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Q75177: FIX: Subtracting Two Externs Gives Incorrect Results

Article: Q75177
Product(s): Microsoft Macro Assembler
Version(s): MS-DOS:6.0
Operating System(s): 
Keyword(s): 
Last Modified: 04-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Macro Assembler (MASM), version 6.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

Subtracting two external variables in the Microsoft Macro Assembler (MASM)
version 6.0 results in an incorrect value, and the assembler does not generate
any errors. It should produce an error similar to the one received in MASM 5.1,
such as:

  A2032: Illegal Use of extern

CAUSE
=====

When subtracting two local variables, the assembler will correctly calculate the
difference in their offsets. However, with external variables, it does not know
the offset at assemble time, so it cannot correctly determine their difference.

RESOLUTION
==========

The difference in offsets for external variables can be calculated by moving the
values into registers or temporary variables and then performing the
calculation. The sample code below illustrates this problem.

STATUS
======

Microsoft has confirmed this to be a problem in MASM version 6.0. This problem
was corrected in MASM version 6.0a.

MORE INFORMATION
================

Sample Code 1
-------------

  ; Assemble options needed: none

  .MODEL small
  .STACK 4096

  .DATA
  extrn Var1 : byte, Var2 : byte ; The extern variables

  .CODE
      .startup
      mov ax, offset Var1   ; Gives correct offset of extern var1
      mov bx, offset Var2   ; Gives correct offset of extern var2
      mov cx, (Var2 - Var1) ; Incorrectly calculate a zero for
                            ; the difference
      sub bx, ax            ; This will calculate the correct value
      .exit 0
  END

Sample Code 2
-------------

  ; Contains the external variable definitions

  PUBLIC   Var1, Var2
  .MODEL small
  .DATA
  Var1     byte   '5'
  Var2     byte   '9'
  END

Additional query words: 6.00 buglist6.00 fixlist6.00a

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600
Version           : MS-DOS:6.0
Solution Type     : kbfix

=============================================================================

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