Interview - Operating System - geeksquiz

http://www.geeksforgeeks.org/operating-systems-set-16/
1) 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?

(a) I/O protection is ensured by operating system routine(s)

Memory mapped I/O means, accessing I/O via general memory access as opposed to specialized IO instructions. An example,

  unsigned int volatile const *pMappedAddress const = (unsigned int *)0x100;

So, the programmer can directly access any memory location directly. To prevent such an access, the OS (kernel) will divide the address space into kernel space and user space. An user application can easily access user application. To access kernel space, we need system calls (traps).

2) What is the swap space in the disk used for?
(b) Saving process data
3) Increasing the RAM of a computer typically improves performance because:
(c) Fewer page faults occur

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

Answer (c)
In the extreme condition, all processes acquire Si-1 resources and need 1 more resource. So following condition must be true to make sure that deadlock never occurs.
 < m

The above expression can be written as following.
 < (m + n)

5) 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?
(c) u + 10 = x and v = y

fork() returns 0 in child process and process ID of child process in parent process.
In Child (x), a = a + 5
In Parent (u), a = a – 5;
Therefore x = u + 10.
The physical addresses of ‘a’ in parent and child must be different. But our program accesses virtual addresses (assuming we are running on an OS that uses virtual memory). The child process gets an exact copy of parent process and virtual address of ‘a’ doesn’t change in child process. Therefore, we get same addresses in both parent and child. 

Operating System | User Level thread Vs Kernel Level thread

USER LEVEL THREAD KERNEL LEVEL THREAD

User thread are implemented by users. kernel threads are implemented by OS.

OS doesn’t recognized user level threads. Kernel threads are recognized by OS.

Implementation of User threads is easy. Implementation of Kernel thread is complicated.

Context switch time is less. Context switch time is more.

Context switch requires no hardware support. Hardware support is needed.

If one user level thread perform blocking operation then entire process will be blocked. If one kernel thread perform blocking operation then another thread can continue execution.

Example : Java thread, POSIX threads.

Process Synchronization | Monitors

Monitor is one of the ways to achieve Process synchronization. Monitor is supported by programming languages to achieve mutual exclusion between processes. For example Java Synchronized methods. Java provides wait() and notify() constructs.

1. It is the collection of condition variables and procedures combined together in a special kind of module or a package.

2. The processes running outside the monitor can’t access the internal variable of monitor but can call procedures of the monitor.

3. Only one process at a time can execute code inside monitors.

Syntax of Monitor

monitors

Condition Variables

Two different operations are performed on the condition variables of the monitor.

Wait.

signal.

let say we have 2 condition variables

condition x, y; //Declaring variable

Wait operation

x.wait() : Process performing wait operation on any condition variable are suspended. The suspended processes are placed in block queue of that condition variable.

Note: Each condition variable has its unique block queue.

Signal operation

x.signal(): When a process performs signal operation on condition variable, one of the blocked processes is given chance.

If (x block queue empty)

  // Ignore signal

else

  // Resume a process from block queue.


http://www.shuatiblog.com/blog/2014/09/12/DNS-communication-protocol/

What protocol is used for communicating with a DNS?

Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most prominently, it translates easily memorized domain names to the numerical IP addresses needed for the purpose of locating computer services and devices worldwide. The Domain Name System is an essential component of the functionality of the Internet.

DNS primarily uses User Datagram Protocol (UDP) on port number 53 to serve requests.

DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server.