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DOS Overview


DOS is pretty simple once you have a  basic understanding of some key elements.

  • DOS is a command line operating system. That means all it's commands are typed at the DOS prompt.  Looks like this normally  C:\

  • DOS sometimes requires drivers for devices in your computer such as a CD-ROM or Sound Card. The drivers are usually provided by the company that makes the device or sells the computer system.

  • DOS requires certain programs to run at startup, like memory managers and CD-ROM extensions.

The purpose of this page is to provide the key information needed to keep your computer running smoothly and help avoid future problems. There are plenty of books on DOS if want to learn more.


 The Basics

When the computer is first started it looks for a disk with an operating system on it. That disk could be a floppy disk or it may be a hard disk inside the computer. While DOS is loading it checks the Config.sys file for any settings or drivers that need to be used. When it's finished it checks the Autoexec.bat for any programs or settings that need to be run at the start. Finally the DOS prompt appears allowing the user to control DOS with specific commands.

Before continuing it's important to mention a few things about the disk drives and software. Most Disk Drives work like a VCR or cassette deck. Instead of tape, the magnetic film is in the shape of a disk. The disk allows the computer to read (play) or write (record) to any part of the disk instantly. Since it's possible to have several drives in your computer DOS assigns each one a letter. The Floppy drives are always given A: and B:, the following drives are each assigned the next available letter. Most computers have A:(floppy), C:(hard drive) and D:(CD-ROM). Note: it is possible to split a disk drive into more then 1 letter using partitions. For example, a 2Gb hard drive could be divided in half to create two 1Gb drives such as C: and D:.

In order for the computer to perform useful tasks it requires a set of instructions. Those instructions are supplied by programmers in the form of Software. Software is stored on the disk drive as files and normally organized into directories (folders). It works exactly a like an office file cabinet. The cabinet is the computer, the drawers are disk drives and each drawer has folders containing files. Technically DOS uses directories and Windows uses folders. They're they exact same thing.

In DOS based systems all files have 8 letter names with a 3 letter extension. The 3 letter extension helps identify the file. For example, .JPG and .GIF are two types of picture files. So house.jpg would probably be a picture of a house. Windows95 allows 255 letter names with a 3 letter extension known as a long filename. Long filenames are cut short by DOS. They appear as the first 6 letters plus ~# and then the 3 letter extension. For example, My Letter.doc in Windows95 would appear mylett~1.doc in DOS.


 The DOS prompt...

The DOS prompt is normally the current drive letter followed by some punctuation, C:\. Since it can be altered using the Prompt command it may look slightly different. The prompt displays the current drive plus your current location on that drive. For example, if the prompt looked like this: C:\games\doom, you are currently on drive C: inside the games folder, inside the doom folder. Folders are separated by a \ mark.

Navigating the DOS prompt only requires a few commands. The commands have additional options if needed. See below for detailed information on these commands.

  • To change drives type the letter of the new drive plus a colon. Example, A:

  • To change directories type CD followed by the name of the directory or directories. Example, CD windows

  • To backup one directory type CD followed by two periods, Example CD..

  • To list the files located in the current directory type DIR. DIR has several options called switches, the most useful being /p and /w. Example, DIR or DIR /p (stops at one page of files) or DIR /w (shows files full screen)

  • To start a program or batch file find it's location and type it's name. Programs end in EXE and COM while batch files end in BAT. Example, to start the game doom from it's directory type Doom or Doom.exe. To start doom at any prompt type it's location and name such as C:\games\doom\doom.exe



There are two symbols that may be used with DOS commands known as Wildcards. They are the " * " (some call it star) and " ? ". When used with commands like DIR they can be used to filter which filenames appear in the list.

The " * " allows entire filenames are parts of filenames to be occupied by any letter in any order. "*.doc" would show every file ending in ".doc". "Let*.doc" would show every file beginning with "Let" and ending in ".doc", such as "Letter.doc, LetsGo.doc, Lettuce.doc, Letter2.doc". "Ship.*" would show files such as "Ship.bmp, ship.doc, ship.jpg".

The " ? " only allows 1 character to be replaced. "?.doc" would show all files with 1 letter names ending in ".doc" such as "1.doc, a.doc, f.doc". "?ouse.bmp" would show "mouse.bmp, house.bmp".

The Wildcards can also be used in combination to further filter filenames. Lets say you wanted to copy all the files ending in EXE and ERR from one directory to another. You could use " *.E?? ". You may want to delete every file that has Temp anywhere in the name and ends in TXT and TOT. You would use " *temp*.T?T ".


 Where is DOS?

DOS itself only requires a few key files to actually start the computer. These files are normally located on drive C: or a floppy boot disk in drive A:. Some of them are hidden and require an extra step to view. The key files are:

DOS also includes a variety of small programs and files used to accomplish other tasks. Older versions are usually located at C:\DOS or if your using Windows95 C:\WINDOWS\COMMAND and C:\WINDOWS. The files needed most are:

Attrib.exe Shows/changes files attributes Text Editor
Emm386.sys Expanded Memory Manager Uncompresses DOS files Used to Format/Partition Hard Drives Formats disks
Himem.sys High Memory Manager
Mem.exe Shows memory free
Memmaker.exe Helps configure DOS to free memory
Mscdex.exe Adds CD support to DOS
Qbasic.exe Basic Language, needed to use
Scandisk.exe Scans disk drive for errors Creates Windows95 boot disk Copies entire Directories


 DOS's Memory Problems...

Back when DOS was first introduced they decided nobody would ever need over 640K of RAM. Well... by time they discovered that was completely wrong, DOS was already being used by everyone. So to stay compatible they added memory managers to help over come the 640K limit.

The first 640K of memory is called Conventional memory. It's extremely important to free as much of this memory area as possible. When DOS is poorly configured it loads itself and everything else into the Conventional memory area. When you go to run your software there's not enough free Conventional memory and you get the dreaded out of memory message. That's normally why a 4Mb DOS game won't run on a 16Mb computer. There's not enough free Conventional memory.

The memory area between 640K and 1Mb is called the Upper memory area. By moving your DOS drivers out of the Conventional area into the Upper area you can free up Conventional memory. You do this by correctly editing the Config.sys file and Autoexec.bat file.



Attrib.exe is a small program included with DOS and Windows. It allows you to view and change the attribute settings of files. All files have certain attributes. Those attributes are Archive, Hidden, System and Read Only. Normally you don't have to worry about the attributes settings but occasional you may need to repair something or remove it. Hidden files won't appear using the DIR command, you can only see them by using Attrib.

Attrib works just like DIR for viewing files. At C:\ type Attrib and you should see a list of the files on C:\ along with their attribute settings. You will also see the hidden DOS files IO.SYS and MSDOS.SYS.

The contents of this page are 2009 Realm
Last Updated (January 1st, 2007)