Assembly Interrupts

date 14.11.2020

Interrupts are simply set of procedures. We're using interrupts with `INT` command. Before running an interrupt we have to store some data on stack to tell interrupt what we want to do. ```asm MOV AH, 09H ;function number for outputing string to the monitor LEA DX, STR ;store the string that we want to display INT 21H ;run interrupt ```

This is an axample interrupt to display string.

Useful Interrupts

INT 10H - Display

This interrupt is looking to AH register. Let's look at it's functions:

06 - clear the screen

- **AH = 06**   ;function number - **AL = 00**   ;page number - **BH = 07**   ;default style (black background with white text) - **CH = 00**   ;row number (start) - **CL = 00**   ;column number (start) - **DH = 00**   ;row number (end) - **DL = 00**   ;column number (end)

02 - setting the cursor

- **AH = 02**   ;function number - **DH = 00**   ;row number for cursor - **DL = 00**   ;column number for cursor - **BH = 00**   ;page number

03 - get the current cursor position

- **AH = 03**   ;function number - **BH = 00**   ;page number - **DH = 00**   ;row number - **DL = 00**   ;column number

0E - output a character to the screen

- **AH = 0E**   ;function number - **AL = 00**   ;character we want to display - **BH = 00**   ;page number - **BL = 00**   ;foreground color

---

INT 21H - Display & Keyboard

09 - outputing a string of data to the monitor

- **AH = 09**   ;function number - **DX = 00**   ;offset address for string array

It's important to have **$** in the end of the array.

02 - outputing a single character to the monitor

- **AH = 02**   ;function number - **DL = 00**   ;ASCII code for character

01 - inputing a single character with an echo

- **AH = 01**   ;function number - **AL = 00**   ;it's stores the data displayed and taken by keyboard after interrupt (ASCII).

0A - inputing a string of data from the keyboard

- **AH = 0A**   ;function number - **DX = 00**   ;offset address for string array we want to store

When we press **ENTER** it stops recording data, after:

- **DS:DX =** buffer size with bytes (character length + 2) - **DS:DX+1 =** character length - **DS:DX+2 =** first character of array - **DS:DX+n =** last character of array

To be able to record array we have to declare buffer storage.

- **BUFFER DB 10,?,10 DUP (FF)**

---

INT 16H - Keyboard

01 - controlling to detect key strokes

- **AH = 01**   ;function number

If we type something **ZF** becomes **0**.

00 - reading keyboard

- **AH = 00**   ;function number

When we type something to keyboard **AL** stores typed character with ASCII format.

---

These interrupts are just examples, there are ton of interrupts we can use.

Example Code

I want to show you how to use interrupts with a simple example. In this example I want to type some string and display it. ```asm .MODEL SMALL

.STACK 64

.DATA MESSAGE DB 'Type your string:', '$' ;welcome message USER_INPUT DB 80 DUP('$') ;user input we want to store NEWLINE DB 10,13,'$' ;move to new line after typing

.CODE MOV AX, @DATA ;required stuff MOV DS, AX

LEA SI, USER_INPUT

MOV AH, 09H LEA DX, MESSAGE ;display welcome message INT 21H

MOV AH, 0AH ;get pressed key MOV DX, SI INT 21H

MOV AH, 09H ;display new line LEA DX, NEWLINE INT 21H

MOV AH, 09H ;display typed string LEA DX, USER_INPUT+2 INT 21H

MOV AH, 4CH ;terminate program INT 21H

END ;exit ```