Frequently Asked OS General Concepts Interview Questions
Q – 1 Explain in which situation we use sjf scheduling algorithm?
Ans- when we want to reduce the average waiting time in non preemitive sjf and response time in pree,tivesjf or srtf
Q – 2 In general what is paging? and segmaentation?
Ans- Paging is the memory mgmt scheme that permits the physical address space of a process to be non contigous.Pages are of
fix sizes.
Segmentationis the memory mgmtscheme that consider adrees space as collectionof segments. Segments are o0fvariable sizes.
Q – 3 How to install operating system through USB pen drive?
Ans- For creating a bootable USB drive with Windows 2000 or XP installer we are going to use a utility called WinSetupFromUSB. Download it from MSFN forums and install it. You are going to need the Windows 2000/XP installation disk.
If you have an ISO, extract the iso. Enter the path to Windows 200/XP installer as Source. After this, plugin in your USB device and format it. You can use the PEtoUSB tool to format USB drives smaller than 2GB as FAT16 drive.
However I would recommend using HP Format Tool to format drives as FAT32 or NTFS (recommended). That is all you need
to do to create a basic installer for Windows 2000 and Windows XP on your pen drive (USB device). Don’t bother yourself with any of the other settings. Click on GO to begin the process.
WinSetupFromUSB is a fairly advanced tool and allows to you create multiboot pen drives that can have Windows Vista, Windows 7 and even Linux installers. If you want to know more about this utility just read through the thread at MSFN forums.
Q – 4 What are multi core operating systems?
Ans- It’s a single processor system in which the processor is divided into more than one parts(core)
Q – 5 What is virtual memory? How we can increase or decrease the virtual memory?
Ans- Virtual memory are a part of operating sysytem in which whenever ram have no space to execute any process than operating system uses some part of hard disk as a ram.
Those part of memory known as virtual memory because this is virtually created by operating system whenever needed.
This memory created by operating system in big companies in which more process executes and ram have no space because
they contain more process already. Most operating systems contains this concept.
Q – 6 What is the full form of “windows xp”?
Ans- XP = Extrme Performance
Q – 7 Who was the discover Mouse and When?
Ans- Douglas Engelbart in the year 1963
Q – 8 What is batch programming in DOS?
Ans- Batch programming is creating a file with DOS commands which will execute with a single click even if it comprises of thousands of DOS commands. It’s much faster than manual execution of commands. And saves the effort of retyping the same commands over and over again if you’re doing a repetitive job with a fixed set of commands.
You can rather type those commands down in notepad, save the file as filename.bat as an “all files” type of file. For example if there are a set of services to start on 100 systems, it’s a waste of time going to each system, and starting all the services manually. You can type down commands to start those services, save it as a batch file, and run this batch file on all the systems.
All the services will start with just a click.
Q – 9 Suppose we have one root domain & two child domain, then in total how many FSMO roles i will have in entire forest?
Ans- My answer is 11 roles.
5 roles for root domain,
3 roles for one child domain &
3 roles for 2nd child domain.
Q – 10 Explain What is the difference between Authoritative restore & Non-Authoritative restore?
Ans- A nonauthoritative restore is the default method for restoring Active Directory. To perform a nonauthoritative restore, you must be able to start the domain controller in Directory Services Restore Mode. After you restore the domain controller from backup, replication partners use the standard replication protocols to update Active Directory and associated information on the restored domain controller.
An authoritative restore brings a domain or a container back to the state it was in at the time of backup and overwrites all changes made since the backup. If you do not want to replicate the changes that have been made subsequent to the last backup operation, you must perform an authoritative restore.
In this one needs to stop the inbound replication first before performing the An authoritative restore.
Q – 11 Explain What is DRAM? In which form does it store data?
Ans- DRAM is the Hershey’s chocolate of readable/writable memory:
it’s not the best, but it’s cheap, does the job, and is available almost everywhere you look. DRAM data resides in a cell made of a capacitor and a transistor. The capacitor tends to lose data unless it’s recharged every couple of milliseconds, and this recharging tends to slow down the performance of DRAM compared to speedier RAM types.
Q – 12 Do you know What is cache memory?
Ans- Cache memory is very high-speed Memory chips which exist between the CPU and main memory. It stores (i.e., caches)
memory accesses by the CPU. Cache ram helps to alleviate the gap between the speed of a CPU’s megahertz rating and the ability of RAM to respond and deliver data. It reduces the frequency that the CPU must wait for data from the main memory.
Q – 13 Differentiate between Complier and Interpreter?
Ans- when ever a program is to be compiled the syntax is necessary and we wont receive the output until a correct syntax is written it shows the errors so debugging becomes easier the best example is C language
where as in interpreter if we miss any syntax it violates that particular line and goes to the next line in this case we may or may not get the required output it does not show any errors even though its violates the syntax the best example is HTML
Q – 14 Tell me What are the different tasks of Lexical analysis?
Ans- The purpose of the lexical analyzer is to partition the input text, delivering a sequence of comments and basic symbols. Comments are character sequences to be ignored, while basic symbols are character sequences that correspond to terminal symbols of the grammar defining the phrase structure of the input
Q – 15 Do you know What are the different functions of Syntax phase, Scheduler?
Ans- Syntax phase sheduler deals with the problem of deciding which of the process in ready queue is to allocate the CPU time for processing.
Q – 16 What is Difference between Primary storage and secondary storage?
Ans- PRIMARY MEMORY STORES THE DATA TEMPORARY.FOR EXAMPLE RAM,ROM.BUT SECONDARY MEMORY STORES THE DATA PERMANENTLY TILL WE REQUIRE.
Q – 17 Explain Demand paging, page faults, replacement algorithms, thrashing?
Ans- demand paging:-
not all of a process’s virtual address space needs to be loaded in main memory at any given time. Each
page can be either:
o In memory (physical page frame)
o On disk (backing store)
loading the page into the memory from disk when required is called as demand paging
page faults:-
when a process references a page that is in the backing store,the page fault occurs
replacement algorithms:-
various approaches to replace pages from disk to memory on occurence of page fault
* Page fetching: when to bring pages into memory.
* Page replacement: which pages to throw out of memory.
Page Replacement
* Once all of memory is in use, will need to throw out
one page each time there is a page fault.
* Random: pick any page at random (works surprisingly well!)
* FIFO: throw out the page that has been in memory longest.
* MIN: The optimal algorithm requires us to predict the
future.
* Least Recently Used (LRU): use the past to predict the
future.
* Strange but true: for some placement policies, such as
FIFO, adding more memory can sometimes cause paging
performance to get worse. This is called “Belady’s Anomaly”
after Les Belady, who was the first person to notice it.
* Implementing LRU: need hardware support to keep track
of which pages have been used recently.
o Perfect LRU?
+ Keep a register for each page, store system clock into that register on each memory reference.
+ To choose page for placement, scan through all pages to find the one with the oldest clock.
+ Hardware costs would have been prohibitive in the early days of paging; also, expensive to scan all
pages during replacement.
o In practice nobody implements perfect LRU.
Instead, we settle for an approximation that is efficient. Just find an old page, not necessarily the oldest.
* Clock algorithm (called second chance algorithm in Silberschatz et al.): keep reference bit for each page
frame, hardware sets the reference bit whenever a page is
read or written. To choose page for placement:
o Start with FIFO approach: cycle through pages in order circularly.
o If the next page has been referenced, then don’t replace it; just clear the reference bit and continue to the next page.
o If the page has not been referenced since the last time we checked it, then replace that page.
* Dirty bit: one bit for each page frame, set by
hardware whenever the page is modified. If a dirty page is
replaced, it must be written to disk before its page frame
is reused.
* The clock algorithm typically gives additional preference to dirty pages. For example, if the reference but
for a page is clear, but the dirty bit is set, don’t replace this page now, but clear the dirty bit and start writing the
page to disk.
* Free page pool: some systems keep a small list of clean pages that are available immediately for replacement.
o During replacement, take the page that has been in the free pool the longest, then run the replacement
algorithm to add a new page to the free pool.
o Pages in the free pool have their exists bit off, so any references to those pages cause a page fault
o If a page fault occurs for a page in the free pool, remove it from the free pool and put it back in service; much faster than reading from disk.
o Provides an extra opportunity for recovery if we make a poor page replacement decision.
Usually thrashing refers to two or more processes accessing a shared resource repeatedly such that serious system performance degradation occurs because the system is spending a disproportionate amount of time just accessing the shared resource. Resource access time may generally be considered as wasted, since it does not contribute to the advancement of any process.
This is often the case when a CPU can process more information than can be held in available RAM; consequently the system spends more time preparing to execute instructions than actually executing them.
Q – 18 Do you know about paged segmentation and segment paging?
Ans- divide program’s logical address space in to segmwnts. each segment will have a page table….each segment pointer
points to page table
Q – 19 Tell me While running DOS on a PC, which command would be used to duplicate the entire diskette?
Ans- diskcopy
Q – 20 Explain the concept of Reentrancy?
Ans- 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.
Q – 21 Explain Beladys Anomaly?
Ans- Belady’s anomaly is in context with the page faults occurring in FIFO page replacement policy. It says that on increasing the number of page frames, the no. of page faults do not necessarily decrease, they may also increase. LRU page replacement algorithm is free from Belady’s anomaly.
Q – 22 List the Coffmans conditions that lead to a deadlock?
Ans- 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.