Consider three concurrent processes P1, P2 and P3 as shown below, which access a shared variable D that has been initialized to 100.

The process are executed on a uniprocessor system running a time-shared operating system. If the minimum and maximum possible values of D after the three processes have completed execution are X and Y respectively, then the value of Y–X is _______.

The following C program is executed on a Unix/Linux system:

            #include <unistd.h>
            int main ()
            {
                  int i ;
                  for (i=0; i<10; i++)
                         if (i%2 == 0) fork ( ) ;
                  return 0 ;
            }

The total number of child processes created is _____.

Assume that in a certain computer, the virtual addresses are 64 bits long and the physical addresses are 48 bits long. The memory is word addressable. The page size is 8 kB and the word size is 4 bytes. The Translation Look-aside Buffer (TLB) in the address translation path has 128 valid entries. At most how many distinct virtual addresses can be translated without any TLB miss?

Consider the following four processes with arrival times (in milliseconds) and their length of CPU bursts (in milliseconds) as shown below:

These processes are run on a single processor using preemptive Shortest Remaining Time First scheduling algorithm. If the average waiting time of the processes is 1 millisecond, then the value of Z is _____.

The index node (inode) of a Unix-like file system has 12 direct, one single-indirect and one double-indirect pointers. The disk block size is 4 kB, and the disk block address is 32-bits long. The maximum possible file size is (rounded off to 1 decimal place) ______ GB.

Consider the following snapshot of a system running n concurrent processes. Process i is holding X_i instances of a resource R, 1 ≤ i ≤ n. Assume that all instances of R are currently in use. Further, for all i, process i can place a request for at most Y_i additional instances of R while holding the X_i instances it already has. Of the n processes, there are exactly two processes p and q such that Y_p = Y_q = 0. Which one of the following conditions guarantees that no other process apart from p and q can complete execution?

Consider a process executing on an operating system that uses demand paging. The average time for a memory access in the system is M units if the corresponding memory page is available in memory and D units if the memory access causes a page fault. It has been experimentally measured that the average time taken for a memory access in the process is X units.

Which one of the following is the correct expression for the page fault rate experienced by the process?

Consider a system with 3 processes that share 4 instances of the same resource type. Each process can request a maximum of K instances. Resource instances can be requested and released only one at a time. The largest value of K that will always avoid deadlock is _________.

Consider the following solution to the producer-consumer synchronization problem. The shared buffer size is N. Three semaphores empty, full and mutex are defined with respective initial values of 0, N and 1. Semaphore empty denotes the number of available slots in the buffer, for the consumer to read from. Semaphore full denotes the number of available slots in the buffer, for the producer to write to. The placeholder variables, denoted by P, Q, R and S, in the code below can be assigned either empty or full. The valid semaphore operations are: wait() and signal().

Which one of the following assignments to P, Q, R and S will yield the correct solution?

Consider a storage disk with 4 platters (numbered as 0, 1, 2 and 3), 200 cylinders (numbered as 0, 1, … , 199), and 256 sectors per track (numbered as 0, 1, … 255). The following 6 disk requests of the form [sector number, cylinder number, platter number] are received by the disk controller at the same time:

[120, 72, 2], [180, 134, 1], [60, 20, 0], [212, 86, 3], [56, 116, 2], [118, 16, 1]

Currently head is positioned at sector number 100 of cylinder 80, and is moving towards higher cylinder numbers. The average power dissipation in moving the head over 100 cylinders is 20 milliwatts and for reversing the direction of the head movement once is 15 milliwatts. Power dissipation associated with rotational latency and switching of head between different platters is negligible.

The total power consumption in milliwatts to satisfy all of the above disk requests using the Shortest Seek Time First disk scheduling algorithm is ______ .

In a system, there are three types of resources: E, F and G. Four processes P₀, P₁, P₂ and P₃ execute concurrently. At the outset, the processes have declared their maximum resource requirements using a matrix named Max as given below. For example, Max[P₂, F] is the maximum number of instances of F that P₂ would require. The number of instances of the resources allocated to the various processes at any given state is given by a matrix named Allocation.

Consider a state of the system with the Allocation matrix as shown below, and in which 3 instances of E and 3 instances of F are the only resources available.

From the perspective of deadlock avoidance, which one of the following is true?

Threads of a process share

Consider the following CPU processes with arrival times (in milliseconds) and length of CPU bursts (in milliseconds) as given below:

If the pre-emptive shortest remaining time first scheduling algorithm is used to schedule the processes, then the average waiting time across all processes is __________ milliseconds.

A multithreaded program P executes with x number of threads and used y number of locks for ensuring mutual exclusion while operating on shared memory locations. All locks in the program are non-reentrant, i.e., if a thread holds a lock l, then it cannot re-acquire lock l without releasing it. If a thread is unable to acquire a lock, it blocks until the lock becomes available. The minimum value of x and the minimum value of y together for which execution of P can result in a deadlock are:

Which of the following is/are shared by all the threads in a process?

I. Program counter

II. Stack

III. Address space

IV. Registers

In a file allocation system, which of the following allocation scheme(s) can be used if no external fragmentation is allowed?

I. Contiguous

II. Linked

III. Indexed

A system shares 9 tape drives. The current allocation and maximum requirement of tape drives for three processes are shown below:

Which of the following best describe current state of the system?

Consider the set of processes with arrival time (in milliseconds), CPU burst time (in milliseconds), and priority (0 is the highest priority) shown below. None of the processes have I/O burst time.

The average waiting time (in milliseconds) of all the processes using preemptive priority scheduling algorithm is __________.

Consider an arbitrary set of CPU-bound processes with unequal CPU burst lengths submitted at the same time to a computer system. Which one of the following process scheduling algorithms would minimize the average waiting time in the ready queue?

Consider a computer system with 40-bit virtual addressing and page size of sixteen kilobytes. If the computer system has a one-level page table per process and each page table entry requires 48 bits, then the size of the per-process page table is __________ megabytes.

Consider a disk queue with requests for I/O to blocks on cylinders 47, 38, 121, 191, 87, 11, 92, 10. The C-LOOK scheduling algorithm is used. The head is initially at cylinder number 63, moving towards larger cylinder numbers on its servicing pass. The cylinders are numbered from 0 to 199. The total head movement (in number of cylinders) incurred while servicing these requests is ___________.

Consider a computer system with ten physical page frames. The system is provided with an access sequence (a₁,a₂,…,a₂₀,a₁,a₂,…,a₂₀), where each a_i is a distinct virtual page number. The difference in the number of page faults between the last-in-first-out page replacement policy and the optimal page replacement policy is ___________.

Consider the following proposed solution for the critical section problem. There are n processes: P₀...P_(n-1). In the code, function pmax returns an integer not smaller than any of its arguments. For all i, t[i] is initialized to zero.

Code for Pi:

do {

      c[i]=1; t[i] = pmax(t[0],...,t[n-1])+1; c[i]=0;
      for every j ≠ i in {0,...,n-1} {
           while (c[j]);
           while (t[j] != 0 && t[j]<=t[i]);
      }
      Critical Section;
      t[i]=0;
      Remainder Section;
} while (true);

Which one of the following is TRUE about the above solution?

In which one of the following page replacement algorithms it is possible for the page fault rate to increase even when the number of allocated frames increases?

Consider the following processes, with the arrival time and the length of the CPU burst given in milliseconds. The scheduling algorithm used is preemptive shortest remaining-time first.

The average turn around time of these processes is _________ milliseconds.

Consider the following two-process synchronization solution.

Process 0                                     Process 1
---------                                     ---------
Entry: loop while (turn == 1);                Entry: loop while (turn == 0);
  (critical section)                            (critical section)
Exit: turn = 1;                               Exit: turn = 0;

The shared variable turn is initialized to zero. Which one of the following is TRUE?

Consider a non-negative counting semaphore S. The operation P(S) decrements S, and V(S) increments S. During an execution, 20 P(S) operations and 12 V(S) operations are issued in some order. The largest initial value of S for which at least one P(S) operation will remain blocked is _________.

The following two functions P1 and P2 that share a variable B with an initial value of 2 execute concurrently.

P1() 
{ 
   C = B – 1; 
   B = 2*C;  
}

P2()
{
   D = 2 * B;
   B = D - 1; 
}

The number of distinct values that B can possibly take after the execution is

Consider a uniprocessor system executing three tasks T₁, T₂ and T₃, each of which is composed of an infinite sequence of jobs (or instances) which arrive periodically at intervals of 3, 7 and 20 milliseconds, respectively. The priority of each task is the inverse of its period and the available tasks are scheduled in order of priority, with the highest priority task scheduled first. Each instance of T₁, T₂ and T₃ requires an execution time of 1, 2 and 4 milliseconds, respectively. Given that all tasks initially arrive at the beginning of the 1^st millisecond and task preemptions are allowed, the first instance of T₃ completes its execution at the end of ______________ milliseconds.

Suppose the following disk request sequence (track numbers) for a disk with 100 tracks is given: 45, 20, 90, 10, 50, 60, 80, 25, 70. Assume that the initial position of the R/W head is on track 50. The additional distance that will be traversed by the R/W head when the Shortest Seek Time First (SSTF) algorithm is used compared to the SCAN (Elevator) algorithm (assuming that SCAN algorithm moves towards 100 when it starts execution) is _________ tracks.

Consider a main memory with five page frames and the following sequence of page references: 3, 8, 2, 3, 9, 1, 6, 3, 8, 9, 3, 6, 2, 1, 3. Which one of the following is true with respect to page replacement policies First-In-First Out (FIFO) and Least Recently Used (LRU)?

A system has 6 identical resources and N processes competing for them. Each process can request atmost 2 resources. Which one of the following values of N could lead to a deadlock?

A computer system implements a 40-bit virtual address, page size of 8 kilobytes, and a 128-entry translation look-aside buffer (TLB organized into 32 sets each having four ways. Assume that the TLB tag does not store any process id. The minimum length of the TLB tag in bits is _________.

A computer system implements 8 kilobyte pages and a 32-bit physical address space. Each page table entry contains a valid bit, a dirty bit, three permission bits, and the translation. If the maximum size of the page table of a process is 24 megabytes, the length of the virtual address supported by the system is ________ bits.

The maximum number of processes that can be in Ready state for a computer system with n CPUs is

Consider the following policies for preventing deadlock in a system with mutually exclusive resources.

I. Processes should acquire all their resources at the beginning of execution. If any resource is not available, all resources acquired so far are released.
II. The resources are numbered uniquely, and processes are allowed to request for resources only in increasing resource numbers.
III. The resources are numbered uniquely, and processes are allowed to request for resources only in decreasing resource numbers.
IV. The resources are numbered uniquely. A process is allowed to request only for a resource with resource number larger than its currently held resources.

Which of the above policies can be used for preventing deadlock?

For the processes listed in the following table, which of the following scheduling schemes will give the lowest average turnaround time?

Suppose a disk has 201 cylinders, numbered from 0 to 200. At some time the disk arm is at cylinder 100, and there is a queue of disk access requests for cylinders 30, 85, 90, 100, 105, 110, 135 and 145. If Shortest-Seek Time First (SSTF) is being used for scheduling the disk access, the request for cylinder 90 is serviced after servicing ____________ number of requests.

Which one of the following is FALSE?

An operating system uses the Banker’s algorithm for deadlock avoidance when managing the allocation of three resource types X, Y, and Z to three processes P0, P1, and P2. The table given below presents the current system state. Here, the Allocation matrix shows the current number of resources of each type allocated to each process and the Max matrix shows the maximum number of resources of each type required by each process during its execution.

There are 3 units of type X, 2 units of type Y and 2 units of type Z still available. The system is currently in a safe state. Consider the following independent requests for additional resources in the current state:

REQ1: P0 requests 0 units of X,  
      0 units of Y and 2 units of Z
REQ2: P1 requests 2 units of X, 
      0 units of Y and 0 units of Z

Which one of the following is TRUE?

Consider the following set of processes that need to be scheduled on a single CPU. All the times are given in milliseconds.

Using the shortest remaining time first scheduling algorithm, the average process turnaround time (in msec) is ________.

Assume that there are 3 page frames which are initially empty. If the page reference string is 1, 2, 3, 4, 2, 1, 5, 3, 2, 4, 6, the number of page faults using the optimal replacement policy is__________.

A FAT (file allocation table) based file system is being used and the total overhead of each entry in the FAT is 4 bytes in size. Given a 100 × 10⁶ bytes disk on which the file system is stored and data block size is 10³ bytes, the maximum size of a file that can be stored on this disk in units of 10⁶ bytes is ________.

Consider the procedure below for the Producer-Consumer problem which uses semaphores:

semaphore n=0;
semaphore s=1;
void producer()          void consumer()
{                        {
     while(true)              while(true)
     {                        {
     produce();               semWait(s);
     semWait(s);              semWait(n);
     addToBuffer();           removeFromBuffer();
     semSignal(s);            semSignal();
     semSignal(n);            consume();
     }                        }
}                         }

Which one of the following is TRUE?

Three processes A, B and C each execute a loop of 100 iterations. In each iteration of the loop, a process performs a single computation that requires tc CPU milliseconds and then initiates a single I/O operation that lasts for tio milliseconds. It is assumed that the computer where the processes execute has sufficient number of I/O devices and the OS of the computer assigns different I/O devices to each process. Also, the scheduling overhead of the OS is negligible. The processes have the following characteristics:

 Process id      tc      tio
     A        100 ms    500 ms
     B        350 ms    500 ms
     C        200 ms    500 ms

The processes A, B, and C are started at times 0, 5 and 10 milliseconds respectively, in a pure time sharing system (round robin scheduling) that uses a time slice of 50 milliseconds. The time in milliseconds at which process C would complete its first I/O operation is ___________.

A computer has twenty physical page frames which contain pages numbered 101 through 120. Now a program accesses the pages numbered 1, 2, …, 100 in that order, and repeats the access sequence THRICE. Which one of the following page replacement policies experiences the same number of page faults as the optimal page replacement policy for this program?

A system contains three programs and each requires three tape units for its operation. The minimum number of tape units which the system must have such that deadlocks never arise is _________.

An operating system uses shortest remaining time first scheduling algorithm for pre-emptive scheduling of processes. Consider the following set of processes with their arrival times and CPU burst times (in milliseconds):

The average waiting time (in milliseconds) of the processes is

Consider a paging hardware with a TLB. Assume that the entire page table and all the pages are in the physical memory. It takes 10 milliseconds to search the TLB and 80 milliseconds to access the physical memory. If the TLB hit ratio is 0.6, the effective memory access time (in milliseconds) is  _________.

Consider the basic block given below.

a = b + c
c = a + d
d = b + c
e = d - b
a = e + b

The minimum number of nodes and edges present in the DAG representation of the above basic block respectively are

The memory access time is 1 nanosecond for a read operation with a hit in cache, 5 nanoseconds for a read operation with a miss in cache, 2 nanoseconds for a write operation with a hit in cache and 10 nanoseconds for a write operation with a miss in cache. Execution of a sequence of instructions involves 100 instruction fetch operations, 60 memory operand read operations and 40 memory operand write operations. The cache hit-ratio is 0.9.  The average memory access time (in nanoseconds) in executing the sequence of instructions is   __________.

A scheduling algorithm assigns priority proportional to the waiting time of a process. Every process starts with priority zero (the lowest priority). The scheduler re-evaluates the process priorities every T time units and decides the next process to schedule. Which one of the following is TRUE if the processes have no I/O operations and all arrive at time zero?

Three concurrent processes X, Y, and Z execute three different code segments that access and update certain shared variables. Process X executes the P operation (i.e., wait) on semaphores a, b and c; process Y executes the P operation on semaphores b, c and d; process Z executes the P operation on semaphores c, d, and a before entering the respective code segments. After completing the execution of its code segment, each process invokes the V operation (i.e., signal) on its three semaphores. All semaphores are binary semaphores initialized to one. Which one of the following represents a deadlock-free order of invoking the P operations by the processes?

Consider a hard disk with 16 recording surfaces (0-15) having 16384 cylinders (0-16383) and each cylinder contains 64 sectors (0-63). Data storage capacity in each sector is 512 bytes. Data are organized cylinder-wise and the addressing format is <cylinder no., surface no., sector no.>. A file of size 42797 KB is stored in the disk and the starting disk location of the file is <1200, 9, 40>. What is the cylinder number of the last sector of the file, if it is stored in a contiguous manner?

A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes W and X reads x from memory, increments by one, stores it to memory, and then terminates. Each of the processes Y and Z reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P operation (i.e., wait) on a counting semaphore S and invokes the V operation (i.e., signal) on the semaphore S after storing x to memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution?

A certain computation generates two arrays a and b such that a[i]=f(i) for 0 ≤ i < n and b[i]=g(a[i]) for 0 ≤ i < n. Suppose this computation is decomposed into two concurrent processes X and Y such that X computes the array a and Y computes the array b. The processes employ two binary semaphores R and S, both initialized to zero. The array a is shared by the two processes. The structures of the processes are shown below.

Process X:                         Process Y:
private i;                         private i;
for (i=0; i < n; i++) {            for (i=0; i < n; i++) {
   a[i] = f(i);                       EntryY(R, S);
   ExitX(R, S);                       b[i]=g(a[i]);
}                                 }

Which one of the following represents the CORRECT implementations of ExitX and EntryY?

A computer uses 46-bit virtual address, 32-bit physical address, and a three-level paged page table organization. The page table base register stores the base address of the first-level table (T₁), which occupies exactly one page. Each entry of T₁ stores the base address of a page of the second-level table (T₂). Each entry of T₂ stores the base address of a page of the third-level table (T₃). Each entry of T₃ stores a page table entry (PTE). The PTE is 32 bits in size. The processor used in the computer has a 1 MB 16-way set associative virtually indexed physically tagged cache. The cache block size is 64 bytes.

What is the size of a page in KB in this computer?

A computer uses 46-bit virtual address, 32-bit physical address, and a three-level paged page table organization. The page table base register stores the base address of the first-level table (T₁), which occupies exactly one page. Each entry of T₁ stores the base address of a page of the second-level table (T₂). Each entry of T₂ stores the base address of a page of the third-level table (T₃). Each entry of T₃ stores a page table entry (PTE). The PTE is 32 bits in size. The processor used in the computer has a 1 MB 16-way set associative virtually indexed physically tagged cache. The cache block size is 64 bytes.

What is the minimum number of page colours needed to guarantee that no two synonyms map to different sets in the processor cache of this computer?

A process executes the code

fork();
fork();
fork();

The total number of child processes created is

Consider the 3 processes, P1, P2 and P3 shown in the table.

Process           Arrival time         Time Units Required
   P1                0                         5
   P2                1                         7
   P3                3                         4

The completion order of the 3 processes under the policies FCFS and RR2 (round robin scheduling with CPU quantum of 2 time units) are

Fetch_And_Add(X,i) is an atomic Read-Modify-Write instruction that reads the value of memory location X, increments it by the value i, and returns the old value of X. It is used in the pseudocode shown below to implement a busy-wait lock. L is an unsigned integer shared variable initialized to 0. The value of 0 corresponds to lock being available, while any non-zero value corresponds to the lock being not available.

  AcquireLock(L){
         while (Fetch_And_Add(L,1))
               L = 1;
   }
  ReleaseLock(L){
         L = 0;
   }

This implementation

A file system with 300 GByte disk uses a file descriptor with 8 direct block addresses, 1 indirect block address and 1 doubly indirect block address. The size of each disk block is 128 Bytes and the size of each disk block address is 8 Bytes. The maximum possible file size in this file system is

Consider the virtual page reference string

1, 2, 3, 2, 4, 1, 3, 2, 4, 1  

On a demand paged virtual memory system running on a computer system that main memory size of 3 pages frames which are initially empty. Let LRU, FIFO and OPTIMAL denote the number of page faults under the corresponding page replacements policy. Then

Let the page fault service time to 10 ms in a computer with average memory access time being 20 ns. If one page fault is generated for every 10⁶ memory accesses, what is the effective access time for the memory?

A thread is usually defined as a "light weight process" because an operating system (OS) maintains smaller data structures for a thread than for a process. In relation to this, which of the following is TRUE?

Let the time taken to switch between user and kernel modes of execution be t₁ while the time taken to switch between two processes be t₂. Which of the following is TRUE?

Consider the following table of arrival time and burst time for three processes P0, P1 and P2.

The pre-emptive shortest job first scheduling algorithm is used. Scheduling is carried out only at arrival or completion of processes. What is the average waiting time for the three processes?

Consider the methods used by processes P1 and P2 for accessing their critical sections whenever needed, as given below. The initial values of shared boolean variables S1 and S2 are randomly assigned.

Which one of the following statements describes the properties achieved?

A system uses FIFO policy for page replacement. It has 4 page frames with no pages loaded to begin with. The system first accesses 100 distinct pages in some order and then accesses the same 100 pages but now in the reverse order. How many page faults will occur?

Which of the following statements are true?

I. Shortest remaining time first scheduling may cause starvation
II. Preemptive scheduling may cause starvation
III. Round robin is better than FCFS in terms of response time

The following program consists of 3 concurrent processes and 3 binary semaphores.The semaphores are initialized as S0 = 1, S1 = 0, S2 = 0.

How many times will process P0 print '0'?

A system has n resources R₀,...,R_n-1,and k processes P₀,....P_k-1.The implementation of the resource request logic of each process P_i is as follows:

if (i % 2 == 0) {
      if (i < n) request Ri
      if (i+2 < n) request Ri+2
}
else {
      if (i < n) request Rn-i
      if (i+2 < n) request Rn-i-2
}

In which one of the following situations is a deadlock possible?

In which one of the following page replacement policies, Belady’s anomaly may occur?

The essential content(s) in each entry of a page table is/are

Consider a system with 4 types of resources R1 (3 units), R2 (2 units), R3 (3 units), R4 (2 units). A non-preemptive resource allocation policy is used. At any given instance, a request is not entertained if it cannot be completely satisfied. Three processes P1, P2, P3 request the sources as follows if executed independently.

Which one of the following statements is TRUE if all three processes run concurrently starting at time t=0?

Consider a disk system with 100 cylinders. The requests to access the cylinders occur in following sequence:

    4, 34, 10, 7, 19, 73, 2, 15, 6, 20 

Assuming that the head is currently at cylinder 50, what is the time taken to satisfy all requests if it takes 1ms to move from one cylinder to adjacent one and shortest seek time first policy is used?

In the following process state transition diagram for a uniprocessor system, assume that there are always some processes in the ready state:

Now consider the following statements:

I. If a process makes a transition D, it would result in another process making transition A immediately.
II. A process P2 in blocked state can make transition E while another process P1 is in running state.
III. The OS uses preemptive scheduling.
IV. The OS uses non-preemptive scheduling.

Which of the above statements are TRUE?

The enter_CS() and leave_CS() functions to implement critical section of a process are realized using test-and-set instruction as follows:

void enter_CS(X)
{
    while test-and-set(X) ;
}
void leave_CS(X)
{
   X = 0;
}

In the above solution, X is a memory location associated with the CS and is initialized to 0. Now consider the following statements:

I. The above solution to CS problem is deadlock-free.
II. The solution is starvation free.
III. The processes enter CS in FIFO order.
IV More than one process can enter CS at the same time.

Which of the above statements is TRUE?

A multilevel page table is preferred in comparison to a single level page table for translating virtual address to physical address because

The data blocks of a very large file in the Unix file system are allocated using

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 P_b and V_b the wait and signal operations on binary semaphores are provided. Two binary semaphores X_b and Y_b 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 X_b and Y_b are respectively

Which of the following statements about synchronous and asynchronous I/O is NOT true?

Which of the following is NOT true of deadlock prevention and deadlock avoidance schemes?

A process executes the following code

for(i=0; i<n; i++) for();

The total number of child processes created is

A processor uses 36 bit physical addresses and 32 bit virtual addresses, with a page frame size of 4 Kbytes. Each page table entry is of size 4 bytes. A three level page table is used for virtual to physical address translation, where the virtual address is used as follows

• Bits 30-31 are used to index into the first level page table
• Bits 21-29 are used to index into the second level page table
• Bits 12-20 are used to index into the third level page table, and
• Bits 0-11 are used as offset within the page

The number of bits required for addressing the next level page table (or page frame) in the page table entry of the first, second and third level page tables are respectively.

Group 1 contains some CPU scheduling algorithms and Group 2 contains some applications. Match entries in Group 1 to entries in Group 2.

     Group I                          Group II
(P) Gang Scheduling              (1) Guaranteed Scheduling
(Q) Rate Monotonic Scheduling    (2) Real-time Scheduling
(R) Fair Share Scheduling        (3) Thread Scheduling

Consider the following statements about user level threads and kernel level threads. Which one of the following statements is FALSE?

An operating system uses Shortest Remaining Time first (SRT) process scheduling algorithm. Consider the arrival times and execution times for the following processes:

 
        Process     Execution time     Arrival time
          P1             20                0
          P2             25                15
          P3             10                30
          P4             15                45

What is the total waiting time for process P2?

A virtual memory system uses First In First Out (FIFO) page replacement policy and allocates a fixed number of frames to a process. Consider the following statements:

P: Increasing the number of page frames allocated to a 
   process sometimes increases the page fault rate.
Q: Some programs do not exhibit locality of reference.

Which one of the following is TRUE?

A single processor system has three resource types X, Y and Z, which are shared by three processes. There are 5 units of each resource type. Consider the following scenario, where the column alloc denotes the number of units of each resource type allocated to each process, and the column request denotes the number of units of each resource type requested by a process in order to complete execution. Which of these processes will finish LAST?

 
    alloc      request
    X Y Z       X Y Z
P0  1 2 1       1 0 3
P1  2 0 1       0 1 2
P2  2 2 1       1 2 0

Two processes, P1 and P2, need to access a critical section of code. Consider the following synchronization construct used by the processes:

  /* P1 */
while (true) {
  wants1 = true;
  while (wants2 == true);
  /* Critical
    Section */
  wants1=false;
}
/* Remainder section */       


/* P2 */
while (true) {
  wants2 = true;
  while (wants1==true);
  /* Critical
    Section */
  wants2 = false;
}
/* Remainder section */ 

Here, wants1 and wants2 are shared variables, which are initialized to false. Which one of the following statements is TRUE about the above construct?

A process has been allocated 3 page frames. Assume that none of the pages of the process are available in the memory initially. The process makes the following sequence of page references (reference string): 1, 2, 1, 3, 7, 4, 5, 6, 3, 1

If optimal page replacement policy is used, how many page faults occur for the above reference string?

A process has been allocated 3 page frames. Assume that none of the pages of the process are available in the memory initially. The process makes the following sequence of page references (reference string): 1, 2, 1, 3, 7, 4, 5, 6, 3, 1

Least Recently Used (LRU) page replacement policy is a practical approximation to optimal page replacement. For the above reference string, how many more page faults occur with LRU than with the optimal page replacement policy?

Consider three CPU-intensive processes, which require 10, 20 and 30 time units and arrive at times 0, 2 and 6, respectively. How many context switches are needed if the operating system implements a shortest remaining time first scheduling algorithm? Do not count the context switches at time zero and at the end.

The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.

void P (binary_semaphore *s)
{
    unsigned y;
    unsigned *x = &(s->value);
    do
    {
        fetch-and-set x, y;
    }
    while (y);
}
void V (binary_semaphore *s)
{
    S->value = 0;
}

Which one of the following is true?

A CPU generates 32-bit virtual addresses. The page size is 4 KB. The processor has a translation look-aside buffer (TLB) which can hold a total of 128 page table entries and is 4-way set associative. The minimum size of the TLB tag is:

A computer system supports 32-bit virtual addresses as well as 32-bit physical addresses. Since the virtual address space is of the same size as the physical address space, the operating system designers decide to get rid of the virtual memory entirely. Which one of the following is true?

Consider three processes (process id 0, 1, 2 respectively) with compute time bursts 2, 4 and 8 time units. All processes arrive at time zero. Consider the longest remaining time first (LRTF) scheduling algorithm. In LRTF ties are broken by giving priority to the process with the lowest process id. The average turnaround time is:

Consider three processes, all arriving at time zero, with total execution time of 10, 20 and 30 units, respectively. Each process spends the first 20% of execution time doing I/O, the next 70% of time doing computation, and the last 10% of time doing I/O again. The operating system uses a shortest remaining compute time first scheduling algorithm and schedules a new process either when the running process gets blocked on I/O or when the running process finishes its compute burst. Assume that all I/O operations can be overlapped as much as possible. For what percentage of time does the CPU remain idle?

Consider the following snapshot of a system running n processes. Process i is holding x_i instances of a resource R, 1≤i≤n. Currently, all instances of R are occupied. Further, for all i, process i has placed a request for an additional y_i instances while holding the x_i instances it already has. There are exactly two processes p and q such that y_p = y_q = 0. Which one of the following can serve as a necessary condition to guarantee that the system is not approaching a deadlock?

Barrier is a synchronization construct where a set of processes synchronizes globally i.e. each process in the set arrives at the barrier and waits for all others to arrive and then all processes leave the barrier. Let the number of processes in the set be three and S be a binary semaphore with the usual P and V functions. Consider the following C implementation of a barrier with line numbers shown on left.

void barrier (void) {
1:   P(S);
2:   process_arrived++;
3.   V(S);
4:   while (process_arrived !=3);
5:   P(S);
6:   process_left++;
7:   if (process_left==3) {
8:      process_arrived = 0;
9:      process_left = 0;
10:  }
11:  V(S);
}

The variables process_arrived and process_left are shared among all processes and are initialized to zero. In a concurrent program all the three processes call the barrier function when they need to synchronize globally.

The above implementation of barrier is incorrect. Which one of the following is true?

Barrier is a synchronization construct where a set of processes synchronizes globally i.e. each process in the set arrives at the barrier and waits for all others to arrive and then all processes leave the barrier. Let the number of processes in the set be three and S be a binary semaphore with the usual P and V functions. Consider the following C implementation of a barrier with line numbers shown on left.

void barrier (void) {
1:   P(S);
2:   process_arrived++;
3.   V(S);
4:   while (process_arrived !=3);
5:   P(S);
6:   process_left++;
7:   if (process_left==3) {
8:      process_arrived = 0;
9:      process_left = 0;
10:  }
11:  V(S);
} 

The variables process_arrived and process_left are shared among all processes and are initialized to zero. In a concurrent program all the three processes call the barrier function when they need to synchronize globally.

Which one of the following rectifies the problem in the implementation?

Normally user programs are prevented from handling I/O directly by I/O instructions in them. For CPUs having explicit I/O instructions, such I/O protection is ensured by having the I/O instructions privileged. In a CPU with memory mapped I/O, there is no explicit I/O instruction. Which one of the following is true for a CPU with memory mapped I/O?

Increasing the RAM of a computer typically improves performance because:

Suppose n processes, P₁, …., P_n share m identical resource units, which can be reserved and released one at a time. The maximum resource requirement of process P_i is s_i, where s_i>0. Which one of the following is a sufficient condition for ensuring that deadlock does not occur?

Consider the following code fragment:

  if (fork() == 0)
      { a = a + 5; printf(“%d, %d\n”, a, &a); }
  else { a = a – 5; printf(“%d, %d\n”, a, &a); }

Let u, v be the values printed by the parent process, and x, y be the values printed by the child process. Which one of the following is TRUE?

Consider the following statements with respect to user-level threads and kernel supported threads

(i) context switch is faster with kernel-supported threads
(ii) for user-level threads, a system call can block the entire process
(iii) Kernel supported threads can be scheduled independently
(iv) User level threads are transparent to the kernel

Which of the above statements are true?

Consider an operating system capable of loading and executing a single sequential user process at a time. The disk head scheduling algorithm used is First Come First Served (FCFS). If FCFS is replaced by Shortest Seek Time First (SSTF), claimed by the vendor to give 50% better benchmark results, what is the expected improvement in the I/O performance of user programs?

he minimum number of page frames that must be allocated to a running process in a virtual memory environment is determined by

Consider the following set of processes, with the arrival times and the CPU-burst times given in milliseconds.

  Process   Arrival Time    Burst Time
     P1          0              5
     P2          1              3
     P3          2              3
     P4          4              1 

What is the average turnaround time for these processes with the preemptive shortest remaining processing time first (SRPT) algorithm?

Consider a system with a two-level paging scheme in which a regular memory access takes 150 nanoseconds, and servicing a page fault takes 8 milliseconds. An average instruction takes 100 nanoseconds of CPU time, and two memory accesses. The TLB hit ratio is 90%, and the page fault rate is one in every 10,000 instructions. What is the effective average instruction execution time?

Consider two processes P₁ and P₂ accessing the shared variables X and Y protected by two binary semaphores S_X and S_Y respectively, both initialized to 1. P and V denote the usual semaphone operators, where P decrements the semaphore value, and V increments the semaphore value. The pseudo-code of P₁ and P₂ is as follows:

P1 :
 While true do {
   L1 : ................
   L2 : ................
   X = X + 1;
   Y = Y - 1;
   V(SX);
   V(SY);             
 }
P2 :
 While true do {
   L3 : ................   
   L4 : ................
   Y = Y + 1;
   X = Y - 1;
   V(SY);
   V(SX);            
}

In order to avoid deadlock, the correct operators at L₁, L₂, L₃ and L₄ are respectively

A Unix-style i-node has 10 direct pointers and one single, one double and one triple indirect pointers. Disk block size is 1 Kbyte, disk block address is 32 bits, and 48-bit integers are used. What is the maximum possible file size?

In a system with 32 bit virtual addresses and 1 KB page size, use of one-level page tables for virtual to physical address translation is not practical because of

A uni-processor computer system only has two processes, both of which alternate 10ms CPU bursts with 90ms I/O bursts. Both the processes were created at nearly the same time. The I/O of both processes can proceed in parallel. Which of the following scheduling strategies will result in the least CPU utilization (over a long period of time) for this system?

A processor uses 2-level page tables for virtual to physical address translation. Page tables for both levels are stored in the main memory. Virtual and physical addresses are both 32 bits wide. The memory is byte addressable. For virtual to physical address translation, the 10 most significant bits of the virtual address are used as index into the first level page table while the next 10 bits are used as index into the second level page table. The 12 least significant bits of the virtual address are used as offset within the page. Assume that the page table entries in both levels of page tables are 4 bytes wide. Further, the processor has a translation look-aside buffer (TLB), with a hit rate of 96%. The TLB caches recently used virtual page numbers and the corresponding physical page numbers. The processor also has a physically addressed cache with a hit rate of 90%. Main memory access time is 10 ns, cache access time is 1 ns, and TLB access time is also 1 ns.

Assuming that no page faults occur, the average time taken to access a virtual address is approximately (to the nearest 0.5 ns)

A processor uses 2-level page tables for virtual to physical address translation. Page tables for both levels are stored in the main memory. Virtual and physical addresses are both 32 bits wide. The memory is byte addressable. For virtual to physical address translation, the 10 most significant bits of the virtual address are used as index into the first level page table while the next 10 bits are used as index into the second level page table. The 12 least significant bits of the virtual address are used as offset within the page. Assume that the page table entries in both levels of page tables are 4 bytes wide. Further, the processor has a translation look-aside buffer (TLB), with a hit rate of 96%. The TLB caches recently used virtual page numbers and the corresponding physical page numbers. The processor also has a physically addressed cache with a hit rate of 90%. Main memory access time is 10 ns, cache access time is 1 ns, and TLB access time is also 1 ns.

Suppose a process has only the following pages in its virtual address space: two contiguous code pages starting at virtual address 0x00000000, two contiguous data pages starting at virtual address 0×00400000, and a stack page starting at virtual address 0×FFFFF000. The amount of memory required for storing the page tables of this process is:

Suppose we want to synchronize two concurrent processes P and Q using binary semaphores S and T. The code for the processes P and Q is shown below.

Process P:
while (1) {
W:
   print '0';
   print '0';
X:
}
	
Process Q:
while (1) {
Y:
   print '1';
   print '1';
Z:
}

Synchronization statements can be inserted only at points W, X, Y and Z.

Which of the following will always lead to an output staring with '001100110011'?

Suppose we want to synchronize two concurrent processes P and Q using binary semaphores S and T. The code for the processes P and Q is shown below.

Process P:
while (1) {
W:
   print '0';
   print '0';
X:
}
	
Process Q:
while (1) {
Y:
   print '1';
   print '1';
Z:
}

Synchronization statements can be inserted only at points W, X, Y and Z.

Which of the following will ensure that the output string never contains a substring of the form 01ⁿ0 or 10ⁿ1 where n is odd?

Which of the following scheduling algorithms is non-preemptive?

The optimal page replacement algorithm will select the page that

Dynamic linking can cause security concerns because

Which combination of the following features will suffice to characterize an OS as a multi-programmed OS?

(a) More than one program may be loaded into main memory at the same time for execution.
(b) If a program waits for certain events such as I/O, another program is immediately scheduled for execution.
(c) If the execution of program terminates, another program is immediately scheduled for execution.

In the index allocation scheme of blocks to a file, the maximum possible size of the file depends on

(a) Draw the process state transition diagram of an OS in which (i) each process is in one of the five states: created, ready, running, blocked (i.e. sleep or wait), or terminated, and (ii) only non-preemptive scheduling is used by the OS. Label the transitions appropriately.

(b) The functionality of atomic TEST-AND-SET assembly language instruction is given by the following C function.

 int TEST-AND-SET (int *x)
 {
    int y;
    A1:y=*x;
    A2:*x=1;
    A3:return y;
 } 

(i) Complete the following C functions for implementing code for entering and leaving critical sections based on the above TEST-AND-SET instruction.

 int mutex=0;
 void enter-cs()
 {
 while (…………………………………);
 }
void leave-cs()
 {
 …………………………………..;
 } 

(ii) Is the above solution to the critical section problem deadlock free and starvation-free?

(iii) For the above solution, show by an example that mutual exclusion is not ensured if TEST-AND-SET instruction is not atomic.

A computer system uses 32-bit virtual address, and 32-bit physical address. The physical memory is byte addressable, and the page size is 4 kbytes. It is decided to use two level page tables to translate from virtual address to physical address. Equal number of bits should be used for indexing first level and second level page table, and the size of each page table entry is 4 bytes.

(a) Give a diagram showing how a virtual address would be translated to a physical address.
(b) What is the number of page table entries that can be contained in each page?
(c) How many bits are available for storing protection and other information in each page table entry?

The following solution to the single producer single consumer problem uses semaphores for synchronization.

 #define BUFFSIZE 100
 buffer buf[BUFFSIZE];
 int first=last=0;
 semaphore b_full=0;
 semaphore b_empty=BUFFSIZE;
 
void producer()
 {
 while (1) {
     produce an item;
     p1: …………………..;
     put the item into buff (first);
     first=(first+1)%BUFFSIZE;
     p2: …………………..;
        }
 }
 void consumer()
 { 
while (1) {
             c1:……………………..
             take the item from buf[last];
             last=(last+1)%BUFFSIZE;
             c2: ……………………..;
             consume the item;
          }
 } 

(a) Complete the dotted part of the above solution.
(b) Using another semaphore variable, insert one line statement each immediately after p1, immediately before p2, immediately after c1, and immediately before c2 so that the program works correctly for multiple procedures and consumers.

Which of the following statements is false?

Consider a set of n tasks with known runtimes r₁, r₂, ..., r_n to be run on a uniprocessor machine. Which of the following processor scheduling algorithms will result in the maximum throughput?

Consider a virtual memory system with FIFO page replacement policy. For an arbitrary page access pattern, increasing the number of page frames in main memory will

Which of the following does not interrupt a running process?

Consider a machine with 64 MB physical memory and a 32-bit virtual address space. If the page size is 4 KB, what is the approximate size of the page table?

Consider Peterson’s algorithm for mutual exclusion between two concurrent processes i and j. The program executed by process is shown below.

   repeat   
      flag[i] = true; 
      turn = j; 
      while (P) do no-op; 
      Enter critical section, perform actions, then exit critical 
      section 
      flag[i] = false; 
      Perform other non-critical section actions. 
   until false; 

For the program to guarantee mutual exclusion, the predicate P in the while loop should be.

Two concurrent processes P1 and P2 want to use two resources R1 and R2 in a mutually exclusive manner. Initially, R1 and R2 are free. The programs executed by the two processes are given below.

Program for P1:
                    S1: While (R1 is busy) do no-op;
                    S2: Set R1 ← busy;
                    S3: While (R2 is busy) do no-op;
                    S4: Set R2 ← busy;
                    S5: Use R1 and R2;
                    S6: Set R1 ← free;
                    S7: Set R2 ← free;
Program for P2:
                    Q1: While (R1 is busy) do no-op;
                    Q2: Set R1 ← busy;
                    Q3: While (R1 is busy) do no-op;
                    Q4: Set R1 ← busy;
                    Q5: Use R1 and R2;
                    Q6: Set R2 ← free;
                    Q7: Set R1 ← free; 

(a) Is mutual exclusion guaranteed for R1 and R2? If not, show a possible interleaving of the statements of P1 and P2 such that mutual exclusion is violated (i.e., both P1 and P2 use R1 or R2 at the same time).
(b) Can deadlock occur in the above program? If yes, show a possible interleaving of the statements of P1 and P2 leading to deadlock.
(c) Exchange the statements Q1 and Q3 and statements Q2 and Q4. Is mutual exclusion guaranteed now? Can deadlock occur?

Which of the following need not necessarily be saved on a context switch between processes?

Let m[0] … m[4] be mutexes (binary semaphores) and P[0] … P[4] be processes. Suppose each process P[i] executes the following:

  wait (m[i]); wait(m[(i+1) mode 4]);
  ------
  release (m[i]); release (m[(i+1)mod 4]);

This could cause:

A graphics card has on board memory of 1MB. Which of the following modes can the card not support?

Suppose the time to service a page fault is on the average 10 milliseconds, while a memory access takes 1 microsecond. Then a 99.99% hit ratio results in average memory access time of

Which of the following is NOT a valid deadlock prevention scheme?

(a) Fill in the boxes below to get a solution for the readers-writers problem, using a single binary semaphore, mutex (initialized to 1) and busy waiting. Write the box numbers (1, 2 and 3), and their contents in your answer book.

     int R = 0, W = 0;
     Reader () {
 L1:        wait (mutex);
            If (W ==0) {
                R = R +1; 
                ▭______(1)
     }
     else {
                ▭______(2)
                goto L1;
                }
                …./* do the read */
                wait (mutex)
                R = R – 1;
                signal (mutex);
          }
          Writer () {
 L2:            wait(mutex);
                If (▭) {______(3)
                    signal (mutex);
                    goto L2;
                }
                W=1;
                signal (mutex);
                …./*do the write*/
                wait(mutex)
                W = 0;
                signal (mutex); 

(b) Can the above solution lead to starvation of writers?

Listed below are some operating system abstractions (in the left column) and the hardware components or mechanism (in the right column) that they are abstractions of. Which of the following matching of pairs is correct?

    A. Thread                      1. Interrupt
    B. Virtual address space       2. Memory
    C. File system                 3. CPU
    D. Signal                      4. Disk 

Which of the following disk scheduling strategies is likely to give the best through put?

System calls are usually invoked by using

A  multi-user,  multi-processing  operating  system  cannot  be  implemented  on hardware that does not support

Which of the following is/are advantage of virtual memory?

Which  of  the  following  actions  is/are  typically  not  performed  by  the  operating system when switching context from process A to process B?

Consider the following program in a language that has dynamic seeping.

   var x: real;
   procedure show:
       begin print(x);end;
   procedure small;
       var x: real;
           begin x: = 0.125; show; end;
   begin x:=0.25
       show; small
       end. 

Then the output of the program is:

A certain computer system has the segmented paging architecture for virtual memory. The memory is byte addressable. Both virtual and physical address spaces contain 2¹⁶ bytes each. The virtual address space is divided into 8 non-overlapping equal size segments. The memory management unit (MMU) has a hardware segment table, each entry of which contains the physical address of the page table for the segments. Page tables are stored in the main memory and consists of 2 byte page table entries.

(a) What is the minimum page size in bytes so that the page table for a segment requires at most one page to store it? Assume that the page size can only be a power of 2.

(b) Now suppose that the pages size is 512 bytes. It is proposed to provide a TLB (Transaction look-aside buffer) for speeding up address translation. The proposed TLB will be capable of storing page table entries for 16 recently referenced virtual pages, in a fast cache that will use the direct mapping scheme. What is the number of tag bits that will need to be associated with each cache entry?

(c) Assume that each page table entry contains (besides other information) 1 valid bit, 3 bits for page protection and 1 dirty bit. How many bits are available in page table entry for storing the aging information for the page? Assume that the page size is 512 bytes.

(a) A certain processor provides a 'test and set' instruction that is used as follows:

 TEST register, flag 

This instruction atomically copies flag to register and sets flag to 1. Give pseudo-code for implementing the entry and exit code to a critical region using this instruction.

(b) Consider the following solution to the producer-consumer problem using a buffer of size 1. Assume that the initial value of count is 0. Also assume that the testing of count and assignment to count are atomic operations.

            Producer:      
                Repeat 
                         Produce an item;
                         if count = 1 then sleep;
                         place item in buffer.
                         Count = 1;
                         Wakeup(Consumer);
               Forever 

            Consumer:
               Repeat
                         if count = 0 then sleep;
                         Remove item from buffer;
                         Count = 0;
                         Wakeup(Producer);
                         Consume item;
              Forever; 

Show that in this solution it is possible that both the processes are sleeping at the same time.

A linker reads four modules whose lengths are 200, 800, 600 and 500 words, respectively. If they are loaded in that order, what are the relocation constants?

Which of the following is an example of a spooled device?

When  the  result  of  a  computation  depends  on  the  speed  of  the  processes involved there is said to be

A counting semaphore was initialized to 10. Then 6P (wait) operations and 4V (signal) operations were completed on this semaphore. The resulting value of the semaphore is

A  computer  has  six  tape  drives,  with  n  processes  competing  for  them.  Each process may need two drives. What is the maximum value of n for the system to be deadlock free?