Interacting with BIOS interrupts in 8086 assembly language is a crucial technique for handling low-level hardware operations. This blog post explores an 8086 assembly language program that invokes BIOS interrupts to manipulate the screen cursor position and handle user input.

code segment
assume cs:code
start:
mov sp, 7000h
mov ah, 01
mov ch, 00
mov cl, 14
int 10h
mov ah, 02
mov bh, 00
mov dh, 23
mov dl, 10
int 10h
mov ax, 4c00h
int 21h
int 3
code ends
end start

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Step-by-Step Explanation

1. Stack Initialization
   • mov sp, 7000h: Sets up the stack pointer to a specific memory address for program execution.
2. Checking Keyboard Buffer
   • mov ah, 01: Selects BIOS function 01h to check for a keystroke in the keyboard buffer.
   • mov ch, 00 and mov cl, 14: Adjusts cursor shape settings.
   • int 10h: Calls BIOS video interrupt to execute the function.
3. Setting Cursor Position
   • mov ah, 02: Selects BIOS function 02h to set cursor position.
   • mov bh, 00: Specifies the active display page.
   • mov dh, 23: Defines the row coordinate (starting from 0).
   • mov dl, 10: Defines the column coordinate.
   • int 10h: Calls BIOS interrupt to move the cursor to the defined position.
4. Program Termination
   • mov ax, 4c00h: Prepares termination routine.
   • int 21h: Calls DOS interrupt to exit the program.
   • int 3: Generates a breakpoint interrupt for debugging.

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Overall Process

• Stack Setup: It initializes the stack pointer to a specific memory location.
• Keyboard Buffer Check: It checks whether a key has been pressed using a BIOS interrupt.
• Cursor Positioning: It moves the cursor to a specific row (23) and column (10) on the screen.
• Program Termination: It exits the program using a DOS interrupt and sets a breakpoint for debugging.

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Output

C:\TASM>masm an_bios.asm
Microsoft (R) Macro Assembler Version 5.00
Copyright (C) Microsoft Corp 1981-1985, 1987.  All rights reserved.
 
Object filename [an_bios.OBJ]:
Source listing  [NUL.LST]:
Cross-reference [NUL.CRF]:
 
  50406 + 450250 Bytes symbol space free
 
      0 Warning Errors
      0 Severe  Errors
 
C:\TASM>link an_bios.obj
 
Microsoft (R) Overlay Linker  Version 3.60
Copyright (C) Microsoft Corp 1983-1987.  All rights reserved.
 
Run File [AN_BIOS.EXE]:
List File [NUL.MAP]:
Libraries [.LIB]:
LINK : warning L4021: no stack segment
 
C:\TASM>debug an_bios.exe
-g
Program terminated normally
-q

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Summary of Output

• The program successfully interacts with BIOS interrupts.
• The cursor position is manipulated before the program exits.
• The program terminates normally after execution.