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DOS
 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: MSDOS.
• MS-DOS is a disk operating system for IBM PC–compatible computers.
• In its day, it was easily the most popular operating system in the world.
 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.
 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.
Internals of DOS
BIOS
DOS
Kernel
Command
processor
DOS Loading
• When a system is started or reset, program execution begins at address FFF0H.
• The system start-up routine of ROM runs a test called Power On Self Test (POST) which
check whether peripherals connected to the computer are working or not.
• 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.
• The boot tries to load the DOS into the memory by reading the two hidden files IO.SYS,
MSDOS.SYS, If these two are found, they are loaded along with the DOS command
interpreter COMMAND.COM.
Continue…
• The IO.SYS file consists of two separate modules.
• The first module consist of BIOS which consist of system initialization program
init which determines the linked set of resident device drivers for the console,
auxiliary port, printer, clock devices and some hardware specific initialization code.
• Second module consists of system initialization program which determines the
Ram size in the PC and based on this information moves itself to high memory.
• Then, it loads the MSDOS.SYS program to its final memory location or shift it
from its original to the final one. The final location of the DOS kernel program.
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The DOS kernel initializes its tables and sets up various work areas.
It sets up the interrupt vector table.
It then loads and executes the device drivers.
These driver function determine device status, perform necessary hardware initialization and
set up vectors for any external hardware interrupts.
• After initialization of DOS kernel and all device drivers are avialable, SYSINIT calls the
normal MS-DOS file service to open the CONFIG.SYS file.
• The CONFIG.SYS contains a list of additional device drivers that the user wants in his
system.
Continue…
• After loading all installable device drivers, SYSINIT reopens the console devices as
a standard output device and standard input device.
• Finally, SYSINIT calls the EXEC function load the command interpreter.
• Once the interpreter is loaded , it displays a prompt and waits for the user to enter a
command.
ROM bootstrap routine
Transient part of
COMMAND.COM
Transient Program Area
Resident part of
COMMAND.COM
Installable drivers
File Control Blocks
Disk Buffer Cache
DOS Kernel
BIOS
00400H
00000H Interrupt Vector
DOS Memory Map
Top of RAM
DOS Internal Commands
• TIME
•
•
CLS
Displays current time and allows it to be changed.
Clears the screen.
Syntax: TIME
Syntax: CLS
DATE
Displays current date and allows it to be changed.
Syntax: DATE
•
DIR
Shows directory information of a diskette: name,
size, and the
date and time stamp of files.
Syntax: DIR [d:][path]
DOS External Commands
• FORMAT
Prepares a diskette for use by DOS.
Syntax: FORMAT [d:]
• CHKDSK
Analyzes disk or diskette (first and only parameter) and displays
disk and memory status report.
Syntax: CHKDSK [d:] [/f] [/v]
• DISKCOPY
Makes an exact copy of a diskette, including hidden system files if
they are present.
Syntax: DISKCOPY [d1:] [d2:]
BIOS MODULE
• Bios is located in an 8 k byte ROM at the top of memory.
• The bios provides the most direct , lowest level interaction with the various
devices in the system.
• Bios contains the default hardware-dependent device drivers for I/o devices.
• These drivers are built into the Bios, which is written in read only memory and
supplied by the manufacturer , they are called resident drivers.
• The Bios is loaded into random-access memory from the read only memory
during system initialization part of file named IO.SYS.