OS final

Question 1

Deadlocks
Multiprogramming environment characterized by several process competing for a limited number of resources

Answer

true

Question 2

Deadlock can arise if four conditions hold simultaneously

Accepted Answer

Mutual exclusion

Accepted Answer

Hold and wait

Accepted Answer

No preemption

Accepted Answer

Circular wait

Answer

Recursive wait

Answer

Mutual Inclusion

Question 3

Deadlock Characterization

[blank_start]Mutual exclusion[blank_end] - Only one process at a time can use a resource

[blank_start]Hold and wait[blank_end] - A process holding at least one resource is waiting to acquire additional resources held by other processes

[blank_start]No preemption[blank_end] - A resource can be released only voluntarily by the process holding it, after that process has completed its task

[blank_start]Circular wait[blank_end] - There exists a set {P0, P1, ..., Pn} of waiting processes

Accepted Answer

Mutual exclusion

Accepted Answer

Hold and wait

Accepted Answer

No preemption

Accepted Answer

Circular wait

Question 4

Rule of Thumb for Deadlock

If graph contains no cycles  = no deadlock

If graph contains a cycle (closed cycle):
if only one instance per resource type = Deadlock
if several instances per resource type = Possibility of deadlock

Answer

true

Question 5

.

Accepted Answer

Y

Answer

.

Question 6

.

Accepted Answer

Y

Answer

.

Question 7

The deadlock-avoidance algorithm is based on resource-allocation state

The # of available resources + allocated resources + maximum demands of the processes

Answer

true

Question 8

Safe State
If a system is in safe state = no deadlocks
If a system is in unsafe state = possibility of deadlock

Answer

true

Question 9

https://www.youtube.com/watch?v=w0LwGqffUkg

Bankers Algorithm - Safety:

1) Let Work and Finish be vectors of length ‘m’ and ‘n’ respectively.
Initialize: Work = Available
Finish[i] = false; for i=1, 2, 3, 4….n

2) Find an i such that both
a) Finish[i] = false
b) Needi <= Work
if no such i exists goto step (4)

3) Work = Work + Allocation[i]
Finish[i] = true
goto step (2)

4) if Finish [i] = true for all i
then the system is in a safe state

Answer

true

Question 10

Bankers Algorithm - resource request

1) If Requesti <= Needi
Goto step (2) ; otherwise, raise an error condition, since the process has exceeded its maximum claim.

2) If Requesti <= Available
Goto step (3); otherwise, Pi must wait, since the resources are not available.

3) Have the system pretend to have allocated the requested resources to process Pi by modifying the state as
follows:
Available = Available – Requesti
Allocationi = Allocationi + Requesti
Needi = Needi– Requesti

Answer

true

Question 11

Several Instances of a Resource Type

Data structure
[blank_start]Available[blank_end]:
Vector of length m indicates the number of available resources of each type

[blank_start]Allocation[blank_end]:
n x m matrix defines the number of resources of each type currently allocated to each process

[blank_start]Request[blank_end]:
n x m matrix indicates the current request  of each process

Accepted Answer

Available

Accepted Answer

Allocation

Accepted Answer

Request

Question 12

Detection Algorithm:

Let Work and Finish be vectors of length m and n respectively. Initialize Work= Available. For i=0, 1, …., n-1, if Requesti = 0, then Finish[i] = true; otherwise, Finish[i]= false.

Find an index i such that both
a) Finish[i] == false
b) Requesti <= Work
If no such i exists go to step 4.

Work= Work+ Allocationi
Finish[i]= true
Go to Step 2.

If Finish[i]== false for some i, 0<=i<n, then the system is in a deadlocked state. Moreover, if Finish[i]==false the process Pi is deadlocked.

Answer

true

Question 13

Abort all deadlocked processes
Costly since it results in loss of all computations performed up to the time of the processes termination

Answer

true

Question 14

Abort one process at a time until the deadlock cycle is eliminated
Significant overhead:
every time a process is terminated the deadlock-detection algorithm should run again to find if the system is in a deadlocked state

Answer

true

Question 15

Base and Limit Registers:
User processes are restricted to this limit
A pair of base and limit registers define the logical address space

CPU must check every memory access generated in user mode to be sure it is between base and limit for that user

Answer

true

Question 16

Address Space

[blank_start]Logical[blank_end] address:
Generated by the CPU
Also referred to as virtual address
set of all logical addresses generated by a process

[blank_start]Physical[blank_end] address:
Address seen by the memory unit
set of all _____ addresses corresponding to these _____ addresses generated by a process

Accepted Answer

Logical

Accepted Answer

Physical

Question 17

The user program deals with physical addresses
It never sees the real logical addresses

Answer

true

Question 18

Memory-Management Unit (MMU)
Hardware device that maps virtual to physical address

Answer

true

Question 19

processes who are not currently using the CPU may have their memory swapped out to a fast local disk called the backing store

not used by modern OS such as UNIX, Linux, and Windows

Answer

true

Question 20

Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory.

[blank_start]Page number(p)[blank_end]: Number of bits required to represent the pages in Logical Address Space or Page number

[blank_start]Page offset(d)[blank_end]: Number of bits required to represent particular word in a page or page size of Logical Address Space or word number of a page or page offset.

[blank_start]Frame number(f)[blank_end]: Number of bits required to represent the frame of Physical Address Space or Frame number.

[blank_start]Frame offset(d)[blank_end]: Number of bits required to represent particular word in a frame or frame size of Physical Address Space or word number of a frame or frame offset.

Accepted Answer

Page number(p)

Accepted Answer

Page offset(d)

Accepted Answer

Frame number(f)

Accepted Answer

Frame offset(d)

Question 21

Virtual memory–separation of user logical memory from physical memory

Only part of the program needs to be in memory for execution

Answer

true

Question 22

What happens if there’s no free frame on the physical memory to store the needed page?

Accepted Answer

Page replacement – find some page in memory that is not in use and replace it

Answer

Page algorithm - find the least used page not in use and swap with the most used page

Question 23

First in First Out

Initially all slots are empty, so when 1, 3, 0 came they are allocated to the empty slots —> 3 Faults.
when 3 comes, it is already in  memory so —> 1 Hit.
Then 5 comes, it is not available in  memory so it replaces the oldest page slot i.e 1. —>1 Fault
6 comes, it is also not available in memory so it replaces the oldest page slot i.e 3 —>1 Fault.
Finally when 3 come it is not available so it replaces 0 --> 1 fault

total faults: 6
total hits: 1

Accepted Answer

Y

Answer

.

Question 24

Optimal

Initially all slots are empty, so when 7 0 1 2 are allocated to the empty slots —> 4 faults
0 is already there so —> 1 hit.
when 3 came it will take the place of 7 because it is not used for the longest duration of time in the future.—>1 fault
0 is already there so —> 1 hit.
4 will takes place of 1 —> 1 Fault.
Now for the further page reference string —> all hits because they are already available in the memory

total faults: 6
total hits: 8

Accepted Answer

Y

Answer

.

Question 25

LRU least recently used

Initially all slots are empty, so when 7 0 1 2 are allocated to the empty slots —> 4 faults
0 is already their so —> 1 hit
when 3 came it will take the place of 7 because it is least recently used —>1 fault
0 is already in memory so —> 1 hit.
4 will takes place of 1 —> 1 Fault
the further page reference string —> hits because they are already available in the memory.

total faults: 6
total hits: 8

Accepted Answer

Y

Answer

.

Question 26

Principles of Protection

[blank_start]Principle of least privilege[blank_end]:
Programs, users and systems should be given just enough privileges to perform their tasks

[blank_start]“Need to know” principle[blank_end]:
Process should be able to access only those resources that it currently requires to complete its tasks

Accepted Answer

Principle of least privilege

Accepted Answer

“Need to know” principle

Question 27

Domain Structure

[blank_start]Domains as users[blank_end]:
The set of objects depends on the user ID
Domain switching occurs when user changes

[blank_start]Domains as processes[blank_end]:
The set of objects depends on the PID.
Domain switching occurs when a processes send a message to another process and then waits for a response

[blank_start]Domains as procedure[blank_end]:
The set of objects can be the local variables
Domain switching occurs when a procedure call is invoked

Accepted Answer

Domains as users

Accepted Answer

Domains as processes

Accepted Answer

Domains as procedure

Question 28

Access Matrix

Accepted Answer

Y

Answer

.

Question 29

ACL - Access control list

Accepted Answer

Y

Answer

.

Question 30

Capability list

Accepted Answer

Y

Answer

.

Question 31

File Attributes

[blank_start]Name[blank_end] – only information kept in human-readable form
[blank_start]Identifier[blank_end] – unique tag (number) identifies file within file system
[blank_start]Type[blank_end] – needed for systems that support different types
[blank_start]Location[blank_end] – pointer to file location on device
Size – current file size
[blank_start]Protection[blank_end] – controls who can do reading, writing, executing
Time, date, and user identification – data for protection, security, and usage monitoring

Accepted Answer

Name

Accepted Answer

Identifier

Accepted Answer

Location

Accepted Answer

Protection

Accepted Answer

Type

Question 32

Access Methods

Sequential Access:

Simplest access method
Supports a few operations:

[blank_start]read next[blank_end] - read a record and advance the tape to the next position.

[blank_start]write next[blank_end] - write a record and advance the tape to the next position. 
rewind
[blank_start]skip n records[blank_end] - May or may not be supported.

Accepted Answer

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