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CE01000-3 Operating Systems
Lecture 3
Overview of OS functions and
structure
Overview of OS functions and
structure
In this lecture we shall give an overview of
1. The functions of an operating system
2. Operating system components and services
3. Different models of the structure of an
operating system

What do programs need in
order to run?
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CPU to execute the program instructions
Memory to hold copies of
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Access to disk
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the program instructions
data to be used by program
to hold program and data between runs of the
program
Access to input devices
Access to output devices
What do Operating Systems
do?
As we saw last week in a operating system
programs are not allowed direct access to any
of the resources except CPU and memory and
even then the use has limits placed on it. Thus
1. OS handles requests from programs for
various resources to do things e.g. input or
output something and enforces the limits
placed on the use of the CPU and memory.

What do Operating Systems
do? (cont.)
Resources cannot be used by different
programs at the same time. Thus
2. OS needs to control sharing of resources
between programs so that they do not interfere
with each other
 Programs sometimes need to communicate
with each other. Thus
3. OS needs to provide a mechanism by which
one program can communicate with another.

What do Operating Systems do?
(cont.)
The users need to be able to tell the OS what
programs to run, when and with what data.
Thus
4. OS needs to provide some form of interface to
allow users to issue commands to the OS.

Functions of OS
So functions of OS have to do with:
1. Handling requests
2. Sharing resources
3. Communication
4. User interface

Function 1 - system call interface
and device drivers
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
In order to fulfil function 1 the OS must
provide a mechanism by which programs can
pass on requests to the OS to do something for
the program.
This is the system call (remember from last
week - software interrupt).
System call interface and device
drivers (Cont.)

To allow a program to make a wide variety
of requests then the OS needs to provide a
large variety of system calls. This is the
system call interface, often called the
Application Programmer’s Interface (API).

This is essentially a library of code modules that
perform various operations on behalf of the
application program
System call interface and
device drivers (Cont.)

To get various hardware components (e.g. I/O
or disk) to do what the OS wants them to do
(on behalf of the user’s program) then the OS
must have programs of its own that tell the
hardware what to do i.e. to manage and
control the hardware. These are the device
drivers.
Function 2 - Schedulers, mutual
exclusion, deadlock and security

In order to fulfil function 2, for each type of
resource, the OS needs to share the use of the
resource between the different programs that
want to use it. To do this fairly between the
different programs it must allocate resources to a
program i.e. give it the right to use a resource and
then schedule the use of the resource between
the different programs.
Schedulers, mutual exclusion,
deadlock and security (Cont.)

Thus for each different resource the OS will
have a program called a scheduler that
determines which program gets what resource
next.
Schedulers, mutual exclusion,
deadlock and security (Cont.)

OS also needs to ensure that the use of
resources by programs is safe. Safe from
software faults and malicious action and safe
from errors that the operating system may
produce as a result of its actions. Thus the OS
needs to provide software that is part of OS for
security and protection to prevent
unauthorised use of resources and to monitor
its own actions for safety e.g. management of
deadlock.
Schedulers, mutual exclusion,
deadlock and security (Cont.)

OS must also ensure that when a program is
using a resource the earlier or later use of that
resource by another program does not corrupt
or interfere with that program. This requires
the OS to provide the programs with software
that is part of OS that implements the notion
of an atomic action

(also known as mutual exclusion – stops
programs from executing some code, until other
programs have finished executing the same
code).
Function 3 - Interprocess
communication and co-ordination

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In order to fulfil function 3, the OS must
provide a mechanism by which programs can
communicate with each other.
This is called interprocess communication.
The OS may provide a number of different
ways of doing this.
Interprocess communication
and co-ordination (Cont.)

Interprocess communication then allows user
programs to co-operate with each other and
with the OS in order to complete some task.
This co-ordination of activities sometimes
requires synchronisation of actions between
programs.
Function 4 - User interface

OS needs software that is part of OS that can
process user requests and initiate the
execution of programs on the user’s behalf commands are given to the operating system
by
 control actions (mouse clicks) or
 typed commands
User interface (Cont.)

Since the users can permanently store
programs and data on the computer then the
OS must provide software that is part of OS
that the user can use to manage the files that
the user owns.
User interface (Cont.)

The program that receives, interprets and
executes the next command action/statement
may be:
 command-line interpreter or shell - to
process typed commands
 Graphical User Interface (Windows,
Icons, Menus, Pointers) based system - to
process control actions
OS components

Thus OS components consist of a number of
programs that have the tasks described in the
previous slides:
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System call interface
Device drivers
Schedulers for each type of resource
Deadlock management
Security & protection
Implementation of mutual exclusion
Interprocess communication and co-ordination
User interface
System Calls

System calls provide the interface between a
running program and the operating system.

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In assembly language – they are implemented by
traps or software interrupts
In programming languages that were developed to
replace assembly language for systems
programming (e.g., C, Perl, BCPL), they are
implemented like calls to library routines.

The system calls are compiled into software interrupts
System Programs

System programs are provided by system to
allow users to carry out O/S maintenance
functions and program development
directly. Typical functions include,
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File/directory management
Programming language support
Program loading and execution
Communications
System Programs (Cont.)

Most users’ view of the operating system is
defined by system programs, not the actual
system calls.
System Design Goals

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User goals – operating system should be
convenient to use, easy to learn, reliable, safe,
and fast.
System goals – operating system should be
easy to design, implement, and maintain, as
well as flexible, reliable, error-free, and
efficient.
Mechanisms and Policies
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Mechanisms determine how to do something;
policies decide what will be done.
The separation of policy from mechanism is a
very important principle; it allows maximum
flexibility if policy decisions are to be
changed later.
System Implementation
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Originally written in assembly language,
operating systems now largely written in
higher level languages, except for small
amount of code which is still in assembly
language
Code written in a high level language:
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can be written faster.
is easier to understand and debug.
System Implementation
(Cont.)

An operating system is far easier to port
(move to some other hardware platform) if it
is written in a high level language.
System Generation

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Operating systems are designed to run on any
of a class of machines; the system must be
configured for each specific computer site or
machine.
A system generation program obtains
information concerning the specific
configuration of the hardware system. Then it
could either,
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Recompile whole kernel, or
link together precompiled components
Operating system models

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An operating system model is a framework that
characterises how the various system features
and services are combined together into a
structure. Essentially it defines a general
approach to the structure of the OS
Popular models:
Monolithic operating systems
 Layered operating systems
 Client/Server operating systems

Monolithic operating systems
Application
Program
OS
Procedures
Application
Program
System Services
hardware
User Mode
Kernel Mode
Monolithic operating systems
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operating system code organised as a set of
procedures
each procedure can call any other procedures
all operating system data structures are
visible to all operating system code
code in procedures are dependent upon
knowledge of structure/operation of other
procedures
Monolithic operating
systems (Cont.)
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very difficult to change code
typical of smaller and older operating systems
e.g. MS-Dos and Unix
Layered operating systems
Application
Program
Application
Program
System Services
File System
Memory and I/O Device Management
Process Management
Hardware
User Mode
Kernel Mode
Layered operating systems
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operating system code is organised into a
series of layers
each layer is responsible for performing some
given task
each layer provides an interface - set of
functions that other layers can call
can only access functionality of given layer
through calls to functions that make up the
interface
Layered operating systems
(Cont.)
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code in one layer can only access interface
functions of lower layers
much better structure - easier to implement,
test and modify
Increases modularity of design, but
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overhead in moving through many layers means it
is less efficient than a monolithic OS
problem with mapping functions to layers
(ordering of layers)
Client/Server operating
systems
Client
Application
Process
Server
Network
Server
Memory
Server
File
Server
Microkernel
Send
Reply
Hardware
Print
Server
User Mode
Kernel Mode
Client/Server operating
systems
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operating system code is organised into a
series of independent processes called servers
each server provides a set of services to other
parts of the operating system or to user
programs via a defined interface of service
requests
requesting process (client) requests service by
sending a message to server
service usually involves messages being
passed back from server to client
Client/Server operating
systems (Cont.)
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microkernel is privileged operating system
code that is responsible for acting as
postmaster for messages passed between
servers and clients
servers are small and self-contained, hide
details of their implementation, etc. - easy to
change
But, like layered OS there is an overhead
produced by the need to communicate
between clients and servers.
References
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Operating System Concepts. Chapter 2.