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

Operating System

BIOS

Hardware

Application

Device Drivers

DOS Windows

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 <Enter>

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 DirectoryC:\

DataApps Stuff

FebJan

MyFile01MyFile01

Mar

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 DirectoryC:\

DataApps Stuff

FebJan

MyFile01MyFile01

MarThis is legal since fully qualified file name is:C:\Data\Jan\MyFile01

File name here is:C:\Data\Feb\MyFile01

Path

• Fully Qualified Name is also the Path (to the file):

• C:\Data\Jan\MyFile01

Path

File Name

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

C:\>_

Drive Letter

Focus Directory (Root)

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

• <Action> [on what] [where]

• 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

Minimum of one space

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