printing in assembly programming using FASM

Question

I am trying to print a message by using the code below:

org 100h
start:
    jmp begin

begin:
    mov ah, 9
    mov dx, msg
    msg db 'Ascii sign:.$'
    int 21h

finish:
    mov ax, 4c00h
    int 21h

It is able to compile but it display nothing at all. But if I move the line "msg db 'Ascii sign:.$'" below "jmp begin", the message is able to display.

I want to know the logic behind this. Does that make a difference where I declare the message ?

This is just out of curiosity, thank you!

Answer 1

Yes. Right now, msg is defined in the middle of the code, where the CPU will attempt to execute it. You normally want to define data separately, in the data segment. I don't remember the syntax for FASM, but with MASM or TASM, you'd normally do something like this:

.model small
.data

msg db 'ASCII sign: .$'

.code
main proc
     mov ah, 9
     mov dx, offset msg
     int 21h
     mov ax, 4c00h
     int 21h
main endp
     end main

Answer 2

If you really must have your strings in the code section, then just jump over them.

begin:
    mov ah, 9
    mov dx, msg
    jmp overstring
    msg db 'Ascii sign:.$'
overstring:
    int 21h

finish:
    mov ax, 4c00h
    int 21h