Chapter 1: Operating Systems Overview

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Chapter 1 Operating Systems Overview
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Content

• Introduction
• What is Operating Systems
• What Operating Systems Do
• Computer-System Organization
• Computer-System Architecture
• Operating Systems structure
• User Operating System Interface
• Types of Operating System
• Virtual Machines
• System Boot
• Function of Operating Systems
• As a User/Computer Interface
• As Resource Manager
• Characteristic of the Operating Systems
• Process Management
• Memory Management
• Storage Management
• Protection and Security

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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.

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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

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Layers of Computer Systems
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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

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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

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Computer System Organization

• 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

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Operating Systems Structure

• User Operating System Interface
• Types of Operating System
• Virtual Machines
• System Boot

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User Operating System Interface - CLI

• CLI allows direct command entry
• Sometimes implemented in kernel, sometimes by
  systems program
• Sometimes multiple flavours implemented shells
• Primarily fetches a command from user and
  executes it
• Sometimes commands built-in, sometimes just names
  of programs
• If the latter, adding new features doesnt
  require shell modification

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User Operating System Interface - GUI

• User-friendly desktop metaphor interface
• Usually mouse, keyboard, and monitor
• Icons represent files, programs, actions, etc
• Many systems now include both CLI and GUI
  interfaces
• Microsoft Windows is GUI with CLI command shell
• Apple Mac OS X as Aqua GUI interface with UNIX
  kernel underneath and shells available
• Linux/Solaris is CLI with optional GUI interfaces
  (GNOME, KDE,etc)

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Types of OS

• Serial Processing - preOS
• Batch Processing
• Multiprogramming
• Time sharing
• Distributed system Chapter 6
• Real time System Chapter 7

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Serial Processing

• Programmer interacted directly with computer
  hardware
• Actually there is no OS
• Machines run from console consist of display
  light, toggle switch...
• Program in machine code loaded directly into the
  machine
• No scheduling
• Setup included loading the compiler, source
  program, saving compiled program, and loading and
  linking

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Batch Processing

• User no longer has direct access to the machines
• Simple Batch Systems FIRST OS
• Monitors
• Software that controls the sequence of events
• Batch jobs together
• Program branches back to monitor when finished

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Example MS-DOS

• MS-DOS written to provide the most
  functionality in the least space
• Not divided into modules
• Although MS-DOS has some structure, its
  interfaces and levels of functionality are not
  well separated

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Uniprogramming

• Processor must wait for I/O instruction to
  complete before preceding

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Multiprogramming

• When one job needs to wait for I/O, the processor
  can switch to the other job

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Multiprogramming
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Utilization Histograms
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Example
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Time Sharing

• Using multiprogramming to handle multiple
  interactive jobs
• Processors time is shared among multiple users
• Multiple users simultaneously access the system
  through terminals

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Virtual Machines

• A virtual machine takes the layered approach to
  its logical conclusion. It treats hardware and
  the operating system kernel as though they were
  all hardware
• A virtual machine provides an interface identical
  to the underlying bare hardware
• The operating system creates the illusion of
  multiple processes, each executing on its own
  processor with its own (virtual) memory

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Virtual Machines (Cont.)

• The resources of the physical computer are shared
  to create the virtual machines
• CPU scheduling can create the appearance that
  users have their own processor
• Spooling and a file system can provide virtual
  card readers and virtual line printers
• A normal user time-sharing terminal serves as the
  virtual machine operators console

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Virtual Machines (Cont.)

• (a) Nonvirtual
  machine (b) virtual machine

Non-virtual Machine
Virtual Machine
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The Java Virtual Machine
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System Boot

• Operating system must be made available to
  hardware so hardware can start it
• Small piece of code bootstrap loader, locates
  the kernel, loads it into memory, and starts it
• Sometimes two-step process where boot block at
  fixed location loads bootstrap loader
• When power initialized on system, execution
  starts at a fixed memory location
• Firmware used to hold initial boot code

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Function of Operating Systems

• As a User/Computer Interface
• Controls execution of programs to prevent errors
  and improper use of the computer
• As Resource Manager
• Manages all resources

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User/Computer Interface

• User only a computer systems in term of set of
  applications
• OS provides services in the following areas
• Program development - text editor, compiler,
  debugger, utilities,..
• Program execution - Scheduling duties
• Access to I/O devices - Simple reads and writes
• Controlled access to files structure of data,
  protection mechanisms

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User/Computer Interface .cont

• OS provides services in the following areas
• System access protection of resource, resolve
  conflicts
• Error detection response by ending the
  application, retrying the operation, simply
  reporting the error
• Accounting collect usage statistics, Monitor
  performance, anticipate future enhancement,
  billing purpose , etc

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Resource manager

• Responsible for managing resources
• Decides between conflicting requests for
  efficient and fair resource use
• Functions same way as ordinary computer software
• It is program that is executed
• Operating system relinquishes control of the
  processor

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Characteristic of the Operating Systems

• Process Management
• Memory Management
• Storage Management
• Protection and Security

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Process Management

• A process is a program in execution. It is a 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
• Initialization data
• 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

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Process Management Activities

• The operating system is responsible for the
  following activities in connection with process
  management
• 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

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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

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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

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Mass-Storage Management

• Usually disks used to store data that does not
  fit in main memory or 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
• Some storage need not be fast
• Tertiary storage includes optical storage,
  magnetic tape
• Still must be managed
• Varies between WORM (write-once, read-many-times)
  and RW (read-write)

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I/O Subsystem

• One purpose of OS is to hide peculiarities of
  hardware devices from the user
• I/O subsystem responsible for
• Memory management of I/O including buffering
  (storing data temporarily while it is being
  transferred), caching (storing parts of data in
  faster storage for performance), spooling (the
  overlapping of output of one job with input of
  other jobs)
• General device-driver interface
• Drivers for specific hardware devices

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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
• Privilege escalation allows user to change to
  effective ID with more rights

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End of Chapter 1