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(FAQ) OS and UNIX interview questions

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From: "aruna priyadarshini" <arunapriyadarshini5@gmail.com>Date: Thu Jun 29, 2006 3:03 amSubject: (FAQ) OS and UNIX interview questions hi all............ here are a few OS and UNIX interview questions with answers that i found. do check it......... its in the files attached....1. What is MUTEX ?2. What isthe difference between a ‘thread’ and a ‘process’?3. What is INODE?4. Explain the working of Virtual Memory.5. How does Windows NT supports Multitasking?6. Explain the Unix Kernel.7. What is Concurrency? Expain with example Deadlock and Starvation.8. What are your solution strategies for “Dining Philosophers Problem” ?9. Explain Memory Partitioning, Paging, Segmentation.10. Explain Scheduling.11. Operating System Security.12. What is Semaphore?13. Explain the following file systems : NTFS, Macintosh(HPFS), FAT .14. What are the different process states?15. What is Marshalling?16. Define and explain COM?17. What is Marshalling?18. Difference - Loading and Linking ?19. Q1. What is mutex?Ans: - Mutex is a program object that allows multiple program threads to sharethe same resource, such as file access, but not simultaneously. When a programis started a mutex is created woth a unique name. After this stage, any threadthat needs the resource must lock the mutex from other threads while it is usingthe resource. the mutex is set to unlock when the data is no longer needed orthe routine is finished.Tech Interviews comment by Nikesh Gupta20. Q2. What isthe difference between a ‘thread’ and a ‘process’?Ans. A process is a collection of virtual memory space, code, data, and systemresources. A thread is code that is to be serially executed within a process. Aprocessor executes threads, not processes, so each application has at least oneprocess, and a process always has at least one thread of execution, known as theprimary thread. A process can have multiple threads in addition to the primarythread. Prior to the introduction of multiple threads of execution, applicationswere all designed to run on a single thread of execution.When a thread begins to execute, it continues until it is killed or until it isinterrupted by a thread with higher priority (by a user action or the kernel’sthread scheduler). Each thread can run separate sections of code, or multiplethreads can execute the same section of code. Threads executing the same blockof code maintain separate stacks. Each thread in a process shares that process’sglobal variables and resources.Tech Interviews comment by Nikesh Gupta21. What isthe difference between ‘convenient’ and ‘efficient’?Tech Interviews comment by daniel22. Q12: What is Semaphore?Ans: Locking Mechanism used inside resource mangers and resourse dispensers.Tech Interviews comment by Naveen23. 15Q: What is Marshalling?Ans: The process of packaging and sending interface method parameters acrossthread or process boundaries.Tech Interviews comment by Naveen24. Q16:Define and Explain COM?Ans: COM is a specification(Standards). COM has two aspectsa: COM specifications provide a definition for what object isb: COM provides services or blue prints for creation of object and comminicationbetween client and server.COM is loaded when the first object of the component is created.Tech Interviews comment by Naveen25. #14.A process may be in anyone of the following states1.NEW2.READY3.WAIT4.RUNNING5.TERMINATETech Interviews comment by elumalai26. Answer to Question:What isthe difference between ‘convenient’ and ‘efficient’?Answer:Convenient programming is the way of programming using the available resourceslike Library APIs, Macros etc without caring for efficiency.Let me explain this with an example. Suppose we need to convert a character intoan upper case, only if the character is an alphabet in lower case.A convinient way of achieving this is as followsif( isalpha(ch) ){ch = toupper(ch);}However, the overhead with this is as follows1. Involves the use of ctype.h library and2. Takes high MIPS (million instructions per second) since it involves functioncalls.An efficient way to this is as followsif( ‘a’Tech Interviews comment by Shrinidhi KaranthFollowing are a few basic questions that cover the essentials of OS:1. When is a system in safe state?The set of dispatchable processes is in a safe state if there exists at leastone temporal order in which all processes can be run to completion withoutresulting in a deadlock.2. What is cycle stealing?We encounter cycle stealing in the context of Direct Memory Access (DMA). Eitherthe DMA controller can use the data bus when the CPU does not need it, or it mayforce the CPU to temporarily suspend operation. The latter technique is calledcycle stealing. Note that cycle stealing can be done only at specific breakpoints in an instruction cycle.3. What is meant by arm-stickiness?If one or a few processes have a high access rate to data on one track of astorage disk, then they may monopolize the device by repeated requests to thattrack. This generally happens with most common device scheduling algorithms(LIFO, SSTF, C-SCAN, etc). High-density multisurface disks are more likely to beaffected by this than low density ones.4. What are the stipulations of C2 level security?C2 level security provides for:> Discretionary Access Control> Identification and Authentication> Auditing> Resource reuse5. What is busy waiting?The repeated execution of a loop of code while waiting for an event to occur iscalled busy-waiting. The CPU is not engaged in any real productive activityduring this period, and the process does not progress toward completion.6. What are short-, long- and medium-term scheduling?Long term scheduler determines which programs are admitted to the system forprocessing. It controls the degree of multiprogramming. Once admitted, a jobbecomes a process.Medium term scheduling is part of the swapping function. This relates toprocesses that are in a blocked or suspended state. They are swapped out ofreal-memory until they are ready to execute. The swapping-in decision is basedon memory-management criteria.Short term scheduler, also know as a dispatcher executes most frequently, andmakes the finest-grained decision of which process should execute next. Thisscheduler is invoked whenever an event occurs. It may lead to interruption ofone process by preemption.7. What are turnaround time and response time?Turnaround time is the interval between the submission of a job and itscompletion. Response time is the interval between submission of a request, andthe first response to that request.8. What are the typical elements of a process image?> User data: Modifiable part of user space. May include program data, user stackarea, 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.9. What is the Translation Lookaside Buffer (TLB)?In a cached system, the base addresses of the last few referenced pages ismaintained in registers called the TLB that aids in faster lookup. TLB containsthose page-table entries that have been most recently used. Normally, eachvirtual memory reference causes 2 physical memory accesses-- one to fetchappropriate page-table entry, and one to fetch the desired data. Using TLBin-between, this is reduced to just one physical memory access in cases ofTLB-hit.10. What is the resident set and working set of a process?Resident set is that portion of the process image that is actually inreal-memory at a particular instant. Working set is that subset of resident setthat is actually needed for execution. (Relate this to the variable-window sizemethod for swapping techniques.)11. 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 ofa program during the same period. Reentrancy has 2 key aspects: The program codecannot modify itself, and the local data for each user process must be storedseparately. Thus, the permanent part is the code, and the temporary part is thepointer back to the calling program and local variables used by that program.Each execution instance is called activation. It executes the code in thepermanent part, but has its own copy of local variables/parameters. Thetemporary 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 interruptingprogram, and still execute correctly on returning to the procedure.12. Explain Belady's Anomaly.Also called FIFO anomaly. Usually, on increasing the number of frames allocatedto a process' virtual memory, the process execution is faster, because fewerpage faults occur. Sometimes, the reverse happens, i.e., the execution timeincreases even when more frames are allocated to the process. This is Belady'sAnomaly. This is true for certain page reference patterns.13. 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 toimplement mutual exclusion and synchronize concurrent processes.14. What is thrashing?It is a phenomenon in virtual memory schemes when the processor spends most ofits time swapping pages, rather than executing instructions. This is due to aninordinate number of page faults.15. 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 forothers.> 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 holdsat least one resource needed by another process in the chain.16. Explain the popular multiprocessor thread-scheduling strategies.> Load Sharing: Processes are not assigned to a particular processor. A globalqueue of threads is maintained. Each processor, when idle, selects a thread fromthis queue. Note that load balancing refers to a scheme where work is allocatedto processors on a more permanent basis.> Gang Scheduling: A set of related threads is scheduled to run on a set ofprocessors at the same time, on a 1-to-1 basis. Closely related threads /processes may be scheduled this way to reduce synchronization blocking, andminimize process switching. Group scheduling predated this strategy.> Dedicated processor assignment: Provides implicit scheduling defined byassignment of threads to processors. For the duration of program execution, eachprogram is allocated a set of processors equal in number to the number ofthreads in the program. Processors are chosen from the available pool.> Dynamic scheduling: The number of thread in a program can be altered duringthe course of execution.17. When does the condition 'rendezvous' arise?In message passing, it is the condition in which, both, the sender and receiverare blocked until the message is delivered.18. What is a trap and trapdoor?Trapdoor is a secret undocumented entry point into a program used to grantaccess without normal methods of access authentication. A trap is a softwareinterrupt, usually the result of an error condition.19. What are local and global page replacements?Local replacement means that an incoming page is brought in only to the relevantprocess' address space. Global replacement policy allows any page frame from anyprocess to be replaced. The latter is applicable to variable partitions modelonly.20. 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 therequired sector to reach the head. Sum of seek time (if any) and latency is theaccess time. Time taken to actually transfer a span of data is transfer time.21. Describe the Buddy system of memory allocation.Free memory is maintained in linked lists, each of equal sized blocks. Any suchblock is of size 2^k. When some memory is required by a process, the block sizeof next higher order is chosen, and broken into two. Note that the two suchpieces 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. Ifso, it is rejoined, and put into the original free-block linked-list.22. What is time-stamping?It is a technique proposed by Lamport, used to order events in a distributedsystem without the use of clocks. This scheme is intended to order eventsconsisting of the transmission of messages. Each system 'i' in the networkmaintains a counter Ci. Every time a system transmits a message, it incrementsits counter by 1 and attaches the time-stamp Ti to the message. When a messageis received, the receiving system 'j' sets its counter Cj to 1 more than themaximum of its current value and the incoming time-stamp Ti. At each site, theordering of messages is determined by the following rules: For messages x fromsite i and y from site j, x precedes y if one of the following conditionsholds....(a) if Ti<Tj or (B) if Ti=Tj and i<j.23. How are the wait/signal operations for monitor different from those forsemaphores?If a process in a monitor signal and no task is waiting on the conditionvariable, the signal is lost. So this allows easier program design. Whereas insemaphores, every operation affects the value of the semaphore, so the wait andsignal operations should be perfectly balanced in the program.24. In the context of memory management, what are placement and replacementalgorithms?Placement algorithms determine where in available real-memory to load a program.Common methods are first-fit, next-fit, best-fit. Replacement algorithms areused when memory is full, and one process (or part of a process) needs to beswapped out to accommodate a new program. The replacement algorithm determineswhich are the partitions to be swapped out.25. In loading programs into memory, what is the difference between load-timedynamic linking and run-time dynamic linking?For load-time dynamic linking: Load module to be loaded is read into memory. Anyreference to a target external module causes that module to be loaded and thereferences are updated to a relative address from the start base address of theapplication module.With run-time dynamic loading: Some of the linking is postponed until actualreference during execution. Then the correct module is loaded and linked.26. What are demand- and pre-paging?With demand paging, a page is brought into memory only when a location on thatpage is actually referenced during execution. With pre-paging, pages other thanthe one demanded by a page fault are brought in. The selection of such pages isdone based on common access patterns, especially for secondary memory devices.27. Paging a memory management function, while multiprogramming a processormanagement function, are the two interdependent?Yes.28. What is page cannibalizing?Page swapping or page replacements are called page cannibalizing.29. What has triggered the need for multitasking in PCs?> Increased speed and memory capacity of microprocessors together with thesupport fir virtual memory and> Growth of client server computing30. What are the four layers that Windows NT have in order to achieveindependence?> Hardware abstraction layer> Kernel> Subsystems> System Services.31. What is SMP?To achieve maximum efficiency and reliability a mode of operation known assymmetric multiprocessing is used. In essence, with SMP any process or threadscan be assigned to any processor.32. What are the key object oriented concepts used by Windows NT?> Encapsulation> Object class and instance33. 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 languageand the data structures reside within one executive component and are notrepresented as objects and it does not support object oriented capabilities .34. What is a drawback of MVT?It does not have the features like> ability to support multiple processors> virtual storage> source level debugging35. What is process spawning?When the OS at the explicit request of another process creates a process, thisaction is called process spawning.36. How many jobs can be run concurrently on MVT?15 jobs37. 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.38. What are the reasons for process suspension?> swapping> interactive user request> timing> parent process request39. What is process migration?It is the transfer of sufficient amount of the state of process from one machineto the target machine40. What is mutant?In Windows NT a mutant provides kernel mode or user mode mutual exclusion withthe notion of ownership.41. What is an idle thread?The special thread a dispatcher will execute when no ready thread is found.42. What is FtDisk?It is a fault tolerance disk driver for Windows NT.43. What are the possible threads a thread can have?> Ready> Standby> Running> Waiting> Transition> Terminated.44. What are rings in Windows NT?Windows NT uses protection mechanism called rings provides by the process toimplement separation between the user mode and kernel mode.45. What is Executive in Windows NT?In Windows NT, executive refers to the operating system code that runs in kernelmode.46. What are the sub-components of I/O manager in Windows NT?> Network redirector/ Server> Cache manager.> File systems> Network driver> Device driver47. What are DDks? Name an operating system that includes this feature.DDks are device driver kits, which are equivalent to SDKs for writing devicedrivers. Windows NT includes DDks.48. What level of security does Windows NT meets?C2 level security.UNIX ConceptsSECTION - IFILE MANAGEMENT IN UNIX1. How are devices represented in UNIX?All devices are represented by files called special files that are locatedin/dev directory. Thus, device files and other files are named and accessed inthe same way. A 'regular file' is just an ordinary data file in the disk. A'block special file' represents a device with characteristics similar to a disk(data transfer in terms of blocks). A 'character special file' represents adevice with characteristics similar to a keyboard (data transfer is by stream ofbits in sequential order).2. What is 'inode'?All UNIX files have its description stored in a structure called 'inode'. Theinode contains info about the file-size, its location, time of last access, timeof last modification, permission and so on. Directories are also represented asfiles and have an associated inode. In addition to descriptions about the file,the inode contains pointers to the data blocks of the file. If the file islarge, inode has indirect pointer to a block of pointers to additional datablocks (this further aggregates for larger files). A block is typically 8k.Inode consists of the following fields:> File owner identifier> File type> File access permissions> File access times> Number of links> File size> Location of the file data3. Brief about the directory representation in UNIXA Unix directory is a file containing a correspondence between filenames andinodes. A directory is a special file that the kernel maintains. Only kernelmodifies directories, but processes can read directories. The contents of adirectory are a list of filename and inode number pairs. When new directoriesare created, kernel makes two entries named '.' (refers to the directory itself)and '..' (refers to parent directory).System call for creating directory is mkdir (pathname, mode).4. What are the Unix system calls for I/O?> open(pathname,flag,mode) - open file> creat(pathname,mode) - create file> close(filedes) - close an open file> read(filedes,buffer,bytes) - read data from an open file> write(filedes,buffer,bytes) - write data to an open file> lseek(filedes,offset,from) - position an open file> dup(filedes) - duplicate an existing file descriptor> dup2(oldfd,newfd) - duplicate to a desired file descriptor> fcntl(filedes,cmd,arg) - change properties of an open file> ioctl(filedes,request,arg) - change the behaviour of an open fileThe difference between fcntl anf ioctl is that the former is intended for anyopen file, while the latter is for device-specific operations.5. How do you change File Access Permissions?Every file has following attributes:> owner's user ID ( 16 bit integer )> owner's group ID ( 16 bit integer )> File access mode word'r w x -r w x- r w x'(user permission-group permission-others permission)r-read, w-write, x-executeTo change the access mode, we use chmod(filename,mode).Example 1:To change mode of myfile to 'rw-rw-r--' (ie. read, write permission for user -read,write permission for group - only read permission for others) we give theargs as:chmod(myfile,0664) .Each operation is represented by discrete values'r' is 4'w' is 2'x' is 1Therefore, for 'rw' the value is 6(4+2).Example 2:To change mode of myfile to 'rwxr--r--' we give the args as:chmod(myfile,0744).6. What are links and symbolic links in UNIX file system?A link is a second name (not a file) for a file. Links can be used to assignmore than one name to a file, but cannot be used to assign a directory more thanone name or link filenames on different computers.Symbolic link 'is' a file that only contains the name of another file.Operationon the symbolic link is directed to the file pointed by the it.Both thelimitations of links are eliminated in symbolic links.Commands for linking files are:Link ln filename1 filename2Symbolic link ln -s filename1 filename27. What is a FIFO?FIFO are otherwise called as 'named pipes'. FIFO (first-in-first-out) is aspecial file which is said to be data transient. Once data is read from namedpipe, it cannot be read again. Also, data can be read only in the order written.It is used in interprocess communication where a process writes to one end ofthe pipe (producer) and the other reads from the other end (consumer).8. How do you create special files like named pipes and device files?The system call mknod creates special files in the following sequence.1. kernel assigns new inode,2. sets the file type to indicate that the file is a pipe, directory or specialfile,3. If it is a device file, it makes the other entries like major, minor devicenumbers.For example:If the device is a disk, major device number refers to the disk controller andminor device number is the disk.9. Discuss the mount and unmount system callsThe privileged mount system call is used to attach a file system to a directoryof another file system; the unmount system call detaches a file system. When youmount another file system on to your directory, you are essentially splicing onedirectory tree onto a branch in another directory tree. The first argument tomount call is the mount point, that is , a directory in the current file namingsystem. The second argument is the file system to mount to that point. When youinsert a cdrom to your unix system's drive, the file system in the cdromautomatically mounts to /dev/cdrom in your system.10. How does the inode map to data block of a file?Inode has 13 block addresses. The first 10 are direct block addresses of thefirst 10 data blocks in the file. The 11th address points to a one-level indexblock. The 12th address points to a two-level (double in-direction) index block.The 13th address points to a three-level(triple in-direction)index block. Thisprovides a very large maximum file size with efficient access to large files,but also small files are accessed directly in one disk read.11. What is a shell?A shell is an interactive user interface to an operating system services thatallows an user to enter commands as character strings or through a graphicaluser interface. The shell converts them to system calls to the OS or forks off aprocess to execute the command. System call results and other information fromthe OS are presented to the user through an interactive interface. Commonly usedshells are sh,csh,ks etc.SECTION - IIPROCESS MODEL and IPC1. Brief about the initial process sequence while the system boots up.While booting, special process called the 'swapper' or 'scheduler' is createdwith Process-ID 0. The swapper manages memory allocation for processes andinfluences CPU allocation. The swapper inturn creates 3 children:> the process dispatcher,> vhand and> dbflushwith IDs 1,2 and 3 respectively.This is done by executing the file /etc/init. Process dispatcher gives birth tothe shell. Unix keeps track of all the processes in an internal data structurecalled the Process Table (listing command is ps -el).2. What are various IDs associated with a process?Unix identifies each process with a unique integer called ProcessID. The processthat executes the request for creation of a process is called the 'parentprocess' whose PID is 'Parent Process ID'. Every process is associated with aparticular user called the 'owner' who has privileges over the process. Theidentification for the user is 'UserID'. Owner is the user who executes theprocess. Process also has 'Effective User ID' which determines the accessprivileges for accessing resources like files.getpid() -process idgetppid() -parent process idgetuid() -user idgeteuid() -effective user id3. Explain fork() system call.The `fork()' used to create a new process from an existing process. The newprocess is called the child process, and the existing process is called theparent. We can tell which is which by checking the return value from `fork()'.The parent gets the child's pid returned to him, but the child gets 0 returnedto him.4. Predict the output of the following program codemain(){fork();printf("Hello World!");}Answer:Hello World!Hello World!Explanation:The fork creates a child that is a duplicate of the parent process. The childbegins from the fork().All the statements after the call to fork() will beexecuted twice.(once by the parent process and other by child). The statementbefore fork() is executed only by the parent process.5. Predict the output of the following program codemain(){fork(); fork(); fork();printf("Hello World!");}Answer:"Hello World" will be printed 8 times.Explanation:2^n times where n is the number of calls to fork()6. List the system calls used for process management:System calls Descriptionfork() To create a new processexec() To execute a new program in a processwait() To wait until a created process completes its executionexit() To exit from a process executiongetpid() To get a process identifier of the current processgetppid() To get parent process identifiernice() To bias the existing priority of a processbrk() To increase/decrease the data segment size of a process7. How can you get/set an environment variable from a program?Getting the value of an environment variable is done by using `getenv()'.Setting the value of an environment variable is done by using `putenv()'.8. How can a parent and child process communicate?A parent and child can communicate through any of the normal inter-processcommunication schemes (pipes, sockets, message queues, shared memory), but alsohave some special ways to communicate that take advantage of their relationshipas a parent and child. One of the most obvious is that the parent can get theexit status of the child.9. What is a zombie?When a program forks and the child finishes before the parent, the kernel stillkeeps some of its information about the child in case the parent might need it -for example, the parent may need to check the child's exit status. To be able toget this information, the parent calls `wait()'; In the interval between thechild terminating and the parent calling `wait()', the child is said to be a`zombie' (If you do `ps', the child will have a `Z' in its status field toindicate this.)10. What are the process states in Unix?As a process executes it changes state according to its circumstances. Unixprocesses have the following states:Running : The process is either running or it is ready to run .Waiting : The process is waiting for an event or for a resource.Stopped : The process has been stopped, usually by receiving a signal.Zombie : The process is dead but have not been removed from the process table.11. What Happens when you execute a program?When you execute a program on your UNIX system, the system creates a specialenvironment for that program. This environment contains everything needed forthe system to run the program as if no other program were running on the system.Each process has process context, which is everything that is unique about thestate of the program you are currently running. Every time you execute a programthe UNIX system does a fork, which performs a series of operations to create aprocess context and then execute your program in that context. The steps includethe following:> Allocate a slot in the process table, a list of currently running programskept by UNIX.> Assign a unique process identifier (PID) to the process.> iCopy the context of the parent, the process that requested the spawning ofthe new process.> Return the new PID to the parent process. This enables the parent process toexamine or control the process directly.After the fork is complete, UNIX runs your program.12. What Happens when you execute a command?When you enter 'ls' command to look at the contents of your current workingdirectory, UNIX does a series of things to create an environment for ls and therun it: The shell has UNIX perform a fork. This creates a new process that theshell will use to run the ls program. The shell has UNIX perform an exec of thels program. This replaces the shell program and data with the program and datafor ls and then starts running that new program. The ls program is loaded intothe new process context, replacing the text and data of the shell. The lsprogram performs its task, listing the contents of the current directory.13. What is a Daemon?A daemon is a process that detaches itself from the terminal and runs,disconnected, in the background, waiting for requests and responding to them. Itcan also be defined as the background process that does not belong to a terminalsession. Many system functions are commonly performed by daemons, including thesendmail daemon, which handles mail, and the NNTP daemon, which handles USENETnews. Many other daemons may exist. Some of the most common daemons are:> init: Takes over the basic running of the system when the kernel has finishedthe boot process.> inetd: Responsible for starting network services that do not have their ownstand-alone daemons. For example, inetd usually takes care of incoming rlogin,telnet, and ftp connections.> cron: Responsible for running repetitive tasks on a regular schedule.14. What is 'ps' command for?The ps command prints the process status for some or all of the runningprocesses. The information given are the process identification number (PID),theamount of time that the process has taken to execute so far etc.15. How would you kill a process?The kill command takes the PID as one argument; this identifies which process toterminate. The PID of a process can be got using 'ps' command.16. What is an advantage of executing a process in background?The most common reason to put a process in the background is to allow you to dosomething else interactively without waiting for the process to complete. At theend of the command you add the special background symbol, &. This symbol tellsyour shell to execute the given command in the background.Example: cp *.* ../backup& (cp is for copy)17. How do you execute one program from within another?The system calls used for low-level process creation are execlp() and execvp().The execlp call overlays the existing program with the new one , runs that andexits. The original program gets back control only when an error occurs.execlp(path,file_name,arguments..); //last argument must be NULLA variant of execlp called execvp is used when the number of arguments is notknown in advance.execvp(path,argument_array); //argument array should be terminated by NULL18. What is IPC? What are the various schemes available?The term IPC (Inter-Process Communication) describes various ways by whichdifferent process running on some operating system communicate between eachother. Various schemes available are as follows:Pipes:One-way communication scheme through which different process can communicate.The problem is that the two processes should have a common ancestor(parent-child relationship). However this problem was fixed with theintroduction of named-pipes (FIFO).Message Queues :Message queues can be used between related and unrelated processes running on amachine.Shared Memory:This is the fastest of all IPC schemes. The memory to be shared is mapped intothe address space of the processes (that are sharing). The speed achieved isattributed to the fact that there is no kernel involvement. But this schemeneeds synchronization.Various forms of synchronisation are mutexes, condition-variables, read-writelocks, record-locks, and semaphores.SECTION - IIIMEMORY MANAGEMENT1. 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 iseasier to implementation and overhead to the system. Swapping systems does nothandle 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 forexecution. Process size does not matter. Gives the concept of the virtualmemory.It provides greater flexibility in mapping the virtual address space into thephysical memory of the machine. Allows more number of processes to fit in themain memory simultaneously. Allows the greater process size than the availablephysical memory. Demand paging systems handle the memory more flexibly.2. What is major difference between the Historic Unix and the new BSD release ofUnix System V in terms of Memory Management?Historic Unix uses Swapping – entire process is transferred to the main memoryfrom the swap device, whereas the Unix System V uses Demand Paging – only thepart of the process is moved to the main memory. Historic Unix uses one SwapDevice 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-coreresident,> Monitors the available main memory and periodically write the processes intothe 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 swapdevice that are allocatable resources, and the number of the resource unitsavailable there.This allows First-Fit allocation of contiguous blocks of a resource. Initiallythe Map contains one entry – address (block offset from the starting of the swaparea) and the total number of resources.Kernel treats each unit of Map as a group of disk blocks. On the allocation andfreeing of the resources Kernel updates the Map for accurate information.5. What scheme does the Kernel in Unix System V follow while choosing a swapdevice among the multiple swap devices?Kernel follows Round Robin scheme choosing a swap device among the multiple swapdevices in Unix System V.6. What is a Region?A Region is a continuous area of a process’s address space (such as text, dataand stack). The kernel in a ‘Region Table’ that is local to the processmaintains region. Regions are sharable among the process.7. What are the events done by the Kernel after a process is being swapped outfrom the main memory?When Kernel swaps the process out of the primary memory, it performs thefollowing:> Kernel decrements the Reference Count of each region of the process. If thereference 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 operationis 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 donot bother about the empty slots while swapping the process out.After swapping the process resides in the swap (secondary memory) device. Theregions swapped out will be present but only the occupied region slots but notthe empty slots that were present before assigning.While swapping the process once again into the main memory, the Kernel referringto the Process Memory Map, it assigns the main memory accordingly taking care ofthe 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 islocal 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 swappingthe process out of the main memory?All memory space occupied by the process, process’s u-area, and Kernel stack areswapped out, theoretically.Practically, if the process’s u-area contains the Address Translation Tables forthe 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 callsfork() system call, the child process is created and if there is short of memorythen the child process is sent to the read-to-run state in the swap device, andreturn to the user state without swapping the parent process. When the memorywill 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 currentlyallocated, the Kernel performs Expansion swap. To do this Kernel reserves enoughspace in the swap device. Then the address translation mapping is adjusted forthe new virtual address space but the physical memory is not allocated. At lastKernel swaps the process into the assigned space in the swap device. Later whenthe Kernel swaps the process into the main memory this assigns memory accordingto the new address translation mapping.13. How the Swapper works?The swapper is the only process that swaps the processes. The Swapper operatesonly in the Kernel mode and it does not uses System calls instead it usesinternal Kernel functions for swapping. It is the archetype of all kernelprocess.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 theSleeping processes.15. What are the requirements for a swapper to work?The swapper works on the highest scheduling priority. Firstly it will look forany sleeping process, if not found then it will look for the ready-to-runprocess for swapping. But the major requirement for the swapper to work theready-to-run process must be core-resident for at least 2 seconds beforeswapping out. And for swapping in the process must have been resided in the swapdevice for at least 2 seconds. If the requirement is not satisfied then theswapper will go into the wait state on that event and it is awaken once in asecond by the Kernel.16. What are the criteria for choosing a process for swapping into memory fromthe swap device?The resident time of the processes in the swap device, the priority of theprocesses 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 memoryto 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 ofthe process. This value that is returned by the nice () system call. Theequation 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 worksfor the running process only. Nice value of one process cannot affect the nicevalue of the other process.19. What are conditions on which deadlock can occur while swapping theprocesses?> 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 areswapped 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 thetotal data space of the process. i.e. the process frequently calls the samesubroutines 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 theprocess had referred the set of pages in the working set of the process24. What is called a page fault?Page fault is referred to the situation when the process addresses a page in theworking set of the process but the process fails to locate the page in theworking set. And on a page fault the kernel updates the working set by readingthe 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.26. What are the bits that support the demand paging?Valid, Reference, Modify, Copy on write, Age. These bits are the part of thepage table entry, which includes physical address of the page and protectionbits.Page addressAgeCopy on writeModifyReferenceValidProtection27. How the Kernel handles the fork() system call in traditional Unix and in theSystem V Unix, while swapping?Kernel in traditional Unix, makes the duplicate copy of the parent’s addressspace and attaches it to the child’s process, while swapping. Kernel in System VUnix, manipulates the region tables, page table, and pfdata table entries, byincrementing the reference count of the region table of shared regions.28. Difference between the fork() and vfork() system call?During the fork() system call the Kernel makes a copy of the parent process’saddress 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 ofthe vfork() system call executes exec() system call. The child process fromvfork() system call executes in the parent’s address space (this can overwritethe parent’s data and stack ) which suspends the parent process until the childprocess exits.29. What is BSS(Block Started by Symbol)?A data representation at the machine level, that has initial values when aprogram starts and tells about how much space the kernel allocates for theun-initialized data. Kernel initializes it to zero at run-time.30. What is Page-Stealer process?This is the Kernel process that makes rooms for the incoming pages, by swappingthe 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 invokesit throughout the lifetime of the system. Kernel locks a region when a processfaults on a page in the region, so that page stealer cannot steal the page,which is being faulted in.31. 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 toother virtual address space.32. What are the phases of swapping a page from the memory?> Page stealer finds the page eligible for swapping and places the page numberin the list of pages to be swapped.> Kernel copies the page to a swap device when necessary and clears the validbit in the page table entry, decrements the pfdata reference count, and placesthe pfdata table entry at the end of the free list if its reference count is 0.33. What is page fault? Its types?Page fault refers to the situation of not having a page in the main memory whenany process references it.There are two types of page fault :> Validity fault,> Protection fault.34. In what way the Fault Handlers and the Interrupt handlers are different?Fault handlers are also an interrupt handler with an exception that theinterrupt handlers cannot sleep. Fault handlers sleep in the context of theprocess that caused the memory fault. The fault refers to the running processand no arbitrary processes are put to sleep.35. What is validity fault?If a process referring a page in the main memory whose valid bit is not set, itresults 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 physicaladdress is assigned to it.36. What does the swapping system do if it identifies the illegal page forswapping?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 sendsa “Segmentation violation” signal to the offending process. This happens whenthe swapping system identifies any invalid memory reference.37. 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”.38. 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.39. At what mode the fault handler executes?At the Kernel Mode.40. What do you mean by the protection fault?Protection fault refers to the process accessing the pages, which do not havethe access permission. A process also incur the protection fault when itattempts to write a page whose copy on write bit was set during the fork()system call.41. 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 pageis shared by more than one process, the Kernel allocates new page and copies thecontent to the new page and the other processes retain their references to theold page. After copying the Kernel updates the page table entry with the newpage number. Then Kernel decrements the reference count of the old pfdata tableentry.In cases like, where the copy on write bit is set and no processes are sharingthe page, the Kernel allows the physical page to be reused by the processes. Bydoing so, it clears the copy on write bit and disassociates the page from itsdisk copy (if one exists), because other process may share the disk copy. Thenit removes the pfdata table entry from the page-queue as the new copy of thevirtual page is not on the swap device. It decrements the swap-use count for thepage and if count drops to 0, frees the swap space.42. 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 thatthe page is invalid (valid bit is clear), the validity fault handler returnsimmediately, and the process incur the validity page fault. Kernel handles thevalidity fault and the process will incur the protection fault if any one ispresent.43. In what way the protection fault handler concludes?After finishing the execution of the fault handler, it sets the modify andprotection bits and clears the copy on write bit. It recalculates theprocess-priority and checks for signals.44. 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 thememory. If the sum of the working sets of all processes is greater that thephysical memory then the fault handler will usually sleep because it cannotallocate pages for a process. This results in the reduction of the systemthroughput because Kernel spends too much time in overhead, rearranging thememory in the frantic pace.

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I would also recommend you to go through the unix book by Bach to get a good hold of OS concepts.

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