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1
OPERATING SYSTEMS
Networks and
Communication
Department
Lecture 3: we will explore the role of the
operating system in a computer
Outline
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Define the operating system
Discuss the process of bootstrapping
Operating system evolution
Identify the components of an operating system
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Memory manager
Process manager
Device manager
File manager
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Computer System
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7.4
Figure 7.1 A computer system
Computer System

A computer is a system composed of 2 major
components:

hardware and software.

Computer hardware: is the physical equipment.

Software is the collection of programs that
allows the hardware to do its job.
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Computer System
Computer software is divided into 2 broad
categories:
 The
operating system and application
programs
 Application
programs: use the computer
hardware to solve users’ problems.
 The operating system: controls the access to
hardware by users.

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Operating System Introduction
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Operating System
An operating system: is an interface between the
hardware of a computer and the user (programs or
humans) that facilitates the execution of other
programs and the access to hardware and software
resources
Two major design goals of an operating system are:

Efficient use of hardware.

Ease of use of resources.
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Bootstrap Process
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Bootstrap Process


The operating system is responsible for loading
other programs into memory for execution.
However, the operating system itself is a program
that needs to be loaded into the memory and be
run. How is this dilemma solved?
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What is the Bootstrap process?
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
So a very small section of memory is made of ROM
and holds a small program called the bootstrap
program (Firmware).
The program is only responsible for loading the
operating system itself, or that part of it required to
start up the computer, into RAM memory.
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Bootstrap Process
1.
2.
When the computer is turned on, the CPU counter
(a type of the CPU registers) is set to the first
instruction of this bootstrap program and
executes the instructions in this program.
When loading is done, the program counter is set
to the first instruction of the operating system in
RAM.
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7.13
Figure 7.2 The bootstrap process
Operating System Evolution
Operating systems have gone through a long
history of evolution
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Operating System Evolution
Batch operating systems were designed in the
1950s to control mainframe computers.
At that time, computers were large machines that
used :
 punched cards for input
 line printers for output
 tape drives for secondary storage media
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Operating System Evolution
Punched Cards
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Operating System Evolution
Tape
Drives
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Operating System Evolution
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Operating System Evolution
In Batch operating systems each program to be
executed was called a job.
 A programmer who wished to execute a job sends
a request to the operating room along with
punched cards.
 The punched cards were fed to the computer by
an operator.
 If the program was successful, a print out of the
result was sent to the programmer.
 If not, a print out of the error was sent.

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Batch System Example
The idea behind it was to collect a tray full of jobs in the
input room and then read them onto a magnetic tape
using a small (relatively) inexpensive computer, such as
the IBM 1401, which was very good at reading cards,
copying tapes, and printing output, but not at all good at
numerical calculations. Other, much more expensive
machines, such as the IBM 7094, were used for the real
computing.
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Batch System Example
(d) 7094 does computing.
(a) Programmers bring cards to 1401.
(b) 1401 reads batch of jobs onto tape. (e) Operator carries output
(c) Operator carries input tape to 7094. tape to 1401.
(f) 1401 prints output.
Operating System Evolution
Time Sharing Systems: to use computer system resources
efficiently, multiprogramming was introduced. The idea is to
hold several jobs in memory at a time, and only assign a
resource to a job that needs it on the condition that the
resource is available.
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Multiprogramming brought the idea of :
time sharing: resources could be shared between different
jobs, with each job being allocated a portion of time to use
a resource.
Because a computer is much faster than a human, time
sharing is hidden from the user—each user has the
impression that the whole system is serving them exclusively.
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Operating System Evolution
Time Sharing Systems required a more complex operating
system.

They had to do scheduling which is : allocating resources to
different programs and deciding which program should use
which resource and when.

During this era, a new term was coined which is a process.

A job: is a program to be run.

But a process: is a program that is in memory and waiting
for resources.
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Operating System Evolution

Personal Systems: When personal computers were
introduced, there was a need for an operating system for
this new type of computer.

During this era, single-user operating systems such as DOS
(Disk Operating System) were introduced.
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Operating System Evolution
The need for more speed and efficiency led to the design
of:

parallel systems: multiple CPUs on the same machine.

Each CPU can be used to serve one program or a part of
a program, which means that many tasks can be
accomplished in parallel instead of serially.

The operating systems required for this are more
complex than those that support single CPUs.
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Operating System Evolution

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Distributed Systems: Networking and internetworking
have created a new dimension in operating systems.
A job that was previously done on one computer can
now be shared between computers that may be
thousands of miles apart.
A program can be run partially on one computer and
partially on another if they are connected through a
network.
Resources can be distributed too.
Distributed systems combine features of the previous
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generation with new duties such as Networks
controlling
security.
Operating System Evolution

Real-Time Systems: A real-time system is expected to do
a task within a specific time constraint. They are used
with real-time applications, which monitor, respond to or
control external processes or environments.

Such as : traffic control, patient monitoring and military
control systems.
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The typical components of an operating
system
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Operating System Components
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Today’s operating systems are very complex.
An operating system needs to manage different
resources in a computer system.
It resembles an organization with several managers
at the top level.
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Operating System Components
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
Each manager is responsible for managing
their department, but also needs to cooperate
with others and coordinate activities.
A modern operating system has at least four
duties: memory manager, process manager,
device manager and file manager.
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OS Components- User interface
Each operating system has a :
user interface: a program that accepts requests
from users (processes) and interprets them for the
rest of the operating system.
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A user interface in some operating systems, such
as UNIX, is called a shell. In others, it is called a
window to denote that it is menu driven and has
a GUI (graphical user interface) component.
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User Operating System Interface - CLI

Command Line Interface (CLI) or command
interpreter allows direct command entry
 Which
is an operating system shell that uses
alphanumeric characters typed on a keyboard to
provide instructions and data to the operating system,
interactively.
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User Operating System Interface - GUI

User-friendly desktop metaphor interface
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Usually mouse, keyboard, and monitor
Icons represent files, programs, actions, etc
Various mouse buttons over objects in the interface cause various
actions (provide information, options, execute function, open
directory (known as a folder)
Many systems now include both CLI and GUI interfaces
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Microsoft Windows is GUI with CLI “command” shell
Apple Mac OS X as “Aqua” GUI interface with UNIX kernel
underneath and shells available
Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)
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OS Components- Memory manager
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Although the memory size of computers has
increased tremendously in recent years, so has the
size of the programs and data to be processed.
Memory allocation must be managed to prevent
applications from running out of memory.
Operating systems can be divided into two broad
categories
of
memory
management:
monoprogramming and multiprogramming.
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Monoprogramming
In monoprogramming, most of the memory
capacity is dedicated to a single program.
 only a small part is needed to hold the
operating system.
 In this configuration, the whole program is in
memory for execution.
 When
the program finishes running, the
program area is occupied by another
program.

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Monoprogramming
Figure shows memory in
Monoprogramming a environment
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Monoprogramming (Cont.)

There are several problems with this technique:
If the size of memory is less than the size of the program,
can the program run?
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the program cannot be run
Inefficient use of memory and CPU time
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Multiprogramming
In multiprogramming:
 more than one program is in memory at the
same time and they are executed concurrently
 with the CPU switching rapidly between the
programs

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Multiprogramming
Figure shows memory in a multiprogramming
environment
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Categories of multiprogramming
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Multiprogramming (Cont.)
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Multiprogramming (Cont.)

Nonswapping category:
 This
means that the program remains in
memory for the duration of execution
 Two
techniques belong to the this category:
 Partitioning
 paging
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Multiprogramming (Cont.)

Swapping category:
 This
means that, during execution, the
program can be swapped between memory
and disk one or more times.
 Two
techniques belong to the this category:
 Demand
paging
 Demand
segmentation
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Partitioning

The first technique used in multiprogramming.
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Memory is divided into variable length sections
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Each section or partition holds one program
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The CPU switches between programs

With this technique, each program is entirely in
memory and occupying contiguous locations
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Partitioning- how its work ?!
1.
2.
3.
4.
The CPU starts with one program, executing some
instructions until it either encounters an input/output
operation or the time allocated for that program has
expired
Then, it saves the address of the memory location
where the last instruction was executed and moves to
the next program.
The same procedure is repeated with the second
program
After all the programs have been served, the CPU
moves back to the first program
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Partitioning- how its work ?! (Cont.)


Priority levels can be used to control the amount of
CPU time allocated to each program
Its improves the efficiency of the CPU, but there are
still some issues
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Partitioning- how its work ?! (Cont.)
Figure shows memory with
partitioning technique
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Partitioning issues

The size of the partitions has to be determined
beforehand by the memory manager
what will happen if partition sizes are small or large?!

The memory manager can compact the partitions to
remove the holes and creates new partitions, but this
creates extra overhead on the system
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Paging
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Paging improves the efficiency of partitioning
Memory is divided into equally sized sections called
frames
Programs are also divided, into equally sized
sections called pages
The size of a page and frame is usually the same
and equal to the size of the block used by the
system to retrieve information from a storage
device
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Paging - how its work!!
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A page is loaded into a frame in memory
The program does not have to be contiguous in
memory
There is no need for the new program to wait
until set of contiguous frames are free before
being loaded into memory
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Paging - how its work!!
Figure shows memory with paging technique
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Paging issues

The whole program still needs to be in memory
before being executed

Ex: A program has six pages, cannot be loaded into
memory if there are only four unoccupied frames
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Demand paging

In this technique the program is divided into
pages

but the pages can be loaded into memory one
by one, executed, and replaced by another
page.

In addition, a page can be loaded into any
free frame
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Demand paging (Cont.)

An example of demand paging is shown in figure
below. Two pages from program A, one page from
program B, and one page from program C are in
the memory
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Demand segmentation

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A technique similar to paging is segmentation
A programmer thinks in terms of modules
 A program is usually made up of a main program
and subprograms
In demand segmentation, the program is divided into
segments that match the programmer’s view
These are loaded into memory, executed, and
replaced by another module from the same or a
different program
since segments in memory are of equal size, part of a
segment may remain empty
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Demand segmentation
An example of demand
segmentation is shown in figure
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Demand paging and segmentation
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Demand paging and segmentation can be
combined to further improve the efficiency of the
system
A segment may be too large to fit any available
free space in memory
Memory can be divided into frames, and module
can be divided into pages
The pages of a module can then be loaded into
memory one by one and executed
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Virtual Memory
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Virtual memory

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Demand paging and demand segmentation mean
that, when a program is being executed, part of
the program is in memory and part is on disk.
This means that, for example, a memory size of
10 MB can execute 10 programs, each of size
3 MB, for a total of 30 MB.
At any moment, 10 MB of the 10 programs are in
memory and 20 MB are on disk.
There is therefore an actual memory size of
10 MB, but a virtual memory size of 30 MB
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Virtual memory (Cont.)
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Virtual memory, which implies demand paging,
demand segmentation or both, is used in almost all
operating systems today.
Figure below shows the concept.
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OS Components- Process manager

A second function of an operating system is process management,
but before discussing this concept, we need to define some terms.

A program is a non-active set of instructions stored on disk. It may
or may not become a job

A program becomes a job from the moment it is selected for
execution until it has finished running and becomes a program
again.

A process is a program in execution. It is a program that has started
but has not finished.
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State diagrams
The relationship between a program, a job and
a process becomes clearer if we consider how
a program becomes a job and how a job
becomes a process.
 This can be illustrated with a state diagram
that shows the different states of each of these
entities.

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State diagrams (Cont.)
Figure shows a State diagram with boundaries between program, job and process
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State diagrams (Cont.)

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A program becomes a job when selected by the operating system
and brought to the hold state
It remains in this state until it can be loaded into memory
When there is memory space available to load the program totally
or partially, the job moves to the ready state. It now becomes a
process
It remains in memory and in this state until the CPU can execute it,
moving it to the running state at this time
When in the running state, one of three things can happen:
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The process executes until it needs I/O resources, it goes into the waiting
state
The process exhausts its allocated time slot, it goes into the ready state
The process terminates, it goes into the terminated state
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Schedulers
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
To move a job or process from one state to
another,
The process manager uses two schedulers
 The
job scheduler
 The process scheduler
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Schedulers (Cont.)
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
Job scheduler:
Moves a job from the hold state to the ready state or
from the running state to the terminated state
(Figure shows job scheduler)
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Schedulers (Cont.)



Process scheduler:
Moves a process between the running, waiting, and
ready states many times before it goes to the
terminated state and is no longer a process
As shown in the figure below
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Queuing
In reality, there are many jobs and many
processes competing with each other for
computer resources.
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Queuing
To handle multiple processes and jobs, the
process manager uses queues (waiting lists).
 A job control block or process control block is
associated with each job or process.
 This is a block of memory that stores
information about that job or process.
 The process manager stores the job or process
control block in the queues instead of the job
or process itself.

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Queuing (Cont.)

Figure shows the circulation of jobs and processes
through three queues ( the job queue, the ready
queue , and the I/O queue)
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Queuing (Cont.)

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
The job queue holds the jobs that are waiting for
memory
The ready queue holds the processes that are in
memory, ready to be run and waiting for the CPU
The I/O queue holds the processes that are waiting
for an I/O device (one queue for each I/O device)
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Selecting next job or process


The process manager can have different policies for
selecting the next job or process from a queue:
It could be first in first out (FIFO), shortest length
first, highest priority first, and so on
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Process synchronization
The whole idea behind process management is
to synchronize different processes with
different resources.
 Whenever resources can be used by more than
one user (or process, in this case), we can have
two problematic situations: deadlock and
starvation.

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Deadlock on a narrow bridge
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Deadlock
Deadlock occurs when the operating system does not
put resource restrictions on processes.
 There are four necessary conditions for deadlock to
occur:
Mutual exclusion only one process can hold a resource
 Resource holding A process holds a resource even though it
cannot use it until other resources are available
 No preemption The OS cannot temporarily reallocate a
resource
 Circular waiting All processes and resources involved form a
loop

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Deadlock example
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Assume that there are two processes, A and B
Process A is holding File 1, and it has requested File
2 and cannot release File 1 until it acquires File 2
Process B is holding File 2, and it has requested File
1 and cannot release File 2 until it acquires File 1
Files in most system are not sharable
If there is no provision in this situation to force a
process to release a file, deadlock is created (as
shown in figure below)
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Deadlock example (Cont.)
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How the system preventing deadlock?


One solution is not to allow a process to start running
until the required resources are free, but this creates
another problem (Starvation)
The second solution is: do not allow one of deadlock
conditions to happen (i.e. limit the time a process can
hold a resource)
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The dining philosophers problem
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Starvation
Starvation is the opposite of deadlock. It can happen
when the operating system puts too many resource
restrictions on a process.
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Starvation example


For example, imagine an operating system that
specifies that a process must have possession of its
required resources before it can be run
In Figure below,
Imagine that process A needs two files, File1 and File2
 File1 is being used by process B and File2 is being used by
process E
 Process B terminates and releases File1
 Process C, which needs only File1, is allowed to run
 Process E terminates and releases File2
 Process D, which needs only File2, is allowed to run

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Starvation example (Cont.)
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OS Components- Device manager
The device manager, or input/output manager,
is responsible for access to input/ output
devices.
 There are limitations on the number and speed
of input/output devices in a computer system.
 The device manager is responsible for the
efficient use of input/output devices

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Device manager responsibilities



The device manager monitors every input/output
device constantly to ensure that the device is
functioning properly.
The device manager maintains a queue for each
input/output device or one or more queues for
similar input/output devices.
The device manager controls the different policies
for accessing input/output devices. i.e. it may use
FIFO for one device and shortest length first for
another
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OS Components- File manager

Operating systems today use a file manager to
control access to files. The responsibilities of a file
manager:

Controls access to files. The type of access can vary
(read, write, execute)

Supervises the creation, deletion, and modification of
files.

Controls the naming of files.

Supervises the storage of files. ( how, where,.. etc)

is responsible for archiving and backups.
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Any Questions ?
86
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References

Behrouz Forouzan and Firouz Mosharraf,
“Foundations of computer science”, Second edition,
chapter7, pp. 188-203
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