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Chapter 10
Operating Systems
Software Categories
Software
Application Software
System Software
Utlity Software
Operating System
Shell
Kernel
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Software Categories

Application software is written to address
our specific needs—to solve problems in the
real world.


Word processing programs, games, inventory
control systems, automobile diagnostic programs,
and missile guidance programs are all application
software.
System software manages a computer system at a
fundamental level.

It provides the tools and an environment in which
application software can be created and run.
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System Software
Within the class of system software are two
categories:
 Utility software


programs for performing various activities
fundamental to computer installations, but not
part of the OS. (Examples include formating a
disk, networking, copying files, using a modem,
and data compression.)
Operating Systems
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Operating System

An operating system also consists of two
parts.



The kernel manages computer resources, such as
memory and input/output devices.
The shell provides an interface through which a
human can interact with the computer.
An operating system also allows application
programs to interact with the other system
resources.
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Operating System
Figure 10.1
An operating system
interacts with many
aspects of a computer
system.
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Operating System


The various roles of an operating system
generally revolve around the idea of “sharing
nicely”.
An operating system manages resources, and
these resources are often shared in one way or
another among programs that “want” to use
them.
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Managing Resources
Resource management consists of:
 Memory management
 Process management
 CPU scheduling
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Memory Management


Memory management keeps track of what is
stored in memory and where in memory it is.
Multiprogramming is the technique of
keeping multiple programs in main memory at
the same time.
These programs compete for access to the
CPU so that they can execute.
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Memory
Figure 10.3
Memory is a continuous
set of bits referenced by
specific addresses
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Logical and Physical Addresses
A program may include instructions that transfer control. For
example, in BASIC a programmer can say
GOTO 200
where 200 is the line number of the instruction to be executed
next.
This line number is relative to the start of the program and so is
a logical address.
However, the physical address is the actual location in memory
where this instruction is stored.
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Memory Management



A logical address (sometimes called a virtual or
relative address) is a value that specifies a generic
location, relative to the program but not to the reality
of main memory.
A physical address is an actual address in the main
memory device.
Operating systems must employ techniques to:


Track where and how a program resides in memory.
Convert logical program addresses into actual memory
addresses.
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Memory Management

There are three approaches to memory
management depending on how we conceive
of memory being organised:



Single Contiguous Memory
Partitioned Memory
Paged Memory
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Single Contiguous Memory
Management

There are only two
programs in memory


Figure 10.4
Main memory
divided into two
sections

The operating system
The application
program
This approach is called
single contiguous
memory management.
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Single Contiguous Memory
Management


In this system, a logical address is simply an
integer value relative to the starting point of
the program.
To produce a physical address, we add a
logical address to the starting address of the
program in physical main memory.
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Single Contiguous Memory
Management
Figure 10.5
binding a logical address
to a physical one
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Partition Memory Management


When using fixed partitions, main memory is
divided into a particular number of partitions.
When using dynamic partitions, the partitions
are created to fit the need of the programs.
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Partition Memory Management



Figure 10.6
Address resolution
in partition memory
management
At any point in time, memory
is divided into a set of
partitions, some empty and
some allocated to programs.
The Base register holds the
beginning address of the
current partition.
The Bounds register holds the
length of the current partition.
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Partition Selection

First fit


Best fit

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Program is allocated to the first partition big
enough to hold it.
Program is allocated to the smallest partition big
enough to hold it.
Worst fit

Program is allocated to the largest partition big
enough to hold it.
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Paged Memory Management

Paged memory technique: main memory is
divided into small fixed-size blocks of storage
called frames.


A program is divided into pages that (for the sake
of our discussion) we assume are the same size as
a frame.
The operating system maintains a separate
page-map table (PMT) for each program in
memory.
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Paged Memory Management


Figure 10.7
A paged memory
management
approach
To produce a physical
address, you first look up
the page in the PMT to
find the frame number in
which it is stored.
Then multiply the frame
number by the frame size
and add the offset to get
the physical address.
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Paged Memory Management

An important extension is demand paging.


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Not all parts of a program actually have to be in
memory at the same time.
In demand paging, the pages are brought into
memory on demand.
The act of bringing in a page from secondary
memory, which often causes another page to be
written back to secondary memory, is called a
page swap.
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Paged Memory Management


The demand paging approach gives rise to the
idea of virtual memory, the illusion that there
are no restrictions on the size of a program.
Too much page swapping, however, is called
thrashing and can seriously degrade system
performance.
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Resource Management


A process can be defined as a program in
execution.
The operating system performs process
management to carefully track the progress of
each process and all of its intermediate states.
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Batch Processing
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Figure 10.2 In early systems, human operators would organize jobs into batches
Timesharing



Multiprogramming allowed multiple processes to be
active at once, which gave rise to the ability for
programmers to interact with the computer system
directly, while still sharing its resources.
A timesharing system allows multiple users to
interact with a computer at the same time.
In a timesharing system, each user has his or her
own virtual machine, in which all system resources
are (in effect) available for use.
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Process Management

The Process States
Figure 10.8 The process life cycle
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The Process Control Block


The operating system must manage a large
amount of data for each active process.
Usually that data is stored in a data structure
called a process control block (PCB).

Each state is represented by a list of PCBs, one
for each process in that state.
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The Process Control Block


Keep in mind that there is only one CPU and therefore only
one set of CPU registers.

These registers contain the values for the currently executing
process.

The values define the state of the machine at any given time.
Each time a process is moved to the running state:

Register values for the interrupted process are stored into its PCB.

Register values of the process admitted to the running state are
loaded into the CPU from its waiting state PCB.

This exchange of information is called a context switch.
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CPU Scheduling


The act of determining which process in the
ready state should be moved to the running
state.
That is, decide which process should be given
over to the CPU.
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CPU Scheduling

Nonpreemptive scheduling occurs when the currently
executing process gives up the CPU voluntarily.

Preemptive scheduling occurs when the operating system
decides to favour another process, preempting the currently
executing process.

Turnaround time for a process is the amount of time between
when the process arrives in the ready state to the time it exits
the running state for the last time.
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CPU Scheduling
In each of the following
examples we will
consider 5 processes
arriving in the Ready
state. The service time
for each is listed in this
table.
How does the dispatcher
decide their order?
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First-Come, First-Served


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The first ordering structure that comes to
mind is the queue.
Processes are moved to the CPU in the order
in which they arrive in the Ready state.
FCFS scheduling is nonpreemptive – one
process completes before the next begins.
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First-Come, First-Served

Page 336
The Gantt Chart
below shows the
order and turn
around time for the
5 processes.
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First-Come, First-Served
Page 336
Process
Turn Around
Time
p1
140
p2
215
p3
535
p4
815
p5
940
Average
529
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Shortest Job Next


This technique looks at all processes in the
Ready state and dispatches the one with the
shortest service time.
It is also generally implemented as a
nonpreemptive algorithm.
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Shortest Job Next
The same 5 processes
produce a much smaller
average turn around time.
SJN is provably optimal as a
strategy.
It’s weakness is that it relies
on knowledge of the future.
Process
Turn Around
Time
p2
75
p5
200
p1
340
p4
620
p3
940
Average
435
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Round Robin Scheduling



…distributes the processing time equitably
among all ready processes.
The algorithm establishes a particular time
slice (or quantum), which is the amount of
time each process receives before being
preempted. It is then returned to the ready
state to allow another process its turn.
The Round-robin algorithm is preemptive.
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Round Robin Scheduling
Suppose the time
slice is 50.
This Gannt Chart
shows how the
processes will be
scheduled.
Process
Turn Around
Time
p1
515
p2
325
p3
940
p4
920
p5
640
Average
668
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Round Robin Scheduling


Notice that Round Robin is much less
efficient in principle.
However, multiprogramming requires a preemptive strategy. Can you think of a reason?
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