Operating Systems | Set 10

Following questions have been asked in GATE 2008 CS exam.

1) The data blocks of a very large file in the Unix file system are allocated using
(A) contiguous allocation
(B) linked allocation
(C) indexed allocation
(D) an extension of indexed allocation

Answer (D)
The Unix file system uses an extension of indexed allocation. It uses direct blocks, single indirect blocks, double indirect blocks and triple indirect blocks. Following diagram shows implementation of Unix file system.

2) The P and V operations on counting semaphores, where s is a counting semaphore, are defined as follows:

P(s) : s =  s - 1;
  if (s  < 0) then wait;
V(s) : s = s + 1;
  if (s <= 0) then wakeup a process waiting on s;

Assume that Pb and Vb the wait and signal operations on binary semaphores are provided. Two binary semaphores Xb and Yb are used to implement the semaphore operations P(s) and V(s) as follows:

P(s) : Pb(Xb);
  s = s - 1;
  if (s < 0) {
   Vb(Xb) ;
   Pb(Yb) ;
  else Vb(Xb); 

V(s) : Pb(Xb) ;
  s = s + 1;
  if (s <= 0) Vb(Yb) ;
  Vb(Xb) ;

The initial values of Xb and Yb are respectively
(A) 0 and 0
(B) 0 and 1
(C) 1 and 0
(D) 1 and 1

Answer (C)
Both P(s) and V(s) operations are perform Pb(xb) as first step. If Xb is 0, then all processes executing these operations will be blocked. Therefore, Xb must be 1.
If Yb is 1, it may become possible that two processes can execute P(s) one after other (implying 2 processes in critical section). Consider the case when s = 1, y = 1. So Yb must be 0.

3) Which of the following statements about synchronous and asynchronous I/O is NOT true?
(A) An ISR is invoked on completion of I/O in synchronous I/O but not in asynchronous I/O
(B) In both synchronous and asynchronous I/O, an ISR (Interrupt Service Routine) is invoked after completion of the I/O
(C) A process making a synchronous I/O call waits until I/O is complete, but a process making an asynchronous I/O call does not wait for completion of the I/O
(D) In the case of synchronous I/O, the process waiting for the completion of I/O is woken up by the ISR that is invoked after the completion of I/O

Answer (A)
In both Synchronous and Asynchronous, an interrupt is generated on completion of I/O. In Synchronous, interrupt is generated to wake up the process waiting for I/O. In Asynchronous, interrupt is generated to inform the process that the I/O is complete and it can process the data from the I/O operation. See this for more details.

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