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. BIOS Interrupt 10H Option 0H Sets video mode. Registers used: 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 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. BIOS Interrupt 10H
Ex: MOV AH,2 MOV BH,0 MOV DH,12 MOV DL,39 INT 10H 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. 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. 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. 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. 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. Attribute Definition Blinking D7 D6 Background D5 D4 Intensity D3 D2 Monochrome display attributes
Blinking 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 Foreground D1
D0 Attribute Definition R D6 B linking D7 B ackground G D5 B D4 Color display attributes Blinking Intensity
D7 = 0 - Non-blinking D7 = 1 - Blinking 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. R D2 Inte ns ity D3 I 0 0 0
0 0 0 0 0 1 1 1 1 1 1 1 1 Fore ground G D1 R 0 0 0 0 1 1 1 1 0 0
0 0 1 1 1 1 G 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 B 0 1 0
1 0 1 0 1 0 1 0 1 0 1 0 1 B D0 Color B lack B lue Gre e n Cyan Re d M age nta B rown White Gray Light blue Light gre e n
Light cyan Light re d Light mage nta Ye llow High inte nsity white 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 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 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 DOS Interrupt 21H Option 0AH Inputs a string of data from the keyboard. Registers used: AH =9 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. DOS Interrupt 21H Ex: .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
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
