Operating System Concepts - Covers fundamental Questions in OS

Following are a few basic questions that cover the essentials of Operating System Concepts  :

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.)


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Comments (21)

Said this on 1-11-2008 At 10:31 am
paper is good...
i think more questions can be added...
i will expect more questions in near future...
Said this on 2-9-2008 At 05:49 pm
its really a useful stuff......!!!!!!!
really did a lot for my friends n me
Said this on 5-13-2008 At 11:57 am
Said this on 11-18-2008 At 02:18 pm
it was really good and quite useful too... i still expexct lot more important questions.
Said this on 12-1-2008 At 02:25 pm
Said this on 12-12-2008 At 01:00 pm
Really Good One
Said this on 2-4-2009 At 11:49 am
quite nice effort!!!!
keeeeeeeep it updated with more nd more questions....
vikkas gupta
Said this on 7-10-2009 At 08:58 am
very good
Said this on 8-27-2009 At 03:45 pm
Said this on 10-31-2009 At 11:36 am

this information is really good

Said this on 12-15-2009 At 11:32 pm

hats off...............brilliant effort..............looking forward 4 the similar ones.............

Said this on 1-31-2010 At 09:05 am

it is very useful to me...and i need lot of info about this..

Said this on 2-22-2010 At 03:58 pm

it is very useful for me.. really good... we need some more tutorials...

Said this on 3-16-2010 At 09:25 pm

its toooo nice

Said this on 3-28-2010 At 07:23 am

it's nice...

Said this on 5-23-2010 At 06:25 am

informaction about operting sysrem

Said this on 6-1-2010 At 01:44 pm
paper is good...
i think more questions can be added...
i will expect more questions in near future...

sudhir kumar dhiraj(impscet)
Said this on 8-30-2010 At 09:16 pm

i think it is not enough,

please add some more best question with answer

ur these question is too much goo there is no dought

thank you

Said this on 11-3-2010 At 11:24 am

really good.....i got this questions in interview

                   thanks to author

Said this on 11-26-2010 At 07:17 pm

very good answers

Said this on 3-16-2011 At 03:59 pm

this is my first semester of bca


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