Sunday, January 20, 2008

Operating Systems Interview Questions

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?

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


1. What is the difference between Swapping and Paging?
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.
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.

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.

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 AgeCopy on writeModifyReferenceValidProtection
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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.)

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.

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.

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.

What are the stipulations of C2 level security?
C2 level security provides for:
• Discretionary Access Control
• Identification and Authentication
• Auditing
• Resource reuse

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

Paging a memory management function, while multiprogramming a processor management function, are the two interdependent?

What is page cannibalizing?
Page swapping or page replacements are called page cannibalizing.

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

What are the four layers that Windows NT have in order to achieve independence?
• Hardware abstraction layer
• Kernel
• Subsystems
• System Services.

What is SMP?
To achieve maximum efficiency and reliability a mode of operation known as symmetric multiprocessing is used. In essence, with SMP any process or threads can be assigned to any processor.

What are the key object oriented concepts used by Windows NT?
• Encapsulation
• Object class and instance

Is Windows NT a full blown object oriented operating system? Give reasons.
No Windows NT is not so, because its not implemented in object oriented language and the data structures reside within one executive component and are not represented as objects and it does not support object oriented capabilities .

What is a drawback of MVT?
It does not have the features like
• ability to support multiple processors
• virtual storage
• source level debugging

What is process spawning?
When the OS at the explicit request of another process creates a process, this action is called process spawning.

How many jobs can be run concurrently on MVT?
15 jobs

List out some reasons for process termination.
• Normal completion
• Time limit exceeded
• Memory unavailable
• Bounds violation
• Protection error
• Arithmetic error
• Time overrun
• I/O failure
• Invalid instruction
• Privileged instruction
• Data misuse
• Operator or OS intervention
• Parent termination.

What are the reasons for process suspension?
• swapping
• interactive user request
• timing
• parent process request

What is process migration?
It is the transfer of sufficient amount of the state of process from one machine to the target machine

What is mutant?
In Windows NT a mutant provides kernel mode or user mode mutual exclusion with the notion of ownership.

What is an idle thread?
The special thread a dispatcher will execute when no ready thread is found.

What is FtDisk?
It is a fault tolerance disk driver for Windows NT.

What are the possible threads a thread can have?
• Ready
• Standby
• Running
• Waiting
• Transition
• Terminated.

What are rings in Windows NT?
Windows NT uses protection mechanism called rings provides by the process to implement separation between the user mode and kernel mode.

What is Executive in Windows NT?
In Windows NT, executive refers to the operating system code that runs in kernel mode.

What are the sub-components of I/O manager in Windows NT?
• Network redirector/ Server
• Cache manager.
• File systems
• Network driver
• Device driver

What are DDks? Name an operating system that includes this feature.
DDks are device driver kits, which are equivalent to SDKs for writing device drivers. Windows NT includes DDks.

What level of security does Windows NT meets?
C2 level security.

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