UNIX Interview Questions with Answers Page V

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1. 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.

2. 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.

3. n 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.

4. 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.

5. 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.

6. 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”.

7. 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.

8. At what mode the fault handler executes?

At the Kernel Mode.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14.Explain different types of Unix systems.

The most widely used are: 1. System V (AT&T) 2. AIX (IBM) 3. BSD (Berkeley) 4. Solaris (Sun) 5. Xenix ( A PC version of Unix)

15. Explain kernal and shell.

Kernal: It carries out basic operating system functions such as allocating memory, accessing files and handling communications. Shell:A shell provides

the user interface to the kernal.There are 3 major shells : C-shell, Bourne shell , Korn shell

16. Is it possible to count number char, line in a file; if so, How?

Yes, wc-stands for word count.

wc -c for counting number of characters in a file.

wc -l for counting lines in a file.

17. Name the data structure used to maintain file identification?

‘inode’, each file has a separate inode and a unique inode number.

18. How many prompts are available in a UNIX system?

Two prompts, PS1 (Primary Prompt), PS2 (Secondary Prompt).

19. How does the kernel differentiate device files and ordinary files?

Kernel checks 'type' field in the file's inode structure.

20. How to switch to a super user status to gain privileges?

Use ‘su’ command. The system asks for password and when valid entry is made the user gains super user (admin) privileges.