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Introduction to the Course
 Operating systems – essential part of any
computer system
 Course discusses:
 What they are
 What they do
 How they are designed and structures
 Common features
 Processes, Threads, CPU-scheduling, Synchronization, Deadlocks, Memory
Management, Virtual Memory, File system interface
 Book:
 Silberschatz, Galvin and Gagne, Operating System
Concepts – 9th Edition

Main Themes
Topic
#
Introduction
1
Operating System Structures
2
Processes
3
Threads
4
CPU Scheduling
5
Process Synchronization
6
Deadlocks
7
Memory Management
8
Virtual Memory
9
File-System Interface
10
File-System Implementation
11

Chapter 1: Objectives
 To provide a grand tour of the major
operating systems components
 To provide coverage of basic computer
system organization

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

Example – MS-Paint over Windows
 Assume we are using MS-Paint over Windows -
when do we need to access the OS?
 Loading the application / terminating the application
 Memory allocation / management (e.g., paging)
 Access to IO devices – keyboard, mouse, printer,
monitor
 CPU allocation
 Copy / Paste (inter-process communication)

Operating System Design & Goals
 Each OS has different goals and design:
 Mainframe – maximize HW utilization/efficiency
 PC – maximum support to user applications
 Handheld – convenient interface for running applications,
performance per amount of battery life
convenience
efficiency
performance,
resource utilization
ease of use

Mainframe, PC, Handheld
 Supercomputer - computer at the frontline of current
processing capacity, particularly speed of calculation
 Mainframe – powerful computers used mainly by large
organizations for critical applications (the term originally referred to the large
cabinets that housed the central processing unit and main memory of early computers. Later the term was used to
distinguish high-end commercial machines from less powerful units)
 Personal Computer (PC) - any general-purpose
computer whose size, capabilities, and original sales
price make it useful for individuals (and which is intended to be operated directly by
an end-user with no intervening computer operator)
 Handheld - pocket-sized computing device, typically
having a display screen with touch input and/or a
miniature keyboard.
 Of course, one generation's "supercomputer" is the next
generation's "mainframe"

Computer System Structure
 Computer system can be divided into four components
 Hardware – provides basic computing resources
 CPU, memory, I/O devices
 Operating system
 Controls and coordinates use of hardware among various
applications and users
 Application programs – define the ways in which the system
resources are used to solve the computing problems of the
users
 Word processors, compilers, web browsers, database
systems, video games
 Users
 People, machines, other computers

Four Components of a Computer System

Operating System Definition
 OS is a resource allocator
 Manages all resources
 Decides between conflicting requests for efficient and fair
resource use
 OS is a control program
 Controls execution of programs to prevent errors and improper
use of the computer

Operating System Definition (Cont.)
 No universally accepted definition
 “Everything a vendor ships when you order an operating system”
is good approximation
 But varies wildly
 “The one program running at all times on the computer” is the
kernel. Everything else is either a system program (ships with
the operating system) or an application program
 Along with the kernel, there are two other types of programs:
 system programs, which are associated with the
operating system but are not necessarily part of the kernel,
and
 application programs, which include all programs not
associated with the operation of the system.

Computer Startup
 bootstrap program is loaded at power-up or reboot
 Typically stored in ROM or EPROM, generally known as
firmware
 Initializates all aspects of system
 Loads operating system kernel and starts execution

Computer System Organization and Architecture
 Computer-system operation
 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

Device Controller
 Each device controller is in charge of a particular
device type (thus competing on memory cycles)
 Each device controller has a local buffer
 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

Lecture_01 Operating System Course Introduction

Computer-System Architecture
 Most systems use a single general-purpose processor
(PDAs through mainframes)
 Most systems have special-purpose processors as well
 Multiprocessors systems (two or more processors in close communication,
sharing bus and sometimes clock and memory) growing in use and
importance
 Also known as parallel systems, tightly-coupled systems
 Advantages include
1. Increased throughput
2. Economy of scale
3. Increased reliability – graceful degradation or fault tolerance

Multiprocessors systems
Two types of
Multiprocessing:
1. Asymmetric
Multiprocessing - assigns
certain tasks only to certain processors.
In particular, only one processor may be
responsible for handling all of the
interrupts in the system or perhaps even
performing all of the I/O in the system
2. Symmetric Multiprocessing
- treats all of the processing elements in
the system identically
Key role – the scheduler

Clustered Systems
 Like multiprocessor systems, but multiple systems working
together
 Usually sharing storage via a storage-area network (SAN)
 Provides a high-availability service which survives failures
 Asymmetric clustering has one machine in hot-standby
mode
 Symmetric clustering has multiple nodes running
applications, monitoring each other
 Some clusters are for high-performance computing (HPC)
 Applications must be written to use parallelization

Interrupts
 Interrupt transfers control to the interrupt service routine
generally, through the interrupt vector, which contains the
addresses of all the service routines
 Interrupt architecture must save the address of the
interrupted instruction (and the state of registers if about to change)
 Incoming interrupts are disabled while another interrupt is
being processed to prevent a lost interrupt
 A trap is a software-generated interrupt caused either by an
error or a user request
 An operating system is interrupt driven

Interrupt Handling
 The operating system preserves the state of the
CPU by storing registers and the program counter
 Determines which type of interrupt has occurred:
 polling
 vectored interrupt system
 Separate segments of code determine what action
should be taken for each type of interrupt
Ideally, we would have used a generic code for analyzing the
interrupt information and deciding what code to run, however
speed is critical here…

I/O Structure
 After I/O starts, control returns to user program only upon I/O
completion
 Wait instruction idles the CPU until the next interrupt
 Wait loop (contention for memory access)
 At most one I/O request is outstanding at a time, no
simultaneous I/O processing
 After I/O starts, control returns to user program without waiting
for I/O completion
 System call – request to the operating system to allow
user to wait for I/O completion
 Device-status table contains entry for each I/O device
indicating its type, address, and state
 Operating system indexes into I/O device table to
determine device status and to modify table entry to
include interrupt
Synchronous
Asynchronous

Two I/O Methods
Synchronous Asynchronous

Memory Management
 All data in memory before and after processing
 All instructions in memory in order to execute
 Memory management determines what is in memory when
 Optimizing CPU utilization and computer response to
users
 Memory management activities
 Keeping track of which parts of memory are currently
being used and by whom
 Deciding which processes (or parts thereof) and data to
move into and out of memory
 Allocating and deallocating memory space as needed

Storage Management
 OS provides uniform, logical view of information storage
 Abstracts physical properties to logical storage unit - file
 Each medium is controlled by device (i.e., disk drive, tape
drive)
 Varying properties include access speed, capacity,
data-transfer rate, access method (sequential or
random)
 File-System management
 Files usually organized into directories
 Access control on most systems to determine who can
access what
 OS activities include
 Creating and deleting files and directories
 Primitives to manipulate files and dirs
 Mapping files onto secondary storage
 Backup files onto stable (non-volatile) storage media

Mass-Storage Management
 Main memory – only large storage media that the CPU can
access directly
 Why using disks?
 Store data that does not fit in main memory
 Store data that must be kept for a “long” period of time
 Proper management is of central importance
 Entire speed of computer operation hinges on disk subsystem
and its algorithms
 OS activities
 Free-space management
 Storage allocation
 Disk scheduling

Mass Storage Management (2)
 Some storage need not be fast
 Includes optical storage, magnetic tape
 Not critical to the computer performance but still must be
managed
 Varies between WORM (write-once, read-many-times)
and RW (read-write)

Storage Structure
 Secondary storage:
 Extension of main memory
 Provides large nonvolatile storage capacity
 Magnetic disks – rigid metal or glass platters
covered with magnetic recording material
 Disk surface is logically divided into tracks, which are
subdivided into sectors
 The disk controller determines
the logical interaction
between the device and the
computer

Storage Hierarchy
 Storage systems organized in hierarchy
 Speed
 Cost
 Volatility
It takes some time (several CPU
cycles) to read/write to main
memory – in the meantime the
processor needs to stall because it
doesn’t have the necessary data
Expensive
but
faster

Caching
 Important principle, performed at many levels in a computer (in
hardware, operating system, software)
 Information in use copied from slower to faster storage temporarily
 Faster storage (cache) checked first to determine if information is
there
 If it is, information used directly from the cache (fast)
 If not, data copied to cache and used there
 Cache smaller than storage being cached
 Cache management important design problem
 Cache size and replacement policy

Performance of Various Levels of Storage
 Movement between levels of storage hierarchy can be explicit or
implicit

Migration of Integer A from Disk to Register
 Multitasking environments must be careful to use most recent
value, no matter where it is stored in the storage hierarchy
 Multiprocessor environment must provide cache coherency in
hardware such that all CPUs have the most recent value in their
cache
 Distributed environment situation even more complex
 Several copies of a datum can exist
 Various solutions covered in Chapter 17

Direct Memory Access Structure
 Used for high-speed I/O devices able to
transmit information at close to memory
speeds
 Good example: tape, disk
 Bad example: keyboard
 Device controller transfers blocks of data
from buffer storage directly to main
memory without CPU intervention
 Only one interrupt is generated per block,
rather than the one interrupt per byte

Operating System Structure
 Multiprogramming needed for efficiency
 Single user cannot keep CPU and I/O devices busy at all times
 Multiprogramming organizes jobs (code and data) 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 program executing in memory process
 If several jobs ready to run at the same time  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

Memory Layout for Multiprogrammed System

Operating-System Operations
 Interrupt driven by hardware
 Software error or request creates exception or trap
 Division by zero, request for operating system service
 Other process problems include infinite loop, processes modifying
each other or the operating system
 Dual-mode operation allows OS to protect itself and other system
components
 User mode and kernel mode
 Mode bit provided by hardware
 Provides ability to distinguish when system is running user
code or kernel code
 Some instructions designated as privileged, only
executable in kernel mode
 System call changes mode to kernel, return from call resets
it to user

Transition from User to Kernel Mode
 Timer to prevent infinite loop / process hogging resources
 Set interrupt after specific period
 Operating system decrements counter
 When counter zero generate an interrupt
 Set up before scheduling process to regain control or terminate
program that exceeds allotted time

Process Management
 Process and Program:
 A process is a program in execution (unit of work within the system).
 Program is a passive entity, process is an active entity.
 Process needs resources to accomplish its task
 CPU, memory, I/O, files (received upon creation and along execution)
 Initialization data (e.g., a process for presenting the status of a file)
 Process termination requires reclaim of any reusable resources
 Single-threaded process has one program counter
specifying location of next instruction to execute
 Process executes instructions sequentially, one at a time, until
completion
 Multi-threaded process has one program counter per thread
 Typically system has many processes, some user, some operating
system running concurrently on one or more CPUs
 Concurrency by multiplexing the CPUs among the processes /
threads

Process Management Activities
The operating system is responsible for the following
activities:
 Creating and deleting both user and system
processes
 Suspending and resuming processes
 Providing mechanisms for process
synchronization
 Providing mechanisms for process communication
 Providing mechanisms for deadlock handling

Protection and Security
 Protection – any mechanism for controlling access of processes or
users to resources defined by the OS
 Security – defense of the system against internal and external attacks
 Huge range, including denial-of-service, worms, viruses, identity
theft, theft of service
 Systems generally first distinguish among users, to determine who can
do what
 User identities (user IDs, security IDs) include name and
associated number, one per user
 User ID then associated with all files, processes of that user to
determine access control
 Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file

Computing Environments
 Traditional computer
 Blurring over time
 Office environment
 PCs connected to a network, terminals attached to
mainframe or minicomputers providing batch and
timesharing
 Now portals allowing networked and remote systems
access to same resources
 Home networks
 Used to be single system, then modems
 Now firewalled, networked

Computing Environments (Cont)
 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

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 and Gnutella

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 XP, which can be clients and
servers

Open-Source Operating Systems
 Operating systems made available in source-code
format rather than just binary closed-source
 Counter to the copy protection and Digital Rights
Management (DRM) movement
 Started by Free Software Foundation (FSF), which
has “copyleft” GNU Public License (GPL)
 Examples include GNU/Linux, BSD UNIX
(including core of Mac OS X), and Sun Solaris

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Lecture_01 Operating System Course Introduction

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