernel which run services those are minimal for operating system
performance. In this kernel all other operations are performed by
processor.

Macro Kernel is a
combination of micro and monolithic kernel. In monolithic kernel all
operating system code is in single executable image.

6. What is dead lock?

Deadlock is a
situation or condition where the two processes are waiting for each
other to complete so that they can start. This result both the processes
to hang.

7. What is a process?

A program in execution is called a process.

Processes are of two types:
1. Operating system processes
2. User processes

8. What are the states of a process?

1. New
2. Running
3. Waiting
4. Ready
5. Terminated

9. What is starvation and aging?

Starvation is
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 is a technique to avoid starvation in a scheduling system.

10. What is semaphore?

Semaphore is a
variable, whose status reports common resource, Semaphore is of two
types one is Binary semaphore and other is Counting semaphore.

11. What is context switching?

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.

12. What is a thread?

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

13. What is process synchronization?

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.

14. What is virtual memory?

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.

15. What is thrashing?

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.

16. What is fragmentation? Tell about different types of fragmentation?

When many of free
blocks are too small to satisfy any request then fragmentation occurs.
External fragmentation and internal fragmentation are two types of
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.  Internal fragmentation is the
space wasted inside of allocated memory blocks because of restriction on
the allowed sizes of allocated blocks.

17. What are necessary conditions for dead lock?

1. Mutual exclusion (where at least one resource is non-sharable)

2. Hold and wait (where a process holds one resource and waits for other resource)

3. No preemption (where the resources can’t 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 )

18. What is cache memory?

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 from larger memory.

 

19. What is logical and physical addresses space?

Logical address
space is generated from CPU; it bound to a separate physical address
space is central to proper memory management. Physical address space is
seen by the memory unit. Logical address space is virtual address space.
Both these address space will be same at compile time but differ at
execution time.

20. Differentiate between Complier and Interpreter?

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

21. What is Throughput, Turnaround time, waiting time and Response time?

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)

22. What is Memory-Management Unit (MMU)?

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

23. What is a Real-Time System?

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

24. What is a trap and trapdoor?

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.

25. When is a system in safe state?

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.

26. Explain the concept of the Distributed systems?

Distributed systems work in a network. They can share the network resources, communicate with each other.

27. What is cache-coherency?

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.

 

28. What is a long term scheduler & short term schedulers?

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 from the ready queue and allocate
the CPU to one of them.

29. Explain the meaning of mutex.

Mutex is the short
form for ‘Mutual Exclusion object’. A mutex allows multiple threads for
sharing the same resource. The resource can be file. A mutex with a
unique name is created at the time of starting a program. A mutex must
be locked from other threads, when any thread that needs the resource.
When the data is no longer used / needed, the mutex is set to unlock.

30. What is cycle stealing?

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.

31. What is Marshalling?

The process of packaging and sending interface method parameters across thread or process boundaries.

32. What is a daemon?

Daemon is a program
that runs in the background without user’s interaction. A daemon runs in
a multitasking operating system like UNIX. A daemon is initiated and
controlled by special programs known as ‘processes’.

33. What is pre-emptive and non-preemptive scheduling?

Preemptive
scheduling: The preemptive scheduling is prioritized. The highest
priority process should always be the process that is currently
utilized.

Non-Preemptive
scheduling: When a process enters the state of running, the state of
that process is not deleted from the scheduler until it finishes its
service time.

34. What is busy waiting?

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.

35. What is page cannibalizing?

Page swapping or page replacements are called page cannibalizing.

36. What is SMP?

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.

37. What is process migration?

It is the transfer of sufficient amount of the state of process from one machine to the target machine.

38. Difference between Primary storage and secondary storage?

Primary memory is the main memory (Hard disk, RAM) where the operating system resides.

Secondary memory can
be external devices like CD, floppy magnetic discs etc. secondary
storage cannot be directly accessed by the CPU and is also external
memory storage.

39. Define compactions.

Compaction is a process in which the free space is collected in a large memory chunk to make some space available for processes.

40. What are residence monitors?

Early operating systems were called residence monitors.

41. What is dual-mode operation?

In order to protect
the operating systems and the system programs from the malfunctioning
programs the two mode operations were evolved
System mode
User mode.

42. What is a device queue?

A list of processes waiting for a particular I/O device is called device queue.

43. What are the different types of Real-Time Scheduling?

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.

44. What is relative path and absolute path?

Absolute path-- Exact path from root directory.
Relative path-- Relative to the current path.

45. What are the disadvantages of context switching?

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.

46. What is a data register and address register?

Data registers - can
be assigned to a variety of functions by the programmer. They can be
used with any machine instruction that performs operations on data.
Address registers - contain main memory addresses of data and
instructions or they contain a portion of the address that is used in
the calculation of the complete addresses.

47. What is DRAM?  

Dynamic Ram stores the data in the form of Capacitance, and Static RAM stores the data in Voltages.

48. What are local and global page replacements?  

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.

49. Explain the concept of the batched operating systems?  

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.

50. What is SCSI?

SCSI - Small
computer systems interface is a type of interface used for computer
components such as hard drives, optical drives, scanners and tape
drives. It is a competing technology to standard IDE (Integrated Drive
Electronics).

51.When is a system in safe state?

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.

52. What is cycle stealing?

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.

53. What is an idle thread?

The special thread a dispatcher will execute when no ready thread is found.

54. What is FtDisk?

It is a fault tolerance disk driver for Windows NT.

55.What is  Dispatcher?

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 – time it takes for the dispatcher to stop one
process and start another running.

56. When does the condition 'rendezvous' arise?

In message passing, it is the condition in which, both, the sender and receiver are blocked until the message is delivered.

57. What is process spawning?

When the OS at the explicit request of another process creates a process, this action is called process spawning

58. What are the reasons for process suspension?
1) swapping
2) interactive user request
3) timing
4) parent process request

59. What are the sub-components of I/O manager in Windows NT?
1) Network redirector/ Server
2) Cache manager.
3) File systems
4) Network driver
5) Device driver

60. What is a drawback of MVT?
1) ability to support multiple processors
2) virtual storage
3) source level debugging