CODEWITHSUNDEEP
assemblyx86dos
user1091856
user1091856

Reputation: 3158

16bit/ASM: Simple function using int 21h?

Why isn't my 'show_msg' function working properly?

org 100h

push str1
call show_msg
pop ax

mov ah, 4Ch
int 21h

show_msg:
mov ah, 9
mov bx, sp ;in 16bit bx is the only register that can act as a pointer (besides di and si)
mov dx, [bx]
int 21h
ret


str1 db 'Hello world!$'

Upvotes: 1

Views: 898

Answers (3)

Storm-Eyes
Storm-Eyes

Reputation: 821

The program would not work properly because DX does NOT point to the address of "str1".

First, you push the address of str1 into the stack, which SP points to. And that is no problem.

But after you call a function "show_str" to print str1, SP will no longer point to str1, because the value of original IP is pushed into the stack, on which the SP points, now.

You did not realize the change of SP and still tried to pass the value of SP to DX, which is supposed to store the address of the string to be printed.

My advice is that you should use "normal" style to write programs, because that will make our life easier unless you are exploring the secrets of the language itself.

This is my program. 

    ;file: showmsg.asm (.COM)
    ;nasm -fbin showmsg.asm -o showmsg.com 

    org 0x100
    mov ax, cs
    mov ds, ax

    mov ax, str1     ;Transmit parameter to show_msg through AX
    call show_msg

mov ax, 4c00h
int 21h

show_msg:            ;the address offset of the string is stored in ax
    mov dx, ax       ;DS:DX=string address
    mov ax, 0900h    ;AH=09 
    int 21h
    ret
str1: db "Hello, world!$"

Upvotes: 1

Skizz
Skizz

Reputation: 71070

Your code has a fundamental flaw in it.

Firstly, bp can also be used as a pointer.

You should index the stack using the bp register and not the bx register. The [bp] form uses the SS segment by default whereas the [bx] form uses DS as the default segment. This means that if DS != SS you will be not be reading the value pushed onto the stack if you use [bx] but some other undefined value.

So, the correct version is:-

show_msg:
  mov bp,sp
  mov ah,9
  mov dx,[bp+2]
  int 21h
  ret

If your function needs some local storage, then the general form is:-

function:
  mov bp,sp
  sub sp,amount of space for local storage

  some code, parameters are [bp+offset], local data are [bp-offset]

  mov sp,bp
  ret

Upvotes: 2

Daniel Kamil Kozar
Daniel Kamil Kozar

Reputation: 19286

Most probably because [sp] upon entry to the function contains the address of the return code, which - in case of this program - will be whatever address pop ax at the beginning is under. Try changing mov dx, [bx] to mov dx, [bx+2], which will cause the value of the argument pushed before the entry to the function to be retrieved.

Upvotes: 5

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