Disk Operating System (DOS) (Unit III)

1. In the 1980s or early 1990s, the operating system that shipped with most PCs was a version of the Disk Operating System (DOS) created by Microsoft: MS-DOS.
2. MS-DOS is a disk operating system for IBM PC–compatible computers.
3. In its day, it was easily the most popular operating system in the world. 
4. As with any other operating system, its function is to oversee the operation of the system by providing support for executing programs, controlling I/O devices, handling errors, and providing the user interface.
5. MS-DOS is a disk-based, single-user, single-task operating system. These qualities make it one of the easiest disk operating systems to understand.

DOS FILES

• The main portions of MS-DOS are the IO.SYS, MSDOS.SYS, and COMMAND.COM files.

•  IO.SYS and MSDOS.SYS are special, hidden system files

• The IO.SYS file moves the system’s basic I/O functions into memory and then implements the MS-DOS default control programs, referred to as device drivers, for various hardware components.

• These include the following:

1.  The boot disk drive
2. The console display and keyboard
3. The system’s time-of-day clock
4. The parallel and serial communications port

IO.SYS & MSDOS.SYS

 In the PC bootup sequence, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector, it loads the first three sectors of IO.SYS into memory and transfers control to it. IO.SYS then:

1. Loads the rest of itself into memory.
2. Initializes each default device driver in turn (console, disk, serial port, etc..). At this point, the default devices are available.
3. Loads the DOS files and calls its initialization routine.
4. Processes the MSDOS.SYS file .
5. Processes the CONFIG.SYS file, in MS-DOS 2.0 .
6. Loads COMMAND.COM (or other operating system shell if specified).

COMMAND.COM 

• The COMMAND.COM command interpreter accepts commands issued through the keyboard, or other input device, and carries them out according to the commands definition.
•  When DOS runs an application, COMMAND.COM finds the program, loads it into memory, and then gives it control of the system. When the program is shut down, it passes control back to the command interpreter.

Common MS-DOS Commands
Command	Description	Usage
dir	Lists all files in current directory	Command: C:\>dir
		Result: List of directories Homework, Personal, Work and the file info.doc
md	Creates a new directory in the directory specified in the command, or in the current working directory if none other is specified		Command: md <directory name>
cd	Changes the current directory to the one specified.	Command: C:\>cd <directory name>
cd ..	The command changes the directory to the parent directory of the current directory.	Command: cd ..
cd \	The command changes to the root directory of the current drive.		Command:  cd \
move	This command moves files or directories from the original directory specified to the new directory. To move a file or files, you specify the name and location of the file or files you want to move followed by the destination.		Examples: move <source> <destination>
copy	This command copies files from one location to another.		Command: copy <source> <destination>
rename	Changes the name under which files and/or directories are stored.		Command: rename <original name> <new name>
rd	Removes (deletes) a directory. Note: to remove a directory, directory must be empty.  You must delete all files and subdirectories in order to delete a parent directory.		Command:  rd <directory name>
del	Used to delete files. If a directory name is given in the command, all files in the directory are deleted.		Command: del <file/directory name>
*	Wildcard character, a special symbol that stands for one or more characters.  Operating systems support the use of wild cards for identifying Files and Directories.		Usage: M* searches for all files and directories that start with the letter M.
edit	Command used to run the DOS Text Editor program.		Command: edit
exit	Closes the DOS window		Command: exit