1) 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.
2)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.
3)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.
4) 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.
5)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.
6)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.
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.
8) 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.
9)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.
10)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.
11) 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.
12)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.
13)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.
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.
15)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.
16)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.
17)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.
18)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.
19)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.
20)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.
21)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.
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.
22)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.
23)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.
24)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"
25)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.
26) At what
mode the fault handler executes?
At the Kernel Mode.
27) 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.
28)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.
29)Define
session?
A session is a collection of
one or more process groups. A process establishes a new session by calling
setsid function. This function returns process group id if OK.
30)What do you
mean by signal?
Signals are software
interrupts. Signals provide a way of handling asynchronous events: a user at a
terminal typing the interrupt key to stop a program or the next program in the
pipeline terminating prematurely.
31)What is
unix?
UNIX is the most popular
operating system on multi-user systems. This operating system originated as a
single-user system. It started off on a cast-off DEC PDP-7 at Bell laboratories
in 1969. Ken Thompson, with ideas and help from Dennis Ritchie, and others,
wrote a small, general-purpose operating system.
31)Define
message queue?
message queue Message queue
is a linked list of messages. It is used to make a communication between the
related or unrelated processes.
32)What is the
use of break and continue statements?
The continue statement
suspends execution of all statements following it, and switches control to the
top of the loop for the next iteration. The break statement, on the other hand,
causes control to break out of the loop.
36. What is
single users system?
The personal computer (PC)
is a small, general-purpose system that can execute programs to perform a wide
variety of tasks. The PC, however, was designed for use by one person at a
time; that is, it is Single-User oriented with MS-DOS as the de facto standard
operating system for this range of machines. Single user systems became very
popular due to the low cost hardware and wide range of software available for
these machines.
37)What is
Kernel?
Kernel is core part of unix
o/s. It is a group of hundreds of system calls.
38) What are
the different security features in Unix?
Password
protection
File permissions
Encryption.
39)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.
40) How do you
find out the current directory you’re in?
pwd