Software

1
Software

• Operating System

Civilization advances by extending the number of
important operations which we can perform without
thinking about them. -Alfred North
Whitehead, An Introduction to Mathematics, 1911
2
Software Classification

• Software
• Applications System
• MS-Excel
• Quake II Utility Operating System
• WordPerfect Windows
• Linux, Unix
• Solaris
• Shell Kernel

3
Application and System Software

• Application Software
• User needs
• System Model
• Source Code
• Machine instructions
• Programs to help USER perform tasks that utilize
  the machines capabilities

• System Software
• Machines needs
• Services
• Hardware management
• Machine instructions
• Programs that help the MACHINE perform tasks that
  manage the machines functions

4
System Software

• Operating Systems (OS) Utility Tools
• User interface disk formatting
• Manage hardware file-copy
• Manage storage modem functionality
• Manage CPU data compression utilities
• Manage I/O network software
• Distinction between Utility tools and Application
  software is rather vague...
• A Utility software in first release can become
  part of OS in the next release

5
Evolution of Operating Systems

• Early programs (including data) were input in the
  form of punch cards
• Each programmer must have his set of punch cards
  and sign up for using the computer
• The computer operator submits the job (program)
  in the order it was received and leaves the
  printed output for the programmer
• If something goes wrong, operator has no clue
  about the program, and, programmer has no access
  to interactively fix the program

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Batch processing

• Traditional batch processing
• is quite efficient for applications like payroll
  system involving fixed or static computing
  environment,
• in which all data and processing decisions are
  clearly established in advance
• User domain separate from Machine domain

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Interactive and Real-time processing

• Interactive and dynamic computing environment
• Batch processing is highly inefficient for
  applications like airline reservation need some
  form of user-interface to communicate with system
  as it executes
• Real-time processing Time used by machine to
  perform task must be compatible with user needs
• Example Word-processing would be frustrating if
  machine doesnt keep up with user typing

8
Multi-User Environment

• Interactive, real-time processing would be ideal
  for single-user environment, but, many
  applications are multi-user environments
• Airline reservation system would be slow and
  frustrating if only one ticketing agent can make
  a reservation at one time
• Often, different users run different programs on
  the same computer leading to a need for
  handling programs in rotations and time-sharing
  mode
• Multitasking single-user, many programs
• Multi-user environment involves some form of
  time-sharing scheme

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Points to note about early processing

• Each program inputs other instructions is
  submitted as a job JCL explained how the job
  should be run
• User and equipment are separate job is
  submitted as a batch with computer operator
  physically doing the actual computer operations
• A job queue FIFO decides the order of job
  processing
• Order of processing the jobs can be changed
  physically bump up priority for a particular job
• Job Control Language
• First In First Out

10
User Input Format

• Yesterdays input
• Punch cards, card readers, switches
• Todays input
• Keyboard, Mouse, Microphone, scanner, camera
• Tomorrows input?
• Voice recognition, Agents? Virtual environments
  (like Star Treks Holodeck)? Thought
  recognition?!
• Future systems need to be designed to handle such
  user inputs via different user-interface

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Single-processor von Neumann machines

• Can execute more than one program without
  re-wiring
• Cannot execute more than one program at a time,
  though i.e., simultaneously
• Execution of multiple programs is via
  CPU-time-sharing in the form of interleaving
• Batch processing is still in use in certain
  environments like nightly processing of checks in
  a bank, preparing credit card statements
• Todays trend is to have the machine handle as
  many tasks as it can by letting the Operating
  System perform most of the management and control
  issues to get the tasks done.

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User environments

• Single-tasking vs. Multi-tasking
• Single-user vs. Multi-user
• Batch vs. Time-sharing
• How can we accommodate such varied functionality?
• Design a set of layered software that can
• manage processes
• manage users
• manage resources
• manage hardware
• control overall working of the machine

13
Birth of Operating System
14
Layered Software

• Why is OS designed as a layered software?
• It breaks a larger problem to smaller, more
  manageable, subtasks
• Each subtask can be implemented independently
• A layer can be replaced with a different
  implementation without affecting other layers
• Example Command layer can be upgraded or
  modified with a different implementation without
  affecting the Service layer functionality
• Each layer considers the other layer as an
  abstract tool

15
Getting it all started

• Remember the Program Counter in BSM Simulator?
• You had to set the program counter to the cell
  address where your program resides if you didnt
  start at location 00 by default, else, CPU cannot
  do its fetch-decode-execute cycle for that
  program
• How does the CPU know where to start when
  switched on each time?
• It is hard-wired to look for contents at a
  particular pre-determined cell address every time
  it is switched on
• Recall that main memory is volatile meaning,
  its contents are lost when power is turned off.
• How can we ensure that the cell address that CPU
  is hard-wired to look for in main memory retains
  its contents all the time even when power is
  turned off?

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Boot Strapping

• ROM A portion of main memory is constructed such
  that information is permanently burned in and is
  known as Read-Only Memory
• The start-up program for the machine resides in
  this ROM portion of memory which has the same
  cell address that CPU is hard-wired to look for
  when power is turned on
• When CPU is turned on, it looks for this small
  program at its hard-wired cell address always,
  which happens to be the ROM portion of memory
  that always retains its contents and cannot be
  changed
• The small program that is permanently stored in
  ROM is called the Bootstrap.
• Bootstrap programs in PCs usually instruct the
  machine to load the OS from floppy first, and if
  it fails load it from hard-drive.

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Bootstrapping Step1
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Bootstrapping Step 2
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So, what does an Operating System do?

• A better question is, What doesnt an OS do?
• OS acts as the interpreter between the user and
  the hardware and software components of a
  computer
• OS also acts as a traffic police among various
  software components in the computer
• OS acts as a manager by controlling the devices
  attached to the computer system like printer,
  scanner, camera

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Operating System Components

• CPU Management
• Process Management
• Memory Management
• File Management
• I/O System Management
• Interface to the User
• Shells
• Graphical User Interface
• Persistent Storage Management
• Distributed Systems and Networks

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Functional Overview of Modern OS
22
OS Terminology

• Lets understand some key OS terms
• Kernel and Shell
• Process and Program
• Scheduler and Dispatcher
• CPU Interval Timer and Time slices
• Interrupt and Interrupt Handler
• Process Administration
• Interprocess Communication

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OS Kernel and Shell

• Kernel is the core of OS consisting of software
  for very basic computer functions like file
  manager, memory manager and so on.
• Shell is the interface between the OS and the
  user. Shell interface can be textual (as in UNIX)
  or graphical (as in Windows)

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Program and Process

• Program is a static set of instructions that need
  to be executed to accomplish a specific task
• Process the actual execution of such a program
• Process State is a snapshot of the machine at a
  given instant of time, showing such information
  as
• Program counter value
• Data of the program the registers in use, the
  memory cells pertinent to this program

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Scheduler

• Scheduler maintains a record of the processes
  present in the computer system in the form of a
  Process Table stored in main memory
• Each time a new task is assigned to the machine,
  Scheduler adds a new entry into the Process Table
  corresponding to this new task storing such
  information as
• Memory address where the program resides
  (obtained from memory manager)
• Priority of the task
• Ready or waiting

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Dispatcher

• Dispatcher is the component of kernel that
  ensures the actual execution of scheduled
  processes
• In time-sharing machines, dispatcher does this by
• Dividing CPU cycles into time slices using an
  interrupt timer,
• And assigning one time slice per process in turn
  till the process completes and another can start

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CPU Protection The Interval Timer

• Interval Timer interrupts the CPU after specified
  period to ensure operating system maintains
  control.
• Timer is decremented on every clock tick
• When timer reaches the value 0, an interrupt
  occurs
• Timer is commonly used to implement time sharing
• Timer is also used to compute the current time.

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Process Management

• A process is a program in execution. A process
  needs certain resources, including CPU time,
  memory, files, and I/O devices, to accomplish its
  task.
• The operating system is responsible for the
  following activities in connection with process
  management
• Process creation and deletion
• Process suspension and resumption
• Provision of mechanisms for
• Process synchronization
• Process communication

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Processes Administration Time-sharing
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Handling competition among processes

• Semaphores
• Critical Region A set of instructions that can
  be executed by only one process at a time
• Mutual Exclusion Requirement that only one
  process execute such Critical Region instructions
  at any time
• How to implement Mutual Exclusion?
• Use a Semaphore, as a single-instruction
  Test-and-Set flag
• Check for the semaphore flag GO before entering
  the Critical Region set the flag to STOP as
  soon as you enter re-set the flag to GO when
  you exit.

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Handling Competition among processes

• Deadlock When two processes are in time-sharing
  mode, each locks its resource while it is
  executing. This could lead to blocking of
  non-shareable resources. Example
• Process A has access to printer but is waiting
  for access to tape drive which is locked by
  Process B
• And, Process B has access to tape drive and locks
  it, waiting for access to printer.

Deadlock usually can be avoided using Deadlock
Avoidance Schemes, by planning for it and
preventing such occurrences. Deadlocks that are
not avoided, can be detected and corrected
forcibly.
32
Interprocess Communication

• Various processes within the OS must communicate
  with each other constantly to ensure smooth and
  efficient operation
• One form of interprocess communication is the
  Client/Server model
• Each process needing attention or resource, acts
  as the Client and makes it requests
• The OS manager responsible for fulfilling that
  request acts as the Server and serves the request.

33
Memory Management

• Memory is a large array of cells containing words
  or bytes, each with its own address. It holds
  quickly accessible data shared by the CPU and I/O
  devices.
• Main memory is a volatile storage device. It
  loses is contents in the case of a system
  failure.
• The operating system is responsible for the
  following activities in connection with memory
  management
• Keep track of which parts of memory are currently
  being used and by whom.
• Decide which process to load when memory space
  becomes available.
• Allocate and deallocate memory space as needed.

34
Persistent Storage Management

• Main memory is volatile and too small to
  accommodate all data and programs permanently
• Computer system must provide persistent storage
  to back up main memory.
• Disks are the principle on-line storage medium.
• The operating system is responsible for the
  following activities in relation to disk
  management
• Free-space management
• Storage allocation
• Disk scheduling

35
Virtual memory

• In a multi-user, multi-tasking environment, main
  memory space is limited and cannot accommodate
  all the programs
• OS creates an illusion of larger memory
• Memory manager divides the required space into
  pages and store these pages in the hard disk
• As and when the next page is needed, memory
  manager will exchange the new page for the old
  one that is not needed anymore for program
  execution
• Thus programs continue to execute as though there
  was no space crunch to begin with
• Such an illusionary memory space is called
  Virtual Memory

36
File Management

• A file is a collection of related information
  defined by the files creator. Files can be
  collected and grouped in folders or directories
• Usually, files represent programs (both source
  and object forms) and data.
• The operating system does the following
  activities in connection with file management
• File creation and deletion
• Directory creation and deletion
• Support of primitives for manipulating files and
  directories
• Mapping files onto persistent storage
• File backup on off-line media

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I/O System Management

• The I/O system consists of
• A buffer caching system
• A general device-driver interface
• Drivers for specific hardware devices

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User Interface

• A command shell or a window-based GUI provides
• the ability to execute application programs, and,
  
• convenient access to invoking systems programs.
• Shell Interface of an OS allows users to perform
• Process creation and management.
• I/O handling, monitoring, and management
• Secondary storage management
• Main memory monitoring and management
• File-system access
• Protection
• Networking
• Programming Language support

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OS Components Common Requirements

• Any reasonable OS implementation should satisfy
  the following requirements
• Resource Management
• Protection
• Exception Handling
• Asynchronous Operation and I/O
• Scheduling
• Concurrency

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How many kinds of OS are there?

• Too many to list here individually
• MS Windows, Apple MacOS, for single-user desktop
  PCs
• IBMs Vax/VM/CMS for mainframes
• Various implementations (flavors) of Unix like
  Sun/Solaris, HP/UX for multi-user environments
• Different OS are designed for different machine
  architectures, and different groups of systems.
• A Network Operating System (NOS) allows users to
  share files across a network and to access shared
  devices such as printers, scanners, servers and
  gateways.
• A desktop OS, such as Windows 2000, runs a single
  computer NOS, such as Novell NetWare, controls
  how groups of individual computers and other
  devices communicate.

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Communicating with other devices

• CPU and Main Memory together form the Core of a
  machine, which we will call the computer, or the
  processor
• How does the computer communicate with other
  devices?
• Through intermediary devices called Controllers

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Device Controllers

• Each device has its own controller by design
• In modern PCs, the controllers plug into
  motherboard and connect via cables to their
  devices
• Each controller if often a computer in its own
  right, with a CPU and memory circuitry
• Controllers communicate with the processor via
  dedicated controller bus or system bus
• Controllers pass messages and data between their
  respective devices and the processor by
  converting the messages and data into forms
  compatible with the main processor design

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Pictorial View Device Controllers
CPU
Cache
Processor
Printer
Network Medium
Disk
Disk
DiskController
NetworkController
PrinterController
Main Memory
System Bus
CD-drive Controller
Modem Controller
CD-drive
Modem
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Controllers DMA, Ports

• DMA Some controllers have access to system bus
  and hence access to processor main memory, known
  as Direct Memory Access controllers
• Useful when CPU can delegate tasks to controllers
  that can be performed simultaneously and
  independent of CPU execution
• Certain machines have special op-codes designed
  for handling Controller commands, known as I/O
  instructions
• I/O instructions for each controller reside in
  special addresses called I/O addresses, known as
  Ports

45
Controllers and 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.
• CPU moves data from/to main memory to/from the
  local buffers.
• I/O is from the device to the local buffer of the
  controller.
• Device controller informs CPU that it has
  finished an operation or needs attention by
  causing an interrupt.

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Interruptions

• Interruption, as we saw earlier, is an event in a
  computer system disrupting the normal instruction
  execution flow.
• An Interrupt Handler is designed to handle each
  type of interruption
• Interrupts can originate from various sources in
  a system
• From Programs, called Exception or Trap
• Synchronous with the instruction stream
• Arithmetic overflow, divide by zero, illegal
  instruction, illegal memory reference, etc.
• From Timer, called Interrupt
• Asynchronous
• Interval timer
• From I/O, called Interrupt
• Asynchronous
• I/O normal completion or error condition
  (abnormal completion)
• From Hardware Failure

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Summary

• OS is a system software designed to manage the
  various components of a computer system with
  kernel being its core and shell being its
  interface to the user
• OS Kernel has several components like memory
  manager, i/o manager, file manager that perform
  the appropriate functions
• Bootstrapping loads the OS into processor main
  memory at start-up
• Process administration in time-share systems
  involve scheduler, dispatcher and interrupts set
  up by interval timer
• Competing processes can be handled via Deadlock
  detection and avoidance, or Semaphores
• Device Controllers handle communication between
  processor and other devices attached to the system

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Distributed Systems and Networks

• A distributed system is a collection of
  processors that do not share memory or a clock.
  Each processor has its own local memory.
• The processors in the system are connected
  through a communication network.
• A distributed system provides user access to
  various system resources.
• Access to a shared resource allows
• Computation speed-up
• Potential for increased data availability
• Potential for enhanced reliability