Operating Systems

1.

ANSWER

2.Explain the concept of Reentrancy

ANSWER
It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution 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

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

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

4.What is thrashing?

ANSWER
It is a phenomenon in virtual memory schemes when the processor spends most of its time swapping pages, rather than executing instructions. This is due to an inordinate number of page faults.

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

ANSWER
Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process. Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on 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.

6.What are turnaround time and response time?

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

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

ANSWER
User data: Modifiable part of user space. May include program data, user stack area, and programs that may be modified. User program: The instructions to be executed. System Stack: Each process has one or more LIFO stacks associated with it. Used to store parameters and calling addresses for procedure and system calls. Process control Block (PCB): Info needed by the OS to control processes.

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

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

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

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

10.When is a system in safe state?

ANSWER
The set of dispatchable processes is in a safe state if there exists at least one temporal order in which all processes can be run to completion without resulting in a deadlock.

11.What is cycle stealing?

ANSWER
We encounter cycle stealing in the context of Direct Memory Access (DMA). Either the DMA controller can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific break points in an instruction cycle.

12.What is meant by arm-stickiness?

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

13.What are the stipulations of C2 level security?

ANSWER
C2 level security provides for: Discretionary Access Control Identification and Authentication Auditing Resource reuse

14.What is busy waiting?

ANSWER
The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The CPU is not engaged in any real productive activity during this period, and the process does not progress toward completion.

15.Explain the popular multiprocessor thread-scheduling strategies.

ANSWER
Load Sharing: Processes are not assigned to a particular processor. A global queue of threads is maintained. Each processor, when idle, selects a thread from this queue. Note that load balancing refers to a scheme where work is allocated to processors on a more permanent basis. 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.

16.What is a trap and trapdoor?

ANSWER
Trapdoor is a secret undocumented entry point into a program used to grant access without normal methods of access authentication. A trap is a software interrupt, usually the result of an error condition.

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

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

18.Describe the Buddy system of memory allocation.

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

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

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

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

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

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

ANSWER
For load-time dynamic linking: Load module to be loaded is read into memory. Any reference to a target external module causes that module to be loaded and the references are updated to a relative address from the start base address of the application module. With run-time dynamic loading: Some of the linking is postponed until actual reference during execution. Then the correct module is loaded and linked

22.What are demand- and pre-paging?

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

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

ANSWER
Yes.

24.What is page cannibalizing?

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Page swapping or page replacements are called page cannibalizing.

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

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

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

ANSWER
Hardware abstraction layer Kernel Subsystems System Services.

27.What is SMP?

ANSWER
To achieve maximum efficiency and reliability a mode of operation known as symmetric multiprocessing is used. In essence, with SMP any process or threads can be assigned to any processor.

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

ANSWER
Encapsulation Object class and instance

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

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

30.What is a drawback of MVT?

ANSWER
It does not have the features like ability to support multiple processors virtual storage source level debugging

31.What is process spawning?

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

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

ANSWER
15 jobs

33.List out some reasons for process termination

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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.

34.What are the reasons for process suspension?

ANSWER
swapping interactive user request timing parent process request

35.What is process migration?

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

36.What is mutant?

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

37.What is an idle thread?

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

38.What is FtDisk?

ANSWER
It is a fault tolerance disk driver for Windows NT.

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

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Ready Standby Running Waiting Transition Terminated.

40.What are rings in Windows NT?

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

41.What is Executive in Windows NT?

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

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

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

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

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

44.What level of security does Windows NT meets?

ANSWER
C2 level security.