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See right interview answers on 30 common job interview questions

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1. Define latency, transfer and seek time with respect to disk I/O.

Seek time is the time required to move the disk arm to the required track. Rotational delay or latency is the time it takes for the beginning of the  required sector to reach the head. Sum of seek time (if any) and latency is the access time. Time taken to actually transfer a span of data is transfer time.

2. Describe the Buddy system of memory allocation.

Free memory is maintained in linked lists, each of equal sized blocks. Any such block is of size 2^k. When some memory  is required by a process, the block size of next higher order  is chosen, and broken into two. Note that the two such pieces differ in address only in their kth bit. Such pieces are called buddies. When any used block is freed, the OS checks to see if its buddy is also free. If so, it is rejoined, and put into the original free-block linked-list.

3. Explain Belady's Anomaly?

Also called FIFO anomaly. Usually, on  increasing  the number of  frames  allocated  to a process  virtual  memory,  the  process  execution  is  faster,  because  fewer  page  faults  occur. Sometimes,  the  reverse  happens,  i.e.,  the execution  time  increases even when more  frames are allocated  to  the  process.  This  is  Belady's  Anomaly.  This  is  true  for  certain  page  reference patterns.

4. Explain the concept of Reentrancy?

It  is  a  useful, memory-saving  technique  for multiprogrammed  timesharing  systems.  A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify  itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but  has  its  own  copy  of  local  variables/parameters.  The  temporary  part  associated with  each activation is the activation record. Generally, the activation record is kept on the stack.
 Note: A  reentrant  procedure  can  be  interrupted  and  called  by  an  interrupting  program, and still execute correctly on returning to the procedure.

5. Explain the popular multiprocessor thread-scheduling strategies.

Load Sharing: Processes are not assigned to a particular processor. A global queue of threads is maintained. Each  processor, when  idle,  selects  a thread from this queue. Note that load balancing refers to a scheme where work is allocated to processors on a more permanent basis.
• Gang Scheduling: A set of  related  threads  is scheduled to run on a set of processors at  the same time, on a 1-to-1 basis. Closely related  threads / processes may be scheduled this way to reduce synchronization blocking, and minimize process switching. Group scheduling predated this strategy.
• Dedicated processor assignment: Provides implicit scheduling defined by assignment of threads to processors. For the duration of program execution, each program is allocated a set of processors equal in number to the number of threads in the program. Processors are chosen from the available pool.
• Dynamic scheduling: The number of thread in a program can be altered during the course of execution. 

6. How are the wait/signal operations for monitor different from those for semaphores?

If a process in a monitor signal and no task is waiting on the condition variable, the signal is lost. So this allows easier program design. Whereas in semaphores, every operation affects the value of the semaphore, so the wait and signal operations should  be  perfectly  balanced  in  the program.

7. How many jobs can be run concurrently on MVT?

15 jobs.

8. In loading programs into memory, what is the difference between load-time dynamic linking and run-time dynamic linking?

For  load-time dynamic linking:  Load module  to  be  loaded  is  read  into memory.  Any 
reference  to  a  target  external module  causes  that module  to  be  loaded  and  the  references  are 
updated to a relative address from the start base address of the application module.

With run-time dynamic loading: Some of  the  linking  is postponed until actual reference 
during execution. Then the correct module is loaded and linked.

9. In the context of memory management, what are placement and replacement algorithms?

Placement  algorithms  determine  where  in  available  real-memory  to  load  a  program. Common methods are first-fit, next-fit, best-fit. Replacement algorithms are used when memory is  full, and one process  (or part of a process) needs  to be swapped out  to accommodate a new program. The replacement algorithm determines which are the partitions to be swapped out.

10. Is Windows NT a full blown object oriented operating system? Give reasons.

No Windows NT  is not so, because  its not  implemented  in object oriented  language and the data structures reside within one executive component and are not represented as objects and it does not support object oriented capabilities.

11. List out some reasons for process termination.

• Normal completion 
• Time limit exceeded
• Memory unavailable
• Bounds violation
• Protection error
• Arithmetic error
• Time overrun
• I/O failure
• Invalid instruction
• Privileged instruction
• Data misuse
• Operator or OS intervention
• Parent termination.

12. List the Coffman's conditions that lead to a deadlock.

Mutual Exclusion: Only one process may use a critical resource at a time.
• Hold & Wait: A process may be allocated some resources while waiting for others. 
• No Pre-emption: No resource can be forcible removed from a process holding it.
• Circular Wait: A closed chain of processes exist such that each process holds at least one resource needed by another process in the chain.

13. Paging a memory management function, while multiprogramming a processor management function, are the two interdependent?


14. What are DDks? Name an operating system that includes this feature.

DDks are device driver kits, which are equivalent to SDKs for writing device drivers. Windows NT includes DDks.

15. What are demand-paging and pre-paging?

With demand paging, a page is brought into memory only when a location on that page is actually referenced during execution. With pre-paging, pages other than the one demanded by a page fault are brought in. The selection of such pages is done based on common access patterns, especially for secondary memory devices.

16. 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.

17. What are rings in Windows NT?

Windows NT uses protection mechanism called rings provides by the process to implement separation between the user mode and kernel mode.

18. What are short, long and medium-term scheduling?

Long term scheduler  determines  which  programs  are  admitted  to  the  system  for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.

Medium  term scheduling  is part of  the swapping  function. This relates  to processes  that 
are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on memory-management criteria.


Short term scheduler, also know as a dispatcher executes most frequently, and makes the finest-grained  decision  of  which  process  should  execute  next.  This  scheduler  is  invoked whenever an event occurs. It may lead to interruption of one process by preemption.

19. What are the four layers that Windows NT have in order to achieve independence?

• Hardware abstraction layer 
• Kernel
• Subsystems
• System Services.

20. What are the key object oriented concepts used by Windows NT?

Encapsulation, Object class and instance.

21. What are the possible threads a thread can have?

• Ready
• Standby
• Running
• Waiting
• Transition
• Terminated

22. What are the reasons for process suspension?

• swapping
• interactive user request
• timing
• parent process request

23. What are the stipulations of C2 level security?

C2 level security provides for: 

• Discretionary Access Control
• Identification and Authentication
• Auditing
• Resource reuse

24. What are the sub-components of I/O manager in Windows NT?

• Network redirector/ Server
• Cache manager.
• File systems
• Network driver
• Device driver

25. What are the typical elements of a process image?

User data: Modifiable  part  of  user  space. May  include  program  data,  user  stack  area,  and 
programs that may be modified.

User program: The instructions to be executed.

System Stack: Each process has one or more LIFO  stacks associated with  it. Used  to  store parameters and calling addresses for procedure and system calls.

Process control Block (PCB): Info needed by the OS to control processes.

26. What are turnaround time and response time?

Turnaround  time  is  the  interval  between  the  submission  of  a  job  and  its  completion. Response  time  is  the  interval  between  submission  of  a  request,  and  the  first  response  to  that request.

27. What has triggered the need for multitasking in PCs?

• Increased speed and memory capacity of microprocessors together with the support fir virtual memory and 
• Growth  of client server computing

28. What is a binary semaphore? What is its use?

A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.

29. What is a drawback of MVT?

It does not have the features like  

• ability to support multiple processors 
• virtual storage
• source level debugging

30. 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.

31. What is an idle thread?

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

32. 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.

33. 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.

34. What is Executive in Windows NT?

In Windows NT, executive refers to the operating system code that runs in kernel mode.

35. What is FtDisk?

It is a fault tolerance disk driver for Windows NT.

36. What is meant by arm-stickiness?

If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track. This generally happens with most common device scheduling algorithms (LIFO,  SSTF,  C-SCAN,  etc). High-density multisurface disks are more likely to be affected by this than low density ones.

37. What is mutant?

In Windows NT a mutant provides kernel mode or user mode mutual exclusion with the notion of ownership.

38. What is page cannibalizing?

Page swapping or page replacements are called page cannibalizing.

39. What is process migration?

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

40. What is process spawning?

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

41. 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.

42. What is the resident set and working set of a process?

Resident  set  is  that  portion  of  the  process  image  that  is  actually  in  real-memory  at  a particular instant. Working set is that subset of resident set that is actually needed for execution. (Relate this to the variable-window size method for swapping techniques.)

43. What is the Translation Lookaside Buffer (TLB)?

In a cached system,  the base addresses of  the  last  few referenced pages  is maintained  in registers  called  the TLB  that  aids  in  faster  lookup. TLB  contains  those  page-table  entries  that have  been  most  recently  used.  Normally,  each  virtual  memory  reference  causes  2  physical memory accesses- one  to  fetch appropriate page-table entry, and one  to  fetch  the desired data. Using TLB in-between, this is reduced to just one physical memory access in cases of TLB-hit.

44. 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.

45. What is time-stamping?

It  is  a  technique  proposed  by  Lamport,  used  to  order  events  in  a  distributed  system without the use of clocks. This scheme is intended to order events consisting of the transmission of  messages.  Each  system  'i'  in  the  network  maintains  a  counter  Ci.  Every  time  a  system transmits  a  message,  it  increments  its  counter  by  1  and  attaches  the  time-stamp  Ti  to  the message. When a message  is received, the receiving system  'j' sets  its counter Cj to 1 more than the maximum of  its current value and the  incoming time-stamp Ti. At each site, the ordering of messages  is determined by  the  following  rules: For messages x  from site  i and y  from site  j, x precedes y if one of the following conditions holds....(a) if Ti<Tj or  (b) if Ti=Tj and i<j.

46. What level of security does Windows NT meets?

C2 level security.

47. 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.

48. 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.