UGC-NET COMPUTER SCIENCE PAPER-2 DECEMBER 2004 Answer Key with Explanation

Q::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

Answer: 
Explanation:

A semaphore, in its most basic form, is a protected integer variable that can facilitate and restrict access to shared resources in a multi-processing environment. The two most common kinds of semaphores are counting semaphores and binary semaphores. Counting semaphores represent multiple resources, while binary semaphores, as the name implies, represents two possible states (generally 0 or 1; locked or unlocked). Semaphores were invented by the late Edsger Dijkstra.

Semaphores can be looked at as a representation of a limited number of resources, like seating capacity at a restaurant. If a restaurant has a capacity of 50 people and nobody is there, the semaphore would be initialized to 50. As each person arrives at the restaurant, they cause the seating capacity to decrease, so the semaphore in turn is decremented. When the maximum capacity is reached, the semaphore will be at zero, and nobody else will be able to enter the restaurant. Instead the hopeful restaurant goers must wait until someone is done with the resource, or in this analogy, done eating. When a patron leaves, the semaphore is incremented and the resource becomes available again.

A semaphore can only be accessed using the following operations: wait() and signal(). wait() is called when a process wants access to a resource. This would be equivalent to the arriving customer trying to get an open table. If there is an open table, or the semaphore is greater than zero, then he can take that resource and sit at the table. If there is no open table and the semaphore is zero, that process must wait until it becomes available. signal() is called when a process is done using a resource, or when the patron is finished with his meal. 

Q::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

Answer: C

Q::38 Remote computing system involves the use of timesharing systems and:

(A) Real time processing        (B) Batch processing

(C) Multiprocessing                 (D) All of the above

Answer: B

Q::39 Non modifiable procedures are called

(A) Serially useable procedures      

(B) Concurrent procedures

(C) Reentrant procedures                 

(D) Topdown procedures

Answer: C
Explanation:

In computing, a computer program or subroutine is called reentrant if it can be interrupted in the middle of its execution and then safely called again ("re-entered") before its previous invocations complete execution. The interruption could be caused by an internal action such as a jump or call, or by an external action such as a hardware interrupt or signal. Once the reentered invocation completes, the previous invocations will resume correct execution.

This definition originates from single-threaded programming environments where the flow of control could be interrupted by a hardware interrupt and transferred to an interrupt service routine (ISR). Any subroutine used by the ISR that could potentially have been executing when the interrupt was triggered should be reentrant. Often, subroutines accessible via the operating system kernel are not reentrant. Hence, interrupt service routines are limited in the actions they can perform; for instance, they are usually restricted from accessing the file system and sometimes even from allocating memory.

A subroutine that is directly or indirectly recursive should be reentrant. This policy is partially enforced by structured programming languages. However a subroutine can fail to be reentrant if it relies on a global variable to remain unchanged but that variable is modified when the subroutine is recursively invoked.
Reentrant code may not hold any static (or global) non-constant data.

Reentrant functions can work with global data. For example, a reentrant interrupt service routine could grab a piece of hardware status to work with (e.g. serial port read buffer) which is not only global, but volatile. Still, typical use of static variables and global data is not advised, in the sense that only atomic read-modify-write instructions should be used in these variables (it should not be possible for an interrupt or signal to come during the execution of such an instruction).

Reentrant code may not modify its own code.

The operating system might allow a process to modify its code. There are various reasons for this (e.g., blitting graphics quickly) but this would cause a problem with reentrancy, since the code might not be the same next time.

It may, however, modify itself if it resides in its own unique memory. That is, if each new invocation uses a different physical machine code location where a copy of the original code is made, it will not affect other invocations even if it modifies itself during execution of that particular invocation (thread).

Reentrant code may not call non-reentrant computer programs or routines.

Multiple levels of 'user/object/process priority' and/or multiprocessing usually complicate the control of reentrant code. It is important to keep track of any access and or side effects that are done inside a routine designed to be reentrant.

Q::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-3, c-2, d-1

(C) a-2, b-4, c-1, d-3   

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

Answer: C

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