Operating Systems Sample Paper 2

Part 1. Mark True or False for each of the following questions. (40 points)

eg) Operating systems execute user programs and make solving user problems easier.  True

1. Hand­held device operating systems are designed primarily to maximize resource utilization.

False ---

2. A process control block (PCB) contains CPU usage and clock time elapsed since start.

False  --- true

3. Privileged instructions can run in any of the dual modes ­ user mode and kernel mode.

False can be done only in kernel mode

4. Multiprogramming operating systems is designed to improve response time for multiple users.

True -- false

5. Data section of process in memory contains function parameters, return addresses, local variables

False

6. Dual mode of operating in a system consists of logical mode and physical mode.

False

7. Threads inside a process share global variables.

True

8. Parent can access its child process’ global variables.

9. One­to­one mapping between user thread and kernel thread provides more concurrency than many­to­one mapping.

True

10. A race condition may result from multiple processes trying to read the same data concurrently.

True

Part 2 (11­13). Please answer to the following questions. (40 points)

11. Suppose a husband and a wife are accessing the same bank account they share. The current balance is $100. The husband tries to deposit $100 to the account while the wife tries to withdraw $50 from the account at the same time. Assuming those transactions are not atomic and no mutual exclusion is used for transactions, list all the possible values of the final balance. Explain briefly how each value is possible.

(20 points)

12. Message passing may be either blocking (synchronous) or non­blocking (asynchronous). Please explain briefly how the followings work. (10 points)

(a)  Blocking send

(b) Non­blocking send

(c) Blocking receive

(d) Non­blocking receive

13. Solutions to critical section problems are required to satisfy the following conditions

­    mutual exclusion

­    progress

­    bounded waiting

Please explain briefly about the requirement “bounded waiting”. (10 points)

Part 3 (14­15). Check out the programs and answer to questions. (30 points)

14. Assuming every fork() succeeds, what output will be displayed on the screen? Note that each process display one line so there will be as many lines of output as there are processes. The order of processes’ output doesn’t matter. (15 points)

int value = 0;

int main()

{

pid_t   pid; int i;

pid = fork();

if(pid == 0) {

value = value + 10 ;

pid = fork();

}

fork();

printf(“value=%d\n”, value);

return 0;

}

15. Many modern computer systems provide special hardware instructions that allow us either to test and modify the content of a word or to swap the contents of two words atomically. The following shows the hardware instruction compare_and_swap()and to use it in a code to solve a critical section problem.

int compare_and_swap(int *value, int expected, int new_value) {

int temp = *value;

if (*value == expected)

*value = new_value;

return temp;

}

int lock = 0;

int main(){

...

while (compare_and_swap(&lock, 1, 0) != 0)  /* Line P */

; /* do nothing */

/* critical section */

lock = 0;

/* remainder section */

}

But the above code does not provide mutual exclusion properly. Fix the problem by rewriting line P. (15 points)

while(                                                                                                 )       /* Line P */