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Chapter
Disk Operating System (DOS)
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Finding DOS and Understanding its Strengths
and Weaknesses
Installing DOS
How the FAT File System Works
Working with the DOS Command Prompt
Understanding the DOS Bootup Process
Troubleshooting Common DOS Problems
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Learning Objectives
 Measure the value, strengths, and weaknesses of DOS
 Install DOS
 Use the FAT file system
 Use DOS commands for file management and other tasks
 Describe the DOS bootup process and create startup disks
 Troubleshoot common DOS problems
 DOS is Text base Operating System
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Finding DOS and Understanding
its Strengths and Weaknesses
 Versions of DOS
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CP/M was an important predecessor to DOS
Many versions of DOS, several introduced by Microsoft
MS created PC DOS for IBM
Later licensed DOS as MS-DOS to other manufacturers
MS-DOS 5.0 first version available as a separate product
PC DOS now works on Microsoft/Intel compatible computers
Digital Research (of CP/M fame) introduced
DR-DOS in 1987
DR-DOS 8.0 introduced in 2004 by DeviceLogics
FreeDOS distributed without charge under
GNU GPL license
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Finding DOS and Understanding
its Strengths and Weaknesses
 DOS’ Strengths
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DOS for backward compatibility with DOS apps
DOS when you need a small OS
DOS is more compact than Windows and hence useful for
embedded systems or for portability
DOS is popular as an operating system due to its ability to pack all
startup files on a single floppy disk
The floppy disk is also known as the startup disk
Startup disk accommodates additional files, such as drivers and
utilities
Startup disk can be used for booting up a computer and running
special diagnostic programs
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Finding DOS and Understanding
its Strengths and Weaknesses
 DOS Weaknesses (continued)
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Processor Mode Limits
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Only supports real mode of operation of Intel processors
 Newer Intel processors start in the real mode, and
newer OSs (Windows, Linux, UNIX) switch the
processor from the real mode to the protected mode
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Memory Limits
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Intel processors in real mode use only 1 MB
640 KB of RAM (conventional memory) is workspace
for the OS, applications and data
384 KB of addresses reserved for system BIOS and
RAM and ROM on adapters
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Finding DOS and Understanding
its Strengths and Weaknesses
 DOS Weaknesses (continued)
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Multitasking Limits
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DOS is a single-tasking OS
Hard Drive Limits
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DOS only supports hard drives up to 7.8 GB
DOS uses logical drives that are 2 GB or less and
within the 7.8 GB total disk space limit
A logical drive is a portion of the physical hard
drive with a letter assigned to it
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Installing DOS
 DOS Hardware Requirements
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The general DOS hardware requirements for
MS-DOS 6.22 and IBM PC-DOS
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An IBM or compatible personal computer
6MB of free hard disk space for the DOS utilities
512KB of memory
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Installing DOS
 Preparing for DOS Installation
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Assure that the computer is a complete system
with all the necessary components
Setup program partitions and formats the hard disk
Partition: an area of a physical hard disk that
contains one or more logical drives
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Installing DOS
 Partitioning a Hard Disk in MS-DOS
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Physical format is done at the factory
Master Boot Record (MBR), the 1st sector
on disk, contains the partition table
Partitioning is the first step to prepare a
hard disk
Each partition needs a logical format within
each logical drive
Partitioning and formatting included in OS
installation
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Installing DOS
 Partitioning a Hard Disk in MS-DOS (continued)
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FDISK – the partitioning program of MS-DOS
Primary partition has only one logical drive
2GB is maximum primary partition size in MS-DOS
Extended partition contains one or more logical
drives
After partitioning, then format logical drives
Step-by-Step 2.01 Installing DOS on a Hard Disk Page 55
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How the FAT File System Works
 FAT File System Components
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FAT Table
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File allocation table (FAT) – DOS records disk space
usage
Cluster – minimum space allocated to a file
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How the FAT File System Works
 FAT File System Components (continued)
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FAT12, FAT16, and FAT32
FAT16 – used on hard disks
 FAT32 – available since Windows 95 OEM SR2
 FAT12 – used on floppy disks
The number refers to the size of each entry in the
FAT table. Length of entry limits the number of
entries the FAT table can hold.
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How the FAT File System Works
 FAT File System Components (continued)
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How Cluster Size Affects Partitions
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FAT16 file system limited to 65,525 clusters
Maximum cluster size 32,768 bytes (32K)
65,525 x 32,768 = 2GB (maximum partition size
supported by the FAT16 file system)
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How the FAT File System Works
 FAT File System Components (continued)
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Root Directory and Other Directories
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The FAT and the root directory are the two
primary components of the FAT file system
DOS uses the FAT to record the location of a
file on the disk
A directory is a place where DOS stores
information about files, including a reference
to the FAT table
Root directory is the top level directory
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How the FAT File System Works
 FAT File System Components (continued)
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Root Directory and Other Directories (continued)
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Parent directory contains other directories
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Child directory (subdirectory) is within a parent
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Each directory entry contains the name of a file
or directory, the time and date of its creation or
modification, its size, attributes, and beginning
cluster information
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How the FAT File System Works
 Using a Directory and the FAT Table to find a File
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Directory is like a phone directory for DOS
Finds file name and listing in the directory
Reads the starting cluster number in directory
Looks in the FAT table (like a map of city) for
location on disk
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How the FAT File System Works
 DOS file-naming rules:
 A file name is up to eight characters, followed
by a period, and extension of up to three characters
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A file name and extension can include alphanumeric characters and a few special characters,
but no spaces
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Use the wildcard characters asterisk (*) and
question mark (?) to locate files and folders
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How the FAT File System Works
 DOS File Types
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BAK, BAS, BAT, COM, DOS, EXE, SYS, and TXT
are some common file extensions and file types
recognized by DOS
 Executable Files
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COM, EXE, and BAT are the three types of files
that can be executed by DOS
Step-by-Step 2.02 Creating a Simple Batch File Page 63
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How the FAT File System Works
•DOS File Attributes (Demo)
•Determine the manner in which DOS handles files
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Read-only
Archive
System
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Hidden
Volume Label
Directory
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How the FAT File System Works
 LABEL command
 Creates or changes a volume label (name)
 If a volume has a label, FORMAT requires
the name before reformatting
 ATTRIB command
 Displays and modifies attributes
 Works on read-only, archive, system, and
hidden files
 Works in both MS-DOS and Windows on
FAT and NTFS
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Working with the DOS
Command Prompt
 Success at the DOS Prompt
 Success means correctly entering commands
and getting desired results
 Important for working with DOS and other OSs
 What is the Correct Syntax?
 Syntax is a set of rules for correctly entering a
specific command at the command line
 The HELP command can be used to find the
syntax for a command
 COMMAND /?
Step-by-Step 2.03 Using the Online Help in DOS Page 68
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Working with the DOS
Command Prompt
 Success at the DOS Prompt (continued)
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How is a Command Interpreted?
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COMMAND.COM interprets the command
entered at the prompt
COMMAND.COM loads each command into
the memory, and issues additional instructions
to the command
COMMAND.COM parses the command entry
based on special delimiter characters
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Working with the DOS
Command Prompt
 How is a Program Found and Loaded?
 COMMAND.COM loads the command named
at the beginning of the command line
 Must find the actual program code
 First checks its own list of internal commands
 Then looks for an external command in the
current directory, and then in a list called the
search path
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Working with the DOS
Command Prompt
 Learning to Manage Files and Directories
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File management in DOS is centered on the
abilities and limits of the FAT file system
It is essential to design a directory structure
to save and organize files
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Working with the DOS
Command Prompt
 Learning to Manage Files and Directories (continued)
 Designing a Directory Structure for File Management
 DOS directory/folder structure is hierarchical
 Drive | Directory | File Name
 TREE command can be used to view the entire directory
structure
 Store data in a separate directory from applications
 Creating and Removing Directories
 MD (Make Directory) will create a directory
 CD or CHDIR (Change Directory) allows movement between
directories
 RD (Remove Directory) deletes an empty directory
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Working with the DOS
Command Prompt
 Learning to Manage Files and Directories (continued)
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Use File Management Commands
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DOS commands are divided into internal commands and
external commands
Internal commands are loaded into the memory
along with DOS
Use File management commands (continued):
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DIR (Directory), MD, CD, CLS (CLear Screen),
COPY, REN (REName), RD, and TYPE are some
of the internal commands
XCOPY, DISKCOPY, and DELTREE are often-used external
commands
Step-by-Step 2.04 Managing Files and Directories Page 73
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Working with the DOS
Command Prompt
DOS Command
(dir /a)
Function
DIR
Displays a list of files and folders.
DIR /?
Lists the available syntax for the DIR
command.
DIR /P
List data one page at a time.
CD ..
Moves to the parent directory from the
present directory.
CLS
Clears the screen.
VER
Displays the MS-DOS version.
XCOPY
Used to copy files and entire directories.
COPY
Used to copy only files.
DELTREE
Used to delete an entire directory and its
contents.
DOS
commands
and their
functionality
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Understanding the DOS Boot-Up
Process and Startup Disks
 Why You Should Learn the DOS Bootup Process
 Personal computers are multi-purpose devices.
 Understanding the normal startup process of
the system helps troubleshoot problems that
occur during boot-up.
 DOS system files
 IO.SYS handles hardware interaction and loading
of drivers
 MSDOS.SYS is the kernel of DOS
 COMMAND.COM is the command interpreter
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Understanding the DOS Boot-Up
Process and Startup Disks
 DOS system files (continued)
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DOS Configuration Files
CONFIG.SYS – adds device drivers and modifies
DOS settings
BUFFERS
DEVICE
DEVICEHIGH
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DOS
FILES
STACKS
AUTOEXEC.BAT batch file loaded during bootup
PATH
PROMPT
SET
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Understanding the DOS Boot-Up
Process and Startup Disks
 The DOS Bootup Process
 A PC can be booted up by:
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Turning on the power switch of the PC (a cold boot)
Using the Ctrl-Alt-Delete key combination to reboot the
system (a warm boot)
 The DOS Bootup Process (continued)
 The processor loads a special ROM-based program,
called Power-On Self-Test (POST)
 POST runs a series of small diagnostic tests on the
hardware, and loads the bootstrap loader
 The bootstrap loader is a small program in the ROM
BIOS
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Understanding the DOS Boot-Up
Process and Startup Disks
 The DOS Bootup Process (continued)
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On a hard drive, the bootstrap loader looks into
the MBR (master boot record) and reads the
partition table for the primary active partition
The bootstrap loader loads the boot sector from
the logical drive, which, in turn, runs the operating
system loading program
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Understanding the DOS Boot-Up
Process and Startup Disks
 Creating a DOS Startup Floppy Disk
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Using FORMAT to create a startup disk
Using SYS to create a startup disk
Using Windows to create a startup disk
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Understanding the DOS Boot-Up
Process and Startup Disks
 Using FORMAT to Create a Startup Disk
 The FORMAT command prepares the diskette and
places a new root directory and FAT on the disk
 The /S switch option places the system files on the
diskette
Step-by-Step 2.05 Create a Startup Floppy Disk Using
FORMAT Page 83
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Understanding the DOS Boot-Up
Process and Startup Disks
 Using SYS to Create a Startup Disk
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The SYS command places the DOS system files
on a previously formatted floppy disk
To put the DOS system files on a floppy disk
enter sys a:
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Understanding the DOS Boot-Up
Process and Startup Disks
 Using Windows to Create a Startup Disk
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In Windows 95 and 98 go to Control Panel |
Add/Remove Programs | Startup Disk
In Windows XP format a disk from within My
Computer or Windows Explorer and select the
option to create a MS-DOS startup disk
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Troubleshooting Common DOS
Problems
 “Non-System Disk” error message
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The OS loader program (boot sector) is unable
to locate the IO.SYS, or MSDOS.SYS, or both
Rectify by ensuring that the OS is booted from
the disk that holds the OS
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Troubleshooting Common DOS
Problems
 “Bad or Missing Command Interpreter”
error message
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Implies that the file COMMAND.COM is
missing or that the version is different than
IO.SYS and MSDOS.SYS
Rectify by copying the COMMAND.COM file
with the correct date and time from the floppy
disk to the hard disk
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Troubleshooting Common DOS
Problems
 “Bad Command or File Name” error message
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Implies that the command name or the file name
(or a directory name) used in the command line
is incorrect
Rectify by identifying typos and re-entering the
command correctly
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Troubleshooting Common DOS
Problems
 A request to enter the current date and time
 Means that DOS did not find an AUTOEXEC.BAT file
 If you previously had one, investigate why and
how you don’t now
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Chapter Summary
Finding DOS and Understanding Its Strengths
and Weaknesses
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One reason DOS is still in limited use today is its
small size, which makes it a choice for embedded
systems
Another reason is that its system files easily fit on
a floppy disk, leaving room for other small
programs
Technicians can run diagnostic utilities from a
DOS floppy disk
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Chapter Summary
Finding DOS and Understanding Its Strengths
and Weaknesses
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Windows can run DOS in a virtual DOS machine
Some applications run too slowly or not at all
in a virtual machine, so they must be run on a
computer running DOS
DOS can only access real-mode of the Intel
processors
DOS is a single-tasking operating system
DOS provides limited services for DOS
applications
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Chapter Summary
Finding DOS and Understanding Its Strengths
and Weaknesses
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DOS applications can only use conventional
memory
Microsoft no longer supports or sells MS-DOS
IBM sells PC DOS 2000
Digital Research sold DR-DOS to Novell
DR-DOS is now sold through DeviceLogics
Versions of DOS can be found for free on the
Internet
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Chapter Summary
Installing DOS
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The MS-DOS Setup program will partition
and format a hard drive, if needed
Hardware requirements for MS-DOS are
minimal:
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An IBM or compatible PC
6MB of free hard disk space
512KB of memory
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Chapter Summary
How the FAT File System Works
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DOS uses the FAT16 file system, and only
uses conventional memory
The FAT file system uses the 8.3 naming
convention for files and directories
It is best to use only alphanumeric characters
A file’s extension can indicate the type of file
File attributes determine how DOS handles a
file or directory
File attributes are read-only, archive, system,
hidden, volume label, and directory
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Chapter Summary
Working with the DOS Command Prompt
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The ATTRIB command allows you to view
and manipulate the read-only, archive, system,
and hidden attributes
Internal command are part of COMMAND.COM
and are always available and fast to access
CLS, COPY, REN, DEL, MD, RD, CD, and TYPE
are internal DOS commands
External commands are in individual files, which
must be where DOS can find them
XCOPY, DELTREE, FORMAT, FDISK, and
DISKCOPY are external commands
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Chapter Summary
Describe the DOS Bootup Process and
Create Startup Disks
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It is important to understand the bootup
process of an OS in order to troubleshoot
failures that occur during bootup
A hard boot of a PC occurs when you turn
on the power switch
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Chapter Summary
Describe the DOS Bootup Process and
Create Startup Disks
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A soft boot of a PC occurs when you press
CTRL-ALT-DELETE
Many computers have a Reset button, which
resets a running computer without a powerdown and power-up cycle
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Chapter Summary
Describe the DOS Bootup Process and
Create Startup Disks
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The order of events during bootup of DOS is:
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Cold or warm boot
POST
Bootstrap loader looks on A: or C: drive and
loads the boot record
IO.SYS and MSDOS.SYS are loaded
CONFIG.SYS (if it exists) is read and used by
MSDOS.SYS
COMMAND.COM is loaded
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Chapter Summary
Describe the DOS Bootup Process and Create
Startup Disks
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DOS bootup order of events (continued)
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AUTOEXEC.BAT (if it exists) is read and
used by COMMAND.COM
The DOS prompt is displayed
To create a DOS startup disk, you can:
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Use the DOS Setup program
Use the SYS or FORMAT command
Use the Startup Disk option in Windows 9x
Format a floppy disk in Windows XP
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Chapter Summary
Troubleshoot Common DOS Problems
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A “Non-System Disk” error message means that the
OS loader cannot find IO.SYS or MSDOS.SYS
Most likely cause is a data floppy disk left in drive A:
Remove the floppy and reboot the computer
A “Bad or Missing Command Interpreter” message
means that COMMAND.COM is missing or is a
different version than IO.SYS and MSDOS.SYS
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Boot from a DOS floppy disk of the correct version,
and copy COMMAND.COM to the root of C:
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Chapter Summary
Troubleshoot Common DOS Problems
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A “Bad Command or File Name” error
message means you need to check your
spelling and reenter the command
If you are required to enter the date and time
when you boot up a computer there is
probably no AUTOEXEC.BAT on the root of
the boot disk. Create one, even if it is empty,
and most PCs will use the internal clock for
the time
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