Transcript Chapter 1 - Introduction ()

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.
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Computer System Components
• Application Software : Bank automation system,
airline reservations, payroll etc.
• System Software : OS, data base, compilers,
editors etc.
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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
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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
I/O Devices
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Hardware
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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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History:
Second 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
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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
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History:
Third 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 VDU’s)
• Multics OS - original UNIX Minicomputers
- Cheaper than mainframes but with limited
hardware (eg. DEC PDPx)
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History:
Fourth Generation (1980-1990)
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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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Important Points
• OS provides
– a simpler, more powerful interface
– higher level services
• OS services only accessed via system
calls
• Users and programs can’t 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
Compile
Executable
Link
Libraries and
other Object files
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A Simple Program to print a directory
#include
#include
#include
<sys/types.h>
<dirent.h>
"ourhdr.h"
int main(int argc, char *argv[])
{
DIR
*dp;
struct dirent *dirp;
Functions supplied by system
libraries.
These functions will contain a
trap instruction.
if (argc != 2)
err_quit("a single argument (the directory name) is required");
if ( (dp = opendir(argv[1])) == NULL)
err_sys("can't open %s", argv[1]);
while ( (dirp = readdir(dp)) != NULL)
printf("%s\n", dirp->d_name);
closedir(dp);
exit(0);
}
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User Program #2
RAM
User Program #1
trap 002
4
1
3
User Mode
1. Program performs trap
2. OS determines service
number
3. Service is located and
executed.
4. Control returns to user
program.
Based on a diagram from
“Modern Operating Systems” by
Andrew Tanenbaum.
2
Kernel
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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) {
type_prompt( );
read_command (command, parameters)
/* repeat forever */
/* display prompt */
/* input from terminal */
if (fork() != 0) {
/* Parent code */
waitpid( -1, &status, 0);
} else {
/* Child code */
execve (command, parameters, 0);
}
}
/* fork off child process */
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/* wait for child to exit */
/* execute command */
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OS Structures
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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)
SVC
Program
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Interrupt
Handler
Service routine
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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
User Programs
OS1 OS2 OS3 OS4
Virtual Machine
Physical Hardware
• 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
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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
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Makes use of message passing
Easy to replace server processes
Easier to write and port OS
Design is perfect for distributed systems
• Less performance
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