Chapter 1 – Introduction to OS
What is an Operating System?
 Operating System is a Resource Manager.
– Handles multiple computer resources: CPU,
Internal/External memory, Processes, Tasks,
Applications, Users, etc…
– Manages and allocates resources to multiple
users or multiple jobs running at the same time
(e.g., processor time, memory space, I/O
devices)
– Arranges to use the computer hardware in an
efficient
manner
(maximize
throughput,
minimize response time) and in a fair manner.
 It is a Control Program.
– Manages all the components of a complex
computer system in an integrated manner.
– Controls the execution of user programs and
I/O devices to prevent errors and improper use
of the computer resources.
– Looks over and protects the computer.
 It is an extended/virtual machine
 An interface between the user and hardware
that hides the details of the hardware (e.g., I/O).
 Constructs higher-level (virtual) resources out
of lower-level (physical) resources (e.g., files).
 Definition: Is a collection of software
enhancements, executed on the bare
hardware, culminating in a high-level virtual
machine that serves as an advanced
programming environment
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Why Operating System?
 Computer hardware is developed to execute user
programs and make solving user problems easier.
 An operating system makes a computer more
convenient to use.
 It acts as an interface between user and
computer hardware. Therefore, the end-users
are not particularly concerned with the
computer’s architecture, and they view the
computer system in terms of an application.
 To programmers, it provides some basic utilities
to assist him in creating programs, the
management of files, and the control of I/O
devices.
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Operating System Objectives
 Convenience
 Makes the computer more convenient to use
 Efficiency
 Allows computer system resources to be used
in an efficient manner
 Ability to evolve
 Permit effective development, testing, and
introduction of new system functions without
interfering with service
Services Provided by Operating Systems
 Facilities for program creation
 Editors, compilers, linkers, debuggers, etc.
 Program execution
 Loading in memory, I/O and file initialization.
 Access to I/O and files
 Deals with the specifics of I/O and file formats.
 System access
 Resolves conflicts for resource contention.
 Protection in access to resources and data.
 Error detection and response
 internal and external hardware errors
 memory error
 device failure
 software errors
 arithmetic overflow
 access forbidden memory locations
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 operating system cannot grant request of
application
 Accounting
 collect statistics
 monitor performance
 used to anticipate future enhancements
 used for billing users
Computer System Components
 A computer system can be divided in to four
components.
 The Hardware: Provides basic computing
resources (CPU, memory, I/O devices).
 The
Operating
System:
Controls
and
coordinates the use of the hardware among the
various application programs for the various
users.
 The Application 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).
 The Users: Users (people, machines, other
computers).
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 These components can be viewed as layers, where
each layer uses the services provided by the layer
beneath it.
A Static View of System Components
Dynamic View of System Components
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End
User
Programmer
Application
Programs
Utilities
Operating-System
OperatingSystem
Designer
Computer Hardware
Another view of computer system components
History of Operating Systems
 Let’s see how operating systems evolve over time.
 This will help us to identify some common features
of operating systems and how and why these
systems have been developed as they are.
Evolution of Operating Systems
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Early Systems (1950)
Simple Batch Systems (1960)
Multiprogrammed Batch Systems (1970)
Time-Sharing and Real-Time Systems (1970)
Personal/Desktop Systems (1980)
Multiprocessor Systems (1980)
Networked/Distributed Systems (1980)
Handheld Systems (1990)
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Early Systems
 Structure
 Single user system.
 Large machines run from
console.
 Programmer/User as operator.
 Paper Tape or Punched cards.
 No tapes/disks in computer.
 Early software: Assemblers,
Libraries of common subroutines, Device Drivers,
Compilers, Linkers.
 Significant amount of setup time.
 Low CPU utilization.
 But very secure.
Simple Batch Systems
 Mainframe machines. Input devices were card
readers. Output devices were line printer, tape
drives, and card punch.
 A job (a single program+ associated data + control
information) usually on the punch cards submitted to
the operator.
 The output consisted of the results of the program
or memory dump in case of error.
 The operator used to batch together similar
programs and run as a group to reduce setup time.
 No user interaction while the job is executing.
 Current examples include .bat files under Dos –
Windows and shell files under Unix/Linux.
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Example of card deck of a job
 The operating systems (called resident monitor)
manages the execution of each program in the
batch.
 Monitor utilities are loaded when needed.
 Resident monitor is always in main memory
and available for execution.
 The resident monitor usually has the following
part.
 Control card interpreter – responsible for
reading and carrying out instructions on the
cards.
 Loader – loads systems programs and
applications programs into memory.
 Device
drivers
–
know
special
characteristics and properties for each of
the system’s I/O devices.
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 In batch systems:
 Initial control is in monitor.
 Load next program and transfer control to it.
 When a job completes, the control transfers
back to monitor.
 Automatically transfer control from one job to
another (Automatic job sequencing).
Problems
 Slow Performance – I/O and CPU could not overlap;
card reader very slow.
 CPU was often idle.
Solutions
1. Off-line Operation
 Speed up computation by loading jobs into
memory from tapes while card reading and line
printing is done off-line using smaller machines.
2. Use spooling (Simultaneous Peripheral Operation
On Line).
 Cards are read directly from the card reader
onto a disk and location of card images are
kept in a table by the operating system.
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 The output is sent to the disk and when the job
is completed then the output was actually
printed.
 I/O and computations were overlapped. While
executing one job, the OS:
 Reads next job from card reader into a
storage area on the disk (job queue).
 Outputs printout of previous job from disk
to printer.
Uniprogramming Until Now
 I/O operations are exceedingly slow (compared to
instruction execution).
 A program containing even a very small number of
I/O operations will spend most of its time waiting for
them.
 Hence: poor CPU usage when only one program is
present in memory.
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Memory Layout of Uniprogramming
Memory layout of a simple batch processing system
Multiprogrammed Batch Systems
 Several jobs are kept in main memory at the same
time, and the CPU is multiplexed among them.
 If memory can hold several programs, then CPU
can switch to another one whenever a program is
waiting for an I/O to complete – This is
multiprogramming.
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OS Features Needed for Multiprogramming
 I/O routine supplied by the system.
 Memory management – the system must allocate
the memory to several jobs.
 CPU scheduling – the system must choose among
several jobs ready to run.
 Allocation of devices.
Time Sharing Systems (Interactive Systems)
 TSS extends Batch multiprogramming to handle
multiple interactive jobs – It’s Interactive
Multiprogramming.
 Multiple users simultaneously access the system
through terminals.
 Processor’s time is shared among multiple users,
that is, 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).
 On-line communication between the user and the
system is provided; when the operating system
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finishes the execution of one command, it seeks the
next “control statement” from the user’s keyboard.
 TS system provides each user with her/her own
virtual machine.
Multitasking
 TS eventually supports multitasking.
 A time share system that supports multiple
processes (program in execution) per user is called
a multitasking system.
Why Does Time Sharing Work?
 Because of slow human reaction time, a typical user
needs 2 seconds of processing time per minute.
 Then many users should be able to share the same
system without noticeable delay in the computer
reaction time.
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Batch Multiprogramming Vs. Time Sharing
Batch Multi-Prog.
Principle obj.
Max. Processor use
Source of inst. JCL provided with
To OS
the job
Time Sharing
Min. response time
Commands entered
at the terminal
OS Features Needed for Time Sharing Systems
 On-line file system must be available for users to
access data and code.
 Should do memory management
 Should do CPU scheduling
 Should do job synchronization and have
communication facilities.
 Should ensure that dead lock and indefinite waiting
does not occur.
 Should allow sharing of computer resources.
Personal Computer Systems
 Personal computers – computer system dedicated
to a single user.
 Have a wide variety of I/O devices – keyboards,
mice, display screens, small printers.
 User convenience and responsiveness are of prime
importance.
 Can adopt technology developed for larger
operating system.
 Often individuals have sole use of computer and do
not need advanced CPU utilization of protection
features.
 May run several different types of operating systems
(Windows, MacOS, UNIX, Linux)
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Two Categories of Computer Systems
 Single Instruction Single Data (SISD)
 Single processor executes a single instruction
sequence to operate on data stored in a single
memory.
 This is a Uniprocessor.
 Multiple Instruction Multiple Data (MIMD)
 A set of processors simultaneously execute
different instruction sequences on different data
sets.
 This is a Multiprocessor.
Multiprocessor Systems
 Multiprocessor systems have more than one CPU in
close communication.
 Tightly coupled system – processors share
memory and a clock; communication usually
takes place through the shared memory.
 Advantages of parallel system:
 Increased throughput
 Economical
 Increased reliability
 Graceful degradation
Multiprocessor architecture
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Symmetric Multiprocessing (SMP)
 Each processor runs an identical copy of the
operating system.
 Each processor can perform the same functions and
share same main memory and I/O facilities
(symmetric).
 The OS schedules processes/threads across all the
processors (real parallelism).
 Existence of multiple processors is transparent to
the user.
 Incremental growth: just add another CPU!
 Robustness: a single CPU failure does not halt the
system, only the performance is reduced.
 Many processes can run at once without
performance deterioration.
 Most modern operating systems support SMP
Asymmetric multiprocessing
 Each processor is assigned a specific task; master
processor schedules and allocated work to slave
processors.
 More common in extremely large systems
Distributed Systems
 Distribute the computation among several physically
separated processors.
 Loosely coupled system – each processor has
its own local memory; processors communicate
with
one
another
through
various
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communications lines, such as high-speed
buses or telephone lines.
 Advantages of distributed systems.
 Resources Sharing
 Computation speed up – load sharing
 Reliability and fault tolerance
 Communications
 Requires networking infrastructure - Local area
networks (LAN) or Wide area networks (WAN)
 May be either client-server or peer-to-peer systems.
General structure of client-server
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Peer-to-peer systems
Network Operating System
 Provides file sharing
 Provides communication scheme
 Runs independently from other computers on the
network
Distributed Operating System
 Less autonomy between computers
 Gives the impression there is a single operating
system controlling the network.
Clustered Systems
 Clustering allows two or more systems to share
external storage and balance CPU load.
 Asymmetric clustering: one server runs the
application while other servers standby.
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 Symmetric clustering: all N hosts are running the
application.
Real-Time Systems
 Note that not all Operating Systems are generalpurpose systems.
 Real-Time (RT) systems are dedicated systems that
need to adhere to deadlines, i.e., time constraints.
 Correctness of the computation depends not only on
the logical result but also on the time at which the
results are produced.
 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.
 Real-Time systems may be either hard or soft realtime.
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Hard Real-Time System
 Must meet its deadline.
 Conflicts with time-sharing systems, not supported
by general-purpose operating systems.
 Often used as a control device in a dedicated
application such as industrial control and robotics
 Secondary storage limited or absent, data stored in
short term memory, or read-only memory (ROM).
Soft Real-Time System
 A critical real-time task gets priority over the other
tasks (Deadline desirable but not mandatory).
 Limited utility in industrial control of robotics
 Useful in applications (multimedia, virtual reality)
requiring advanced operating-system features.
Hand Held Systems
 Personal Digital Assistants (PDAs)
 Cellular telephones
 Issues:
 Limited memory
 Slow processors
 Small display screens.
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Migration of Operating-System Concepts and
Features
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