Transcript Ceng 334 - Operating Systems

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
1
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.
2
Computer System Components
• Application Software : Bank automation system,
airline reservations, payroll etc.
• System Software : OS, data base, compilers,
editors etc.
3
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
4
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
5
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
6
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
7
Where does the OS Fit?
System Calls
Users and User Programs
Operating System
CPU & Memory
I/O Devices
Hardware
8
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.)
9
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)
10

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
11
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
12
A Deck of Cards (Program)
13


Applications are mostly scientific and
engineering calculations (eg., solution of partial
differential equations)
High level languages such as FORTRAN and
COBOL
14
History:
Third Generation (1965-1980)


Integrated circuits (small scale) packed as chips
I/O processors (channels) which can work in
parallel with CPU - Multiprogramming
15
Multiprogramming system - three jobs in
memory
16



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)
17
History:
Fourth Generation (1980-1990)




Large scale integration
Personal computers
CP/M, MS DOS, Unix operating systems
Networks
18
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)
19
Links for More History



http://www.old-computers.com
http://www.hitmill.com/computers/history/index.html
http://www.computerhistory.org/
20
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.

21
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
22
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.
23
Producing an Executable
Source Code
Object File
Compile
Executable
Link
Libraries and
other Object files
24
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);
}
25
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
System/Kernel Mode
26
Steps in Making a System Call
There are 11 steps in making the system call :
read (fd, buffer, nbytes)
27
Some System Calls For Process
Management
28
Some System Calls For File
Management
29
Some System Calls For Directory
Management
30
Some System Calls For Miscellaneous
Tasks
31
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 */
/* wait for child to exit */
/* execute command */
32
OS Structures




Monolithic systems
Hierarchy of layers
Virtual machines
Micro-kernel (client/server) model
33
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
Interrupt
Handler
Service routine
34
Simple structuring model for a monolithic
system
35
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

36
Layered System

Structure of “THE” OS (a batch OS)
37
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
38
Layered System (Cont.)


Synchronisation between layers : Hardware and
software (semaphores) interrupts
Each layer provides some sort of a “virtual
machine”
39
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
40
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
41
Micro-Kernel (Client/Server)
Model
 OS is a minimal core known as Kernel.
42

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
43
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



44