ch1.ppt

Chapter 1: Introduction
Operating System Concepts

What happens when a program runs?
Well, a running program does one very simple thing: it executes instructions. Many
millions (and these days, even billions) of times every second, the processor fetches
an instruction from memory, decodes it (i.e., figures out which instruction this is),
and executes it (i.e., it does the thing that it is supposed to do, like add two numbers
together, access memory, check a condition, jump to a function, and so forth). After
it is done with this instruction, the processor moves on to the next instruction, and
so on, and so on, until the program finally completes.
While a program runs, a lot of other wild things are going on with the primary goal of
making the system easy to use.
There is a body of software, in fact, that is responsible for making it easy to run
programs, allowing programs to share memory, enabling programs to interact with
devices.
This body is operating system (OS)

What is an Operating System?
 A program that acts as an intermediary between a user of
a computer and the computer hardware.
 A program that virtualize resources
 Operating system goals:
 Make the computer system convenient to use.
 Efficient use of the computer system’s resources
 Prevention of interference with users’ activities.
Virtualization: That is, the OS takes a physical resource (such as the processor,
or memory, or a disk) and transforms it into a more general, powerful, and easy-
to-use virtual form of itself. Thus, sometimes operating system refer to as a virtual
machine.

Right Combination while Designing OS
Efficient use is more important when a computer system
is dedicated to specific app Like Main Frame
User convenience is more important in personal
computers
While both are equally important when a computer
system is shared by several users
Hence, the designer aims for the right combination of
efficient use and user convenience for the operating
system’s environment.
Prevention of interference is mandatory in all environments.

HOW TO VIRTUALIZE RESOURCES
Why the OS does this is not the main question, as the answer is obvious:
it makes the system easier to use.
The objective of the subject is to find the answer of HOW ?
How does the operating system virtualize resources? This is the crux of
our Subject.
Thus, focus on the how: what mechanisms and policies are implemented by
the OS to attain virtualization? How does the OS do so efficiently? What
hardware support is needed?

Computer System Components
1. Hardware – provides basic computing resources (CPU,
memory, I/O devices).
2.Operating system – controls and coordinates the use of
the hardware among the various application programs
for the various users.
3.Applications programs – define the ways in which the
system resources are used to solve the computing
problems of the users (compilers, database systems,
video games, business programs).
4.Users (people, machines, other computers).

Abstract View of System
Components

User View:
Different Abstract Views
The user's view of the computer varies according to the
interface being used.
Users sitting in front of a PC
Mostly for Ease of Use
Some attention to performance
None to Resource Utilisation
Terminal connected to a
Mainframe or Minicomputer
Some to Ease of Use,
Some attention to performance
Maximum to Resource Utilisation
Workstation connected to
other workstations and Server
Designed to compromise between
individual usability and resource utilization
Handheld computers
Designed mostly for individual usability,
but performance per unit of battery life is
important
Embedded computers
Designed primarily to run without user
intervention

System View
A computer system has many resources that may be required to solve a problem:
CPU time, memory space, file-storage space, I/0 devices, and so on
From the computer's point of view, the operating system is the program most
intimately involved with the hardware. In this context, it view an operating system as
a Resource Allocator or more specific Resource Manager
The operating system acts as the manager of these resources.
Manages the Resources so that it can operate
the computer system efficiently and fairly.
A slightly different view of an operating system emphasizes the need to control the
various I/0 devices and user programs. An operating system is a Control Program. A
Control Program manages the execution of user programs to prevent errors and
improper use of the computer.

Operating System Definitions
 Resource allocator – manages and allocates
resources.
 Control program – controls the execution of
user programs and operations of devices .
 Kernel – The program that executes forever
(everything else is an application with respect
to the kernel) with shell.

Components of Operating System
Protection and Security

Process Management
The operating system is responsible for the following activities in
connection with process management:
• Scheduling processes and threads on the CPUs
• Creating and deleting both user and system processes
• Suspending and resuming processes
• Providing mechanisms for process synchronization
• Providing mechanisms for process communication

Memory Management
• Keeping track of which parts of memory are currently
being used and by whom
• Deciding which processes and data to move into and out
of memory
• Allocating and deallocating memory space as needed

File-System Management
• Creating and deleting files
• Creating and deleting directories to organize files
• Supporting primitives for manipulating files and
directories
• Mapping files onto secondary storage
• Backing up files on stable (nonvolatile) storage media

Mass-Storage Management
(Secondary Storage)
• Free-space management
• Storage allocation
• Disk scheduling

I/O Devices
OS hides the peculiarities of specific hardware devices from the user
•Hide the details of H/W devices
• Manage main memory for the devices using
cache, buffer, and spooling
•Maintain and provide custom drivers for each
device.

Network Management
Network management is the process of keeping your network healthy for an
efficient communication between different computers.
•Network administration
•Network maintenance
•Network operation
•Network provisioning
•Network security

Protection and Security
Mechanisms ensure that files, memory segments, CPU, and other resources can
be operated on by only those processes that have gained proper authorization
from the operating system
For example, memory-addressing hardware ensures that a process can execute
only within its own address space.
Device-control registers are not accessible to users, so the integrity of the various
peripheral devices is protected
while security measures are focused on external threats to a system,
protection is concerned with those that are internal.
Authentication information is stolen
Attacks spread across a huge range and include viruses and worms
denial-of service attacks (which use all of a system's resources and so keep
legitimate users out of the system), identity theft, and theft of service
(unauthorized use of a system).

Operating System Services to User
and Programs

Services
User Interfaces - Means by which users can issue commands to the system.
Program Execution - The OS must be able to load a program into RAM, run the program,
and terminate the program, either normally or abnormally.
I/O Operations - The OS is responsible for transferring data to and from I/O devices,
including keyboards, terminals, printers, and storage devices. For efficiency and protection,
users usually cannot control I/0 devices directly. Therefore, the operating system must
provide a means to do I/0
File-System Manipulation - In addition to raw data storage, the OS is also responsible for
maintaining directory and subdirectory structures, mapping file names to specific blocks of
data storage, and providing tools for navigating and utilizing the file system.
Communications - Inter-process communications, IPC, either between processes running on
the same processor, or between processes running on separate processors or separate
machines. May be implemented as either shared memory or message passing.
Error Detection - Both hardware and software errors must be detected and handled
appropriately, with a minimum of harmful repercussions.
Errors may occur in the CPU and memory hardware: such as a memory error or a power
failure. I/0 devices: such as a parity error on tape, a connection failure on a network, or lack of
paper in the printer. User program :such as an arithmetic overflow, an attempt to access an
illegal memory location, or a too-great use of CPU time.
Debugging and diagnostic tools aid users and administrators in tracing down the cause of problems.

Another set of operating-system functions exists not for helping the
user but rather for ensuring the efficient operation of the system itself
Resource allocation: When there are Multiple users or multiple jobs running at the
same time, resources must be allocated to each of them CPU Memory I/O Devices
etc..
Accounting: We want to keep track of which users use how much and what kinds of
computer resources. This record keeping may be used for accounting, Billing and
usage statistics which may be useful to researchers who wish to reconfigure the
system to improve computing services.
Protection and Security: Protection involves ensuring that all access to system
resources is controlled. Security of the system from outsiders is also important. Such
security starts with requiring each user to authenticate himself or herself to the
system, usually by means of a password, to gain access to system resources.

System Calls
•System calls provide a means for user or application programs to call upon the services of the
operating system.
•Generally written in C or C++, although some are written in assembly for optimal
performance.
The sequence of system calls required to copy a file:

The use of APIs instead of direct system calls provides for greater program portability
between different systems. The API then makes the appropriate system calls through
the system call interface, using a table lookup to access specific numbered system calls

Types Of System Calls

Types Of OS/Evolution Of OS

Memory Layout
The problems with Batch Systems are as
follows:
Lack of interaction between the user and
the job.
CPU is often idle, because the speed of
the mechanical I/O devices is slower than
the CPU.
Difficult to provide the desired priority.
Simple Batch Operating System
● Single user system Programmer/User as
operator
● Paper tape or punched cards
● Reduce setup time by batching jobs
with similar requirements.
● Add a card reader, Hire an operator
● User is NOT the operator
● Automatic job sequencing

How to speed up
●Use disk (random access device) as large storage for reading as many input files
as possible and storing output files until output devices are ready to accept
them.
●Allows overlap - I/O of one job with computation of another.
● Job pool that allows OS choose next job to run so as to increase CPU utilization.
Spooling

Multiprogrammed Batch Systems
Several jobs are kept in main memory at the same time, and the CPU is
multiplexed among them.

OS Features Needed for Multiprogramming
 I/O routine supplied by the system.
 Memory management – the system must
allocate the secure memory to several jobs.
 CPU scheduling – the system must choose
among several jobs ready to run.
 Processor's time is shared among multiple
users simultaneously
Multiprogramming operating systems were developed to provide efficient
resource utilization in a noninteractive environment.

Architectural Support for Multiprogramming
Feature Description
DMA The CPU initiates an I/O operation when
an I/O instruction is executed. The DMA
implements the data transfer involved in
the I/O operation without involving the CPU
and raises an I/O interrupt when the data
transfer completes.
Memory protection A program can access only the part of
memory defined by contents of the base
register and size .
Kernel and user modes of CPU Certain instructions, called privileged
instructions, can be performed only when
the CPU is in the kernel mode. A program
interrupt is raised if a program tries to
execute a privileged instruction when the
CPU is in the user mode.

Time-Sharing Operating Systems–Interactive Computing
● Interactive (action/response) Highly responsive
 The CPU is multiplexed among several jobs that
are kept in memory and on disk (the CPU is
allocated to a job only if the job is in memory).
 A job swapped in and out of memory to the disk.
 On-line communication between the user and the
system is provided; when the operating system
finishes the execution of one command, it seeks
the next “control statement” from the user’s
keyboard.

Desktop Systems
 Personal computers – computer system
dedicated to a single user.
 I/O devices – keyboards, mice, display
screens, small printers.
 User convenience and responsiveness.
 Sole use of computer and do not need
advanced CPU utilization or protection
features.
 May run several different types of operating
systems (Windows, MacOS, UNIX, Linux)

Parallel Computing Systems
 Multiprocessor systems with more than on CPU in
close communication.
 Advantages of parallel system:
 Increased throughput
 Economical
 Increased reliability
 graceful degradation
 fail-soft systems

Symmetric Multiprocessing Architecture
In these types of systems, each processor contains a similar copy of the
operating system and they all communicate with each other. All the processors
are in a peer to peer relationship i.e. no master - slave relationship exists
between them.
Asymmetric Multiprocessors Architecture
In asymmetric systems, each processor is given a predefined task. There is a
master processor that gives instruction to all the other processors. Asymmetric
multiprocessor system contains a master slave relationship.

Distributed Systems
 Distribute the computation among several
physical processors.
 Loosely coupled system – each processor has its
own local memory; processors communicate with
one another through various communications
lines, such as high-speed buses or telephone lines.
 Advantages of distributed systems.
 Resources Sharing
 Computation speed up – load sharing
 Reliability
 Communications

Distributed Systems (cont)
 Requires networking infrastructure.
 Local area networks (LAN) or Wide area
networks (WAN)
 May be either client-server or peer-to-peer
systems.

1. Client-Server
Client
Read File
Request Server
Take
Action
Sends
Content
1.2 The provides a file-system interface where clients can create, update, read, and
delete files
1.1

Peer To Peer
• Clients and servers are not distinguished from one another.
• All nodes within the system are considered peers, and each may act as either a
client or a server, depending on whether it is requesting or providing a service.
• Once a node has joined the network, it can begin providing services to-and
requesting services from -other nodes in the network:
1. When a node joins a network, it registers its service with a centralized
lookup service on the network. Any node desiring a specific service first
contacts this centralized lookup service to determine which node provides
the service. The remainder of the communication takes place between the
client and the service provider.
2. A peer acting as a client must first discover what node provides a
desired service by broadcasting a request for the service to all other
nodes in the network.

Clustered Systems
 Clustering allows two or more systems to share
storage.
 Provides high reliability.
 Asymmetric clustering: one server runs the application
while other servers standby.
 Symmetric clustering: all N hosts are running the
application.

Real-Time Systems
 Often used as a control device in a dedicated
application such as controlling scientific
experiments, medical imaging systems,
industrial control systems, and some display
systems.
 Well-defined fixed-time constraints.
 Real-Time systems may be either hard or soft
real-time.

Real-Time Systems (Cont.)
 Hard real-time:
 Secondary storage limited or absent, data stored in
short term memory, or read-only memory (ROM)
 Conflicts with time-sharing systems, not supported
by general-purpose operating systems.
 Soft real-time
 Limited utility in industrial control of robotics
 Useful in applications (multimedia, virtual reality)
requiring advanced operating-system features.

Handheld Systems
 Personal Digital Assistants (PDAs)
 Cellular telephones
 Issues:
 Limited memory
 Slow processors
 Small display screens.

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