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: MS-DOS.

• 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.

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• 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.

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• 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.

DOS Memory MapROM 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

Interrupt Vector00400H00000H

Top of RAM

DOS Internal Commands

• TIME

Displays current time and allows it to be changed.

Syntax: TIME

• DATE

Displays current date and allows it to be changed.

Syntax: DATE

• CLS

Clears the screen.

Syntax: CLS

• 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.