Chapter 1: Introduction

1
Chapter 1 Introduction

• What is the aim of the subject?
• Why are OSes important?
• What is an OS?
• A bit of history
• Some basic OS concepts
• How does an OS work?
• OS Structures

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The Aim of the Subject

• WILL NOT TEACH YOU HOW TO USE AN OPERATING
  SYSTEM.
• It will examine
• the way in which an OS works
• the algorithms and data structures inside an OS
• the problems, solutions and trade offs in
  designing an OS
• TO ACHIEVE AN UNDERSTANDING OF HOW AN OPERATING
  SYSTEM WORKS.

3
Computer System Components

• Application Software Bank automation system,
  airline reservations, payroll etc.
• System Software OS, data base, compilers,
  editors etc.

4
What is SYSTEM SOFTWARE?

• System software provides the environment and the
  tools to create the application software (sort of
  virtual machine)
• It is also the interface between the hardware and
  the applications

5
Why is the OS Important?

• The operating system is the foundation upon which
  all computing work is performed.
• Knowledge of the internals of an OS is essential
  to achieve efficiency in
• building software applications
• deciding upon a computing platform

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What is an Operating System?

• A big, complex program (sometimes many)
• It has two main purposes in life
• An interface between the user and the hardware
  (provides a virtual machine)
• Provide efficient, safe management of computing
  resources

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Life without an OS

• Every programmer would
• have to know the hardware
• be able to access the hardware
• Every program
• would contain code to do the same thing
• probably do something wrong

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Where does the OS Fit?
System Calls
Users and User Programs
Operating System
CPU Memory
Hardware
I/O Devices
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History First Generation (1945-1955)

• Vacuum tubes
• No operating system
• Programming is done by wiring a plug board
• Applications are mostly numerical calculations
  (trajectory computations, computation of tables
  such as sine, cosine etc.)

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HistorySecond Generation (1955-1965)

• Transistors
• Commercially produced computers
• Very expensive and very slow computers compared
  with your old PC at home
• Batch operation (collect jobs, run in one go,
  print all outputs)

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• Spooling (Simultaneous Peripheral Operation
  On-line)
• off-line spooling
• on-line spooling
• Off-line spooling replace slow I/O devices with
  I/O dedicated computers so that the main system
  sees these machines as its I/O devices

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Early Batch Systems

• bring cards to 1401
• read cards to tape
• put tape on 7094 which does computing
• put tape on 1401 which prints output

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A Deck of Cards (Program)
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• Applications are mostly scientific and
  engineering calculations (eg., solution of
  partial differential equations)
• High level languages such as FORTRAN and COBOL

15
HistoryThird Generation (1965-1980)

• Integrated circuits (small scale) packed as chips
• I/O processors (channels) which can work in
  parallel with CPU - Multiprogramming

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Multiprogramming system - three jobs in memory
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• On-line spooling (using channels)
• Time-sharing (TTY terminals and VDUs)
• Multics OS - original UNIX Minicomputers -
  Cheaper than mainframes but with limited hardware
  (eg. DEC PDPx)

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HistoryFourth Generation (1980-1990)

• Large scale integration
• Personal computers
• CP/M, MS DOS, Unix operating systems
• Networks

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Now!

• Client/Server computation
• Clients PCs, workstations running under Windows
  and UNIX operating systems
• Servers systems that run under UNIX and Windows
  NT
• Internet and intranet networking (WWW)

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Links for More History

• http//www.old-computers.com
• http//www.hitmill.com/computers/history/index.htm
  l
• http//www.computerhistory.org/

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Important Points

• OS provides
• a simpler, more powerful interface
• higher level services
• OS services only accessed via system calls
• Users and programs cant directly access the
  hardware
• Set of System Calls (APIs) is what programs
  think the operating system is.

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Some OS Concepts

• Kernel
• The main OS program. Contains code for most
  services. Always in primary memory
• Device Drivers
• Programs that provide a simple, consistent
  interface to I/O devices
• Typically part of the kernel

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Some OS Concepts

• Program
• A static file of machine code on a disk
• Process
• A program in execution.
• The collection of OS data structures and
  resources owned by a program while it is running.

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Producing an Executable
Source Code
Object File
Executable
Compile
Link
Libraries and other Object files
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A Simple Program to print a directory
include ltsys/types.hgt include
ltdirent.hgt include "ourhdr.h" int
main(int argc, char argv) DIR
dp struct dirent dirp
if (argc ! 2) err_quit("a single
argument (the directory name) is required") if (
(dp opendir(argv1)) NULL)
err_sys("can't open s", argv1)
while ( (dirp readdir(dp)) ! NULL)
printf("s\n", dirp-gtd_name)
closedir(dp) exit(0)
Functions supplied by system libraries. These
functions will contain a trap instruction.
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User Program 2
User Mode
RAM
1. Program performs trap 2. OS determines
service number 3. Service is located and
executed. 4. Control returns to user
program.
User Program 1
trap 002
4
1
3
Based on a diagram from Modern Operating
Systems by Andrew Tanenbaum.
2
Kernel
System/Kernel Mode
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Steps in Making a System Call
There are 11 steps in making the system call
read (fd, buffer, nbytes)
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Some System Calls For Process Management
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Some System Calls For File Management
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Some System Calls For Directory Management
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Some System Calls For Miscellaneous Tasks
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A System Call Example

• A stripped down shell
• while (TRUE) / repeat forever /
• type_prompt( ) / display prompt /
• read_command (command, parameters) / input
  from terminal /
• if (fork() ! 0) / fork off child process
  /
• / Parent code /
• waitpid( -1, status, 0) / wait for
  child to exit /
• else
• / Child code /
• execve (command, parameters, 0) / execute
  command /

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OS Structures

• Monolithic systems
• Hierarchy of layers
• Virtual machines
• Micro-kernel (client/server) model

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Monolithic System

• OS has no structure but is a collection of
  procedures with well defined calling interfaces
• Program - OS interface is via supervisor calls
  (SVC)

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Simple structuring model for a monolithic system
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Monolithic System (Cont.)

• OS code is a binded object program and its source
  code may be logically divided into
• OS main program
• System call service routines
• Utility procedures which help service routines

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Layered System

• Structure of THE OS (a batch OS)

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Layered System (Cont.)

• 0. Process switching, multi programming, CPU
  scheduling
• 1. Memory and swap space (disk) management
  (segment controller)
• 2. Message interpretation, job control (JCL)
  functions
• 3. I/O management (virtual peripherals)
• 4. User programs
• 5. Operator

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Layered System (Cont.)

• Synchronisation between layers Hardware and
  software (semaphores) interrupts
• Each layer provides some sort of a virtual
  machine

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Virtual Machines

• VM provides n duplicates of physical hardware
  using software. So different Oses can work in the
  same machine at the same time

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What is wrong so far?

• OS is one large program that provides all the
  required services.
• Anytime you add a new device you must
• get a device driver for the device
• recompile the kernel with the new device driver
• reboot the machine so the new kernel will be
  used

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Micro-Kernel (Client/Server) Model

• OS is a minimal core known as Kernel.

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• The kernel contains the minimum of function
• memory management
• basic CPU management
• inter-process communication (messages)
• I/O support
• Other functionality provided by user level
  processes

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Characteristics of Kernel

• Makes use of message passing
• Easy to replace server processes
• Easier to write and port OS
• Design is perfect for distributed systems
• Less performance