SYIT

Operating Systems
 Lecture 1: Introduction to Operating Systems
• An operating system is a program that acts as an
intermediary between a user of a computer and the
computer hardware.
• The purpose of an operating system is to provide an
environment in which a user can execute programs.
The primary goal of an operating system is thus to
make the computer system convenient to use.
• A secondary goal is to use the computer hardware
in an efficient manner.
 Lecture 1: Introduction to Operating Systems
• In brief, an operating system is the set of
that controls a computer. Some examples of
programs
operating systems are UNIX, Mach, MS-DOS, MS-

Windows, Windows/NT, OS/2 and MacOS.

• An operating system is an important part of almost
every computer system.
• A computer system can be divided roughly into four
components: the hardware, the operating system,
the application programs and the users (Figure 1.1).
 Objectives of Operating Systems
• To hide details of hardware by creating abstraction.

• To allocate resources to processes

(Manage resources).

• Provide a pleasant and effective user interface.

 History of Operating Systems
• The 1940's - First Generations
The earliest electronic digital had no operating systems.
computersof the time were so primitive that programs were often entered
Machines
one bit at time on rows of mechanical switches (plug boards).
Programming languages were unknown (not even assembly languages).
Operating systems were unheard of.

• The 1950's - Second Generation

By the early 1950's, the routine had improved somewhat with the
introduction of punch cards. The General Motors Research Laboratories
implemented the first operating systems in early 1950's for their IBM 701.
The system of the 50's generally ran one job at a time.
 History of Operating Systems
• The 1960's - Third Generation
The systems of the 1960's were also batch processing systems, but they
were able to take better advantage of the computer's resources by running
several jobs at once.

• Fourth Generation
With the development of LSI (Large Scale Integration) circuits, chips,
operating system entered in the personal computer and the workstation
age. Microprocessor technology evolved to the point that it becomes
possible to build desktop computers as powerful as the mainframes of the
1970s.
 Lecture 2: Operating Systems Structure

• System Components

• Operating Systems Services

• System Calls and System Programs

 System Components
• Process Management
A process is only ONE instant of a program in execution.
There are many processes can be running the same program.

The five major activitiesof an operating system in regard to

process management are:
• Creation and deletion of user and system processes.
• Suspension and resumption of processes.
• A mechanism for process synchronization.
• A mechanism for process communication.
• A mechanism for deadlock handling.
 System Components
• Main-Memory Management
Main-Memory is a large array of words or bytes. Each word or byte
has its own address. Main memory is a repository of quickly accessible
data shared by the CPU and I/O devices.

The major activities of an operating system in regard to memory-management

are:
• Keep track of which part of memory are currently being used and by whom.
• Decide which processes are loaded into memory when memory
space becomes available.
• Allocate and deallocate memory space as needed.
 System Components
• File Management
A file is a collected of related information defined by its
creator. Computer can store files on the disk (secondary
storage), which provide long term storage.

• The creation and deletion of files.

• The creation and deletion of directions.
• The support of primitives for manipulating files and directions.
• The mapping of files onto secondary storage.
• The backup of files on stable storage media.
 System Components
• I/O System Management

One of the purposes of an operating system is to hide the

peculiarities of specific hardware devices from the user.

• Secondary-Storage Management

Generally speaking, systems have several levels of

storage, including primary storage, secondary storage and
cache storage. Instructions and data must be placed in primary
storage or cache to be referenced by a running program.
 System Components
• Networking
A distributed system is a collection of processors that do not share
memory, peripheral devices, or a clock. The processors communicate with
one another through communication lines called network.

• Protection System
Protection refers to mechanism for controlling the access of
programs, processes, or users to the resources defined by a computer
system.

• Command Interpreter System

A command interpreter is an interface of the operating system with
the user. The user gives commands with are executed by operating system
(usually by turning them into system calls).
 Operating Systems Services
• Program Execution
The system must be able to load a program into memory and to run
it. The program must be able to end its execution, either normally or
abnormally (indicating error).
• I/O Operations
A running program may require I/O. This I/O may involve a file or an
I/O device.
• File System Manipulation
The output of a program may need to be written into new files or
input taken from some files. The operating system provides this service.
• Error Detection
An error is one part of the system may cause malfunctioning of the
complete system. To avoid such a situation the operating system constantly
monitors the system for detecting the errors.
 System Calls and System Programs
• System calls provide the interface between a process and the
operating system. These calls are generally available as
assembly-language instructions, and are usually listed in the
manuals used by assembly-language programmers.
 Lecture 3: Process Management
• The operating system is responsible for the following activities in
connection with process management: the creation and deletion of both
user and system processes; the scheduling of processes; and the provision
of mechanisms for synchronization, communication, and deadlock
handling for processes.

Process, on the other hand, includes:

• Current value of Program Counter (PC)
• Contents of the processors registers
• Value of the variables
• The processes stack (SP) which typically contains temporary data such as
subroutine parameter, return address, and temporary variables.
• A data section that contains global variables.
• Process State
As a process executes, it changes state. The state of a
process is defined in part by the current activity of that
process. Each process may be in one of the following states:

• New State: The process being created.

• Running State: A process is said to be running if it has the CPU, that is,
process actually using the CPU at that particular instant.
• Blocked (or waiting) State: A process is said to be blocked if it is waiting for
some event to happen such that as an I/O completion before it can
proceed. Note that a process is unable to run until some external event
happens.
• Ready State: A process is said to be ready if it is waiting to be assigned to a
processor.
• Terminated state: The process has finished execution.
Figure : Diagram of process states.
• Process Control Block
• Each process is represented in the operating system
by a process control block PCS)—also called a task control
block.
Process state
process number
program counter

Registers
memory limits
list of open files
.
.

Figure : Process control block.

 Lecture 4: CPU Scheduling
• CPU scheduling is the basis of multiprogrammed operating
systems. By switching the CPU among processes, the operating
system can make the computer more productive.

• Basic Concepts

The idea of multiprogramming is relatively simple. A

process is executed until it must wait, typically for the
completion of some I/O request. In a simple computer system,
the CPU would then just sit idle.

Scheduling is a fundamental operating-system

function.
Almost all computer resources are scheduled before use.
• CPU - I/O Burst Cycle
The success of CPU scheduling depends on the following
observed property of processes: Process execution consists of
a cycle of CPU execution and I/O wait. Processes alternate
back and forth between these two states.
• Context Switch
To give each process on a multiprogrammed machine a
fair share of the CPU, a hardware clock generates interrupts
periodically.
This allows the operating system to schedule all processes
in main memory (using scheduling algorithm) to run on the
CPU at equal intervals. Each switch of the CPU from one
process to another is called a context switch.
• Preemptive Scheduling

CPU scheduling decisions may take place under

the following four circumstances:

1. When a process switches from the running state to the waiting state (for.
example, I/O request, or invocation of wait for the termination of one of
the child processes).

2. When a process switches from the running state to the ready state (for
example, when an interrupt occurs).

3. When a process switches from the waiting state to the ready state (for
example, completion of I/O).

4. When a process terminates.

• Dispatcher

• Switching context.
• Switching to user mode.
• Jumping to the proper location in the user program to restart that program

• Scheduling Criteria

• Different CPU scheduling algorithms have different properties and may

favor one class of processes over another. In choosing which algorithm to
use in a particular situation, we must consider the properties of the
various algorithms.
• Many have been suggested for comparing
scheduling
criteria algorithms.
CPU

• Criteria that are used include the following:

• CPU utilization.
• Throughput.
• Turnaround time.
• Waiting time.
• Response time.
 Lecture 5: Scheduling Algorithms

1. First-Come, First-Served Scheduling

2. Shortest-Job-First Scheduling

3. Priority Scheduling

4. Round-Robin Scheduling

5. Multilevel Queue Scheduling

6. Multilevel Feedback Queue Scheduling

First-Come, First-Served Scheduling

Process Burst Time

P1 24
P2 3
P3 3

P1 P2 P3
0 24 27 30
 Shortest-Job-First Scheduling
Process Burst Time

P1 6
P2 8
P3 7
P4 3

P4 P1 P3 P2
0 3 9 16 24
 Priority Scheduling

Process Burst Time Priority

P1 10 3
P2 1 1
P3 2 3
P4 1 4
P5 5 2

P2 P5 P1 P3 P4
Round-Robin Scheduling

Process Burst Time

P1 24
P2 3
P3 3
Multilevel Queue Scheduling
• In a multilevel queue scheduling processes are permanently
assigned to one queues.
• The processes are permanently assigned to one
another, based on some property of the process, such as
• Memory size
• Process priority
• Process type

• Algorithm chooses the process from the occupied queue that

has the highest priority, and run that process either
• Preemptive or
• Non-preemptively
 6. Process Synchronization
• A cooperating process is one that can affect or be
affected by the other processes executing in the
system.
• Cooperating processes may either directly share a
logical address space(that is, both code and data), or
be allowed to share data only through files. The
former case is achieved through the use of
lightweight processes or threads. Concurrent access
to shared data may result in data inconsistency.
• In this lecture, we discuss various mechanisms to
ensure the orderly execution of cooperating
processes that share a logical address space, so that
data consistency is maintained.
 Cooperating Processes
• The concurrent processes executing in the operating
system may be either independent processes or
cooperating processes.
• A process is independent if it cannot affect or be
affected by the other processes executing in the
system.
• On the other hand, a process is cooperating if it can
affect or be affected by the other processes executing
in the system.
• There are several reasons for providing an environment that
allows process cooperation:

Information sharing

Computation speedup

Modularity

Convenience
 Race condition
• When several processes access and
manipulate the same data concurrently and
the outcome of the execution depends on the
particular order in which the access takes
place, is called a race condition.
 The Critical-Section Problem
• 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 cooperate.
• Each process must request permission to enter its
critical section.
• A solution to the critical-section problem must satisfy
the following three requirements:
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 wish to enter their critical
sections, then only those processes that are not executing in
their remainder section can participate in the decision of
which will enter 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.
 DEADLOCK
• A process requests S
resources; if the resources are not
available at that time, the process enters a wait state. It may
happen that waiting processes will never again change state,
• because the resources they have requested are held by other
waiting processes. This situation is called a deadlock.

• In this lecture, we describe methods that an operating system

can use to deal with the deadlock problem.
 Resource
• A process must requestsa resource before using it, and must
release the resource after using it.
• A process may request as many resources as it requires
to carry out its designated task.
• a process may utilize a resource in only the following
sequence:

Request Use

Release
Deadlock Characterization

• In a deadlock, processes never finish executing and

system resources are tied up, preventing other jobs
from ever starting.
• Before we discuss the various methods for dealing
with the deadlock problem, we shall describe
features that characterize deadlocks.
 Necessary Conditions
• A deadlock situation can arise if the following
four conditions hold simultaneously in a system:
• Mutual exclusion
• Hold and wait
• No preemption
• Circular wait
 Methods for Handling Deadlocks
• Principally, there are three different methods
for dealing with the deadlock problem:
• We can use a protocol to ensure that the system will never
enter a deadlock state.
• We can allow the system to enter a deadlock state and then
recover.
• Ignore the problem and pretend that deadlocks never
occur in the system; used by most operating systems,
including UNIX.
 Deadlock Prevention
• By ensuring that at least one of these conditions cannot hold,
we can prevent the occurrence of a deadlock
 Mutual Exclusion – not required for sharable resources; must hold for
nonsharable resources.
 Hold and Wait – must guarantee that whenever a process requests a
resource, it does not hold any other resources.
 No Preemption – o If a process that is holding some resources requests
another resource that cannot be immediately allocated to it, then all
resources currently being held are released.
• Circular Wait – impose a total ordering of all resource types, and require
that each process requests resources in an increasing order of
enumeration.
 Deadlock Avoidance
• Requires that the system has some additional a priori
information available.
 Simplest and most useful model requires that each process declare
the maximum number of resources of each type that it may need.
 The deadlock-avoidance algorithm dynamically examines the
resource-allocation state to ensure that there can never be a
circular-wait condition.
 Resource-allocation state is defined by the number of available and
allocated resources, and the maximum demands of the processes.
 Deadlock Detection
• If a system does not employ either a deadlock-prevention or a
deadlock avoidance algorithm, then a deadlock situation may
occur. In this environment, the system must provide:

 An algorithm that examines the state of the system to

determine whether a deadlock has Occurred.

 An algorithm to recover from the deadlock

Memory Management
 Program must be brought (from disk) into memory
and placed within a process for it to be run.

 Main memory and registers are only storage CPU can

access directly. Register access in one CPU clock (or
less).

 Main memory can take many cycles.

 Cache sits between main memory and CPU registers.

 Protection of memory required to ensure correct

operation.
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