Displaying a string using DOS interrupt 21H function 09H is a straightforward way to print messages to the screen in 8086 assembly language. This blog post explores an 8086 assembly program that prints the string ‘hello’ to the console.

data segment
msg1 db 'hello$'
data ends

code segment
assume cs:code,ds:data
start:
mov ax,data
mov ds,ax
mov sp,0d00h
mov ah,09h
int 21h
mov ax,4c00h
int 21h
int 3
code ends
end start

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Overall Process

The program initializes the necessary segments. It loads the string ‘hello’ into memory and prints it using DOS interrupt 21H function 09H. The program successfully prints ‘hello’ on the console before terminating.

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Step-by-Step Explanation

1. Data Segment Initialization
   • msg1 db 'hello$' stores the string ‘hello’ with $ as a termination character required by DOS.
2. Code Execution Flow
   • mov ax, data and mov ds, ax: Initializes the data segment by loading its address into AX and then into DS.
   • mov sp, 0d00h: Sets up the stack pointer.
   • mov ah, 09h: Prepares for string output using DOS interrupt function 09H.
   • lea dx, msg1: Loads the offset of the string into DX register.
   • int 21h: Calls DOS interrupt 21H to print the string to the console.
   • mov ax, 4c00h and int 21h: Terminates the program.
   • int 3: Generates a breakpoint interrupt.

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Flowchart

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Output

C:\TASM>masm an_hello.asm
Microsoft (R) Macro Assembler Version 5.00
Copyright (C) Microsoft Corp 1981-1985, 1987.  All rights reserved.
 
Object filename [an_hello.OBJ]:
Source listing  [NUL.LST]:
Cross-reference [NUL.CRF]:
 
  50394 + 450262 Bytes symbol space free
 
      0 Warning Errors
      0 Severe  Errors
 
C:\TASM>link an_hello.obj
 
Microsoft (R) Overlay Linker  Version 3.60
Copyright (C) Microsoft Corp 1983-1987.  All rights reserved.
 
Run File [AN_HELLO.EXE]:
List File [NUL.MAP]:
Libraries [.LIB]:
LINK : warning L4021: no stack segment
 
C:\TASM>debug an_hello.exe
-g
hello
Program terminated normally
-q

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Summary of Output

• The program successfully prints ‘hello’ to the console.
• The string output is handled by DOS interrupt 21H with function 09H.
• The program terminates properly after execution.