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Unix Interview Questions and Answers
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”.
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.
At what mode the fault handler executes?
At the Kernel Mode.
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.
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.
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.
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.
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.
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)
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
What is ex and vi ?
ex is Unix line editor and vi is the standard Unix
screen editor.
Which are typical system directories below the root
directory?
(1)/bin: contains many programs which will be executed
by users (2)/etc : files used by administrator (3)/dev:
hardware devices (4)/lib: system libraries (5)/usr:
application software (6)/home: home directories for
different systems.
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