1. DOS and BIOS Interrupts
DOS and BIOS interrupts are used to perform some very useful functions, such as displaying data to the monitor, reading data from keyboard, etc.
They are used by identifying the interrupt option type, which is the value stored in register AH and providing, whatever extra information that the specific option requires.

2. Video Programming
When an application program needs to write characters on the screen in text mode, it can choose between three types of video output:
MS-DOS-level access:
Any computer running or emulating MS-DOS can use INT 21h to write text to video display.
BIOS-level access:
Characters are output using INT 10h function, known as BIOS services. This executes more quickly than int 21h, and permits the control of text color.
Direct video access:
Characters are moved directly to video RAM (screen buffer), so the execution is instantaneous.

3. Screen Buffer
The screen buffer begins at B800:0000 and it has some rows and columns.
Number of columns is usually 80.
Number of rows can be 25 or 40 in most cases.
Each line on screen takes 160 bytes (80 x 2 byte for CHARACTER + ATTRIBUTE).

4. Display Modes
Monitors can display both text and graphics and have different techniques and memory requirements for each. Consequently, video adapters have two display modes: text and graphics.
In text mode, the screen is divided into columns and rows, typically 80 columns by 25 rows, and a character is displayed at each screen position (character cell).
In graphics mode, the screen is again divided into columns and rows, and each screen position is called a pixel (short for picture element). A picture can be displayed by specifying the color of each pixel on the screen.
Text Modes
    Mode   Description                 Adapters
3    80 x 25    16-color text     VGA  (most frequently used mode for text display) Graphics Modes
    Mode(hex) Description             Adapters
12       640 x 480   4 color     VGA 13       320 x 200  256 color    VGA  
Video Display Addresses
B800:0000h Standard display address for text mode

5. BIOS Interrupt 10H Option 0H – Sets video mode. Registers used: Ex:
AH = 0H
AL = Video Mode.
3H - CGA Color text of 80X25
7H - Monochrome text of 80X25
Ex:
MOV AH,0
MOV AL,7
INT 10H

6. BIOS Interrupt 10H Option 2H – Sets the cursor to a specific location.
Registers used:
AH = 2H
BH = 0H selects Page 0.
DH = Row position.
DL = Column position.

7. BIOS Interrupt 10H
Ex:
MOV AH,2
MOV BH,0
MOV DH,12
MOV DL,39
INT 10H

8. BIOS Interrupt 10H
Option 6H – Scroll window up. This interrupt is also used to clear the screen when you set AL = 0.
Registers used:
AH = 6H
AL = number of lines to scroll.
BH = display attribute.
CH = y coordinate of top left.
CL = x coordinate of top left.
DH = y coordinate of lower right.
DL = x coordinate of lower right.

9. BIOS Interrupt 10H Clear Screen Example:
MOV AH,6
MOV AL,0
MOV BH,7
MOV CH,0
MOV CL,0
MOV DH,24
MOV DL,79
INT 10H
The code above may be shortened by using AX, BX and DX registers to move word size data instead of byte size data.

10. BIOS Interrupt 10H
Option 7H – Scroll window down. This interrupt is also used to clear the screen when you set AL = 0.
Registers used:
AH = 7H
AL = number of lines to scroll.
BH = display attribute.
CH = y coordinate of top left.
CL = x coordinate of top left.
DH = y coordinate of lower right.
DL = x coordinate of lower right.

11. Practice/Lab
4. Modify your code so that it performs the following tasks:
- Clear screen
- Set cursor to the middle of screen Display the characters (5) in: CHAR_TBL DB ‘A’ ,’B’, ’C’, ’D’, ’E’
on the middle of the screen
5. Compile and run your code

15. BIOS Interrupt 10H
Option 8H – Read a character and its attribute at the cursor position.
Registers used:
AH = 8H and returned attribute value.
AL = Returned ASCII value.
BH = display page.

16. BIOS Interrupt 10H
Option 9H – Write a character and its attribute at the cursor position.
Registers used:
AH = 9H.
AL = ASCII value.
BH = display page.
BL = attribute.
CX = number of characters to write.

17. Attribute Definition Monochrome display attributes Blinking Intensity
D7 = 0 - Non-blinking
D7 = 1 - Blinking
Intensity
D3=0 - Normal intensity
D3=1 - Highlighted intensity
Background and foreground
D6 D5 D4 and D2 D1 D0
White = 0 0 0
Black = 1 1 1

18. Attribute Definition Color display attributes Blinking Intensity
D7 = 0 - Non-blinking
D7 = 1 - Blinking
Intensity
D3=0 - Normal intensity
D3=1 - Highlighted intensity
Background and foreground
D6 D5 D4 and D2 D1 D0
RGB values defined by the table to the right.

20. DOS Interrupt 21H
Option 1 – Inputs a single character from keyboard and echoes it to the monitor.
Registers used:
AH = 1
AL = the character inputted from keyboard.
Ex:
MOV AH,1
INT 21H

21. DOS Interrupt 21H
Option 2 – Outputs a single character to the monitor.
Registers used:
AH = 2
DL = the character to be displayed.
Ex:
MOV AH,2
MOV DL,’A’
INT 21H

22. INT 21h/ 01H and 02H Input a character
Step 1: Set AH =01H
Step 2: : Call INT 21H to display the character
Step 3: The input character is saved in AL
To display the entered character
Step 1: Set AH=02H
Step 2: Load the character from AL to DL
– Step 3 Call INT 21H to display the character

23. Int 21h/1h 2h

24. Example
Prompt reading a character

26. DOS Interrupt 21H
Option 9 – Outputs a string of data, terminated by a $ to the monitor.
Registers used:
AH = 9
DX = the offset address of the data to be displayed.
Ex:
MOV AH,09
MOV DX,OFFSET MESS1
INT 21H

27. DOS Interrupt 21H
Option 0AH – Inputs a string of data from the keyboard.
Registers used:
AH = 0AH
DX = the offset address of the location where string will be stored.
DOS requires that a buffer be defined in the data segment. It should be defined as follows:
1st byte contains the size of the buffer.
2nd byte is used by DOS to store the number of bytes stored.

28. DOS Interrupt 21H Ex: Assume “Go Tigers!” was entered on the keyboard.
.DATA
BUFFER1 DB 15,?,15 DUP (FF) .
MOV AH,0AH
MOV DX,OFFSET BUFFER1
INT 21H
Assume “Go Tigers!” was entered on the keyboard.
BUFFER1 = 10,10,’Go Tigers!’,CR,255,255,255,255

29. DOS Interrupt 21H
Option 4CH – Terminates a process, by returning control to a parent process or to DOS.
Registers used:
AH = 4CH
AL = binary return code.
Ex:
MOV AH,4CH
INT 21H