1.
Silberschatz, Galvin and Gagne 2002
1.1
Operating System Concepts
What is an Operating System?
 A program that acts as an intermediary between a user of
a computer and the computer hardware.
 Operating system goals:
 Execute user programs and make solving user problems
easier.
 Make the computer system convenient to use.
 Use the computer hardware in an efficient manner.

2.
Silberschatz, Galvin and Gagne 2002
1.2
Operating System Concepts
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 (database systems, video games,
business programs).
4. Users (people, machines, other computers).

3.
Silberschatz, Galvin and Gagne 2002
1.3
Operating System Concepts
Abstract View of System Components

4.
Silberschatz, Galvin and Gagne 2002
1.4
Operating System Concepts
Operating System- User and System
View
 Resource allocator – manages and allocates resources.
 Control program – controls the execution of user
programs and operations of I/O devices .
 Kernel – the one program running at all times (all else
being application programs).

5.
Silberschatz, Galvin and Gagne 2002
1.5
Operating System Concepts
Functions of Operating System
 Memory Management
 Process Management
 Device Management
 File Management
 Security
 Control over system performance
 Job accounting
 Error detecting aids
 Coordination between other software and users

6.
Silberschatz, Galvin and Gagne 2002
1.6
Operating System Concepts
Operating System Operations
 There are two modes of operation in the operating system
to make sure it works correctly. These are user mode and
kernel mode

7.
Silberschatz, Galvin and Gagne 2002
1.7
Operating System Concepts
Operating System Operations
 User Mode
 The system is in user mode when the operating
system is running a user application such as handling
a text editor
 The transition from user mode to kernel mode occurs
when the application requests the help of operating
system or an interrupt or a system call occurs
 The mode bit is set to 1 in the user mode. It is
changed from 1 to 0 when switching from user mode
to kernel mode

8.
Silberschatz, Galvin and Gagne 2002
1.8
Operating System Concepts
Operating System Operations
 Kernel Mode
 The system starts in kernel mode when it boots and
after the operating system is loaded, it executes
applications in user mode
 There are some privileged instructions that can only
be executed in kernel mode
 These are interrupt instructions, input output
management etc
 If the privileged instructions are executed in user
mode, it is illegal and a trap is generated

9.
Silberschatz, Galvin and Gagne 2002
1.9
Operating System Concepts
Operating System Operations
 Operating system can be implemented with the help of
various structures.
 The structure of the OS depends mainly on how the
various common components of the operating system are
interconnected and melded into the kernel
 Depending on this we have following structures of the
operating system
 Simple structure
 Layered structure

10.
Silberschatz, Galvin and Gagne 2002
1.10
Operating System Concepts
Operating System Structure
 Simple Structure
 The system starts in kernel mode when it boots and
after the operating system is loaded, it executes
applications in user mode
 There are some privileged instructions that can only
be executed in kernel mode
 These are interrupt instructions, input output
management etc
 If the privileged instructions are executed in user
mode, it is illegal and a trap is generated

11.
Silberschatz, Galvin and Gagne 2002
1.11
Operating System Concepts
Operating System Structure
 Simple Structure
 A common example for simple structure is MS-DOS. It
was designed simply for a niche amount for people

12.
Silberschatz, Galvin and Gagne 2002
1.12
Operating System Concepts
Operating System Structure
 Advantages of Simple structure:
 It delivers better application performance because of
the few interfaces between the application program
and the hardware.
 Easy for kernel developers to develop such an
operating system.
 Disadvantages of Simple structure:
 The structure is very complicated as no clear
boundaries exists between modules.
 It does not enforce data hiding in the operating system.

13.
Silberschatz, Galvin and Gagne 2002
1.13
Operating System Concepts
Operating System Structure
 Layered Structure
 One way to achieve modularity in the operating system is
the layered approach.
 In this, the bottom layer is the hardware and the topmost
layer is the user interface

14.
Silberschatz, Galvin and Gagne 2002
1.14
Operating System Concepts
Operating System Structure
 Layered Structure
 One way to achieve modularity in the operating system is
the layered approach.
 In this, the bottom layer is the hardware and the topmost
layer is the user interface

15.
Silberschatz, Galvin and Gagne 2002
1.15
Operating System Concepts
Operating System Structure
 Advantages of Layered structure:
 Layering makes it easier to enhance the operating
system as implementation of a layer can be changed
easily without affecting the other layers.
 It is very easy to perform debugging and system
verification.
 Disadvantages of Layered structure:
 In this structure the application performance is
degraded as compared to simple structure.
 It requires careful planning for designing the layers
as higher layers use the functionalities of only the
lower layers.