OPERATING SYSTEM QUESTIONS

OPERATING SYSTEMS

 DECEMBER 2004

36. Semaphores are used to :

(A)  Synchronise critical resources to prevent deadlock

(B)   Synchronise critical resources to prevent contention

(C)   Do I/O

(D)  Facilitate memory management

Ans :- A

Explanation :-

In order to understand semaphores, we should first understand the problem which it is tryig to solve. It is called “The critical-section problem”. What is “Critical-Section Problem”?. Consider a system consisting of n processes. Each process has a segment of code, called a critical section, in which the process may be changing common variables, updating a table, writing a file etc. The important thing is that, when one process is executing in its critical section, no other process is to be allowed to execute in its critical section. Thus the execution of critical sections by the processes is mutually exclusive in time.

The critical section problem is to design a protocol that the processes can use to cooperate. To overcome this difficulty, we can use a synchronization tool called  a semaphore.

37. In which of the following storage replacement strategies, is a program placed in the largest available hole in the memory?

(A) Best fit

(B)  First fit

(C)  Worst fit

(D)  Buddy

First-fit, best-fit and worst-fit are the most common strategies used to select a free hole from the set of available holes.

First-fit : allocate the first hole that is big enough.

Best-fit : Allocate the smallest hole that is big enough.

      Worst-fit : Allocate the largest hole.

Ans :- C

38. Remote computing systems involves the use of timesharing systems and :

(A) Real time processing

(B)  Batch processing

(C)  Multiprocessing

(D) All of the above

Ans:- B

39. Non modifiable procedures are called

(A) Serially usable procedures

(B)  Concurrent procedures

(C)  Reentrant procedures

(D) Topdown procedures

Ans :- C

Explanation : -

We would have guessed that it comes under the “Memory management” topic. Non modifiable procedures are referred to when a discussion on advantages of paging is done. One important advantage of paging is the possibility of sharing common code. This is particularly important in a time-sharing environment. Let us understand this with an example.

Consider a system that supports 40 users, each of whom executes a text editor. If the text editor consists of 150K of code and 50K of data space, we would need 8000K to support the 40 users. If the code is reentrant, however, it can be shared. However, each process will have its own data page.

Reentrant code also called pure code is non-self-modifying code. If the code is reentrant, then it never changes during execution. So, the correct answer for the above question is non modifiable procedures are called reentrant procedures.

40. Match the following:

(a) Disk scheduling	(1) Round robin
(b) Batch processing	(2) Scan
(C)Time sharing	(3) LIFO
(d) Interrupt processing	(4) FIFO

(A) a-3,b-4,c-2,d-1

(B)  a-4,b-2,c-2,d-1

(C)  a-2,b-4,c-1,d-3

(D) a-3,b-4,c-1,d-2

Ans:- C

Explanation :-

This is a easy question. You can first choose the obvious ones. For example, Time sharing is associated with round robin. So, C-1. But there are two options where C is matched with 1. So, let us look at other alternatives. Scan is one of the disk scheduling algorithm and so, we can match a with 2. So, batch processing matches to 4 and the last one matches d with 3. So, the correct answer is C.

JUNE 2005

36.Moving Process from main memory to disk is called :

(A)	Caching	(B)	Termination
(C)	Swapping	(D)	Interruption

Ans :- C

Explanation:-

Moving process from main memory to disk and vice versa is called swapping. Swapping is one of the memory management techniques used by the operating system. Since the size of the RAM is limited and finite, all the processes or programs to be executed cannot be made to fit in it. So the disk is also treated as an extension of the memory and is referred to as virtual memory. Moving a process from main memory to disk is called swapping.

37.The principle of Locality of reference justifies the use of :

(A)	Virtual memory	(B)	Interrupts
(C)	Cache memory	(D)	Secondary memory

Ans :- C

In computer science, locality of reference, also called the principle of locality, is the term applied to situations where the same value or related storage locations are frequently accessed. There are three basic types of locality of reference: temporal, spatial and sequential:

Temporal locality
Here a resource that is referenced at one point in time is referenced again soon afterwards.
Spatial locality
Here the likelihood of referencing a storage location is greater if a storage location near it has been recently referenced.
Sequential locality
Here storage is accessed sequentially, in descending or ascending order.

The reason locality occurs is often because of the manner in which computer programs are created. Generally, data that are related are stored in consecutive locations in storage. One common pattern in computing is that processing is performed on a single item and then the next. This means that if a lot of processing is done, the single item will be accessed more than once, thus leading to temporal locality of reference. Furthermore, moving to the next item implies that the next item will be read, hence spatial locality of reference, since memory locations are typically read in batches.

Locality often occurs because code contains loops that tend to reference arrays or other data structures by indices.

Increasing and exploiting locality of reference are common techniques for optimization. This can happen on several levels of the memory hierarchy. Paging obviously benefits from spatial locality. A cache is a simple example of exploiting temporal locality, because it is a specially designed faster but smaller memory area, generally used to keep recently referenced data and data near recently referenced data, which can lead to potential performance increases. Data in cache does not necessarily correspond to data that is spatially close in main memory; however, data elements are brought into cache one cache line at a time. This means that spatial locality is again important: if one element is referenced, a few neighbouring elements will also be brought into cache. Finally, temporal locality plays a role on the lowest level, since results that are referenced very closely together can be kept in the machine registers. Programming languages such as C allow the programmer to suggest that certain variables are kept in registers.

38.Banker’s algorithm is for.

(A)	Dead lock Prevention	(B)	Dead lock Avoidance
(C)	Dead lock Detection	(D)	Dead lock creation

Ans :- B

Explanation :- Banker’s algorithm is used for dead lock avoidance.

39.Which is the correct definition of a valid process transition in an operating system ?

(A)Wake up : Ready Running

(B)Dispatch : Ready Running

(C)Block : Ready Running

(D)Timer run out : Ready Blocked

Ans :- C