Interview Quesions

FAQ In UNIX-Part 4:

31. Name two paging states for a page in memory?
The two paging states are:
> The page is aging and is not yet eligible for swapping,
> The page is eligible for swapping but not yet eligible for reassignment to other virtual address space.

32. What are the phases of swapping a page from the memory?
> Page stealer finds the page eligible for swapping and places the page number in the list of pages to be swapped.
> Kernel copies the page to a swap device when necessary and clears the valid bit in the page table entry, decrements the pfdata reference count, and places the pfdata table entry at the end of the free list if its reference count is 0.

33. What is page fault? Its types?
Page fault refers to the situation of not having a page in the main memory when any process references it.
There are two types of page fault :
> Validity fault,
> Protection fault.

34. In what way the Fault Handlers and the Interrupt handlers are different?
Fault handlers are also an interrupt handler with an exception that the interrupt handlers cannot sleep. Fault handlers sleep in the context of the process that caused the memory fault. The fault refers to the running process and no arbitrary processes are put to sleep.

35. What is validity fault?
If a process referring a page in the main memory whose valid bit is not set, it results in validity fault.
The valid bit is not set for those pages:
> that are outside the virtual address space of a process,
> that are the part of the virtual address space of the process but no physical address is assigned to it.

36. What does the swapping system do if it identifies the illegal page for swapping?
If the disk block descriptor does not contain any record of the faulted page, then this causes the attempted memory reference is invalid and the kernel sends a “Segmentation violation” signal to the offending process. This happens when the swapping system identifies any invalid memory reference.

37. What are states that the page can be in, after causing a page fault?
> On a swap device and not in memory,
> On the free page list in the main memory,
> In an executable file,
> Marked “demand zero”,
> Marked “demand fill”.

38. In what way the validity fault handler concludes?
> It sets the valid bit of the page by clearing the modify bit.
> It recalculates the process priority.

39. At what mode the fault handler executes?
At the Kernel Mode.

40. What do you mean by the protection fault?
Protection fault refers to the process accessing the pages, which do not have the access permission. A process also incur the protection fault when it attempts to write a page whose copy on write bit was set during the fork() system call.

41. How the Kernel handles the copy on write bit of a page, when the bit is set?
In situations like, where the copy on write bit of a page is set and that page is shared by more than one process, the Kernel allocates new page and copies the content to the new page and the other processes retain their references to the old page. After copying the Kernel updates the page table entry with the new page number. Then Kernel decrements the reference count of the old pfdata table entry.
In cases like, where the copy on write bit is set and no processes are sharing the page, the Kernel allows the physical page to be reused by the processes. By doing so, it clears the copy on write bit and disassociates the page from its disk copy (if one exists), because other process may share the disk copy. Then it removes the pfdata table entry from the page-queue as the new copy of the virtual page is not on the swap device. It decrements the swap-use count for the page and if count drops to 0, frees the swap space.

42. For which kind of fault the page is checked first?
The page is first checked for the validity fault, as soon as it is found that the page is invalid (valid bit is clear), the validity fault handler returns immediately, and the process incur the validity page fault. Kernel handles the validity fault and the process will incur the protection fault if any one is present.

43. In what way the protection fault handler concludes?
After finishing the execution of the fault handler, it sets the modify and protection bits and clears the copy on write bit. It recalculates the process-priority and checks for signals.

44. How the Kernel handles both the page stealer and the fault handler?
The page stealer and the fault handler thrash because of the shortage of the memory. If the sum of the working sets of all processes is greater that the physical memory then the fault handler will usually sleep because it cannot allocate pages for a process. This results in the reduction of the system throughput because Kernel spends too much time in overhead, rearranging the memory in the frantic pace.

FAQ In UNIX-Part 3.

11. What is Fork swap?
fork() is a system call to create a child process. When the parent process calls fork() system call, the child process is created and if there is short of memory then the child process is sent to the read-to-run state in the swap device, and return to the user state without swapping the parent process. When the memory will be available the child process will be swapped into the main memory.

12. What is Expansion swap?
At the time when any process requires more memory than it is currently allocated, the Kernel performs Expansion swap. To do this Kernel reserves enough space in the swap device. Then the address translation mapping is adjusted for the new virtual address space but the physical memory is not allocated. At last Kernel swaps the process into the assigned space in the swap device. Later when the Kernel swaps the process into the main memory this assigns memory according to the new address translation mapping.

13. How the Swapper works?
The swapper is the only process that swaps the processes. The Swapper operates only in the Kernel mode and it does not uses System calls instead it uses internal Kernel functions for swapping. It is the archetype of all kernel process.

14. What are the processes that are not bothered by the swapper? Give Reason.
> Zombie process: They do not take any up physical memory.
> Processes locked in memories that are updating the region of the process.
> Kernel swaps only the sleeping processes rather than the ‘ready-to-run’ processes, as they have the higher probability of being scheduled than the Sleeping processes.

15. What are the requirements for a swapper to work?
The swapper works on the highest scheduling priority. Firstly it will look for any sleeping process, if not found then it will look for the ready-to-run process for swapping. But the major requirement for the swapper to work the ready-to-run process must be core-resident for at least 2 seconds before swapping out. And for swapping in the process must have been resided in the swap device for at least 2 seconds. If the requirement is not satisfied then the swapper will go into the wait state on that event and it is awaken once in a second by the Kernel.

16. What are the criteria for choosing a process for swapping into memory from the swap device?
The resident time of the processes in the swap device, the priority of the processes and the amount of time the processes had been swapped out.

17. What are the criteria for choosing a process for swapping out of the memory to the swap device?
> The process’s memory resident time,
> Priority of the process and
> The nice value.

18. What do you mean by nice value?
Nice value is the value that controls {increments or decrements} the priority of the process. This value that is returned by the nice () system call. The equation for using nice value is:
Priority = (“recent CPU usage”/constant) + (base- priority) + (nice value)
Only the administrator can supply the nice value. The nice () system call works for the running process only. Nice value of one process cannot affect the nice value of the other process.

19. What are conditions on which deadlock can occur while swapping the processes?
> All processes in the main memory are asleep.
> All ‘ready-to-run’ processes are swapped out.
> There is no space in the swap device for the new incoming process that are swapped out of the main memory.
> There is no space in the main memory for the new incoming process.

20. What are conditions for a machine to support Demand Paging?
> Memory architecture must based on Pages,
> The machine must support the ‘restartable’ instructions.

21. What is ‘the principle of locality’?
It’s the nature of the processes that they refer only to the small subset of the total data space of the process. i.e. the process frequently calls the same subroutines or executes the loop instructions.

22. What is the working set of a process?
The set of pages that are referred by the process in the last ‘n’, references, where ‘n’ is called the window of the working set of the process.

23. What is the window of the working set of a process?
The window of the working set of a process is the total number in which the process had referred the set of pages in the working set of the process.

24. What is called a page fault?
Page fault is referred to the situation when the process addresses a page in the working set of the process but the process fails to locate the page in the working set. And on a page fault the kernel updates the working set by reading the page from the secondary device.

25. What are data structures that are used for Demand Paging?
Kernel contains 4 data structures for Demand paging. They are,
> Page table entries,
> Disk block descriptors,
> Page frame data table (pfdata),
> Swap-use table.

26. What are the bits that support the demand paging?
Valid, Reference, Modify, Copy on write, Age. These bits are the part of the page table entry, which includes physical address of the page and protection bits.

Page address

Age
Copy on write
Modify
Reference
Valid
Protection

 
27. How the Kernel handles the fork() system call in traditional Unix and in the System V Unix, while swapping?
Kernel in traditional Unix, makes the duplicate copy of the parent’s address space and attaches it to the child’s process, while swapping. Kernel in System V Unix, manipulates the region tables, page table, and pfdata table entries, by incrementing the reference count of the region table of shared regions.

28. Difference between the fork() and vfork() system call?
During the fork() system call the Kernel makes a copy of the parent process’s address space and attaches it to the child process.
But the vfork() system call do not makes any copy of the parent’s address space, so it is faster than the fork() system call. The child process as a result of the vfork() system call executes exec() system call. The child process from vfork() system call executes in the parent’s address space (this can overwrite the parent’s data and stack ) which suspends the parent process until the child process exits.

29. What is BSS(Block Started by Symbol)?
A data representation at the machine level, that has initial values when a program starts and tells about how much space the kernel allocates for the un-initialized data. Kernel initializes it to zero at run-time.

30. What is Page-Stealer process?
This is the Kernel process that makes rooms for the incoming pages, by swapping the memory pages that are not the part of the working set of a process. Page-Stealer is created by the Kernel at the system initialization and invokes it throughout the lifetime of the system. Kernel locks a region when a process faults on a page in the region, so that page stealer cannot steal the page, which is being faulted in.

FAQ In UNIX-part-2!

7. How can you get/set an environment variable from a program?
Getting the value of an environment variable is done by using `getenv()’.
Setting the value of an environment variable is done by using `putenv()’.

8. How can a parent and child process communicate?
A parent and child can communicate through any of the normal inter-process communication schemes (pipes, sockets, message queues, shared memory), but also have some special ways to communicate that take advantage of their relationship as a parent and child. One of the most obvious is that the parent can get the exit status of the child.

9. What is a zombie?
When a program forks and the child finishes before the parent, the kernel still keeps some of its information about the child in case the parent might need it - for example, the parent may need to check the child’s exit status. To be able to get this information, the parent calls `wait()’; In the interval between the child terminating and the parent calling `wait()’, the child is said to be a `zombie’ (If you do `ps’, the child will have a `Z’ in its status field to indicate this.)

10. What are the process states in Unix?
As a process executes it changes state according to its circumstances. Unix processes have the following states:
Running : The process is either running or it is ready to run .
Waiting : The process is waiting for an event or for a resource.
Stopped : The process has been stopped, usually by receiving a signal.
Zombie : The process is dead but have not been removed from the process table.

11. What Happens when you execute a program?
When you execute a program on your UNIX system, the system creates a special environment for that program. This environment contains everything needed for the system to run the program as if no other program were running on the system. Each process has process context, which is everything that is unique about the state of the program you are currently running. Every time you execute a program the UNIX system does a fork, which performs a series of operations to create a process context and then execute your program in that context. The steps include the following:
> Allocate a slot in the process table, a list of currently running programs kept by UNIX.
> Assign a unique process identifier (PID) to the process.
> iCopy the context of the parent, the process that requested the spawning of the new process.
> Return the new PID to the parent process. This enables the parent process to examine or control the process directly.After the fork is complete, UNIX runs your program.

12. What Happens when you execute a command?
When you enter ‘ls’ command to look at the contents of your current working directory, UNIX does a series of things to create an environment for ls and the run it: The shell has UNIX perform a fork. This creates a new process that the shell will use to run the ls program. The shell has UNIX perform an exec of the ls program. This replaces the shell program and data with the program and data for ls and then starts running that new program. The ls program is loaded into the new process context, replacing the text and data of the shell. The ls program performs its task, listing the contents of the current directory.

13. What is a Daemon?
A daemon is a process that detaches itself from the terminal and runs, disconnected, in the background, waiting for requests and responding to them. It can also be defined as the background process that does not belong to a terminal session. Many system functions are commonly performed by daemons, including the sendmail daemon, which handles mail, and the NNTP daemon, which handles USENET news. Many other daemons may exist. Some of the most common daemons are:

> init: Takes over the basic running of the system when the kernel has finished the boot process.
> inetd: Responsible for starting network services that do not have their own stand-alone daemons. For example, inetd usually takes care of incoming rlogin, telnet, and ftp connections.
> cron: Responsible for running repetitive tasks on a regular schedule.

14. What is ‘ps’ command for?
The ps command prints the process status for some or all of the running processes. The information given are the process identification number (PID),the amount of time that the process has taken to execute so far etc.

15. How would you kill a process?
The kill command takes the PID as one argument; this identifies which process to terminate. The PID of a process can be got using ‘ps’ command.

16. What is an advantage of executing a process in background?
The most common reason to put a process in the background is to allow you to do something else interactively without waiting for the process to complete. At the end of the command you add the special background symbol, &. This symbol tells your shell to execute the given command in the background.

Example:  cp *.* ../backup& (cp is for copy)

17. How do you execute one program from within another?
The system calls used for low-level process creation are execlp() and execvp(). The execlp call overlays the existing program with the new one , runs that and exits. The original program gets back control only when an error occurs.
execlp(path,file_name,arguments..); //last argument must be NULL
A variant of execlp called execvp is used when the number of arguments is not known in advance.
execvp(path,argument_array); //argument array should be terminated by NULL

18. What is IPC? What are the various schemes available?
The term IPC (Inter-Process Communication) describes various ways by which different process running on some operating system communicate between each other. Various schemes available are as follows:

Pipes:
One-way communication scheme through which different process can communicate. The problem is that the two processes should have a common ancestor (parent-child relationship). However this problem was fixed with the introduction of named-pipes (FIFO).

Message Queues :
Message queues can be used between related and unrelated processes running on a machine.

Shared Memory:

This is the fastest of all IPC schemes. The memory to be shared is mapped into the address space of the processes (that are sharing). The speed achieved is attributed to the fact that there is no kernel involvement. But this scheme needs synchronization. 
Various forms of synchronisation are mutexes, condition-variables, read-write locks, record-locks, and semaphores.

SECTION - III
MEMORY MANAGEMENT

1. What is the difference between Swapping and Paging?
Swapping:
Whole process is moved from the swap device to the main memory for execution. Process size must be less than or equal to the available main memory. It is easier to implementation and overhead to the system. Swapping systems does not handle the memory more flexibly as compared to the paging systems.

Paging:
Only the required memory pages are moved to main memory from the swap device for execution. Process size does not matter. Gives the concept of the virtual memory.It provides greater flexibility in mapping the virtual address space into the physical memory of the machine. Allows more number of processes to fit in the main memory simultaneously. Allows the greater process size than the available physical memory. Demand paging systems handle the memory more flexibly.

2. What is major difference between the Historic Unix and the new BSD release of Unix System V in terms of Memory Management?
Historic Unix uses Swapping – entire process is transferred to the main memory from the swap device, whereas the Unix System V uses Demand Paging – only the part of the process is moved to the main memory. Historic Unix uses one Swap Device and Unix System V allow multiple Swap Devices.

3. What is the main goal of the Memory Management?
> It decides which process should reside in the main memory,
> Manages the parts of the virtual address space of a process which is non-core resident,
> Monitors the available main memory and periodically write the processes into the swap device to provide more processes fit in the main memory simultaneously.

4. What is a Map?
A Map is an Array, which contains the addresses of the free space in the swap device that are allocatable resources, and the number of the resource units available there.
This allows First-Fit allocation of contiguous blocks of a resource. Initially the Map contains one entry – address (block offset from the starting of the swap area) and the total number of resources.
Kernel treats each unit of Map as a group of disk blocks. On the allocation and freeing of the resources Kernel updates the Map for accurate information.

5. What scheme does the Kernel in Unix System V follow while choosing a swap device among the multiple swap devices?
Kernel follows Round Robin scheme choosing a swap device among the multiple swap devices in Unix System V.

6. What is a Region?
A Region is a continuous area of a process’s address space (such as text, data and stack). The kernel in a ‘Region Table’ that is local to the process maintains region. Regions are sharable among the process.

7. What are the events done by the Kernel after a process is being swapped out from the main memory?
When Kernel swaps the process out of the primary memory, it performs the following:
> Kernel decrements the Reference Count of each region of the process. If the reference count becomes zero, swaps the region out of the main memory,
> Kernel allocates the space for the swapping process in the swap device,
> Kernel locks the other swapping process while the current swapping operation is going on,
> The Kernel saves the swap address of the region in the region table.

8. Is the Process before and after the swap are the same? Give reason.
Process before swapping is residing in the primary memory in its original form. The regions (text, data and stack) may not be occupied fully by the process, there may be few empty slots in any of the regions and while swapping Kernel do not bother about the empty slots while swapping the process out.
After swapping the process resides in the swap (secondary memory) device. The regions swapped out will be present but only the occupied region slots but not the empty slots that were present before assigning.
While swapping the process once again into the main memory, the Kernel referring to the Process Memory Map, it assigns the main memory accordingly taking care of the empty slots in the regions.

9. What do you mean by u-area (user area) or u-block?
This contains the private data that is manipulated only by the Kernel. This is local to the Process, i.e. each process is allocated a u-area.

10. What are the entities that are swapped out of the main memory while swapping the process out of the main memory?
All memory space occupied by the process, process’s u-area, and Kernel stack are swapped out, theoretically.
Practically, if the process’s u-area contains the Address Translation Tables for the process then Kernel implementations do not swap the u-area.

UNIX Concepts

SECTION - I

FILE MANAGEMENT IN UNIX:

1. How are devices represented in UNIX?
All devices are represented by files called special files that are located in/dev directory. Thus, device files and other files are named and accessed in the same way. A ‘regular file’ is just an ordinary data file in the disk. A ‘block special file’ represents a device with characteristics similar to a disk (data transfer in terms of blocks). A ‘character special file’ represents a device with characteristics similar to a keyboard (data transfer is by stream of bits in sequential order).
2. What is ‘inode’?
All UNIX files have its description stored in a structure called ‘inode’. The inode contains info about the file-size, its location, time of last access, time of last modification, permission and so on. Directories are also represented as files and have an associated inode. In addition to descriptions about the file, the inode contains pointers to the data blocks of the file. If the file is large, inode has indirect pointer to a block of pointers to additional data blocks (this further aggregates for larger files). A block is typically 8k.
Inode consists of the following fields:
> File owner identifier
> File type
> File access permissions
> File access times
> Number of links
> File size
> Location of the file data
3. Brief about the directory representation in UNIX
A Unix directory is a file containing a correspondence between filenames and inodes. A directory is a special file that the kernel maintains. Only kernel modifies directories, but processes can read directories. The contents of a directory are a list of filename and inode number pairs. When new directories are created, kernel makes two entries named ‘.’ (refers to the directory itself) and ‘..’ (refers to parent directory).
System call for creating directory is mkdir (pathname, mode).
4. What are the Unix system calls for I/O?
> open(pathname,flag,mode) - open file
> creat(pathname,mode) - create file
> close(filedes) - close an open file
> read(filedes,buffer,bytes) - read data from an open file
> write(filedes,buffer,bytes) - write data to an open file
> lseek(filedes,offset,from) - position an open file
> dup(filedes) - duplicate an existing file descriptor
> dup2(oldfd,newfd) - duplicate to a desired file descriptor
> fcntl(filedes,cmd,arg) - change properties of an open file
> ioctl(filedes,request,arg) - change the behaviour of an open file
The difference between fcntl anf ioctl is that the former is intended for any open file, while the latter is for device-specific operations.
5. How do you change File Access Permissions?
Every file has following attributes:
> owner’s user ID ( 16 bit integer )
> owner’s group ID ( 16 bit integer )
> File access mode word
‘r w x -r w x- r w x’
(user permission-group permission-others permission)
r-read, w-write, x-execute
To change the access mode, we use chmod(filename,mode).
Example 1:
To change mode of myfile to ‘rw-rw-r–’ (ie. read, write permission for user - read,write permission for group - only read permission for others) we give the args as:
chmod(myfile,0664) .
Each operation is represented by discrete values
‘r’ is 4
‘w’ is 2
‘x’ is 1
Therefore, for ‘rw’ the value is 6(4+2).
Example 2:
To change mode of myfile to ‘rwxr–r–’ we give the args as:
chmod(myfile,0744).
6. What are links and symbolic links in UNIX file system?
A link is a second name (not a file) for a file. Links can be used to assign more than one name to a file, but cannot be used to assign a directory more than one name or link filenames on different computers.
Symbolic link ‘is’ a file that only contains the name of another file.Operation on the symbolic link is directed to the file pointed by the it.Both the limitations of links are eliminated in symbolic links.
Commands for linking files are:
Link ln filename1 filename2
Symbolic link ln -s filename1 filename2
7. What is a FIFO?
FIFO are otherwise called as ‘named pipes’. FIFO (first-in-first-out) is a special file which is said to be data transient. Once data is read from named pipe, it cannot be read again. Also, data can be read only in the order written. It is used in interprocess communication where a process writes to one end of the pipe (producer) and the other reads from the other end (consumer).
8. How do you create special files like named pipes and device files?
The system call mknod creates special files in the following sequence.
1. kernel assigns new inode,
2. sets the file type to indicate that the file is a pipe, directory or special file,
3. If it is a device file, it makes the other entries like major, minor device numbers.
For example:
If the device is a disk, major device number refers to the disk controller and minor device number is the disk.
9. Discuss the mount and unmount system calls
The privileged mount system call is used to attach a file system to a directory of another file system; the unmount system call detaches a file system. When you mount another file system on to your directory, you are essentially splicing one directory tree onto a branch in another directory tree. The first argument to mount call is the mount point, that is , a directory in the current file naming system. The second argument is the file system to mount to that point. When you insert a cdrom to your unix system’s drive, the file system in the cdrom automatically mounts to /dev/cdrom in your system.
10. How does the inode map to data block of a file?
Inode has 13 block addresses. The first 10 are direct block addresses of the first 10 data blocks in the file. The 11th address points to a one-level index block. The 12th address points to a two-level (double in-direction) index block. The 13th address points to a three-level(triple in-direction)index block. This provides a very large maximum file size with efficient access to large files, but also small files are accessed directly in one disk read.
11. What is a shell?
A shell is an interactive user interface to an operating system services that allows an user to enter commands as character strings or through a graphical user interface. The shell converts them to system calls to the OS or forks off a process to execute the command. System call results and other information from the OS are presented to the user through an interactive interface. Commonly used shells are sh,csh,ks etc.

SECTION - II

PROCESS MODEL and IPC:

1. Brief about the initial process sequence while the system boots up.
While booting, special process called the ’swapper’ or ’scheduler’ is created with Process-ID 0. The swapper manages memory allocation for processes and influences CPU allocation. The swapper inturn creates 3 children:
> the process dispatcher,
> vhand and
> dbflush
with IDs 1,2 and 3 respectively.
This is done by executing the file /etc/init. Process dispatcher gives birth to the shell. Unix keeps track of all the processes in an internal data structure called the Process Table (listing command is ps -el).
2. What are various IDs associated with a process?
Unix identifies each process with a unique integer called ProcessID. The process that executes the request for creation of a process is called the ‘parent process’ whose PID is ‘Parent Process ID’. Every process is associated with a particular user called the ‘owner’ who has privileges over the process. The identification for the user is ‘UserID’. Owner is the user who executes the process. Process also has ‘Effective User ID’ which determines the access privileges for accessing resources like files.
getpid() -process id
getppid() -parent process id
getuid() -user id
geteuid() -effective user id
3. Explain fork() system call.
The `fork()’ used to create a new process from an existing process. The new process is called the child process, and the existing process is called the parent. We can tell which is which by checking the return value from `fork()’. The parent gets the child’s pid returned to him, but the child gets 0 returned to him.
4. Predict the output of the following program code
main()
{
fork();
printf(”Hello World!”);
}
Answer:
Hello World!Hello World!
Explanation:
The fork creates a child that is a duplicate of the parent process. The child begins from the fork().All the statements after the call to fork() will be executed twice.(once by the parent process and other by child). The statement before fork() is executed only by the parent process.

5. Predict the output of the following program code
main()
{
fork(); fork(); fork();
printf(”Hello World!”);
}
Answer:

“Hello World” will be printed 8 times.

Explanation:

2^n times where n is the number of calls to fork()

6. List the system calls used for process management:
System calls Description
fork() To create a new process
exec() To execute a new program in a process
wait() To wait until a created process completes its execution
exit() To exit from a process execution
getpid() To get a process identifier of the current process
getppid() To get parent process identifier
nice() To bias the existing priority of a process
brk() To increase/decrease the data segment size of a process

7. How can you get/set an environment variable from a program?
Getting the value of an environment variable is done by using `getenv()’.
Setting the value of an environment variable is done by using `putenv()’.

8. How can a parent and child process communicate?
A parent and child can communicate through any of the normal inter-process communication schemes (pipes, sockets, message queues, shared memory), but also have some special ways to communicate that take advantage of their relationship as a parent and child. One of the most obvious is that the parent can get the exit status of the child.

9. What is a zombie?
When a program forks and the child finishes before the parent, the kernel still keeps some of its information about the child in case the parent might need it - for example, the parent may need to check the child’s exit status. To be able to get this information, the parent calls `wait()’; In the interval between the child terminating and the parent calling `wait()’, the child is said to be a `zombie’ (If you do `ps’, the child will have a `Z’ in its status field to indicate this.)

10. What are the process states in Unix?
As a process executes it changes state according to its circumstances. Unix processes have the following states:

Running : The process is either running or it is ready to run .
Waiting : The process is waiting for an event or for a resource.
Stopped : The process has been stopped, usually by receiving a signal.
Zombie : The process is dead but have not been removed from the process table.

Following are a few basic questions that cover the essentials of OS:

1. When is a system in safe state?
The set of dispatchable processes is in a safe state if there exists at least one temporal order in which all processes can be run to completion without resulting in a deadlock.
2. What is cycle stealing?
We encounter cycle stealing in the context of Direct Memory Access (DMA). Either the DMA controller can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific break points in an instruction cycle.
3. What is meant by arm-stickiness?
If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track. This generally happens with most common device scheduling algorithms (LIFO, SSTF, C-SCAN, etc). High-density multisurface disks are more likely to be affected by this than low density ones.
4. What are the stipulations of C2 level security?
C2 level security provides for:
> Discretionary Access Control
> Identification and Authentication
> Auditing
> Resource reuse
5. What is busy waiting?
The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The CPU is not engaged in any real productive activity during this period, and the process does not progress toward completion.
6. What are short-, long- and medium-term scheduling?
Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.
Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on memory-management criteria.
Short term scheduler, also know as a dispatcher executes most frequently, and makes the finest-grained decision of which process should execute next. This scheduler is invoked whenever an event occurs. It may lead to interruption of one process by preemption.
6. What are short-, long- and medium-term scheduling?
Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.
Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on memory-management criteria.
Short term scheduler, also know as a dispatcher executes most frequently, and makes the finest-grained decision of which process should execute next. This scheduler is invoked whenever an event occurs. It may lead to interruption of one process by preemption.
7. What are turnaround time and response time?
Turnaround time is the interval between the submission of a job and its completion. Response time is the interval between submission of a request, and the first response to that request.
8. What are the typical elements of a process image?
> User data: Modifiable part of user space. May include program data, user stack area, and programs that may be modified.
> User program: The instructions to be executed.
> System Stack: Each process has one or more LIFO stacks associated with it. Used to store parameters and calling addresses for procedure and system calls.
> Process control Block (PCB): Info needed by the OS to control processes.
9. What is the Translation Lookaside Buffer (TLB)?
In a cached system, the base addresses of the last few referenced pages is maintained in registers called the TLB that aids in faster lookup. TLB contains those page-table entries that have been most recently used. Normally, each virtual memory reference causes 2 physical memory accesses– one to fetch appropriate page-table entry, and one to fetch the desired data. Using TLB in-between, this is reduced to just one physical memory access in cases of TLB-hit.
10. What is the resident set and working set of a process?
Resident set is that portion of the process image that is actually in real-memory at a particular instant. Working set is that subset of resident set that is actually needed for execution. (Relate this to the variable-window size method for swapping techniques.)
11. Explain the concept of Reentrancy.
It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.
Note: A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.
12. Explain Belady’s Anomaly.
Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process’ virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady’s Anomaly. This is true for certain page reference patterns.
13. What is a binary semaphore? What is its use?
A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.
14. What is thrashing?
It is a phenomenon in virtual memory schemes when the processor spends most of its time swapping pages, rather than executing instructions. This is due to an inordinate number of page faults.
15. List the Coffman’s conditions that lead to a deadlock.
> Mutual Exclusion: Only one process may use a critical resource at a time.
> Hold & Wait: A process may be allocated some resources while waiting for others.
> No Pre-emption: No resource can be forcible removed from a process holding it.
> Circular Wait: A closed chain of processes exist such that each process holds at least one resource needed by another process in the chain.

Operating Systems - Interview Questions - Part 2

16. Explain the popular multiprocessor thread-scheduling strategies.
> Load Sharing: Processes are not assigned to a particular processor. A global queue of threads is maintained. Each processor, when idle, selects a thread from this queue. Note that load balancing refers to a scheme where work is allocated to processors on a more permanent basis.
> Gang Scheduling: A set of related threads is scheduled to run on a set of processors at the same time, on a 1-to-1 basis. Closely related threads / processes may be scheduled this way to reduce synchronization blocking, and minimize process switching. Group scheduling predated this strategy.
> Dedicated processor assignment: Provides implicit scheduling defined by assignment of threads to processors. For the duration of program execution, each program is allocated a set of processors equal in number to the number of threads in the program. Processors are chosen from the available pool.
> Dynamic scheduling: The number of thread in a program can be altered during the course of execution.
17. When does the condition ‘rendezvous’ arise?
In message passing, it is the condition in which, both, the sender and receiver are blocked until the message is delivered.
18. What is a trap and trapdoor?
Trapdoor is a secret undocumented entry point into a program used to grant access without normal methods of access authentication. A trap is a software interrupt, usually the result of an error condition.
19. What are local and global page replacements?
Local replacement means that an incoming page is brought in only to the relevant process’ address space. Global replacement policy allows any page frame from any process to be replaced. The latter is applicable to variable partitions model only.
20. Define latency, transfer and seek time with respect to disk I/O.
Seek time is the time required to move the disk arm to the required track. Rotational delay or latency is the time it takes for the beginning of the required sector to reach the head. Sum of seek time (if any) and latency is the access time. Time taken to actually transfer a span of data is transfer time.
21. Describe the Buddy system of memory allocation.
Free memory is maintained in linked lists, each of equal sized blocks. Any such block is of size 2^k. When some memory is required by a process, the block size of next higher order is chosen, and broken into two. Note that the two such pieces differ in address only in their kth bit. Such pieces are called buddies. When any used block is freed, the OS checks to see if its buddy is also free. If so, it is rejoined, and put into the original free-block linked-list.
22. What is time-stamping?
It is a technique proposed by Lamport, used to order events in a distributed system without the use of clocks. This scheme is intended to order events consisting of the transmission of messages. Each system ‘i’ in the network maintains a counter Ci. Every time a system transmits a message, it increments its counter by 1 and attaches the time-stamp Ti to the message. When a message is received, the receiving system ‘j’ sets its counter Cj to 1 more than the maximum of its current value and the incoming time-stamp Ti. At each site, the ordering of messages is determined by the following rules: For messages x from site i and y from site j, x precedes y if one of the following conditions holds….(a) if Ti<Tj or (b) if Ti=Tj and i<j.
23. How are the wait/signal operations for monitor different from those for semaphores?
If a process in a monitor signal and no task is waiting on the condition variable, the signal is lost. So this allows easier program design. Whereas in semaphores, every operation affects the value of the semaphore, so the wait and signal operations should be perfectly balanced in the program.
24. In the context of memory management, what are placement and replacement algorithms?
Placement algorithms determine where in available real-memory to load a program. Common methods are first-fit, next-fit, best-fit. Replacement algorithms are used when memory is full, and one process (or part of a process) needs to be swapped out to accommodate a new program. The replacement algorithm determines which are the partitions to be swapped out.
25. In loading programs into memory, what is the difference between load-time dynamic linking and run-time dynamic linking?
For load-time dynamic linking: Load module to be loaded is read into memory. Any reference to a target external module causes that module to be loaded and the references are updated to a relative address from the start base address of the application module.
With run-time dynamic loading: Some of the linking is postponed until actual reference during execution. Then the correct module is loaded and linked.
26. What are demand- and pre-paging?
With demand paging, a page is brought into memory only when a location on that page is actually referenced during execution. With pre-paging, pages other than the one demanded by a page fault are brought in. The selection of such pages is done based on common access patterns, especially for secondary memory devices.
27. Paging a memory management function, while multiprogramming a processor management function, are the two interdependent?
Yes.
28. What is page cannibalizing?
Page swapping or page replacements are called page cannibalizing.
29. What has triggered the need for multitasking in PCs?
> Increased speed and memory capacity of microprocessors together with the support fir virtual memory and
> Growth of client server computing
30. What are the four layers that Windows NT have in order to achieve independence?
> Hardware abstraction layer
> Kernel
> Subsystems
> System Services.

Frequently Asked Questions - Hardware & Software Design

Asked by a famous microprocessor manufacturer.

What is Finite Automata
What is a Turing machine?
How many processors are there in a pentium microprocessor?
In Sparc?
Difference between RISC and CISC
Is RISC always fast?
What is a real time system?
Name some real time OSs.
Is DOS a real time OS?
What is a kernel,shell?
What is binary search, traversal, hashing?
Write a code to count the no. of 1’s in a binary representation of
a number.
Memory taken for char *, int * etc.
char *cp;
int *ip;
cp++;
ip++;

What is the result?

Compare the no. of bytes in unix and Dos for long, char, short, int.
How do you make programs portable on unix and Dos under such
circumstances?
In C++, what is a constructor, destructor?
What is friend?
What is waterfall model, prototype model?
What is testing? What is unit testing, integration testing, etc?
What is indexing when talking about databases?
What is atomicity?
Can recursive programs be written in C++, Write a recursive program
to calculate factorial in C++.
What is best data structure to store the processes info in a real
time operating system?
Give two ways of converting a two input NAND gate to an inverter 
Given a circuit, draw its exact timing response. (I was given a
Pseudo Random Signal Generator; you can expect any sequential ckt)
What are set up time & hold time constraints? What do they signify?
Which one is critical for estimating maximum clock frequency of a
circuit?
Give a circuit to divide frequency of clock cycle by two
Design a divide-by-3 sequential circuit with 50% duty circle.
(Hint: Double the Clock)
Suppose you have a combinational circuit between two registers
driven by a clock.What will you do if the delay of the combinational
circuit is greater than your clock signal? (You can’t resize the
combinational circuit transistors)
The answer to the above question is breaking the combinational
circuit and pipelining it. What will be affected if you do this?
What are the different Adder circuits you studied?
Give the truth table for a Half Adder. Give a gate level
implementation of the same.
Draw a Transmission Gate-based D-Latch.
Design a Transmission Gate based XOR. Now, how do you convert it
to XNOR? (Without inverting the output)
How do you detect if two 8-bit signals are same?
How do you detect a sequence of “1101″ arriving serially from a
signal line?
Design any FSM in VHDL or Verilog.
Explain RC circuit’s charging and discharging.
Explain the working of a binary counter.
Describe how you would reverse a singly linked list.
 

Frequently Asked Questions - Operating System Concepts

• What is MUTEX ?
• What isthe difference between a ‘thread’ and a ‘process’?
• What is INODE?
• Explain the working of Virtual Memory.
• How does Windows NT supports Multitasking?
• Explain the Unix Kernel.
• What is Concurrency? Expain with example Deadlock and Starvation.
• What are your solution strategies for “Dining Philosophers Problem” ?
• Explain Memory Partitioning, Paging, Segmentation.
• Explain Scheduling.
• Operating System Security.
• What is Semaphore?
• Explain the following file systems : NTFS, Macintosh(HPFS), FAT .
• What are the different process states?
• What is Marshalling?
• Define and explain COM?
• What is Marshalling?
• Difference - Loading and Linking ?
• What are the basic functions of an operating system?
• Explain briefly about, processor, assembler, compiler, loader, linker and the functions executed by them.
• What are the difference phases of software development? Explain briefly?
• Differentiate between RAM and ROM?
• What is DRAM? In which form does it store data?
• What is cache memory?
• What is hard disk and what is its purpose?
• Differentiate between Complier and Interpreter?
• What are the different tasks of Lexical analysis?
• What are the different functions of Syntax phase, Sheduler?
• What are the main difference between Micro-Controller and Micro- Processor?
• Describe different job scheduling in operating systems.
• What is a Real-Time System ?
• What is the difference between Hard and Soft real-time systems ?
• What is a mission critical system ?
• What is the important aspect of a real-time system ?
•  If two processes which shares same system memory and system clock in a distributed system, What is it called?
• What is the state of the processor, when a process is waiting for some event to occur?
• What do you mean by deadlock?
• Explain the difference between microkernel and macro kernel.
• Give an example of microkernel.
• When would you choose bottom up methodology?
• When would you choose top down methodology?
• Write a small dc shell script to find number of FF in the design.
• Why paging is used ?
• Which is the best page replacement algorithm and Why? How much time is spent usually in each phases and why?
• Difference between Primary storage and secondary storage?
• What is multi tasking, multi programming, multi threading?
• Difference between multi threading and multi tasking?
• What is software life cycle?
• Demand paging, page faults, replacement algorithms, thrashing, etc.
• Explain about paged segmentation and segment paging
• While running DOS on a PC, which command would be used to duplicate the entire diskette?

How would you find out if one of the pointers in a linked list is corrupted
Write a C program to insert nodes into a linked list in a sorted fashion
How to read a singly linked list backwards?
How do you convert a tree into an array?
What is a void pointer? Why can’t we perform arithmetic on a void *
What do Segmentation fault, access violation, core dump and Bus error
What is the difference between an array of pointers and a pointer to an array ?                                                                        What is a memory leak?
What is the difference between malloc() and calloc?
What is the difference between pointers and array ?
How many different trees can be constructed using n nodes have ?
How will you reverse linked list? Write a C program to do the same.
how do you delete a link list?
what is heap sort?
Give pseudocode for the mergesort algorithm
How to declare a structure of a linked list?
What is a NULL pointer? How is it different from an unitialized pointer?
What is a null pointer assignment error?
Does extern in a function declaration mean anything?
Does C support function overloading?

Database-Frequently Asked Questions!

1. What is database?
A database is a logically coherent collection of data with
some inherent meaning, representing some aspect of real world
and which is designed, built and populated with data for a
specific purpose.
2. What is DBMS?
It is a collection of programs that enables user to create and
maintain a database.In other words it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications.
3. What is a Database system?
The database and DBMS software together is called as Database
system.
4. Advantages of DBMS?
� Redundancy is controlled.
� Unauthorised access is restricted.
� Providing multiple user interfaces.
� Enforcing integrity constraints.
� Providing backup and recovery.
5. Disadvantage in File Processing System?
� Data redundancy & inconsistency.
� Difficult in accessing data.
� Data isolation.
� Data integrity.
� Concurrent access is not possible.
� Security Problems.
6. Describe the three levels of data abstraction?
The are three levels of abstraction:
� Physical level: The lowest level of abstraction describes
how data are stored.
� Logical level: The next higher level of abstraction,describes
what data are stored in database and what relationship among those
data.
� View level: The highest level of abstraction describes only
part of entire database.
7. Define the “integrity rules”
There are two Integrity rules.
� Entity Integrity: States that �Primary key cannot have NULL
value�
� Referential Integrity:States that �Foreign Key can be either
a NULL value or should be Primary Key value of other relation.
8. What is extension and intension?
Extension -
It is the number of tuples present in a table at any instance.This
is time dependent.
Intension -
It is a constant value that gives the name, structure of table and
the constraints laid on it.
9. What is System R? What are its two major subsystems?
System R was designed and developed over a period of 1974-79 at
IBM San Jose Research Center. It is a prototype and its purpose
was to demonstrate that it is possible to build a Relational System
that can be used in a real life environment to solve real life
problems, with performance at least comparable to that of existing
system.
Its two subsystems are
� Research Storage
� System Relational Data System.
10. How is the data structure of System R different from the
relational structure?
Unlike Relational systems in System R
� Domains are not supported
� Enforcement of candidate key uniqueness is optional
� Enforcement of entity integrity is optional
� Referential integrity is not enforced
11. What is Data Independence?
Data independence means that �the application is independent of
the storage structure and access strategy of data�.In other words,
The ability to modify the schema definition in one level should not
affect the schema definition in the next higher level.
Two types of Data Independence:
� Physical Data Independence:Modification in physical level should
not affect the logical level.
� Logical Data Independence: Modification in logical level should
affect the view level.
NOTE: Logical Data Independence is more difficult to achieve
12. What is a view? How it is related to data independence?
A view may be thought of as a virtual table, that is, a table that
does not really exist in its own right but is instead derived from
one or more underlying base table.In other words,there is no stored
file that direct represents the view instead a definition of view is
stored in data dictionary.
Growth and restructuring of base tables is not reflected in views.
Thus the view can insulate users from the effects of restructuring
and growth in the database. Hence accounts for logical data
independence.
13. What is Data Model?
A collection of conceptual tools for describing data, data
relationships data semantics and constraints.
14. What is E-R model?
This data model is based on real world that consists of basic
objects called entities and of relationship among these objects.
Entities are described in a database by a set of attributes.
15. What is Object Oriented model?
This model is based on collection of objects. An object contains
values stored in instance variables with in the object. An object
also contains bodies of code that operate on the object. These
bodies of code are called methods. Objects that contain same types
of values and the same methods are grouped together into classes.
16. What is an Entity?
It is a ‘thing’ in the real world with an independent existence.
17. What is an Entity type?
It is a collection (set) of entities that have same attributes.
18. What is an Entity set?
It is a collection of all entities of particular entity type in
the database.
19. What is an Extension of entity type?
The collections of entities of a particular entity type are grouped
together into an entity set.
20. What is Weak Entity set?
An entity set may not have sufficient attributes to form a primary
key,and its primary key compromises of its partial key and primary
key of its parent entity, then it is said to be Weak Entity set.
21. What is an attribute?
It is a particular property, which describes the entity.
22. What is a Relation Schema and a Relation?
A relation Schema denoted by R(A1, A2, �, An) is made up of the
relation name R and the list of attributes Ai that it contains. A
relation is defined as a set of tuples.Let r be the relation which
contains set tuples (t1, t2, t3, …, tn).Each tuple is an ordered
list of n-values t=(v1,v2, …, vn).
23. What is degree of a Relation?
It is the number of attribute of its relation schema.
24. What is Relationship?
It is an association among two or more entities.
25. What is Relationship set?
The collection (or set) of similar relationships.
26. What is Relationship type?
Relationship type defines a set of associations or a relationship
set among a given set of entity types.
27. What is degree of Relationship type?
It is the number of entity type participating.
25. What is DDL (Data Definition Language)?
A data base schema is specifies by a set of definitions expressed
by a special language called DDL.
26. What is VDL (View Definition Language)?
It specifies user views and their mappings to the conceptual schema.
27. What is SDL (Storage Definition Language)?
This language is to specify the internal schema. This language may
specify the mapping between two schemas.
28. What is Data Storage - Definition Language?
The storage structures and access methods used by database system
are specified by a set of definition in a special type of DDL called
data storage-definition language.
29. What is DML (Data Manipulation Language)?
This language that enable user to access or manipulate data as
organised by appropriate data model.
� Procedural DML or Low level: DML requires a user to specify what
data are needed and how to get those data.
� Non-Procedural DML or High level: DML requires a user to specify
what data are needed without specifying how to get those data.
31. What is DML Compiler?
It translates DML statements in a query language into low-level
instruction that the query evaluation engine can understand.
32. What is Query evaluation engine?
It executes low-level instruction generated by compiler.
33. What is DDL Interpreter?
It interprets DDL statements and record them in tables containing
metadata.
34. What is Record-at-a-time?
The Low level or Procedural DML can specify and retrieve each record
from a set of records.This retrieve of a record is said to be Record-
at-a-time.
35. What is Set-at-a-time or Set-oriented?
The High level or Non-procedural DML can specify and retrieve many
records in a single DML statement.This retrieve of a record is said
to be Set-at-a-time or Set-oriented.
36. What is Relational Algebra?
It is procedural query language.It consists of a set of operations
that take one or two relations as input and produce a new relation.
37. What is Relational Calculus?
It is an applied predicate calculus specifically tailored for
relational databases proposed by E.F. Codd. E.g. of languages based
on it are DSL ALPHA, QUEL.
38. How does Tuple-oriented relational calculus differ from domain-
oriented relational calculus
The tuple-oriented calculus uses a tuple variables i.e., variable
whose only permitted values are tuples of that relation. E.g. QUEL
The domain-oriented calculus has domain variables i.e., variables
that range over the underlying domains instead of over relation. E.g.
ILL, DEDUCE.
39. What is normalization?
It is a process of analysing the given relation schemas based on
their Functional Dependencies (FDs) and primary key to achieve the
properties
� Minimizing redundancy
� Minimizing insertion, deletion and update anomalies.
40. What is Functional Dependency?
A Functional dependency is denoted by X Y between two sets of
ttributes X and Y that are subsets of R specifies a constraint on
the possible tuple that can form a relation state r of R. The
constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X]
then they have t1[Y] = t2[Y]. This means the value of X component
of a tuple uniquely determines the value of component Y.
41. When is a functional dependency F said to be minimal?
� Every dependency in F has a single attribute for its right hand
side.
� We cannot replace any dependency X A in F with a dependency Y A
where Y is a proper subset of X and still have a set of dependency
that is equivalent to F.
� We cannot remove any dependency from F and still have set of
dependency that is equivalent to F.
42. What is Multivalued dependency?
Multivalued dependency denoted by X Y specified on relation schema R,
where X and Y are both subsets of R, specifies the following
constraint on any relation r of R: if two tuples t1 and t2 exist in
r such that t1[X] = t2[X] then t3 and t4 should also exist in r with
the following properties
� t3[x] = t4[X] = t1[X] = t2[X]
� t3[Y] = t1[Y] and t4[Y] = t2[Y]
� t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]
43. What is Lossless join property?
It guarantees that the spurious tuple generation does not occur with
respect to relation schemas after decomposition.
44. What is 1 NF (Normal Form)?
The domain of attribute must include only atomic (simple, indivisible)
values.
45. What is Fully Functional dependency?
It is based on concept of full functional dependency. A functional
dependency X Y is full functional dependency if removal of any
attribute A from X means that the dependency does not hold any more.
46. What is 2NF?
A relation schema R is in 2NF if it is in 1NF and every non-prime
attribute A in R is fully functionally dependent on primary key.
47. What is 3NF?
A relation schema R is in 3NF if it is in 2NF and for every FD X A
either of the following is true
� X is a Super-key of R.
� A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively
dependent on primary key.
48. What is BCNF (Boyce-Codd Normal Form)?
A relation schema R is in BCNF if it is in 3NF and satisfies an
additional constraint that for every FD X A, X must be a candidate
key.
49. What is 4NF?
A relation schema R is said to be in 4NF if for every Multivalued
dependency X Y that holds over R, one of following is true
� X is subset or equal to (or) XY = R.
� X is a super key.
50. What is 5NF?
A Relation schema R is said to be 5NF if for every join dependency
{R1, R2, …, Rn} that holds R, one the following is true
� Ri = R for some i.
� The join dependency is implied by the set of FD, over R in which
the left side is key of R.
51. What is Domain-Key Normal Form?
A relation is said to be in DKNF if all constraints and dependencies
that should hold on the the constraint can be enforced by simply
enforcing the domain constraint and key constraint on the relation.
52. What are partial, alternate,, artificial, compound and natural
key?
Partial Key:
It is a set of attributes that can uniquely identify weak entities
and that are related to same owner entity.It is sometime called as
Discriminator.
Alternate Key:
All Candidate Keys excluding the Primary Key are known as Alternate
Keys.
Artificial Key:
If no obvious key, either stand alone or compound is available,then
the last resort is to simply create a key, by assigning a unique
number to each record or occurrence.Then this is known as developing
an artificial key.
Compound Key:
If no single data element uniquely identifies occurrences within a
construct, then combining multiple elements to create a unique
identifier for the construct is known as creating a compound key.
Natural Key:
When one of the data elements stored within a construct is utilized
as the primary key, then it is called the natural key.
53. What is indexing and what are the different kinds of indexing?
Indexing is a technique for determining how quickly specific data
can be found.
Types:
� Binary search style indexing
� B-Tree indexing
� Inverted list indexing
� Memory resident table
� Table indexing
54. What is system catalog or catalog relation? How is better
known as?
A RDBMS maintains a description of all the data that it contains,
information about every relation and index that it contains. This
information is stored in a collection of relations maintained by
the system called metadata. It is also called data dictionary.
55. What is meant by query optimization?
The phase that identifies an efficient execution plan for evaluating
a query that has the least estimated cost is referred to as query
optimization.
56. What is join dependency and inclusion dependency?
Join Dependency:
A Join dependency is generalization of Multivalued dependency.A JD
{R1, R2, …, Rn} is said to hold over a relation R if
 R1,R2,R3,…, Rn is a lossless-join decomposition of R . There is
no set of sound and complete inference rules for JD.
Inclusion Dependency:
An Inclusion Dependency is a statement of the form that some columns
of a relation are contained in other columns.A foreign key constraint
is an example of inclusion dependency.
57. What is durability in DBMS?
Once the DBMS informs the user that a transaction has successfully
completed, its effects should persist even if the system crashes
before all its changes are reflected on disk. This property is
called durability.
58. What do you mean by atomicity and aggregation?
Atomicity:
Either all actions are carried out or none are. Users should not
have to worry about the effect of incomplete transactions. DBMS
ensures this by undoing the actions of incomplete transactions.
Aggregation:
A concept which is used to model a relationship between a collection
of entities and relationships. It is used when we need to express a
relationship among relationships.
59. What is a Phantom Deadlock?
In distributed deadlock detection, the delay in propagating local
information might cause the deadlock detection algorithms to identify
deadlocks that do not really exist. Such situations are called
phantom deadlocks and they lead to unnecessary aborts.
60. What is a checkpoint and When does it occur?
A Checkpoint is like a snapshot of the DBMS state. By taking
checkpoints, the DBMS can reduce the amount of work to be done
during restart in the event of subsequent crashes.
61. What are the different phases of transaction?
Different phases are
� Analysis phase
� Redo Phase
� Undo phase
62. What do you mean by flat file database?
It is a database in which there are no programs or user access
languages. It has no cross-file capabilities but is user-friendly
and provides user-interface management.
63. What is “transparent DBMS”?
It is one, which keeps its Physical Structure hidden from user.
64. Brief theory of Network, Hierarchical schemas and their
properties?
Network schema uses a graph data structure to organize records
example for such a database management system is CTCG while a
hierarchical schema uses a tree data structure example for such
a system is IMS.
65. What is a query?
A query with respect to DBMS relates to user commands that are
used to interact with a data base. The query language can be
classified into data definition language and data manipulation
language.
66. What do you mean by Correlated subquery?
Subqueries, or nested queries, are used to bring back a set of rows
to be used by the parent query. Depending on how the subquery is
written, it can be executed once for the parent query or it can be
executed once for each row returned by the parent query. If the
subquery is executed for each row of the parent, this is called a
correlated subquery.
A correlated subquery can be easily identified if it contains any
references to the parent subquery columns in its WHERE clause.
Columns from the subquery cannot be referenced anywhere else in the
parent query. The following example demonstrates a non-correlated
subquery.
E.g. Select * From CUST Where ‘10/03/1990′ IN (Select ODATE From
ORDER Where CUST.CNUM = ORDER.CNUM)
67.What are the primitive operations common to all record management
systems?
Addition, deletion and modification.
68. Name the buffer in which all the commands that are typed in are
stored
�Edit� Buffer
69. What are the unary operations in Relational Algebra?
PROJECTION and SELECTION.
70. Are the resulting relations of PRODUCT and JOIN operation the
same?
No.
PRODUCT: Concatenation of every row in one relation with every row
in another.
JOIN: Concatenation of rows from one relation and related rows from
another.
71. What is RDBMS KERNEL?
Two important pieces of RDBMS architecture are the kernel,which is
the software, and the data dictionary,which consists of the system-l
evel data structures used by the kernel to manage the database
You might think of an RDBMS as an operating system (or set of
subsystems), designed specifically for controlling data access; its
primary functions are storing, retrieving, and securing data. An
RDBMS maintains its own list of authorized users and their associated
privileges; manages memory caches and paging; controls locking for
concurrent resource usage; dispatches and schedules user requests;
and manages space usage within its table-space structures.
72. Name the sub-systems of a RDBMS?
I/O, Security, Language Processing, Process Control, Storage
Management,Logging and Recovery,Distribution Control,Transaction
Control, Memory Management, Lock Management
73. Which part of the RDBMS takes care of the data dictionary? How?
Data dictionary is a set of tables and database objects that is
stored in a special area of the database and maintained exclusively
by the kernel.
74. What is the job of the information stored in data-dictionary?
The information in the data dictionary validates the existence of
the objects, provides access to them, and maps the actual physical
storage location.
75. Not only RDBMS takes care of locating data it also
determines an optimal access path to store or retrieve the data.
76. How do you communicate with an RDBMS?
You communicate with an RDBMS using Structured Query Language (SQL).
77. Define SQL and state the differences between SQL and other
conventional programming Languages?
SQL is a nonprocedural language that is designed specifically for
data access operations on normalized relational database structures.
The primary difference between SQL and other conventional programming
languages is that SQL statements specify what data operations should
be performed rather than how to perform them.
78. Name the three major set of files on disk that compose a database
in Oracle?
There are three major sets of files on disk that compose a database.
All the files are binary. These are:
� Database files
� Control files
� Redo logs
The most important of these are the database files where the actual
data resides. The control files and the redo logs support the
functioning of the architecture itself.
All three sets of files must be present, open, and available to
Oracle for any data on the database to be useable. Without these
files, you cannot access the database,and the database administrator
might have to recover some or all of the database using a backup,
if there is one.
79. What is an Oracle Instance?
The Oracle system processes, also known as Oracle background
processes, provide functions for the user processes�functions that
would otherwise be done by the user processes themselves
Oracle database-wide system memory is known as the SGA, the system
global area or shared global area. The data and control structures
in the SGA are shareable, and all the Oracle background processes
and user processes can use them.
The combination of the SGA and the Oracle background processes is
known as an Oracle instance
80. What are the four Oracle system processes that must always be up
and running for the database to be useable
The four Oracle system processes that must always be up and running
for the database to be useable include DBWR (Database Writer), LGWR
(Log Writer), SMON (System Monitor), and PMON (Process Monitor).
81. What are database files, control files and log files. How many
of these files should a database have at least? Why?
Database Files:
The database files hold the actual data and are typically the largest
in size. Depending on their sizes, the tables (and other objects) for
all the user accounts can go in one database file�but that’s not
an ideal situation because it does not make the database structure
very flexible for controlling access to storage for different users,
putting the database on different disk drives, or backing up and
restoring just part of the database.
You must have at least one database file but usually, more than one
files are used. In terms of accessing and using the data in the
tables and other objects, the number (or location) of the files is
immaterial.
The database files are fixed in size and never grow bigger than the
size at which they were created
Control Files
The control files and redo logs support the rest of the architecture.
Any database must have at least one control file, although you
typically have more than one to guard against loss. The control file
records the name of the database, the date and time it was created,
the location of the database and redo logs, and the synchronization
information to ensure that all three sets of files are always in step.
Every time you add a new database or redo log file to the database,
the information is recorded in the control files.
Redo Logs
Any database must have at least two redo logs.These are the journals
for the database; the redo logs record all changes to the user
objects or system objects.If any type of failure occurs, the changes
recorded in the redo logs can be used to bring the database to a
consistent state without losing any committed transactions. In the
case of non-data loss failure, Oracle can apply the information in
the redo logs automatically without intervention from the DBA.
The redo log files are fixed in size and never grow dynamically from
the size at which they were created.
82. What is ROWID?
The ROWID is a unique database-wide physical address for every row
on every table. Once assigned (when the row is first inserted into
the database),it never changes until the row is deleted or the table
is dropped.
The ROWID consists of the following three components,the combination
of which uniquely identifies the physical storage location of the row.
� Oracle database file number, which contains the block with the
rows
� Oracle block address, which contains the row
� The row within the block (because each block can hold many rows)
The ROWID is used internally in indexes as a quick means of
retrieving rows with a particular key value. Application developers
also use it in SQL statements as a quick way to access a row once
they know the ROWID.
83.What is Oracle Block?Can two Oracle Blocks have the same address?
Oracle “formats” the database files into a number of Oracle blocks
when they are first created�making it easier for the RDBMS software
to manage the files and easier to read data into the memory areas.
The block size should be a multiple of the operating system block
size.Regardless of the block size,the entire block is not available
for holding data;Oracle takes up some space to manage the contents
of the block.This block header has a minimum size, but it can grow.
These Oracle blocks are the smallest unit of storage.Increasing the
Oracle block size can improve performance,but it should be done only
when the database is first created.
Each Oracle block is numbered sequentially for each database file
starting at 1. Two blocks can have the same block address if they
are in different database files.
84. What is database Trigger?
A database trigger is a PL/SQL block that can defined to automatically
execute for insert, update, and delete statements against a table.
The trigger can e defined to execute once for the entire statement or
once for every row that is inserted, updated, or deleted.For any one
table, there are twelve events for which you can define database
triggers. A database trigger can call database procedures that are
also written in PL/SQL.
85. Name two utilities that Oracle provides,which are use for backup
and recovery.
Along with the RDBMS software, Oracle provides two utilities that you
can use to back up and restore the database. These utilities are
Export and Import.
The Export utility dumps the definitions and data for the specified
part of the database to an operating system binary file. The Import
utility reads the file produced by an export,recreates the definitions
of objects, and inserts the data.
If Export and Import are used as a means of backing up and recovering
the database,all the changes made to the database cannot be recovered
since the export was performed. The best you can do is recover the
database to the time when the export was last performed.
86. What are stored-procedures? And what are the advantages of using
them?
Stored procedures are database objects that perform a user defined
operation.A stored procedure can have a set of compound SQL statements.
A stored procedure executes the SQL commands and returns the result
to the client. Stored procedures are used to reduce network traffic.
87. How are exceptions handled in PL/SQL? Give some of the internal
exceptions’ name?
PL/SQL exception handling is a mechanism for dealing with run-time
errors encountered during procedure execution. Use of this mechanism
enables execution to continue if the error is not severe enough to
cause procedure termination.
The exception handler must be defined within a subprogram
specification. Errors cause the program to raise an exception with a
transfer of control to the exception-handler block. After the
exception handler executes, control returns to the block in which
the handler was defined. If there are no more executable statements
in the block, control returns to the caller.
User-Defined Exceptions:
PL/SQL enables the user to define exception handlers in the declarations area of subprogram specifications. User accomplishes this by naming an exception as in the following example:
ot_failure EXCEPTION;
In this case, the exception name is ot_failure. Code associated with
this handler is written in the EXCEPTION specification area as follows:
EXCEPTION
when OT_FAILURE then
out_status_code := g_out_status_code;
out_msg := g_out_msg;
The following is an example of a subprogram exception:
EXCEPTION
when NO_DATA_FOUND then
g_out_status_code := ‘FAIL’;
RAISE ot_failure;
Within this exception is the RAISE statement that transfers control
back to the ot_failure exception handler.This technique of raising
the exception is used to invoke all user-defined exceptions.
System-Defined Exceptions
Exceptions internal to PL/SQL are raised automatically upon error.
NO_DATA_FOUND is a system-defined exception. Table below gives a
complete list of internal exceptions.

PL/SQL internal exceptions.

Exception Name Oracle Error
CURSOR_ALREADY_OPEN ORA-06511
DUP_VAL_ON_INDEX ORA-00001
INVALID_CURSOR ORA-01001
INVALID_NUMBER ORA-01722
LOGIN_DENIED ORA-01017
NO_DATA_FOUND ORA-01403
NOT_LOGGED_ON ORA-01012
PROGRAM_ERROR ORA-06501
STORAGE_ERROR ORA-06500
TIMEOUT_ON_RESOURCE ORA-00051
TOO_MANY_ROWS ORA-01422
TRANSACTION_BACKED_OUT ORA-00061
VALUE_ERROR ORA-06502
ZERO_DIVIDE ORA-01476

In addition to this list of exceptions, there is a catch-all
exception named OTHERS that traps all errors for which specific
error handling has not been established.

88. Does PL/SQL support “overloading”? Explain
The concept of overloading in PL/SQL relates to the idea that you
can define procedures and functions with the same name. PL/SQL does
not look only at the referenced name, however, to resolve a procedure
or function call. The count and data types of formal parameters are
also considered.
PL/SQL also attempts to resolve any procedure or function calls in
locally defined packages before looking at globally defined packages
or internal functions.To further ensure calling the proper procedure,
you can use the dot notation.Prefacing a procedure or function name
with the package name fully qualifies any procedure or function
reference.

89. Tables derived from the ERD
a) Are totally unnormalised
b) Are always in 1NF
c) Can be further denormalised
d) May have multi-valued attributes

(b) Are always in 1NF

90. Spurious tuples may occur due to
i. Bad normalization
ii. Theta joins
iii. Updating tables from join
a) i & ii b) ii & iii
c) i & iii d)

Advanced Java

1. What is RMI?
2. Explain about RMI Architecture?
3. What are Servelets?
4. What is the use of servlets?
5. Explain RMI Architecture?
6. How will you pass values from htm page to the servlet?
7. How do you load an image in a Servelet?
8. What is purpose of applet programming?
9. How will you communicate between two applets?
10. What IS the difference between Servelets and Applets?
11. How do you communicate in between Applets and Servlets?
12. What is the difference between applet and application?
13. What is the difference between CGI and Servlet?
14. In the servlets, we are having a web page that is invoking servlets ,username and password? which is checks in database? Suppose the second page also if we want to verify the same information whether it will connect to the database or it will be used previous information?
15. What are the difference between RMI and Servelets?
16. How will you call an Applet using Java Script Function?
17. How can you push data from an Applet to a Servlet?
18. What are 4 drivers available in JDBC? At what situation are four of the drivers used?
19. If you are truncated using JDBC , how can you that how much data is truncated?
20. How will you perform truncation using JDBC?
21. What is the latest version of JDBC? What are the new features added in that?
22. What is the difference between RMI registry and OS Agent?
23. To a server method, the client wants to send a value 20, with this value exceeds to 20 a message should be sent to the client . What will you do for achieving this?
24. How do you invoke a Servelet? What is the difference between doPost method and doGet method?
25. What is difference between the HTTP Servelet and Generic Servelet? Explain about their methods and parameters?
26. Can we use threads in Servelets?
27. Write a program on RMI and JDBC using Stored Procedure?
28. How do you swing an applet?
29. How will you pass parameters in RMI? Why do you serialize?
30. In RMI ,server object is first loaded into memory and then the stub reference is sent to the client. true or false?
31. Suppose server object not loaded into the memory and the client request for it. What will happen?
32. What is the web server used for running the servelets?
33. What is Servlet API used for connecting database?
34. What is bean? Where can it be used?
35. What is the difference between java class and bean?
36. Can we sent objects using Sockets?
37. What is the RMI and Socket?
38. What is CORBA?
39. Can you modify an object in corba?
40. What is RMI and what are the services in RMI?
41. What are the difference between RMI and CORBA?
42. How will you initialize an Applet?
43. What is the order of method invocation in an Applet?
44. What is ODBC and JDBC? How do you connect the Database?
45. What do you mean by Socket Programming?
46. What is difference between Generic Servlet and HTTP Servelet?
47. What you mean by COM and DCOM?
48. what is e-commerce?