CMP325

Operating Systems
Lecture 01
Introduction to Operating Systems
Fall 2021
Arif Butt (PUCIT)
Note:
Some slides and/or pictures are adapted from course text book and Lecture slides of
• Dr Syed Mansoor Sarwar
• Dr Kubiatowicz
• Dr P. Bhat
• Dr Hank Levy
• Dr Indranil Gupta

For practical implementation of operating system concepts discussed in these slides,

students are advised to watch and practice video lectures on the subject of OS with
Linux by Arif Butt available on the following link:
http://www.arifbutt.me/category/os-with-linux/
 Today’s Agenda
• Basic Course information and Class Protocols

• Course Outline

• Operating System Overview

– OS An Abstraction

– Interrupt, Trap and Signal

– Dual mode operation

– I/O, Memory and CPU Protection

– Operating System Services

– Types of operating system

– Computing environments

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 Course Information
• Required Textbook: Operating System Concepts,
10th Edition Silbershatz, Galvin, Gagne
• Grades Website: http://online.pucit.edu.pk
• Resources Website: http://arifbutt.me

• Prerequisites : Nill
• Corequisites : CMP223, CMP210
• Office: New Faculty Building # G7
• Students Counseling hours:
– Mentioned on http://arifbutt.me
• Teaching Assistant info:
– Mentioned on http://arifbutt.me
• 24 hour turnaround for email: arif@pucit.edu.pk

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 Course Information
Visit Course Web Site for following information:
http://www.arifbutt.me
– Lecture Slides
– Video Lectures (OS with Linux)
– Quizzes + Assignments + Labs
– Announcements
– Teaching Assistants
– SOPs and Course related Policies
– Misc Resources

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 Lecture Format
• Please come to class (in time)
• I will mark the attendance in the first five minutes. Students
with more than eight absents will not be allowed to sit in the
class and their names will be struck off the register. (Be
watch full)
• To Help you understand important and hard OS concepts
please read the relevant textbook sections before lecture
• Exam, quiz, PA and HW questions could be from anywhere in
the lecture slides, textbook sections covered, reading
material provided. Your best strategy is to play it safe –
read all material referred to in lecture
• Make sure you also
– Periodically check lecture notes on course web page
– Utilize the student counseling hours

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 Lab Format
• Please come to Labs (in time)
• There are one or two video lectures associated
with each lab, uploaded on course web site. Please
listen to those lectures twice before coming to lab
• To Help you understand important and hard OS
concepts please read the relevant Chapters of
UNIX The Textbook by Dr Mansoor Sarwar
• Quizzes might be taken in class or in Lab, so don’t
miss
• Contents covered in the Lab will come in the
Quizzes as well as in the Mid and Final exams

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 Who cares to get an A?
• Final exam: 40%

• Mid-exam: 35%

• Sessionals: 25%
– Surprise Quizzes: 10%

– Assignments (PA/LW/HW): 5%

– Lab : 10%

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 Surprise Quizzes
• There will be surprise quizzes, given at the start
of a lecture, during any lecture. The total
number of quizzes could be anywhere between 4
and 40.
• NO LATE or MAKEUP SURPRISE QUIZZES,
under any circumstances whatsoever
• Surprise quizzes are completely individual
efforts.
• Your best strategy is to play it safe – attend
every lecture and do the reading/programming
assignments
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 Cheating Policy
• Academic integrity

• Both the cheater and the student who aided the

cheater will be held responsible for the cheating

• The instructor may take actions such as:

– require repetition of the subject work,

– assign 'zero' or may be ‘negative’ marks for the

subject work,

– for serious offenses, assign an F grade for the

course
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 Late Policy for HWs and PAs
• Grade book system details can be checked on :
http://online.pucit.edu.pk
• Late policy for Assignment, Quizzes, and other
deliverables
– No late Assignment submissions!
– No late quizzes or exams!
• Sticking to dates is your responsibility!
– Check announcements on lecture notes regularly
• Your best strategy is to play it safe – submit everything
on time
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 Playing it Safe in CMP325
If you follow these 4 simple rules during the CMP320 class,
you'll make sure that you do well in the course:
1. Attend every lecture + Lab
2. Study the course material (video lectures, textbook
sections assigned + slides + Reading assignments)
3. Submit everything (PAs, HWs, quizzes, exams) on
time - don't be late
4. Don't cheat

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Programming Assignments
&
Linux Shell Commands

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No Pains No Gains

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Outline

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Operating System
Overview

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Operating System–An Abstraction

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Operating System–An Abstraction

CMP320 PUCIT Arif Butt 17

Operating System–An Abstraction

CMP320 PUCIT Arif Butt 18

Operating System–An Abstraction

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Operating System–An Abstraction

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Computer System Organization

Multi-processor Architecture Multi-core Architecture

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 Computer System Organization
• One or more CPUs, device controllers connect
through common bus providing access to shared
memory
• Concurrent execution of CPUs and devices
competing for memory cycles

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 Computer System Operation
• I/O devices and the CPU can execute concurrently
• Each device controller is in charge of a particular
device type
• Each device controller has a local buffer
• Each device controller type has an operating
system device driver to manage it
• CPU moves data from/to main memory to/from
local buffers
• I/O is from the device to local buffer of
controller
• Device controller informs CPU that it has finished
its operation by causing an interrupt
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 What Is an OS?
“Code” that:
• Sits between programs & hardware
• Sits between different programs
• Sits betweens different users
But what does it do?
Provides an orderly and controlled allocation of the
processor(s), memory(ies) and I/O devices among the
various programs competing for them

Loose analogy:
– Government: creates and enforces laws that govern
resources (money, land, houses, vehicles, oil, etc.)
but allows citizens to have freedom with using the
resources (as long as the citizen obeys laws)
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 What Is an OS? (…)
Resources Services
• Allocation • Abstraction
• Protection • Simplification
• Reclamation • Convenience
• Virtualization • Standardization

Resource management includes multiplexing (sharing)

resources in two ways:
• Time Multiplexed (CPU sharing, Printer sharing).
• Space Multiplexed (Memory, Hard Disk).

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 What is an OS? (…)
Objective of an OS is to Tame the h/w so that people can program and can get something useful out of it.

Top Down View: A program that acts as an intermediary between a

user of a computer and the computer hardware. Present the user with the
equivalent of an extended machine or virtual machine that is easier to
program than the underlying hardware. (HOW?)

Bottom Up View: A program that allocates and de-allocates computer

system resources (CPUs, memories, timers, disks, mice, network
interfaces, printers, …) in an efficient, fair and secure manner. (HOW?)

“An OS is a program running at all times on the computer (usually

called the kernel), that controls the execution of application programs
and acts as an interface between the user of a computer and the
computer hardware.”
“Primary goal of OS is convenience of user and secondary goal is
efficient operation of the computer system.”
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 Name some OSs
• UNIX • Android from Google
• Linux • BlackBerry from RIM
• Sun Solaris • iOS from Apple
• Macintosh • Symbian from Nokia
• PC BSD • Windows phone from MS
• MS Windows

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 Interrupt
• Virtually all computers provide a
mechanism by which other modules (I/O,
memory) may interrupt the normal
sequencing of the processor.
Answer
• Provided to improve processor utilization
The
Phone
• Most I/O devices are slower than the .
.

processor and Processor must pause to .

.
.
wait for device .

Resume
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 Sub Routine Call vs Interrupt
Sub routine Call Interrupt
• Occur due to an internal event
• Occur due to execution of
(setting of flag) or an
an instruction
external event (key press)
• Address of subroutine is • Address of ISR is determined
determined by the address by h/w (vectored / non
part of an instruction vectored interrupt)
• Other than contents of PC,
• Stores only the contents of
store the state of processor
PC (return address)
as well (all processor
registers)

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 Interrupts, Traps and Signals
• Interrupt. An event generated by an I/O
device to get the attention of CPU and control
goes to the OS. State of the CPU is saved, ISR
is executed and then state of CPU is restored.
• Trap. An event generated by CPU and control
goes to the OS. Normally when an instruction is
executed that may cause a division by zero or Answer
protection error. State of CPU is not saved and The
Phone
.
TSR is executed.
.
• Signal. A notification given to a process by the .
.
OS because the process did something, the .
user did something, one process wants to tell .

another process something. Resume

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 Multiprogramming vs Multitasking
Multiprogramming (Batch system) needed for efficiency
• Single user cannot keep CPU and I/O devices busy at all
time
• Multiprogramming organizes jobs so CPU always has one
to execute
• A subset of total jobs in system is kept in memory
• One job selected and run via job scheduling
• When it has to wait (for I/O for example), OS switches
to another job

Timesharing (multitasking) is logical extension in which

CPU switches jobs so frequently that users can interact
with each job while it is running, creating interactive
computing
• Response time should be < 1 second
• Each user has at least one process executing in memory
• If several processes are ready to run at the same time,
then OS performs CPU scheduling
• If processes don’t fit in memory, swapping moves them
in and out to run
• Virtual memory allows execution of processes not
completely in memory
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 PROTECTION
In Time Sharing System there are multiple
processes using various resources of the
computer. Sounds great, but like no free
lunch, it has disadvantage as well, i.e. the issue
of protection
▪ Keep user programs from crashing the OS
▪ Keep user programs from crashing each other
▪ Keep parts of OS from crashing other parts
Protection is implemented by keeping two
modes
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 Dual Mode Operation
• To protect the OS and all other programs and
their data from any malfunctioning program,
protection is needed for any shared resource. It
is done by keeping two modes:
• User Mode. Execution done on behalf of a
user program
• Monitor/Kernel/System/Supervisor Mode.
Execution done on behalf of OS
• Mode bit added to computer h/w to indicate the
current mode: Kernel(0), User (1) Interrupt/fault
When an interrupt or fault occurs h/w switches to monitor
mode.
monitor user
Privileged instructions can only be executed in monitor set user mode
mode.
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Transition from user to Kernel Mode
• A user process must not be given an open access to kernel code
• Any user/application request that involves access to any system
resource must be handled by the kernel code
• The mechanism used by an application program to request a
service from Operating System is via a system call (details in
next lecture)
• A System call is usually a request to the OS kernel to do a h/w,
system specific or privileged operation.

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How to protect Processes from one another?
Need three important things:
• Protection of I/O devices
• Every process should not have access to every device
• Protection of memory
• A process should not access another process address
space
• Preemptive switching from task to task
• Use of timer
• Must not be able to disable timer from user code

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 I/O PROTECTION
• All I/O instructions are privileged instruction, no user
process can execute an I/O instruction directly. If he/she
is permitted he/she can always overwrite/delete some
ones else data
• I/O is done using system call. A way that an OS allows a
user process to invoke an operation such as an I/O
operation e.g. opening, reading and writing to a disk file
MOV RAX, 1 ;Place system call number in RAX
MOV RDI, 1 ;Place first argument in RDI
MOV RSI, msg ;Place second argument in RSI
MOV RDX, 26 ;Place third argument in RDX
syscall ;write(1, msg, sizeof(msg))

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 Memory Protection
• Process Address Space. Region in the main
memory that a process can legally access is known
as its address space. A process must not be
allowed to go outside its address space
• Implementation. CPU uses two registers to
determine the range of logical addresses a
program may access
• Base Register holds the smallest/starting
address of the Address Space assigned to a
process
• Limit Register contains the size of the address
space allocated to the process
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Memory Protection (cont…)

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Memory Protection (cont…)

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 CPU Protection
• In multiprogramming or time sharing OS, multiple
processes are running at a time and we want to make sure
that the CPU must not stay with a process for an infinite
amount of time
• Implementation.
• Timer interrupts computer after a specified period to ensure OS
maintains control
• Timer (counter register) is loaded with a predetermined value and
decremented every clock tick
• When the timer reaches the value 0, an interrupt occurs and an
ISR is executed to switch CPU to another process
• How Timer is loaded? This is also a privileged instr, because no
one other than the admin should be allowed to assign a time slice
to various processes
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 Operating System Services
Some important tasks a kernel performs are:
• Process management and IPC
• Signal handling
• Synchronization and Deadlock management
• Memory management
• Mass storage management and File management
• Information management
• User management and security
• Networking services (wired + wireless)
Two methods by which a program can make requests for
services from the Kernel:
• By making a system call
• By calling a library routine that makes use of this system
call
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 Types of
Operating Systems

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 Network Operating Systems
• In NOS, the users are aware of the existence of multiple
computers and can log in to remote machines and copy files
from one machine to another
• Each machine runs its own local OS and has its own local
user(s)
• A NOS is a collection of s/w and associated protocols that
allow a set of autonomous computers which are inter-
connected by a computer NW to be used together in a
convenient and cost effective manner
• Remote Login. Each user normally works on his/her own system; using a
different system requires some kind of remote login, instead of having
the OS dynamically allocate processes to CPU
telnet pucit.edu.pk
• Remote File Transfer. Users are aware of where their files are kept
and must move file from one system to another with explicit file
transfer commands instead of having file placement managed by OS
ftp pucit.edu.pk
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 Distributed OS
• A Distributed System is a collection of independent
computers that appears to its users as a single
coherent system

• The users should not be aware of where their

programs are being run or where their files are
located; that should all be handled automatically
and efficiently by the Operating System
“A distributed system is one where I can’t do work
because some machine (in some other part of the
world) I have never heard of isn’t working”
Lamport 44
 NOS vs DOS
• File System. In NOS, control over file placement
must be done manually by the user, where as in a
DOS it is done automatically by the system itself
• Protection. In NOS, there are various machines,
each with its own user to UID, but in DOS there is
a single system wide mapping that is valid every
where
• Program Execution. In the most distributed case
the system chooses a CPU by looking at the
processing load of the machine, location of file to
be used etc. In the least distributed case, the
system always run the process on one specific
machine. (usually the machine on which the user is logged in)
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 Real Time Systems
• In Real Time systems, the correctness of the system
depends not only on the logical result of computation but
also on the time at which the results are produced
• Examples are Process Control Plants, Robotics, Air Traffic
Control, Telecommunications, Missile Control System
• Hard Real Time System. Output should be produced within
the given time constraints, otherwise, the result is life
threatening; e.g. Plane landing systems, process control in
nuclear power plants, missile control system. Secondary
storage is limited or absent, data stored in short term
memory or ROM. No virtual memory
• Soft Real Time System. Output should be produced within
the given time constraints, but if it is not, the result is not
life threatening; e.g. applications of multimedia and virtual
reality
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 Computing Environment
• Client Server computing
• Peer to Peer computing
• Web based computing
• Multi-processor computing
• SMP architecture
• Asymmetric computing
• Clustered computing
• Embedded computing

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 Computing Environment (…)
Client-Server Computing
Dumb terminals supplanted by smart PCs
Many systems now servers, responding to requests
generated by clients
Compute-server provides an interface to client to
request services (i.e. database)
File-server provides interface for clients to store
and retrieve files

CMP320 PUCIT Arif Butt 48

 Computing Environment (…)
• Peer to Peer Computing
• Another model of distributed system
• P2P does not distinguish clients and servers
– Instead all nodes are considered peers
– May each act as client, server or both
– Node must join P2P network
• Registers its service with central lookup service
on network, or
• Broadcast request for service and respond to
requests for service via discovery protocol
– Examples include Napster , KaZaA, and Gnutella
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 Computing Environment (…)
Web Based Computing
• Web has become ubiquitous
• PCs most prevalent devices
• More devices becoming networked to allow web
access
• New category of devices to manage web
traffic among similar servers: load balancers
• Use of operating systems like Windows 95,
client-side, have evolved into Linux and
Windows 7, which can be clients and servers

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 Computing Environment (…)
Multiprocessor Systems
• Also called parallel systems or tightly coupled
systems
• Multiple CPUs in a single entity, comm using system
bus
• Advantages
• Increased throughput
• Increased reliability (Graceful degradation, fault
tolerance)
• Saves money by not duplicating power supplies,
housings, and peripherals
• Disadvantage
• More complex in both hardware and software
than uni-processor systems
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 Computing Environment (…)
Types of Multiprocessor Systems

• Symmetric multiprocessors Two or more identical

processors connected to a single shared memory,
having full access to all I/O devices, controlled by
a single operating system instance that treats all
processors equally (reserving none for special
purpose) is called SMP architecture. SMP
architecture treats multiple cores as separate
processors

• Asymmetric multiprocessors Master slave

scenario. Master distributes tasks among the
slaves, and I/O is usually done by master only
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 Computing Environment (…)
Clustered Systems
• Constructed by combining multiple computers into a
single system to perform a computational task
distributed across the cluster
• They share storage and are linked via LAN
• Clustered systems communicate using messages, while
processors in a multiprocessor system could
communicate using shared memory
• Used to provide high available service. Can be:
• Asymmetric Cluster One machine is in hot standby
mode, doing nothing, while other is running. Hot
standby mode machine monitors the server. If
server fails hot stand by host become active server
• Symmetric / Parallel Cluster Two or more hosts
are running an application and monitoring each other.
More efficient as it utilizes all of the available
hardware 53
 Embedded OS
• Pervasive Computing
• Cheap processors embedded every where
• How many are on your body now? In your car?
• Cell phones, PDAs, iPads, iPod…
• Typically very constrained h/w resources
• Slow processors
• Small amount of memory
• No disk or tiny disk
• Typically only one dedicated application.
• Limited power
• But technology changes fast
• Embedded CPUs are getting faster
• Storage is growing rapidly
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SUMMARY

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We’re done for now, but Todo’s for
you after this lecture…
• Go through the slides and Book Sections: 1.1-1.12
• Go through Unix The Text Book Chapters 0 - 3
• Get a Secure Shell account in the Lab. Log in using your SSH
account in lab and try executing some basic shell commands.
• Copy following resources from course web site
• Video Lectures Series on Linux by Arif Butt
• UNIX The Text Book
• ITC Lecture Slides (Basic Linux Commands)
• Basic Linux Tutorial
• If you are using MS Windows at your home PC, please dual boot
it with some Linux distribution or install Linux in virtual
environment using Virtual Box

If you have problems visit me in student counseling hours. . . .

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