DOS

1
DOS

• Chapter 9
• Mike Fuszner

2
DOS is Dead!

• A exam does not cover DOS, so why am I studying
  this stuff?
• A still assumes you understand all the commands
  in this chapter
• A does test you on DOS memory usage and
  configuring DOS programs within Windows
• Some concepts will be easier to understand if you
  know a little about DOS to begin with
• If youre using Windows, go to a command prompt
  (Start ? Programs ? Accessories ? Command Prompt
  or Start ? Run ? Command or Cmd)

3
Operating Systems
4
Need for an Operating System

• Characteristics of an operating system
• An operating system works with a particular type
  of processor.
• It always starts after POST, taking control of
  the PC.
• The maker of the OS provides a rule book that
  tells programmers how to write programs for their
  OS. The rule book is called application
  programming interfaces (APIs)

5
Communicating with the Hardware

• The operating system works with the system BIOS
  to communicate with the basic devices.
• Most recent operating systems skip the BIOS and
  talk directly to hardware
• Operating systems use device drivers to
  communicate with hardware devices.

6
Creating a User Interface

• A computers user interface provides the user a
  display of the programs and data on the PC.
• The user interface should be flexible and
  scalable, depending on the system on which it is
  installed.

7
Accessing and Supporting Programs

• An operating system must
• Enable users to start a program.
• Provide access to the hardware while the
  application runs.
• Provide a way to either stop the program, or
  generate an error message if a program loses
  control.
• Return to the user interface once an application
  shuts down.

8
Programs and Data

• The operating system should provide a label for
  each program and each individual piece of data.
• It should provide a naming system for all the
  drives.
• The user should be able to store data and
  programs in distinct groups.
• The operating system user interface must be able
  to interact with each of these groups to open or
  close them or to copy, move, or delete programs
  or data.

9
DOS
10
DOS Concepts

• DOS is a single-tasking operating system designed
  to run on an 8086 processor
• It is text based
• Does not support mice
• Completely case insensitive

11
Files

• DOS manifests each program and piece of data as
  an individual file
• Each file has a name that consists of two parts
  the filename and the extension which are
  separated by a dot
• 8.3 filenames
• Cannot use / \ lt gt , ?
• The extension defines the file as an executable
  (EXE), command (COM), batch (BAT), or data file.
  Data files have extensions that associate them
  with the program that created them.
• Files are stored in a binary format

12
ASCII

• Most programs store their data in their own
  special format
• American Standard Code for Information
  Interchange (ASCII) is a special file format that
  can be read by any program.
• The ASCII standard defines 256 eight-bit
  characters.
• DOS treats the first 32 ASCII characters as
  special commands.
• ASCII files are known as text files

13
ASCII Table
14
Drives and Directories

• DOS uses a hierarchical directory tree to
  organize the contents of the drive.
• Directories are groups in which files are placed.
• Windows calls directories folders
• Subdirectories are directories inside
  directories.
• Two files or subdirectories under the same
  directory cannot have the same name.

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Drives and Directories

• Subdirectories in a directory are displayed by
  adding backslashes and names.
• A path represents the exact location of a file.
• Directories make organization of files easier.

C\ represents the root directory of drive
C C\DOS represents the subdirectory DOS on
drive C C\DOS\IO.SYS represents the path to the
file IO.SYS
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DOS Structure

• The DOS operating system is composed of three
  main files
• IO.SYS It handles communication between the
  BIOS and the hardware.
• MSDOS.SYS It is the primary DOS code.
• COMMAND.COM It interprets the commands typed
  into the PC, and passes the information to
  MSDOS.SYS.
• DOS includes many additional auxiliary files that
  are stored in C\DOS (or c\windows\command in
  Windows 9x)

17
The DOS Prompt

• The prompt is a path followed by a greater-than
  sign and a flashing cursor.
• The prompt indicates the directory on which DOS
  is currently focusing.

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DOS Commands

• The DIR command lists the contents of a
  particular directory.
• The DIR/W command only lists the names of the
  files.
• The /? is used to invoke the help screen for a
  particular command.

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DOS Commands

• The CD (or CHDIR) command is used to change the
  focus to a different directory.
• The CD\ command is used to return to the root
  directory.
• To switch between drives, type the drive letter
  followed by a colon.

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DOS Commands

• The MD (or MKDIR) command is used for creating a
  directory.
• The DEL command is used for deleting files, and
  the RD (RMDIR) command is used for deleting
  directories and subdirectories.
• The DELTREE command is used for deleting
  directories containing files and subdirectories.

21
Running a Program

• To run a program
• Change the DOS focus to the directory where the
  program is stored.
• Type the filename without its extension and press
  ENTER.

22
Using Function Keys

• F1 function key brings back the previous command
  one letter at a time
• F3 function key brings back the entire command at
  once
• The DOSKEY command stores a list of all
  previously typed commands and can be accessed by
  using the up arrow key.
• Type DOSKEY to start the program
• Windows 2000 automatically starts the program

23
Working with Files

• Attributes (H, R, S, A) are special values
  assigned to a file.
• Hidden hides the file from DIR
• Read-only protects a file from being deleted or
  modified
• System identifies system files like IO.SYS
• Archive identifies files that have not been
  backed up
• The ATTRIB.EXE program is used to inspect and
  change file attributes.

24
Working with Files

• Wildcards
• Wildcards are special characters that enable DOS
  commands to act on more than one file at a time.
• represents any number of characters
• ? represents a single character

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Working with Files

• The DEL and ERASE commands are used for deleting
  files.
• The RD and DELTREE commands are used for deleting
  directories.
• The COPY command is used for making a copy of the
  file in a new location.
• The MOVE command is used for moving the file from
  its original location to a new one.
• The XCOPY command is used for working with
  multiple directories.

26
DOS in a Windows Environment

• Windows 9x and Windows 2000 use COMMAND.COM and
  CMD.EXE respectively, to invoke the command
  prompt.
• CMD.EXE is a more advanced command processor in
  Windows 2000

27
DOS in a Windows Environment

• Boot modes in Windows 9x
• The Windows startup menu option can be used to
  invoke the command prompt
• When you see Starting Windows 9x press the F8
  function key to bring up the Startup menu
• The Restart in MS-DOS Mode option from the
  Shutdown menu option can be used for booting to a
  command prompt.

28
DOS in a Windows Environment

• Boot modes in Windows 2000
• Windows 2000 does not have a Command Prompt
  option but you can boot to a Safe Mode Command
  Prompt
• Win2K uses the Recovery Console feature to handle
  boot problems.
• To use a command prompt in Windows 2000 run
  command.exe (16-bit) or cmd.exe (32-bit)

29
Communicating with the Hardware

• DOS talks to hardware thru the BIOS
• The BIOS knows how to communicate with basic
  devices in the PC
• If a new device is added, the BIOS may not know
  how to talk to it
• DOS provides two ways to add more code so that
  DOS will know how to talk to new hardware beyond
  the BIOSs knowledge
• Device drives in config.sys
• TSRs in autoexec.bat

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Device Drivers and CONFIG.SYS

• A device driver is a special file that contains
  the programs necessary to communicate with a new
  device
• The drivers are loaded through a special file
  called CONFIG.SYS
• Copy the device driver file to the hard drive
• All a device or devicehigh line to the
  config.sys file with the location of the file
  following the sign
• To comment out a line in config.sys preface with
  
• Changes to config.sys do not take effect until
  you reboot the PC
• DOS comes with many device drivers, two of the
  most useful ones being SETVER.EXE and ANSI.SYS

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DOS Device Drives

• Setver.exe
• Tells DOS to pretend to be whatever version you
  tell it, which helps some programs run properly
• Devicec\dos\setver.exe

• Ansi.sys
• Enables you to add color and features to the
  prompt
• Enables you to remap the keyboard
• Devicec\dos\ansi.sys

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Device Drivers and CONFIG.SYS

• Configuring commands in CONFIG.SYS
• BUFFERS It defines the size of the buffer
  space.
• STACKS It is the memory set aside by DOS for
  storing the register information temporarily.
• FILES DOS sets aside some memory to keep track
  of all the files on the hard drive that are being
  used. The memory used for one file is called a
  file handle.
• SHELL The SHELL statement defines the shell
  that DOS uses.

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Configuration Commands in Config.sys

• Buffers set aside some space that DOS needs for
  disk I/O. The default is 15.
• Buffersnn,m where nn is the number of buffers
  and m is the number of buffers in the look-ahead
  buffer
• Stacks is an area DOS uses to store register
  information temporarily. If a program is running
  and a device sends an interrupt, DOS needs to use
  the register space. So DOS moves the program
  usage of the registers out temporarily to the
  stack
• Stacksnn,mm where nn is the number of stacks
  and mm is the size of each stack
• Increase the values if you get a Stack Overflow
  error

34
Configuration Commands in Config.sys

• Files is used to increase the number of file
  handles. DOS keeps track of files being used at
  any given timeone file handle per file. The
  default value is 8 but is usually not enough
• Filesn where n is the number of file handles
• Shell defines which command.com to use
• Shellc\dos\command.com /P /E512
• See DOS Help for more information

35
TSRs and AUTOEXEC.BAT

• The (Terminate and Stay Resident) TSR program
  also provides support for devices
• A TSR will immediately return you to a DOS prompt
  (terminate) but will still be in RAM (stay
  resident)
• DOSKEY.COM is a TSR program that keeps track of
  typed commands, creates keyboard macros, and adds
  extra power to the function keys for typed
  commands
• The MODE.COM program is used for configuring and
  redirecting ports and may be used to change the
  look of the monitor

36
TSRs and AUTOEXEC.BAT

• The AUTOEXEC.BAT is a text file that gets
  executed each time the system reboots.
• The SET command creates environment variables for
  DOS programs.
• The PATH environment variable specifies the
  location of programs, which are not in the
  current directory.
• The PROMPT command defines the look of the prompt.

37
TSRs and AUTOEXEC.BAT

• The F5 and F8 keys are used to either completely
  override, or step line by line through the
  CONFIG.SYS and AUTOEXEC.BAT files.
• Batch files enable automation of a series of
  prompt commands.
• Just save the file with a BAT extension

38
Making a Boot Disk

• A boot disk is a vital tool!
• You may make a boot disk from any properly
  running PC (make one for every version of DOS
  youll need to work with)

39
Working with Drives

• VOL - A volume label enables users to create
  descriptive names for the drives.
• VOL lets you view the volume name
• LABEL lets you change the volume name
• SYS - The SYS command copies the three DOS system
  files to a partition, making it bootable.
• LASTDRIVE - The LASTDRIVE command is used for
  allocating memory for the drive letters of
  storage devices other than the hard drive.

40
Working with Drives

• DOS provides utilities to inspect, optimize, and
  repair devices.
• The CHKDSK program is used for identifying and
  repairing lost cluster chains.
• A lost cluster is a cluster that does have a
  proper link or filename
• CHKDSK identifies and repairs lost clusters by
  placing them in the root directory as
  FILExxxx.CHK
• A cross-linked file is when two files are
  claiming the same space on the drive
• CHKDSK identifies but cannot repair cross-linked
  files

41
Working with Drives

• Scandisk
• Repairs lost clusters, cross-linked files,
  directory and file structures, file allocation
  tables, and volume labels

• Defrag
• Simple defragmentation program that defragments
  files without any user intervention

42
Working with Drives

• Disk caching
• Disk cache is a program that monitors drive
  access
• Retrieving data from RAM is much faster than
  retrieving data from a hard drive
• Frequently needed data is stored in RAM in the PC
  in an area called a cache (sometimes it is stored
  in RAM built into the hard drive called caching
  controllers)
• SMARTDRV.EXE is a DOS program that makes an
  excellent disk cache.
• RAM drives are assigned drive letters, and users
  can read, write, copy, or delete to and from a
  RAM drive.
• Devicec\dos\ramdrive.sys 1024

43
Smartdrv

• Run Smartdrv by including it in autoexec.bat
• Tweak smartdrv to work its best by checking the
  hit rate
• Hit rate (number of times data is in the
  cache/number of times data is requested) x 100
• A good hit rate is in range of 75-90
• Use smartdrv /s to get the information needed to
  calculate your hit rate

See pages 520-521 in the book for more about
smartdrv.
44
The Computer Virus

• A virus is a program with two main functions -
  proliferation and activation.
• Boot sector, executable, macro, Trojan, Worm, and
  Bimodal/Bipartite are six common types of viruses
  that exist.

45
The Computer Virus

• Boot sector
• Changes the code in the master boot record
• FDISK /MBR will fix if you dont have a disk
  overlay program
• Executable
• Resides in executable files and copies itself
  into other executable files
• Macro
• Auto-starts when an application is run

46
The Computer Virus

• Trojan
• Sophisticated program that causes severe damage
• Worm
• Makes copies of itself on to the other systems on
  a network
• Bimodal/Bipartite
• Uses both boot-sector and executable functions

47
Antivirus Programs

• Antivirus programs use different techniques to
  combat different types of viruses.
• A library of signatures (code pattern of viruses)
  is used for detecting executable viruses.
• Antivirus programs create a checksum on every
  file in the drive to combat unknown polymorphs
• A polymorph attempts to change its signature to
  prevent detection
• Stealth viruses use various methods to try to
  hide from antivirus software

48
Virus Prevention Tips

• Antivirus programs can be installed to
  automatically scan floppies and downloads.
• Bootable antivirus floppy disks can be used to
  eliminate susceptible viruses.