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.
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.
1. Mutual Exclusion: Only
one process may use a critical resource at a time.
2. Hold & Wait: A
process may be allocated some resources while waiting for others.
3. No Pre-emption: No
resource can be forcible removed from a process holding it.
4. 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.
11. 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.
12. 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.
13. 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.
14. What are the stipulations of C2 level
security?
C2 level security provides for:
1. Discretionary Access
Control
2. Identification and
Authentication
3. Auditing
4. Resource reuse
15. 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.
16. Explain the popular multiprocessor thread-scheduling
strategies.
1. 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.
2. 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.
3. 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.
4. Dynamic scheduling: The
number of thread in a program can be altered during the course of execution.
17. 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.
18. 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.
19. 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.
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 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.
21. 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.
22. 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
23. 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.
24. 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.
25. 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.
26. 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.
27. Paging a memory management function,
while multiprogramming a processor management 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?
1. Increased speed and
memory capacity of microprocessors together with the support fir virtual memory
and
2. Growth of client server
computing
30. What are the four layers that Windows
NT have in order to achieve independence?
1. Hardware abstraction
layer
2. Kernel
3. Subsystems
4. System Services.
31. 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.
32. What are the key object oriented
concepts used by Windows NT?
Encapsulation, Object class and instance.
33. 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.
34. What is a drawback of MVT?
It does not have the features like
1. ability to support
multiple processors
2. virtual storage
3. source level debugging
35. What is process spawning?
When the OS at the explicit request of
another process creates a process, this action is called process spawning.
36. How many jobs can be run concurrently
on MVT?
15 jobs.
37. List out some reasons for process
termination.
1. Normal completion
2. Time limit exceeded
3. Memory unavailable
4. Bounds violation
5. Protection error
6. Arithmetic error
7. Time overrun
8. I/O failure
9. Invalid instruction
10.
Privileged
instruction
11.
Data
misuse
12.
Operator
or OS intervention
13.
Parent
termination.
38. What are the reasons for process
suspension?
1. swapping
2. interactive user request
3. timing
4. parent process request
39. What is process migration?
It is the transfer of sufficient amount of
the state of process from one machine to the target machine.
40. What is mutant?
In Windows NT a mutant provides kernel mode
or user mode mutual exclusion with the 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?
1. Ready
2. Standby
3. Running
4. Waiting
5. Transition
6. Terminated
44. 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.
45. What is Executive in Windows NT?
In Windows NT, executive refers to the
operating system code that runs in kernel mode.
46. 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
47. 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.
48. What level of security does Windows NT
meets?
C2 level security.
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