OPERATING SYSTEM INTERVIEW QUESTIONS-08

What is cache-coherency?
In a multiprocessor system there exist several caches each may containing a copy of same variable A. Then a change in one cache should immediately be reflected in all other caches this process of maintaining the same value of a data in all the caches s called cache-coherency.

What are the benefits of multithreaded programming?
Responsiveness
Resources sharing
Economy
Utilization of multiprocessor architectures.

What is the difference between microkernel and macro kernel?
 Micro-Kernel : A micro-kernel is a minimal operating system that performs only the essential functions of an operating system. All other operating system functions are performed by system processes.
Monolithic : A monolithic operating system is one where all operating system code is in a single executable image and all operating system code runs in system mode.

What is DRAM?
 Dynamic Ram stores the data in the form of Capacitance, and Static RAM stores the data in Voltages.

What are the different functions of Scheduler?
 Scheduler deals with the problem of deciding which of the process in the ready queue is to be allocated the CPU. Short Term Schedulers, Long Term Schedulers

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 is a data register and address register?
 Data registers - can be assigned to a variety of functions by the programmer. They can be used with any     machine instruction that performs operations on data.
Address registers - contain main memory addresses of data and instructions or they contain a portion of the address that is used in the calculation of the complete addresses.

A section of code which can be executed only by one process at a time is called:

Critical section. 

OPERATING SYSTEM INTERVIEW QUESTIONS-07

Differentiate between Complier and Interpreter?
An interpreter reads one instruction at a time and carries out the actions implied by that instruction. It does not perform any translation. But a compiler translates the entire instructions

What is Throughput, Turnaround time, waiting time and Response time?
Throughput – number of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is           produced, not output (for time-sharing environment)

What is Memory-Management Unit (MMU)?
Hardware device that maps virtual to physical address. In MMU scheme, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory.
->The user program deals with logical addresses; it never sees the real physical addresses

What is a Real-Time System?
A real time process is a process that must respond to the events within a certain time period. A real time operating system is an operating system that can run real time processes successfully

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.

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.

Explain the concept of the Distributed systems?
Distributed systems work in a network. They can share the network resources, communicate with each other.

What is cache-coherency?
In a multiprocessor system there exist several caches each may containing a copy of same variable A. Then a change in one cache should immediately be reflected in all other caches this process of maintaining the same value of a data in all the caches s called cache-coherency.

What is a long term scheduler & short term schedulers?
Long term schedulers are the job schedulers that select processes from the job queue and load them into memory for execution.
The short term schedulers are the CPU schedulers that select a process from the ready queue and allocate the CPU to one of them.

Explain the meaning of mutex.
Mutex is the short form for ‘Mutual Exclusion object’. A mutex allows multiple threads for sharing the same resource. The resource can be file. A mutex with a unique name is created at the time of starting a program. A mutex must be locked from other threads, when any thread that needs the resource. When the data is no longer used / needed, the mutex is set to unlock.

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 Marshalling?
The process of packaging and sending interface method parameters across thread or process boundaries.

What is a daemon?
Daemon is a program that runs in the background without user’s interaction. A daemon runs in a multitasking operating system like UNIX. A daemon is initiated and controlled by special programs known as ‘processes’.

What is pre-emptive and non-preemptive scheduling?
Preemptive scheduling: The preemptive scheduling is prioritized. The highest priority process should always be the process that is currently utilized.
Non-Preemptive scheduling: When a process enters the state of running, the state of that process is not deleted from the scheduler until it finishes its service time.

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.

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

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

Difference between Primary storage and secondary storage?
Primary memory is the main memory (Hard disk, RAM) where the operating system resides.
Secondary memory can be external devices like CD, floppy magnetic discs etc. secondary storage cannot be directly accessed by the CPU and is also external memory storage.

Define compactions.
Compaction is a process in which the free space is collected in a large memory chunk to make some space available for processes.

What are residence monitors?
Early operating systems were called residence monitors.

What is dual-mode operation?
In order to protect the operating systems and the system programs from the malfunctioning programs the two mode operations were evolved
System mode
User mode.

What is a device queue?
A list of processes waiting for a particular I/O device is called device queue.

What are the different types of Real-Time Scheduling?
Hard real-time systems required to complete a critical task within a guaranteed amount of time.
Soft real-time computing requires that critical processes receive priority over less fortunate ones.

OPERATING SYSTEM INTERVIEW QUESTIONS-06

What is relative path and absolute path?
Absolute path-- Exact path from root directory.
Relative path-- Relative to the current path.

What are the disadvantages of context switching?
Time taken for switching from one process to other is pure over head. Because the system does no useful work while switching.
So one of the solutions is to go for threading when ever possible.

What is a data register and address register?
Data registers - can be assigned to a variety of functions by the programmer. They can be used with any machine instruction
that performs operations on data.
Address registers - contain main memory addresses of data and instructions or they contain a portion of the address that is
used in the calculation of the complete addresses.

What is DRAM?  
Dynamic Ram stores the data in the form of Capacitance, and Static RAM stores the data in Voltages.

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.

Explain the concept of the batched operating systems?
In batched operating system the users gives their jobs to the operator who sorts the programs according to their requirements
and executes them. This is time consuming but makes the CPU busy all the time.

What is SCSI?
SCSI - Small computer systems interface is a type of interface used for computer components such as hard drives, optical drives,
scanners and tape drives. It is a competing technology to standard IDE (Integrated Drive Electronics).

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 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 is  Dispatcher?
Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: Switching context,
Switching to user mode, Jumping to the proper location in the user program to restart that program, dispatch latency – time it takes
for the dispatcher to stop one process and start another running.

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

What are the reasons for process suspension?
1) swapping
2) interactive user request
3) timing
4) parent process request

What are the sub-components of I/O manager in Windows NT?
1) Network redirector/ Server
2) Cache manager.
3) File systems
4) Network driver
5) Device driver

What is a drawback of MVT?
1) ability to support multiple processors
2) virtual storage
3) source level debugging

OPERATING SYSTEM INTERVIEW QUESTIONS-05

What is an operating system?
 An operating system is a program that acts as an intermediary between the user and the computer hardware. The purpose of an OS is to provide a convenient environment in which user can execute programs in a convenient and efficient manner. It is a resource allocator responsible for allocating system resources and a control program which controls the operation of the computer hardware.

Why paging is used?
 Paging is solution to external fragmentation problem which is to permit the logical address space of a
process to be noncontiguous, thus allowing a process to be allocating physical memory wherever the latter is available.

Explain the concept of the batched operating systems?
In batched operating system the users gives their jobs to the operator who sorts the programs according
to their requirements and executes them. This is time consuming but makes the CPU busy all the time.

What is purpose of different operating systems?
 The machine purpose workstation individual usability &resources utilization mainframe optimize utilization
of hardware PC support complex games, business application Hand held PCs Easy interface & min. power consumption.

What is virtual memory?
Virtual memory is hardware technique where the system appears to have more memory that it actually does. This is done by time-sharing, the physical memory and storage parts of the memory one disk when they are not actively being used.

What is Throughput, Turnaround time, waiting time and Response time?
 Throughput : number of processes that complete their execution per time unit.

Turnaround time : amount of time to execute a particular process.

Waiting time : amount of time a process has been waiting in the ready queue.

Response time : amount of time it takes from when a request was submitted until the firstresponse is produced, not output (for time-sharing environment).

What are the various components of a computer system?
The hardware
The operating system
The application programs
The users.

What is a Real-Time System?
 A real time process is a process that must respond to the eventswithin a certain time period. A real time operating system is an operating system that can run realtime processes successfully.

Explain the concept of the Distributed systems?
 Distributed systems work in a network. They can share the network resources,communicate with each other.

What is SCSI?
 SCSI - Small computer systems interface is a type of interface used for computer components such as hard drives, optical drives, scanners and tape drives. It is a competing technology to standard IDE (Integrated Drive Electronics).

OPERATING SYSTEM INTERVIEW QUESTIONS-04

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.

OPERATING SYSTEM INTERVIEW QUESTIONS-03

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-paging 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?
Yes.

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.

OPERATING SYSTEM INTERVIEW QUESTIONS-02

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.

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<Tj or (b) if Ti=Tj and i<j.

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.

OPERATING SYSTEM INTERVIEW QUESTIONS-01

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 sameperiod. 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.

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