DOS
Understanding what you can do
Operating System Traits
• An OS only works with one type of
processor
– X86 processors for us; Motorola for Mac
– Must understand the CPU’s abilities
• How much memory it can use
• What commands it understands and how to use
them
- You can write “High Level Code” and compile
it for different processors
Traits, cont.
• OS starts running immediately after the
POST test
– Takes control of the computer from the POST
test
– Continues running until computer is rebooted
or shut down
– Can’t stop OS without stopping computer
Traits, cont.
• Application Programs can’t run without an
OS
– Word Processor, Spreadsheet, Web browser
– Programmers write applications to run under
a specific OS
– We can compile application(s) for different
OSs
Microsoft Word
Code base for Word –
Page on Screen, Edit, etc.
Compiler for x86 Compiler for Mac
Application for Windows Application for Mac
Functions of an OS
• Must communicate with the hardware
• Must create a User Interface
• Must present available installed programs
• Allow user to add, move and delete
installed programs
Communicate with Hardware
• Has to access hard disk drive, accept input from
keyboard and output to monitor
• To do this, it works with the System BIOS
• To run a program, we:
– Tell the OS what to run
– OS then accesses the hard disk drive with
appropriate BIOS routines
– OS then loads program into RAM
* Should provide error message(s) if hardware fails
How it works
Application
DOS Windows
Operating System
BIOS Device Drivers
Hardware
Create a User Interface
• The Shoe Store analogy:
– Shoes on display by gender, style, type
– Shoes stored (in the back) by code number
– Display(s) can be changed without affecting inventory
– for example a sale on dress shoes
– What you see is the “User Interface” to the shoe
inventory
• The customers (users) look at display (the User
Interface) and tell the salesperson (OS) what to
do (programs and data)
Accessing and Supporting
Programs
• Must enable user to start a program
– Need a way to tell OS what program to run
• Type it in
• Click with mouse
• OS must get out of the way of the program
– Turn over control to program (DOS)
– Fade to background (Windows)
• Must come back when program is done
Organizing Programs and Data
• To the computer, its all just a file
– The OS is one, or more, file(s)
– Programs are file(s)
– Data is stored in file(s)
* Computer can only run three types of files:
- .COM files (compiled)
- .EXE files (also compiled)
- .BAT files (text files)
Naming Drives
• Use single letter only, followed by a colon
• A: and B: reserved for floppy drive(s)
• C: to Z: for hard disk, or other, drives
• Maximum of 26 drives at any one time
• Change drives (change focus) by typing
drive letter (and colon) and press
Naming Files
• 8.3 convention (DOS):
– Up to eight characters for name
– Up to three characters for file extension
– Can’t use “special characters” – stick to letters
and numbers and no spaces
• My File is not valid
• MyFile01 is valid file name
- File names must be unique
Organizing Files
• Use directories and subdirectories; start
with Root Directory (C:\)
Root Directory
C:\
Apps Data Stuff
Jan Feb Mar
MyFile01 MyFile01
Directories
• Directories can hold data or directories or
both
• Directory names are eight characters (or
less)
• Directory names do not have an extension
(99.99% of the time – Word Perfect used
extensions to baffle users)
Organizing Files
• Use directories and subdirectories; start
with Root Directory (C:\)
Root Directory
C:\
Apps Data Stuff
This is legal since fully
Jan Feb Mar
qualified file name is:
C:\Data\Jan\MyFile01
MyFile01 MyFile01 File name here is:
C:\Data\Feb\MyFile01
Path
• Fully Qualified Name is also the Path (to
the file):
• C:\Data\Jan\MyFile01
File Name
Path
User Interface
• Command line – DOS
– You get to type command(s)
– No mouse, but do get a cursor
• Graphical User Interface (GUI) – Windows
– Uses icons to represent files
– Point and click interface
Building DOS
• Three main files and two optional files
– IO.SYS which is basic device drivers
– MSDOS.SYS more drivers and where optional files
plug into the OS
– COMMAND.COM which is the User Interface
• IO.SYS and MSDOS.SYS are hidden files;
COMMAND.COM is visible file in directory listing
• Optional files are: CONFIG.SYS and
AUTOEXEC.BAT – both are text files
Boot Sequence
• IO.SYS first
• MSDOS.SYS next
– CONFIG.SYS for drivers and variables
– AUTOEXEC.BAT for “automatic”
programming
* COMMAND.COM which provides the User
Interface and command library
The Command Prompt
Focus Directory (Root)
C:\>_
Drive Letter
Command
Prompt
(flashing)
Internal and External
• Internal commands are:
– Found within COMMAND.COM much like
chapters in the textbook
– Can be run from any “location” on any drive
• External commands are:
– Programs in their own file
– Have to be available to DOS via directory or
path
What COMMAND.COM Does
• Convert command to upper case letters
• Search internally for command
• Search the current/active directory
• Search the directories specified in the
Path environment variable
• Return to prompt with error message
A DOS Command
• [on what] [where]
Minimum of one space
• DIR, all by itself, will give you a directory
listing of the current directory
• DIR A:\TEST\LIFE will give you a directory
of the test\life directory on the A: drive
Internal Commands
• DIR for directory listing
• CD to change directory
• MD to make a directory under the current
directory
• COPY to copy a file from one location to
another (and, optionally, to change name
of file)
External Commands
• FDISK to partition a hard disk drive
• FORMAT to format (prepare for data) a
disk drive
• SYS to copy system boot files to the
location specified
Path
• Lists drive locations, in order, to be
searched for external commands
• C:\DOS; C:\WINDOWS; C:\WP51;
C:\WINDOWS\SYSTEM32
• Can specify this in an entry in
AUTOEXEC.BAT
Boot Disk
• Enough files (three for DOS) to load an
operating system
• Windows requires too many files to fit on a
boot disk
• Remember www.bootdisk.com – it might
come in handy one day
Virtual Disk
• The boot disk that Windows 98 makes will
build a virtual disk in RAM, unpack several
files and report to you the drive letter
assigned
• This is a cute way to get ten pounds into a
five-pound bag