Operating Systems

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Operating Systems

• Components

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Operating System Components

• An operating system is conceptually broken into
  three sets of components
• a user interface (which may consist of a
  graphical user interface and/or a command line
  interpreter or "shell"),
• low-level system utilities,
• a kernel - which is the heart of the operating
  system.

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What is a kernel

• The central module of an operating system.
• A piece of software responsible for providing
  secure access to the computer hardware.
• A kernel includes
• an interrupt handler that handles all requests or
  completed I/O operations that compete for the
  kernel's service.
• a scheduler that determines which programs share
  the kernel's processing time in what order.
• a supervisor that actually gives use of the
  computer to each process when it is scheduled.

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Types of kernels

• Monolithic Kernel
• includes all (or at least, most) of its services
  in the kernel proper.
• the amount of code running in kernel space makes
  the kernel more prone to fatal bugs.
• Linux uses a monolithic kernels that allows
  loading and unloading of kernel modules at
  runtime.
• Microkernel
• runs most services - like networking, filesystem,
  etc. - in user space.
• microkernels can be more stable, but require
  additional design work.

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Types of kernels

• Hybrid (Modified Microkernel)
• microkernels that have some "non-essential" code
  in kernelspace in order for that code to run more
  quickly.
• Windows NT/2000 and Macintosh OS X use hybrid
  kernels

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User Interface

• The user interface is a program or set of
  programs that sits as a layer above the operating
  system itself.
• Two common types of user interfaces
• Text-based also called command-line interfaces.
• Graphical

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Command-Line Interface (CLI)

• Method of interacting with a computer by giving
  it lines of textual commands either from the
  keyboard or from a script.
• In its simplest form the user types a command
  after the computer displays a prompt character.
• Programs that implement these interfaces are
  called command-line interpreters.

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Advantages of CLIs

• Skilled users may be able to user a command line
  faster than a GUI for simple tasks.
• All options and operations are invokable in a
  consistent form, one "level" away from the basic
  command.
• All options and operations are controlled in more
  or less the same way.
• Can perform operations in a batch processing mode
  without user interaction.

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The Windows XP Command-Line Interface
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Windows Command Prompt Commands

• Windows uses a program called CMD.EXE to
  interpret user commands.
• Command Format
• command-name options argument
• only the command-name is mandatory
• the options component consists of one or more
  switches flagged by an initial forward-slash (/)
  character.
• Batch files
• Group of commands in a file that run one after
  the other.
• Uses a very simple control language IF, FOR,
  GOTO.
• Has a .CMD or .BAT extension.

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Examples of Windows Commands

• HELP displays list of available commands
• DEL FILE1.TXT deletes a file named FILE1.TXT in
  the current folder.
• COPY \MYSTUFF\FILE2.DOC FILE3.DOC copy
  FILE1.DOC in the MYSTUFF folder to FILE3.DOC in
  the current folder
• DIR /OD displays list of files in current
  folder sorted by date, oldest first
• DIR /X /P displays list of short names of files
  in current folder, pausing after each screenful.
• XCOPY /S .TXT \MYSTUFF copies all files with
  the .TXT extension in the current folder and all
  sub-folders into the MYSTUFF folder.

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Example of a Windows Batch file

• REM This batch file will append a .BAK extension
• REM to all .TXT files in the current directory
• REM Commands will not be echoed
• _at_ECHO OFF
• REM 1 is a positional parameter,
• REM in this case it refers to the first character
  after
• REM the batch file name
• REM Example of an IF-statement
• IF 1N ECHO Skipping

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Windows Batch File continued

• REM Example of a GOTO
• GOTO L1
• REM This is a label
• LY
• DIR /B .TXT
• REM Example of a FOR-loop
• FOR F IN (.TXT) DO REN F F.BAK
• DIR /B .BAK
• LN
• ECHO All Done

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Windows Scripting Host (WSH)

• A Windows administration tool that extends the
  scripting functionality beyond batch files.
• Creates an environment for hosting scripts.
• Scripts can be run from either the Windows
  desktop (double-click on the file) or the command
  prompt (CSCRIPT ltfilenamegt).
• Scripts can be written in VBScript (.VBS) or
  JavaScript (.JS).
• Built into Microsoft Windows 98, 2000, and XP.

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A Simple WSH Script

• REM The Famous HELLO WORLD Program
• WSCRIPT.ECHO Hello World

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A More Complex WSH Script

• REM This script will copy all files whose names
• REM contain .txt to a sub-directory, the name
• REM of the sub-directory will be the current date
  
• REM in YYYYMMDD format
• REM Declare some variables
• DIM fs, today

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WSH Script Example continued

• REM This is an example of a function
• REM This function will output a string
• REM zero-filled to the left up to a specified
  length
• FUNCTION zero_pad(p_in, p_length)
• IF LEN(CSTR(p_in)) gt p_length THEN
• zero_pad CSTR(p_in)
• ELSE
• zero_pad STRING(p_length - LEN(CSTR(p_in)),
  "0") _
• CSTR(p_in)
• END IF
• END FUNCTION

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WSH Script Example continued

• REM Create a file system object which allows
  manipulation
• REM of files and folders
• set fs WSCRIPT.CREATEOBJECT("Scripting.FileSyste
  mObject")
• REM Build the sub-directory name
• today CSTR(YEAR(DATE())) zero_pad(MONTH(DATE()
  ),2) _
• zero_pad(DAY(DATE()),2)
• REM Create the sub-folder if it does not already
  exist
• IF NOT fs.FOLDEREXISTS(today) then
  fs.CREATEFOLDER(today)
• REM Copy files to the sub-folder
• REM over-write if they already exist
• fs.COPYFILE ".txt.", today "\", TRUE

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UNIX Command System

• A program called a shell is used to interpret
  UNIX commands.
• UNIX has numerous shells Bourne Shell, C-Shell,
  Bourne-Again shell, etc.
• UNIX commands are case-sensitive
• Command Format
• command-name options argument comment
• only the command-name is mandatory
• the options component consists of one or more
  switches
• each switch consists of a minus-sign followed by
  one or more characters.
• Scripts
• group of commands in a file that run one after
  the other.
• UNIX has a very powerful control language,
  comparable to other programming systems.

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Examples of UNIX Commands

• pwd displays names of current directory.
• ls -l /tmp list files in /tmp in long format.
• rm -r delete all files in current directory
  and all sub-directories
• cp /tmp/sh /home/class copy all files in the
  /tmp directory whose names in sh to the
  /home/class directory
• mkdir new1 create a sub-directory called new1
  in the current directory

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A Simple Unix Shell Script

• ! /bin/sh
• The preceding line indicates that this is a
• Bourne-shell script
• This shell script will create sub-directories
• in the current directory with names new1 to
  new9
• for d in 1 2 3 4 5 6 7 8 9
• do
• The d indicates that the value of variable d
  is substituted
• mkdir newd
• done

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Graphical User Interface (GUI)

• First GUI developed at the Palo ALTO Research
  Center of Xerox Corporation.
• Apple Macintosh, released in 1984, was the first
  commercial use of a GUI.
• GUIs have a number of common features
• on-screen overlapping windows
• pointing device
• graphical features, such as buttons, icons, etc.
• higher level devices, such as menus, toolbars,
  etc.

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Features of a GUI

• Window main central area used to display and
  interact with user data.
• Scroll bar used to reposition the viewing
  window.
• Title bar indicates the name of the program
  currently being used and its associated document.
• Menu bar list of words which constitute the
  top-level choices of a menu.
• Pop-up/Drop-down menu list of choices that
  appear when a top-level menu item is clicked.
• Toolbar Group of icons that perform a function
  when clicked.

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Some examples of toolbars
Microsoft Word
Microsoft Excel
Microsoft Access
Note the use of icons that are common to all the
toolbars
Fast Save, Print, and Print Preview
Examples include
Help
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Some examples of drop-down menus
Note that options that are not available are
greyed or ghosted
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Some examples of pop-up menus
The Microsoft Windows Start Menu pops up when
the Start button is pressed
The AutoShapes Menu in Microsoft Word pops up
when the AutoShapes button is pressed
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Advantages of GUIs

• Performing tasks in a GUI environment is
  intuitive.
• Applications have the same general appearance and
  operation.
• Applications are flexible, commands can be
  executed using either mouse or keyboard.
• GUIs allow you to cancel or undo operations.
• GUIs often ask you to confirm important
  operations.