Bt0070

February 2011
Bachelor of Science in Information Technology (BScIT) – Semester 2/
Diploma in Information Technology (DIT) – Semester 2
BT0070 – Operating Systems – 4 Credits
(Book ID: B0954)
Assignment Set – 1 (60 Marks)

Answer all questions 10 x 6 = 60
1. Explain multi-programmed Batched Operating System.

Ans:- The concept of spooling introduced an important data structure called the job pool.
Spooling creates a number of jobs on the disk which are ready to be executed / processed. The
operating system now has to choose from the job pool, a job that is to be executed next. This
increases CPU utilization. Since the disk is a direct access device, jobs in the job pool may be
scheduled for execution in any order, not necessarily in sequential order. Job scheduling brings
in the ability of multi-programming. A single user cannot keep both CPU and I/O devices busy.
Multiprogramming increases CPU utilization by organizing jobs in such a manner that CPU
always has a job to execute.

The idea of multi-programming can be described as follows. A job pool on the disk consists of
a number of jobs that are ready to be executed (Figure 1.3). Subsets of these jobs reside in
memory during execution. The operating system picks and executes one of the jobs in memory.
When this job in execution needs an I/O operation to complete, the CPU is idle. Instead of
waiting for the job to complete the I/O, the CPU switches to another job in memory. When the
previous job has completed the I/O, it joins the subset of jobs waiting for the CPU. As long as
there are jobs in memory waiting for the CPU, the CPU is never idle. Choosing one out of
several ready jobs in memory for execution by the CPU is called CPU scheduling.

2. Discuss different types of schedulers.

Ans:- Schedulers:- At any given instant of time, a process is in any one of the new, ready,
running, waiting or terminated state. Also a process moves from one state to another as long as
it is active. The operating system scheduler schedules processes from the ready queue for
execution by the CPU. The scheduler selects one of the many processes from the ready queue
based on certain criteria.

Schedulers could be any one of the following:

· Long-term scheduler

· Short-term scheduler

· Medium-term scheduler

Sometimes processes keep switching between ready, running and waiting states with
termination taking a long time. One of the reasons for this could be an increased degree of
multi-programming meaning more number of ready processes than the system can handle. The
medium-term scheduler(Figure 2.4) handles such a situation. When system throughput falls
below a threshold, some of the ready processes are swapped out of memory to reduce the
degree of multi-programming. Sometime later these swapped processes are reintroduced into
memory to join the ready queue.

3. Discuss First Come First Served scheduling algorithm.

Ans:- First Come First Served scheduling algorithm:- This is one of the very brute force
algorithms. A process that requests for the CPU first is allocated the CPU first. Hence, the
name first come first serve. The FCFS algorithm is implemented by using a first-in-first-out
(FIFO) queue structure for the ready queue. This queue has a head and a tail. When a process
joins the ready queue its PCB is linked to the tail of the FIFO queue. When the CPU is idle, the
process at the head of the FIFO queue is allocated the CPU and deleted from the queue. Even
though the algorithm is simple, the average waiting is often quite long and varies substantially
if the CPU burst times vary greatly,

he algorithm also suffers from what is known as a convoy effect. Consider the following
scenario. Let there be a mix of one CPU bound process and many I/O bound processes in the
ready queue.

The CPU bound process gets the CPU and executes (long I/O burst). In the meanwhile, I/O
bound processes finish I/O and wait for CPU, thus leaving the I/O devices idle. The CPU bound
process releases the CPU as it goes for an I/O. I/O bound processes have short CPU bursts and
they execute and go for I/O quickly. The CPU is idle till the CPU bound process finishes the I/
O and gets hold of the CPU.

The above cycle repeats. This is called the convoy effect. Here small processes wait for one big
process to release the CPU. Since the algorithm is non-preemptive in nature, it is not suited for
time sharing systems.

4. What are semaphores? Explain.

Ans:- Semaphores:- Semaphores are the classic method for restricting access to shared
resources (e.g. storage) in a multi-processing environment. They were invented by Dijkstra and
first used in T.H.E operating system.

A semaphore is a protected variable (or abstract data type) which can only be accessed using
the following operations:

The P operation busy-waits until a resource is available whereupon it immediately claims one.
V is the inverse, it simply makes a resource available again after the process has finished using
it. Init is only used to initialize the semaphore before any requests are made. The P and V
operations must be indivisible, i.e. no other process can access the semaphore during their
execution.

To avoid busy-waiting, a semaphore may have an associated queue of processes. If a process
does a P on a semaphore which is zero, the process is added to the semaphore’s queue. When
another process increments the semaphore by doing a V and there are tasks on the queue, one is
taken off and resumed.

Semaphores are not provided by hardware. But they have several attractive properties:

· Machine independent.

· Simple.

· Powerful. Embody both exclusion and waiting.

· Correctness is easy to determine.

· Work with many processes.

· Can have many different critical sections with different semaphores.

· Can acquire many resources simultaneously (multiple P’s).

· Can permit multiple processes into the critical section at once, if that is desirable.

5. Write a note on Resource Allocation Graph.

Ans:- Resource-Allocation Graph:- Deadlocks can be described by a resource allocation
graph. The resource allocation graph is a directed graph consisting of vertices and directed
edges. The vertex set is partitioned into two types, a subset representing processes and another
subset representing resources. Pictorially, the resources are represented by rectangles with dots
within, each dot representing an instance of the resource and circles represent processes.

A directed edge from a process to a resource signifies a request from a process Pi for
an instance of the resource Rj and Pi is waiting for Rj. A directed edge from a resource to a
process indicates that an instance of the resource Rj has been allotted to process Pi.
Thus a resource allocation graph consists of vertices which include resources and processes and
directed edges which consist of request edges and assignment edges. A request edge is
introduced into the graph when a process requests for a resource. This edge is converted into an
assignment edge when the resource is granted. When the process releases the resource, the
assignment edge is deleted. Consider the following system:

There are 3 processes P1, P2 and P3.

Resources R1, R2, R3 and R4 have instances 1, 2, 1, and 3 respectively.

P1 is holding R2 and waiting for R1.

P2 is holding R1, R2 and is waiting for R3.

P3 is holding R3.

The resource allocation graph for a system in the above situation is as shown:

6. What are the reasons for building distributed system?

Ans:- Distributed Systems:- In this type of system all the computations are distributed among
several processors. Distributed systems are also referred as loosely coupled systems because
here the processors do not share memory or a clock. Instead, each processor has its own local
memory. The processors communicate with one another through various communication lines,
such as high speed buses or telephone lines. The processors in distributed system vary in size,
function and are referred as sites, nodes, computers and so on depending on the context in
which they are mentioned.

There are variety of reasons for building distributed systems, the major ones are :

Resource sharing. If different user are connected to one another with different resources, then
the user at one site may be able to use the resources available at another.

Computation speedup. In distributed system, a particular computation will be partitioned into
number of sub computations that can run concurrently. In addition to this, distributed system
may allow us to distribute the computations among various sites. It is called load sharing.

Reliability. It means in distributed system, if one site fails, the remaining sites will share the
work of failed site.

Communication. When many sites are connected to one another by a communication network,
the processes at different sites have the opportunity to exchange information. A User may
initiate file transfer or communicate with one another via electronic mail. A user can send mail
to another user at the same site or at a different site.

7. Explain with a neat diagram all possible states a process visits during the course of its

execution.

Ans:- Process State:- A process being an active entity, changes state as execution proceeds. A
process can be any one of the following states:

· New: Process being created.

· Running: Instructions being executed.

· Waiting (Blocked): Process waiting for an event to occur.

· Ready: Process waiting for CPU.

· Terminated: Process has finished execution.

Locally, the ‘Running’ and ‘Ready’ states are similar. In both cases the process is willing to
run, only in the case of ‘Ready’ state, there is temporarily no CPU available for it. The
‘Blocked’ state is different from the ‘Running’ and ‘Ready’ states in that the process cannot
run, even if the CPU is available. These above states are arbitrary and very between operating
systems. Certain operating systems also distinguish among more finely delineating process
states. It is important to realize that only one process can be running on any processor at any
instant. Many processes may be ready and waiting. A state diagram (Figure 2.1) is used to
diagrammatically represent the states and also the events that trigger the change of state of a
process in execution.

8. Why are Round-Robin Scheduling algorithm designed for time-sharing systems?

Explain.

Ans:- Round-Robin Scheduling:-The round-robin CPU scheduling algorithm is basically a
preemptive scheduling algorithm designed for time-sharing systems. One unit of time is called
a time slice(Quantum). Duration of a time slice may range between 10 msecs and about 100
msecs. The CPU scheduler allocates to each process in the ready queue one time slice at a time
in a round-robin fashion. Hence the name round-robin.

The ready queue in this case is a FIFO queue with new processes joining the tail of the queue.
The CPU scheduler picks processes from the head of the queue for allocating the CPU. The
first process at the head of the queue gets to execute on the CPU at the start of the current time
slice and is deleted from the ready queue. The process allocated the CPU may have the current
CPU burst either equal to the time slice or smaller than the time slice or greater than the time
slice. In the first two cases, the current process will release the CPU on its own and thereby the
next process in the ready queue will be allocated the CPU for the next time slice. In the third
case, the current process is preempted, stops executing, goes back and joins the ready queue at
the tail thereby making way for the next process.

Consider the same example explained under FCFS algorithm.

Let the duration of a time slice be 4 msecs, which is to say CPU switches between processes
every 4 msecs in a round-robin fashion. The Gantt chart below shows the scheduling of
processes.

Average waiting time = (4 + 7 + (10 – 4)) / 3 = 17/ 3 = 5.66 msecs.

If there are 5 processes in the ready queue that is n = 5, and one time slice is defined to be 20
msecs that is q = 20, then each process will get 20 msecs or one time slice every 100 msecs.
Each process will never wait for more than (n – 1) x q time units.

The performance of the RR algorithm is very much dependent on the length of the time slice. If
the duration of the time slice is indefinitely large then the RR algorithm is the same as FCFS
algorithm. If the time slice is too small, then the performance of the algorithm deteriorates
because of the effect of frequent context switching. A comparison of time slices of varying
duration and the context switches they generate on only one process of 10 time units is shown
below..

The above example shows that the time slice should be large with respect to the context switch
time, else, if RR scheduling is used the CPU will spend more time in context switching.

9. Define the following terms.

a. Mutual Exclusion

b. Busy Waiting

c. Critical Section

Ans:- A critical-section is a part of program that accesses a shared resource (data structure or
device) that must not be concurrently accessed by more than one process of execution.
Consider a system consisting of n processes { p0,p1,……pn-1}. Each process has a segment of
code called a critical-section. The important feature of the system is that, when one process is
executing in its critical-section, no other process is to be allowed to execute in its critical-
section. Thus the execution of critical-sections by the processes is mutually exclusive in time.
The critical-section problem is to design a protocol that the processes can use to co-operate.
Each process must request permission to enter its critical-section. The section of the code

implementing this request is the entry section. The critical-section may be followed by an exit
section. The remaining code is the remainder section.
1. Mutual Exclusion: If process pi is executing in its critical-section, then no other
processes can be executing in their critical-sections.
2. Progress: If no process is executing in its critical-section and there exist some
processes that are not executing in their remainder section can participate in the
decision of which will enter in its critical-section next, and this selection cannot be
postponed indefinitely.
3. Bounded Waiting: There exist a bound on the number of times that other processes are
allowed to enter their critical-sections after a process has made a request to enter its
critical-section and before that request is granted.

10. Explain how Banker’s algorithm is used to check safe state of a system.

Ans:- Banker’s Algorithm:- The resource allocation graph algorithm is not applicable where
resources have multiple instances. In such a case Banker’s algorithm is used. A new process
entering the system must make known a priori the maximum instances of each resource that it
needs subject to the maximum available for each type. As execution proceeds and requests are
made, the system checks to see if the allocation of the requested resources ensures a safe state.
If so only are the allocations made, else processes must wait for resources.

The following are the data structures maintained to implement the Banker’s algorithm:

1. n: Number of processes in the system.

2. m: Number of resource types in the system.

3. Available: is a vector of length m. Each entry in this vector gives maximum instances of a
resource type that are available at the instant. Available[j] = k means to say there are k
instances of the jth resource type Rj.

4. Max: is a demand vector of size n x m. It defines the maximum needs of each resource by the
process. Max[i][j] = k says the ith process Pi can request for atmost k instances of the jth
resource type Rj.

5. Allocation: is a n x m vector which at any instant defines the number of resources of each
type currently allocated to each of the m processes. If Allocation[i][j] = k then ith process P i is
currently holding k instances of the jth resource type Rj.

6. Need: is also a n x m vector which gives the remaining needs of the processes. Need[i][j] = k
means the ith process Pi still needs k more instances of the jth resource type Rj. Thus Need[i][j]
= Max[i][j] – Allocation[i][j].

February 2011
Bachelor of Science in Information Technology (BScIT) – Semester 2/
Diploma in Information Technology (DIT) – Semester 2
BT0070 – Operating Systems – 4 Credits
(Book ID: B0954)
Assignment Set – 2 (60 Marks)

Answer all questions 10 x 6 = 60
1. Explain how virtual addresses are mapped on to physical addresses.

Ans:- Memory management strategies like paging and segmentation helps to implement the
concept of multi-programming. But they have a few disadvantages. One problem with the
above strategies is that they require the entire process to be in main memory before execution
can begin. Another disadvantage is the limitation on the size of the process. Processes whose
memory requirement is larger than the maximum size of the memory available, will never be
able to be run, that is, users are desirous of executing processes whose logical address space is
larger than the available physical address space.

Virtual memory is a technique that allows execution of processes that may not be entirely in
memory. In addition, virtual memory allows mapping of a large virtual address space onto a
smaller physical memory. It also raises the degree of multi-programming and increases CPU
utilization. Because of the above features, users are freed from worrying about memory
requirements and availability.

Virtual to physical memory mapping given below:-

2. Discuss the Optimal Page Replacement algorithm.

Ans:- Optimal Algorithm:- An optimal page replacement algorithm produces the lowest page
fault rate of all algorithms. The algorithm is to replace the page that will not be used for the
longest period of time to come. Given a fixed number of memory frame by allocation, the
algorithm always guarantees the lowest possible page fault rate and also does not suffer from
Belady’s anomaly.

Ignoring the first three page faults that do occur in all algorithms, the optimal algorithm is
twice as better than the FIFO algorithm for the given string. But implementation of the optimal
page replacement algorithm is difficult since it requires future a priori knowledge of the
reference string. Hence the optimal page replacement algorithm is more a benchmark algorithm
for comparison.

3. Explain the concept of File.

Ans:- Concept of a File:-Users use different storage media such as magnetic disks, tapes,
optical disks and so on. All these different storage media have their own way of storing
information. The operating system provides a uniform logical view of information stored in
these different media. The operating system abstracts from the physical properties of its storage
devices to define a logical storage unit called a file. These files are then mapped on to physical
devices by the operating system during use. The storage devices are usually non-volatile,
meaning the contents stored in these devices persist through power failures and system reboots.

The concept of a file is extremely general. A file is a collection of related information recorded
on the secondary storage. For example, a file containing student information, a file containing
employee information, files containing C source code and so on. A file is thus the smallest
allotment of logical secondary storage, that is any information to be stored on the secondary
storage need to be written on to a file and the file is to be stored. Information in files could be
program code or data in numeric, alphanumeric, alphabetic or binary form either formatted or
in free form. A file is therefore a collection of records if it is a data file or a collection of bits /
bytes / lines if it is code. Program code stored in files could be source code, object code or
executable code whereas data stored in files may consist of plain text, records pertaining to an
application, images, sound and so on. Depending on the contents of a file, each file has a pre-
defined structure. For example, a file containing text is a collection of characters organized as
lines, paragraphs and pages whereas a file containing source code is an organized collection of
segments which in turn are organized into declaration and executable statements.

4. Explain the execution of RPC.

Ans:- Remote Procedure Call (RPC):- A distributed environment consists of servers and
clients. Server is a computer that offers services of shared resources. Client is a computer that
requests for a shared resource present on the server through a request. A procedure is present
on the server to locate and retrieve data present on a shared device attached to it. This
procedure is part of the operating system running on the server. When a client requests for

some data on the server this procedure on the server operating system is called remotely from
the client. Hence it is called a remote procedure call

5. Write a note on computer virus.

Ans:- A computer virus is written with an intention of infecting other programs. It is a part of a
program that piggybacks on to a valid program. It differs from the worm in the following ways:

· Worm is a complete program by itself and can execute independently whereas virus does not
operate independently. Worm consumes only system resources but virus causes direct harm to
the system by corrupting code as well as data.

Types of Viruses:- There are several types of computer viruses. New types get added every
now and then. Some of the common varieties are:

· Boot sector infectors.

· Memory resident infectors.

· File specific infectors.

· Command processor infectors.

· General purpose infectors.

6. Explain first-fit, best-fit and worst-fit allocation algorithms with an example.

Ans:-
1. First-fit: Allocate the first hole that is big enough to hold the process. Search can either start
at the beginning of the set of holes or at the point where the last search terminated.
2. Best-fit: Allocate the smallest hole that is big enough to hold the process. Here the entire list
has to be searched or an ordered list of holes by size is to be maintained.
3. Worst-fit: Allocate the largest hole available. Here also, the entire list has to be searched for
the biggest hole or an ordered list of holes by size is to be maintained.

The size of a process is very rarely an exact size of a hole allocated. The best-fit allocation
always produces an optimal allocation where the hole left over after allocation is the smallest.
The first-fit is the fastest method of allocation when compared to others. The worst-fit
allocation is the worst among the three and is seldom used.
Scheduling example is given below:-

Total memory available: 2560K

Resident operating system: 400K

Memory available for user: 2560 – 400 = 2160K

Job queue: FCFS

CPU scheduling: RR (1 time unit)

Given the memory map in the illustration above, P1, P2, P3 can be allocated memory
immediately. A hole of size (2160 – (600 + 1000 + 300)) = 260K is left over which cannot
accommodate P4 (Figure 6.6a). After a while P2 terminates creating the map of Figure 6.6b. P4
is scheduled in the hole just created resulting in Figure 6.6c. Next P 1 terminates resulting in
Figure 6.6d and P5 is scheduled as in Figure 6.6e

7. Explain demand paging virtual memory system.

Ans:- Demand paging:-The protection valid-invalid bit which is used in paging to determine
valid / invalid pages corresponding to a process is used If the valid-invalid bit is set, then the
corresponding page is valid and also in physical memory. If the bit is not set, then any of the
following can occur:

· Process is accessing a page not belonging to it, that is, an illegal memory access.

· Process is accessing a legal page but the page is currently not in memory.

If the same protection scheme as in paging is used, then in both the above cases a page fault
error occurs. The error is valid in the first case but not in the second because in the latter a legal
memory access failed due to non-availability of the page in memory which is an operating
system fault.

1. Check the valid-invalid bit for validity.

2. If valid, then the referenced page is in memory and the corresponding physical address is
generated.

3. If not valid then, an addressing fault occurs.

4. The operating system checks to see if the page is in the backing store. If present, then the
addressing error was only due to non-availability of page in main memory and is a valid page
reference.

5. Search for a free frame.

6. Bring in the page into the free frame.

7. Update the page table to reflect the change.

8. Restart the execution of the instruction stalled by an addressing fault.

In the initial case, a process starts executing with no pages in memory. The very first
instruction generates a page fault and a page is brought into memory.

Page table with valid-invalid bit

· Secondary memory to hold pages not currently in memory, usually a high speed disk known
as a swap space or backing store.

A page fault at any point in the fetch-execute cycle of an instruction causes the cycle to be
repeated

8. Explain different operations possible on Files.

Ans:- Operations on Files:- A file is an abstract data type. Six basic operations are possible on
files. They are:

1. Creating a file: two steps in file creation include space allocation for the file and an entry to
be made in the directory to record the name and location of the file.

2. Writing a file: parameters required to write into a file are the name of the file and the
contents to be written into it. Given the name of the file the operating system makes a search in
the directory to find the location of the file. An updated write pointer enables to write the
contents at a proper location in the file.

3. Reading a file: to read information stored in a file the name of the file specified as a
parameter is searched by the operating system in the directory to locate the file. An updated
read pointer helps read information from a particular location in the file.

4. Repositioning within a file: a file is searched in the directory and a given new value replaces
the current file position. No I/O takes place. It is also known as file seek.

5. Deleting a file: The directory is searched for the particular file, If it is found, file space and
other resources associated with that file are released and the corresponding directory entry is
erased.

6. Truncating a file: file attributes remain the same, but the file has a reduced size because the
user deletes information in the file. The end of file pointer is reset.

Other common operations are combinations of these basic operations. They include append,
rename and copy. A file on the system is very similar to a manual file. An operation on a file is
possible only if the file is open. After performing the operation, the file is closed. All the above
basic operations together with the open and close are provided by the operating system as
system calls.

9. Explain how applications and data can be distributed.

Ans:- Distributed Applications:- Distributed applications mean different programs on
different computers. This scheme allows the possibility of data capture at the place of its origin.
Connections between these computers then allow this data to be shared. Programs / applications
could be distributed in two ways. They are:

· Horizontal distribution

· Vertical / hierarchical distribution

Distribution of Data

In a distributed environment, data can also be distributed similar to distribution of programs.
Data for applications could be maintained as:

· Centralized data

· Replicated data

· Partitioned data

In centralized data, data resides only at one central computer that can be accessed or shared by
all other computers in the network. For example, master database. This central computer must
run an operating system that implements functions of information management. It must keep
track of users and their files and handle data sharing, protection, disk space allocation and other
related issues. It must also run a front-end software for receiving requests / queries from other
computers for data. These requests are then serviced one by one. It is because of this software
that this central computer is called a server. Computers connected to the server can have their
own local data but shared data has to necessarily reside in the server. In a distributed
environment, part of the master database could be centralized and the rest distributed among
the connecting computers. Data could be distributed in a partitioned way. The entire database is
sliced into many parts. Each part of the database then resides on a computer. Processing
depends upon the kind of data distribution. Any other computer wanting to access information
data present not locally but at a remote site must send a query and receive the contents needed.
If such is the case then each computer will run front-end software to receive queries and act a
server for the data stored in it.

10. How the protection is implemented using the concept of domains?

Ans:- Protection is implemented:- System resources need to be protected. Resources include
both hardware and software. Different mechanisms for protection are as follows: Files need to
be protected from unauthorized users. The problem of protecting files is more acute in multi-
user systems. Some files may have only read access for some users, read / write access for
some others, and so on. Also a directory of files may not be accessible to a group of users. For
example, student users do not access to any other files except their own. Like files devices,
databases, processes also need protection. All such items are grouped together as objects. Thus
objects are to be protected from subjects who need access to these objects.

The operating system allows different access rights for different objects. For example, UNIX
has read, write and execute (rwx) rights for owners, groups and others. Possible access rights
are listed below:

· No access

· Execute only

· Read only

· Append only

· Update

· Modify protection rights

· Delete

A hierarchy of access rights is identified. For example, if update right is granted then it is
implied that all rights above update in the hierarchy are granted. This scheme is simple but
creation of a hierarchy of access rights is not easy. It is easy for a process to inherit access
rights from the user who has created it. The system then need maintain a matrix of access rights
for different files for different users.

Bt0070

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