Multimodule assembler program

Question

I'm having troubles with creating simple multimodule program in DOS.

What I want is to create procedure which increments ax with 5 and call it from main procedure. But every time I start debugging I get strange problem with infinite one-instruction cycle:

add [bx:si], al

Down here is my first file:

;(tmp1.bat)
.model small
.386
extrn mytmp

.code
org 100h
start:
  mov ax, 5
  push ax
  call mytmp
  pop ax
  mov dl, al
  mov ah, 06h
  int 21h
  ret
end start
end

And the second file:

;(tmp2.bat)
.model small
.386
public mytmp

.code
mytmp: 
  pop ax
  add ax, 5
  push ax
  ret
END

What am I doing wrong? PS compiling from DOS:

tasm tmp1.bat
tasm tmp2.bat
tlink /t tmp1.obj tmp2.obj

Answer 1

This code:

pop ax
add ax, 5
push ax
ret

Is completely nonsensical and causes a crash. You are popping the return address from the stack, adding 5 to it, putting it back in the stack, and returning. So, the function returns to the original intended return address plus five, which is an arbitrary location five bytes after the location where the call was supposed to return.

Apparently, that's in the middle of some instruction, so the disassembler gets confused, and it is showing you that you are about to execute add [bx:si], al which is a nonsensical, non-existent instruction.

So, whatever it is that you thought you were trying to achieve with those 3 instructions in your mytmp: function, it is wrong, and you should not be doing it.

Answer 2

TASM assembles call mytmp as an indirect call according to a value in mytmp . It doesn't know that mytmp is a procedure.

Change

extrn mytmp

to

extrn mytmp:PROC