DOSSEG
.MODEL TINY
.DATA
TXT DB "Hello world!$"
.CODE
START:
MOV ax, @DATA
MOV ds, ax
MOV ah, 09h ; prepare output function
MOV dx, OFFSET TXT ; set offset
INT 21h ; output string TXT
MOV AX, 4C00h ; go back to DOS
INT 21h
END START