Fundamentals of Operating Systems (COMP355/L) · PDF file1 Fundamentals of Operating Systems (COMP355/L) A Student's Manual for Practice Exercises Text Book: Operating System Concepts

1

Fundamentals of Operating

Systems (COMP355/L)

A Student's Manual

for Practice Exercises

Text Book: Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013

Practice Exercises #1 Chapter 1 Introduction Page 2

Dr. Mohamed Aissa Student's Manual for Practice Exercises COMP355 Fundamentals of Operating Systems

Practice Exercises #1 Introduction

STUDENT LEARNING OUTCOMES:

Upon completion of this assignment, a successful student will be able to:

define an operating system,

define the tasks of an operating system and its elements

1. What is an operating system?

2. What are the operating system goals?

3. What are the main components of a computer system?

Questions



4. What are the three main purposes of an operating system?

5. We have stressed the need for an operating system to make efficient use of the computing

hardware. When is it appropriate for the operating system to forsake this principle and to “waste”

resources? Why is such a system not really wasteful?

6. What is the main difficulty that a programmer must overcome in writing an operating system for a

real-time environment?



7. How does the distinction between kernel mode and user mode function as a rudimentary form of

protection (security) system?

8. Which of the following instructions should be privileged?

a. Set value of timer.

b. Read the clock.

c. Clear memory.

d. Issue a trap instruction.

e. Turn off interrupts.

f. Modify entries in device-status table.

g. Switch from user to kernel mode.

h. Access I/O device.

9. Some early computers protected the operating system by placing it in a memory partition that

could not be modified by either the user job or the operating system itself. Describe two difficulties

that you think could arise with such a scheme.



10. Some CPUs provide for more than two modes of operation. What are two possible uses of these

multiple modes?

11. Timers could be used to compute the current time. Provide a short description of how this could

be accomplished.



12. Answer to the following questions:

a. Give two reasons why caches are useful.

b. What problems do they solve?

c. What problems do they cause?

d. If a cache can be made as large as the device for which it is caching (for instance, a cache as

large as a disk), why not make it that large and eliminate the device?

Practice Exercises #2 Chapter 2 Operating-System Structures Page 7 `


Assignment #2 Operating-System Structures



To describe the services an operating system provides to users, processes, and other systems

To discuss the various ways of structuring an operating system

To explain how operating systems are installed and customized and how they boot

1. What are the fundamental approaches for users to interface with the operating system. Explain these

approaches briefly.

2. What is the purpose of system calls?

Exercises



3. What are the five major activities of an operating system with regard to process

management?

4. What are the three major activities of an operating system with regard to memory

management?

5. What are the three major activities of an operating system with regard to secondary-

storage management?



6. What is the purpose of the command interpreter? Why is it usually separate from the

kernel?

7. What system calls have to be executed by a command interpreter or shell in order to

start a new process?

8. What is the purpose of system programs?



9. What is the main advantage of the layered approach to system design?

10. List five services provided by an operating system, and explain how each creates

convenience for users. In which cases would it be impossible for user-level programs

to provide these services? Explain your answer.



11. Why do some systems store the operating system in firmware, while others store it

on disk?

12. How could a system be designed to allow a choice of operating systems from which

to boot?

What would the bootstrap program need to do?

13. What is the bootstrap program?



14. Operating systems provide an environment for execution of programs and services to

programs and users. Fill in the following table with the appropriate OS service

description.

# Operating

System Services Description

1. User interface

2. Program execution

3. I/O operations

4. File-system

manipulation

5. Communications

6. Error detection

7. Resource allocation

8. Accounting

9. Protection and

security

Practice Exercises #3 Chapter 3 Processes Page 13 `


Assignment #3 Processes



To introduce the notion of a process -- a program in execution, which forms the basis of all

computation

To describe the various features of processes, including scheduling, creation and termination, and

communication

To explore inter-process communication using shared memory and message passing

1. What is a process?

2. What are the main components of a process?

Exercises



3. What is the difference between a program and a process?

4. What are the used technics for loading executable files?

5. What are the process states?

6. What is a process control block (PCB)?



7. A PCB contains many pieces of information associated with a specific process. State

some of these pieces of information?



8. What is the objective of multiprogramming?

9. What is the objective of time sharing?

10. How the objective of time sharing can be met?

11. What is a job queue?



12. What is a ready queue? Where is stored? What does it contain?

13. What is a queueing diagram? Describe it briefly.

14. Describe process scheduling.



15. What is the difference between I/O-bound and CPU-bound processes?

16. How the number of the I/O-bound and CPU-bound processes should selected by the long-

term scheduler? Justify your answer.



17. Fill in the following table with the words "high" or "low".

Process Type Spent Time on I/O Spent Time on Computations

I/O-bound process

CPU-bound process

Practice Exercises #4 Chapter 4 CPU Scheduling Page 20 `


Assignment #4 CPU Scheduling



introduce CPU scheduling, which is the basis for multiprogrammed operating systems

describe various CPU-scheduling algorithms

discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system

examine the scheduling algorithms of several operating systems

1. How many processes can run at a time in a single-processor system?

2. What is the objective of multiprogramming?

Exercises



3. A process is executed until it must wait, typically for the completion of some I/O request.

a. What happens in such situation in a simple computer system?

b. What happens in such situation in a multiprogramming system?

4. Almost all computer resources are scheduled before use. The CPU is, of course, one of the

primary computer resources. How scheduling is assured?

5. Process execution consists of some operations.

a. What are they?

b. How are they organized?



6. Let's consider the Histogram of CPU-burst durations depicted in the following figure:

a. What is the type of the Histogram of CPU-burst durations?

b. Describe short and long CPU bursts.

(Source: Operating System Concepts – 9th

Edition)



c. Describe the distribution of short and long CPU bursts in I/O-bound and CPU-

bound programs.

d. What is the importance of this distribution?



7. Let's consider the case when the CPU becomes idle.

a. From where the operating system must select one of the processes ?

b. What is the role of short-term scheduler?



8. Under which circumstances, CPU-scheduling decisions may take place? When there is a

choice for scheduling?

9. When can we say that the scheduling scheme is nonpreemptive or cooperative?



10. When can we say that the scheduling scheme is preemptive?

11. Explain nonpreemptive scheduling.

12. What are the disadvantages of preemptive scheduling? Give some examples.



13. What is a dispatcher? What are its functions?

14. What is a dispatch latency?

15. What are the criteria for comparing CPU scheduling algorithm?



16. Explain how the criteria for comparing CPU scheduling algorithm should be manipulated?



17. What is CPU scheduling?

18. State the most important scheduling algorithms in a system with a single processor.

Practice Exercises #5 Chapter 5 Deadlocks Page 30 `


Assignment #5 Deadlocks



identify system resources,

identify system resources types,

define deadlock conditions

1. A system consists of a finite number of resources to be distributed among a number of

competing processes. The resources are partitioned into several types. State some examples of these

types.

2. A system consists of a finite number of resources to be distributed among a number of

competing processes. The resources are partitioned into several types. State some examples of these

types. Each consisting of some number of identical instances. State an example of these instances.

3. If a process requests an instance of a resource type, then the allocation of any instance of the

type will satisfy the request. If it will not, then what can you conclude?

Exercises



4. What are the events with which we are mainly concerned when we are with deadlocks?

5. When can we say that a set of processes is in a deadlock state?

6. What type of resources can we have in a system?

7. When a deadlock situation can arise?



Assignment #6 Deadlock Avoidance Safe State



define the conditions of safe and unsafe system

apply the safe and unsafe conditions to examine and identify the state of the system

(safe or unsafe),

define deadlock conditions

A state is safe if the system can allocate resources to each process (up to its maximum) in some order and still

avoid a deadlock.

More formally, a system is in a safe state only if there exists a safe sequence.

A sequence of processes is a safe sequence for the current allocation state if, for each , the

resource requests that can still be satisfied by the currently available resources plus the resources held by all

, with . In this situation, if the resources that needs are not immediately available, then , can wait

until all have finished.

When they have finished, can obtain all of its needed resources, complete its designated task, return its

allocated resources, and terminate. When , terminates, can obtain its needed resources, and so on.

If no such sequence exists, then the system state is said to be unsafe.

Figure 1 Safe, unsafe, and deadlock state spaces

Reminder

Exercises



1. Consider a system with 12 magnetic tape drives and three processes: . The

maximum needs by these processes is depicted in the following table.

Suppose that, at time , these are holding number of tape drives depicted in this table (Thus, there are

3 free tape drives).

a. Explain the state of the system (safe or unsafe) at time . Justify your answer and fill in the

following table.

Processes Maximum Needs Current Needs at Allocated Resources at Available Resources at

4 2

10 5

9 2

Available Tapes: At time , the system is in ______________ state

Processes Maximum Needs Current Needs at Current Needs at

10 5 5

4 2 2

9 2 3

Exercises



b. System at time

Suppose that, at time process requests and is allocated one more tape drive (3).

Processes Maximum Needs Current Needs at Allocated Resources at Available Resources at

10 2

4 5

9 3

Available Tapes: At time , the system is in ______________ state

c. What can you deduce?



2. Consider a system with 12 magnetic tape drives and three processes: . The

maximum needs by these processes is depicted in the following table.

Suppose that, at time , these are holding number of tape drives depicted in this table (Thus, there are

3 free tape drives).

Processes Maximum Needs Current Needs at Current Needs at

10 5 5

4 2 2

9 2 3

d. System at time

At time , the system is in a safe state. The sequence satisfies the safety condition.

Process can immediately be allocated all its tape drives and then return them (the system will then

have 5 available tape drives); then process can get all its tape drives and return them (the system

will then have 10 available tape drives); and finally process can get all its tape drives and return

them (the system will then have all 12 tape drives available).

Processes Maximum

Needs

Current Needs at

Will need Available Resources after allocation

4 2 2 (3-2)+4=5

10 5 5 (5-5)+10=10

9 2 7 (10-7)+9=12

Available Tapes: 3 The sequence t time , the

system is in safe state

e. System at time

Suppose that, at time process requests and is allocated one more tape drive (3).

Processes Maximum

Needs

Current Needs at

Will need

(max –current)

current

Available Resources after allocation

(available-wneed)+max

4 2 2 (2-2)+4=4

10 5 5 needs 5 so cannot be satisfied

9 3 6 needs 6 so cannot be satisfied

Available Tapes: 2 The sequence t time , the

system is in unsafe state

Exercises



(*) The system is no longer in a safe state. At this point, only process

, can be allocated all its tape drives. When it returns them, the system will have only 4 available

tape drives.

Since process , is allocated 5 tape drives but has a maximum of 10, it may request 5 more tape

drives. Since they are unavailable, process must wait.

Similarly, process may request an additional 6 tape drives and have to wait, resulting in a

deadlock.

f. What can you deduce?

The mistake was in granting the request from process for one more tape drive. If we had made

wait until either of the other processes had finished and released its resources, then we could have

avoided the deadlock.



Assignment #7 Banker's Algorithm



Apply Banker's algorithm (Deadlock-avoidance algorithm)

Apply Safety Algorithm for finding out whether or not a system is in a safe state

Apply Resource-Request Algorithm which determines if requests can be safely granted.

1. Consider the following snapshot at for a system that has the following resources depicted in

the following table:

# of Instances

Resource

# of Instances

9 4 6

And the following processes de depicted in the following table:

Processes

Current

Allocation

Maximum

Need

Current

Need

(Max-

Current)

(shown

in order)

Work+Allocation

(According to sequence)

A B C A B C A B C Total A B C

3 0 1 4 2 3 1 2 2 F 3 T 534+301=835 8 3 5

1 1 1 7 2 2 6 1 1 F 4 T 835+111=946 9 4 6

2 2 0 3 3 1 1 1 1 F 2 T 314+220=534 5 3 4

2 1 2 3 1 3 1 0 1 F 1 T 102+212=314 3 1 4

Total 8 4 4

Current Available = 946-844=102

Work=Available= 102

Current Available= 946-844=102

Work=Available=102

Exercises



: 102+212=314

: 314+220=534

: 534+301=835

: 835+111=946

The system is safe at because there is a safety sequence

2. Can the request (1,0,1) by be granted at ? Justify your answer.

(True) (

(True) (

Available=Available- = 102-101=001

= - = 101-101=000

= = 212+101=313

Work=Available= 001

Processes

Current

Allocation

Maximum

Need

Current

Need

(Max-

Current)

(shown in

order)

Work+Allocation


A B C A B C A B C Total

A B C

3 0 1 4 2 3 1 2 2 F 3 T 534+301=835 8 3 5

1 1 1 7 2 2 6 1 1 F 4 T 835+111=946 9 4 6

2 2 0 3 3 1 1 1 1 F 2 T 314+220=534 5 3 4

3 1 3 3 1 3 0 0 0 F 1 T 001+313=314 3 1 4

The request (1,0,1) by can be granted at because the system will remain safe. The safety

sequence is



New Exercise

1. Consider the following snapshot at for a system that has the following resources depicted in

the following table:

# of Instances

Resource

# of Instances

10 5 7

And the following processes de depicted in the following table:

Processes

Current

Allocation

Maximum

Need

Current

Need

(Max-

Current)

(shown

in order)

Work+Allocation



0 1 0 7 5 3 7 4 3 F

2 0 0 3 2 2 1 2 2 F

3 0 2 9 0 2 6 0 0 F

2 1 1 2 2 2 0 1 1 F

Total 7 2 3


Work=Available= 334

Current Available= 946-844=102

Work=Available=102



2-

requests [3,3,0]

Processes

Current

Allocation

Maximum

Need

Current

Need

(Max-

Current)

(shown

in order)

Work+Allocation



0 1 0 7 5 3 7 4 3 F

2 0 0 3 2 2 1 2 2 F

3 0 2 9 0 2 6 0 0 F

5 4 1 2 2 2 0 1 1 F

Total 8 4 2


Work=Available= 215



Assignment #7 Resource-Allocation and Wait-for Graphs



examine and investigate deadlock detection algorithms,

identify the different used methods to detect deadlocks,

apply deadlock detection algorithms and methods to determine the presence of deadlocks

identify the conditions which should be provided in order to draw the corresponding wait-for

graph

use wait-for graph to determine the presence or the absence of deadlocks in a system

Deadlock Detection Algorithm

If all resources have only a single instance, then we can define a deadlock detection algorithm that

uses a variant of the resource-allocation graph, called a wait-for graph.

We obtain this graph from the resource-allocation graph by removing the resource nodes and

collapsing the appropriate edges.

As before, a deadlock exists in the system if and only if the wait-for graph contains a cycle.

To detect deadlocks, the system needs to maintain the wait-for graph and periodically invoke an

algorithm that searches for a cycle in the graph.

An algorithm to detect a cycle in a graph requires an order of operations, where n is the number of

vertices in the graph.

1. We propose that all resources have only a single instance, then we can define a deadlock

detection algorithm that uses a variant of the resource-allocation graph, called a wait-for

graph.

a. What are the conditions which should be verified in order to draw this Resource-Allocation

Graph the corresponding Wait-for Graph.

b. If the conditions are verified, draw the corresponding Wait-for Graph

c. Write the cycles.

Observations

Wait-For Graph (WFG) is the same as the RAG with the resource elements stripped out.

Reminder



The cycles can be seen more clearly in this form of the graph.

There is deadlock in the system if and only if there exists a cycle or a knot in the wait-for-graph.

With about half the number of nodes and edges, the WFG requires only about half the storage of a

SRAG.

However, WFGs can only be used for SRAGs with single-instance resources.

Resources for SRAGs can have any number of instances; However, one cannot create WFGs from

these.

1. We propose the following resource-allocation graph, and we also propose that all resources have

only a single instance.

Resource-Allocation Graph

a. State the conditions which should be provided in order to draw the corresponding wait-for

graph?

Exercises



b. If these condition are provided draw the corresponding wait-for graph?

c. State the cycles if they exist.

d. What can you conclude?



2. We propose the following resource-allocation graph, and we also propose that all resources have

only a single instance.

Resource-Allocation Graph

a. State the conditions which should be provided in order to draw the corresponding wait-for

graph?

b. If these condition are provided draw the corresponding wait-for graph?



c. State the cycles if they exist.

d. What can you conclude?

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Fundamentals of Operating Systems (COMP355/L) · PDF file1 Fundamentals of Operating Systems (COMP355/L) A Student's Manual for Practice Exercises Text Book: Operating System Concepts