Disk Operating System (DOS)

Disk Operating System (DOS)


When the computer starts, it starts the operating system that takes the control of the machine. An Operating System is a set of programs that help in controlling and managing the Hardware and the Software resources of a computer system. A good operating system should have the following features;

1. Help in the loading of programs and data from exter­nal sources into the internal memory before they are executed.

2. Help programs to perform input/output operations, such as;

·         Print or display the result of a program on the printer or the screen.

·         Store the output data or programs written on the comput­er in storage device.

·         Communicate the message from the system to the user through the VDU.

·         Accept input from the user through the keyboard or mouse.

5.2       OBJECTIVES

At the end of this lesson, you would be able to;

·         explain the concept operating system

·         discuss the functions of operating system

·         understand the procedures of loading operating system into the memory

·         use file management features of operating system

·         create separate locations for logically related files

·         copy files from one computer to another

·         use Windows for File Management


As the name suggests, the operating System is used for operating the system or the computer. It is a set of computer programs and also known as DOS (Disk Operating System). The main functions of DOS  are to manage disk files, allocate system resources according to the requirement. DOS provides features essential to control hardware devices such as Keyboard, Screen, Disk  Devices,  Printers, Modems and programs.

Basically, DOS is the medium through which the user and external devices attached to the system communicate with the system. DOS translate the command issued by the user in the format that is understandable by the computer and instruct computer to work accordingly. It also translates the result and any error message in the format for the user to understand.

(a)        Loading of DOS

The BOOT Record into the computer memory loads DOS. BOOT Record in turn is triggered by ROM program already there in the computer.

The system start-up routine of ROM runs a reliability test called Power On  Self Test (POST) which initializes the chips  and  the standard equipment attached to the PC, and check whether  periph­erals connected to the computer are working or not. Then it tests the  RAM memory. Once this process is over, the ROM  bootstrap loader attempts to read the Boot record and if successful, passes the  control  on  to it. The instructions/programs  in  the  boot record  then  load the rest of the program. After  the  ROM  boot strap  loader  turns the control over to boot  record,  the  boot tries to load the DOS into the memory by reading the  two hidden  files IBMBIO.COM and IBMDOS.COM. If these two are  found, they   are   loaded  along with  the   DOS command interpreter COMMAND.COM. COMMAND.COM contains routines that interpret what is typed  in through the keyboard in the DOS command mode.  By com­paring  the input with the list of command, it acts by  executing the  required routines/commands or by searching for the  required routine utility and loads it into the memory.


A  file may contain a program or any other kind  of  information. Generally, a file must be given a name that can be used to  iden­tify it. DOS permits the user to assign a name consisting of  two parts  to a file – primary and secondary names. Primary name  can be  of  a maximum of eight characters consisting  of  Characters, Alphabets, Number  and Hyphen), and the  Secondary  name  should consist of three characters, which is optional. The primary  name and the secondary (or extension) name, if any, are to be separat­ed  by  a  dot (.).

Primary name can be linked  to  proper  name, whereas  extensions are like surnames of people. Using an  exten­sion with the file name is preferable, though optional.  However, once the extension is specified, using the complete name (primary name and extension, with the period separating them can only refer the file). Using extensions can be an excellent way of naming a file so that it can be identified easily.


Filename      Primary Name       Separator      Secondary Name

Employee           Employee            

Employee.Exe   Employee              .               Exe

Employee.Dbf   Employee              .               Dbf

DOS  has a way of showing which disk drive is  currently  active. The floppy disk drives are assigned alphabets A and  B,  whereas the hard disk drive is assigned the alphabet C.  If your PC has  a single floppy drive, the drive would be A and if it has two,  they would  be  termed as A and B.   If your PC includes a  hard  disk,  besides a  FDD (Floppy  Disk Drive),  the  drive names would be A and C.  If the prompt is  A, then  it  implies  that the first floppy disk  drive  is active. Where as the DOS prompt would be C, if the hard disk is active. Data as well as instructions reside in a file stored in a  disk.


The files in the computer come from  various sources.  Some files come with DOS, while other  come  with publications such as a word processor. These files contain  codes and  other information that is necessary to  make  the  computer application operational. Before long, there will be hundreds  or even thousands of files in the computer, which can make it diffi­cult to locate specific files.

The  names of all the files created in a disk are stored  in  its directory. Directory is just like a file folder, which contain all the logically related files. DOS  files  are organized in a  hierarchical or  an inverted tree-like structure. The general analogy is with a file cabinet containing a number of drawers, which in turn may contain folders. The content of these folders is the needed  information.

The file cabinet here is the ROOT DIRECTORY, the drawer is  INDIVIDUAL DIRECTORY, the folders are SUBDIRECTORYand the  informa­tion in these folders may in turn be classified as FILES.

Otherwise,  the large number of files that get created for  var­ious purposes in a disk can make the directory huge and  difficult to view and manage. Therefore, DOS enables the user to organize the files in a disk into directories and sub-directories  in a hierarchical structure. Directories can contain other  directo­ries. A directory within another directory is called  a  sub-directory.

Of course, there may be sub-directories of sub-directories, but  a file name  is the furthest you may descend down  the  (inverted) tree of directories and files. Thus, a file name corresponds to a tree  leaf, a sub-directory to a branch, the  directory  to  the trunk, and the root directory to the root of the tree, hence  the name ROOT DIRECTORY.

Sample of Directory Structure



Command.com    (Personal)  Autoexec.bat  (Accounts)     Pk.bat


         LEAVE     INSURANCE          GPF    PENSION   LOAN

The directory/sub-directory is represented in bold letters.


The content of each of the sub-directory cannot be viewed  unless it is made active, or a sub-directory is specified as   part  of the DIR command. Doing either of these requires an  understanding of the concepts of navigating around the disk.

The directory, the user is in at any point of time, is called the WORKING/PRESENT/CURRENT directory. DOS indicates which direc­tory you are in by displaying the directory’s name in the command prompt. For example, the following command prompt indicate  that you are in the DOS directory: C:\DOS>. Knowing which directory is current  helps  you find files, and to  move  from  one directory  to another more easily. Typically, the ROOT  DIRECTORY (\)  is the initial working directory. The entire specification of directory from  root  is called  a PATH. By itself, the DIR command is applicable  to  the working/present directory. The names of the sub-directories  at adjacent levels are separated by backslash (\), while  specifying the path to be followed while traveling to a sub-directory.


A  path  is the route that leads from the root  directory  of  a drive to the file you want to use.

For example, to access the NOS.LET file in the LETTER  subdirecto­ry of NOS directory, DOS must go from the ROOT (\) directory through  the NOS directory to  the LETTER directory, as shown  in  the  following figure:












To specify the same path at the command prompt, you would type it as shown in the following illustration:


This is the path or route to the file NOS.LET. The first letter and the colon (C:) represent the drive the file is on. The  first  back­ slash (\) represents the root directory. The  second  backslash separates the NOS directory from the LETTER sub-directory.  The third  backslash separates the LETTER sub-directories  from  the file name, NOS.LET.

Note: MS-DOS recognizes path up to 67 characters long (including the

Drive letter, colon, and backslash).

5.8       DIR COMMAND

The DIR command gives the list of is there on the disk  that is mounted on the active drive.

Syntax : C:\> DIR                        A:\> DIR


A:\> DIR

Volume in drive A has no label

Directory of A:\

COMMAND        COM     23612     10-20-88         11.30a

DISKCOPY       COM      4235     10-20-88         12.00p

FORMAT         COM     15780     03-12-89         12.00p

3 file(s)             325013     bytes free


As can be seen, on typing DIR followed by key at DOS  prompt, five  columns of data followed by the number of files  and  bytes that are free in the disk are displayed. The first column  con­tains the primary name of each file resident on the disk.  Howev­er,  most files are named with an extension, which appear in  the second column. Whereas, the third column contains the size of the file in bytes, and the fourth and fifth columns show the date and time  on which the files was created or last modified.  The  last line displays the number of file(s) and remaining disk space  free in  bytes.   It is important to note that the  DIR  command  only displays  the names of the files and not their contents.


All  the names displayed using DIR command that have  be­sides them  are directories. You can see the list  of  files  in another directory by changing to that directory and  then  using the DIR command again.

The  Change Directory (CHDIR) or CD command enables the  user  to travel  around the directories in a disk. Type the CD command  at the command prompt.


A:\> CHDIR {path}    or  a:\> CD {path}

Examples : (Refer to the figure)

# 1.  A:\>CD \NOS

This  command  makes the NOS sub-directory  under  the  root directory (\) active.


The  backslash indicates the root, and LETTERS, which is a sub-directory under the NOS directory, becomes the working directory.

# 3.  A:\> CD \

The  root directory becomes the working directory; i.e.  you  will change  back  to the root or main directory. The slash  typed  in this command is a backslash (\). No matter which  directory  you are in, this command always returns you to the root directory  of a  drive. The root directory does not have a name, it  is  simply referred to by a backslash (\).


As  the  number of files increases in a disk, a need is  felt  to organize them in a meaningful way by creating sub-directories  to store a group of logically related/similar files.

To create a directory, DOS provides the MKDIR (Make Directory) or MD command.


A:\>MKDIR [drive:] {pathname} or  A:\>MD    [drive:] {pathname}

Square brackets indicate that [drive:] entry is optional.

The MD or MKDIR command creates a new empty directory whose  name is the  last item specified in the pathname,  in  the  specified drive. If active, the drive need not be specified. If the  direc­tory is to be created as a sub-directory of the working directory on the active drive, typing MD {directory name} at the DOS prompt or command prompt is sufficient.



makes a SALARY directory in the: drive, under ACCT directo­ry.

# 2. A:\> MD C:\> SALARY

Makes a salary directory in the C: drive, under root directory.


You  may  want to delete or remove a directory to  simplify  your directory structure. DOS provides RD (Remove Directory) to delete a directory.



removes the SALARY sub-directory in ACCT directory.

NOTE: You cannot delete a directory if you are in  it.  Before you can  delete a directory, you must type cd.. at  the  command prompt. At the same point of time, the directory to  be  deleted should be empty.


To copy a file, DOS provides `COPY’ command. When you  use `copy’ command, you must use the following two  parameters;  the location  and the  name of the file you want  to  copy,  or  the source;  and the location and the file name to which you want  to copy the  file  or the target (destination).  You  separate  the source and the destination or target with a space. The syntax of the `COPY’ command is

COPY  {source}  {destination}  or,

COPY [drive:] [path] [filename]  [drive:] [path] [filename]

i.e.  the  first set of drive, path and filename  refers  to  the source  file,  and  the second set of drive,  path  and  filename

refers to the destination file.

(a)        Copying Single File

To  copy the DEBUG.EXE file from the DOS directory to  the  NOS


1. Return to the root directory by typing the following command    prompt: CD\

2.  Change to the DOS directory by typing the following commands at the DOS prompt: CD DOS

3. To copy the file DEBUG.EXE file from the DOS directory to the
    NOS, directory type the following at the command prompt:

   Copy c:\dos\debug.exe c:\nos

   and the following message appears: 1 file (s) copied


# 1. A:\> copy a:\letter\office.doc  \letter\office.bak

makes  a  copy of the office.doc file in the current  or  working directory with a new name office.bak

# 2. A:\> copy office.doc  a:\letters\nos.mem

copy the file office.doc from the root directory to the sub-directory LETTER under root directory with a new name nos.mem.

If the target drive is not specified, the copied file will reside in the disk mounted on the active drive.


If you want to carry out a task for a group of files whose  names have something  in common, you can use  wildcard  characters  to specify groups of files. DOS recognize two wildcard  characters: asterisk  (*) represents one or more characters that a  group  of files  has  in common; and the question mark  (?)  represents  a single character that a group of files has in common. You can use wildcards  to replace all or part of a file’s name or its  exten­sion. The following table shows examples of wildcards:

Wildcard       What it Represents                  Example

*.TXT        All files with a .TXT extension       JULY.TXT


REPORT.*    All files named REPORT with any        REPORT.TXT

            extension                              REPORT.LET


M*.*        All files beginning with letter M      MEMO.TXT

            regardless of their extension          MARCH.XLI

???.*       All files having 3 letter names,       SUN.BMP

            with any or no extension               WIN.LET

You can include the wildcard in the command.

Use of wildcard characters in COPY command

# 1. A:\>COPY \letters\*.COB B:

It means, copy all files with extension *.COB from the  directory LETTERS under the ROOT directory to the working or ROOT directory of the `B’ drive.


The  command is to copy all files with primary name  OPEL  (irre­spective  extension) in the directory COMPANY under ROOT  of  the drive `B’ into the current working directory of the disk  mounted in `A’ drive. Incase  of one drive, the system will ask for the  source  and

target drive.

The command,

#3.A:\>COPY C:\*.*

copies all files of the ROOT directory of the ‘C’ drive into the working directory of the ‘A’ drive.

# 4. A:\> COPY LETTE?.* B:

copies  all files with primary name consisting of 6  charac­ters in total and LETTE as the first five characters (irrespective of extension name) into drive `B’.

# 5. A:\> COPY B:\?.DOC

copies  all files having a primary name of one character with  an extension .DOC from ROOT directory of ‘B’ to the ROOT directory  of `A’ drive.


To  rename  a  file, DOS provides REN command.  The  REN  command stands  for  “Rename”.  When you use the REN  command,  you  must include two parameters. The first is the file you want to rename, and the second is the new name for the file. You separate the two names with a space. The REN command follows this pattern:

     REN oldname newname


Rename the old filename NOS.DOC to a new filename NOS.MEM.


This  section explains how to delete or remove a file that is  no longer  required  in the disk. DOS provides  DEL  command,  which means to delete.

    Syntax : DEL {drive:} {path} {filename}



delete the EDIT.HLP from the DOS directory under ROOT directory.


The  `PRINT’ command of DOS works more or less like  `TYPE’  com­

mand, but at the same time, it enables the content of a text file

to be printed on a paper.


    A:\> PRINT [drive:] {path} {filename}




1. The startup routine runs, when machine boots up is known as

a)      POST

b)      BOOT up

c)    Operating Routine

d)   I/O operation

2. Operating system is also known as:

(a) database

(b) system software

(c) hardware

(d) printer

3.      What is the maximum length allowed for primary name of a computer file under DOS?

(a)    8

(b)   12

(c)    3

(d)   None of the above

4.      Which of the following could be a valid DOS  file  specifica­tion?






5.      How many characters form a secondary name for a file?


(a) 8

(b) 12

(c) 3

(d) None of the above

6.  What  is the name given to something that the  computer  will automatically use unless you tell it otherwise?

(a) a specification

(b) a wildcard

(c) a default

(d) a rule

7.      As per symbolic notation of DOS, which of the following indicates the ROOT directory


(b)   >


(c)    /

(d)   None of the above

8. In wildcard specification `?’ is used as replacement for

 (a) one character

(b) two character

(c) three character

(d) none of the above

9.      With DOS, you may use the `*’ and `?’:


9.(a) when changing the default settings

9.(b) to represent unspecified characters in a filename

9.(c) instead of wildcard characters

9.(d) in the extension but not in the drive name or the file name


10.   DOS system file consists of




(d)   None of the above

11.  The batch file uses the extension


a) .BAT

b) .DOC

c) .PRG

d) .DOS

12.  To display the list of all the file of the disk you would type


(a)    DIR

(b)   COPY

(c)    DIR FILES


13. State whether the following questions are True(T) or False(F).

(a) Command.Com is hidden file.

(b)   Primary name of a file can be of 10 characters.

(c)    The command MKDIR and MD performs the same task.

(d)   Under DOS .EXE is not an executable file.

(e)    DIR command is used to see the content of a specific file.

5.17          WHAT YOU HAVE LEARNT

In this lesson you were introduced to one of the most popular desktop operating system and its working environment. It explained the directory structure, file naming conventions. It also talked in great length about the file management in terms of COPY, DEL, and MOVE. Here you learned the steps involved in loading of operating systems into computer.



1.      Explain in brief what do you understand by Operating system.

2.      Explain the process involved in loading of Operating System.


1. 2.b3.a,4.b,5.b,6.c,7.d,8.a,9.b,10.a,11.a,12.a,13. F,F,T,F,F

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