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Q94587: FIX: DUP Behavior Different with Span-Dependent Value

Article: Q94587
Product(s): Microsoft Macro Assembler
Version(s): MS-DOS:6.0,6.0a,6.0b
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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

- Microsoft Macro Assembler (MASM), versions 6.0, 6.0a, 6.0b 
-------------------------------------------------------------------------------

SYMPTOMS
========

In Microsoft Macro Assembler (MASM) versions 6.0, 6.0a, and 6.0b, if the size of
a DUP directive depends on the difference between the values of two labels (a
span-dependent value), the assembled code differs from that produced by previous
MASM versions.

CAUSE
=====

The size of the DUP depends on label values that are not yet determined in the
first assembly pass.

RESOLUTION
==========

Modify the code to remove span-dependent values in a DUP directive in code
assembled with MASM versions 6.0, 6.0a, or 6.0b.

STATUS
======

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, and
6.0b for MS-DOS and OS/2. This problem was corrected in MASM version 6.1 for
MS-DOS.

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

This problem usually occurs when a forward reference occurs between the two
labels. The assembler adds padding bytes for the forward reference and
eliminates these bytes during a subsequent assembly pass. When this occurs, the
second label has a larger value during the first assembly pass than it does in
subsequent passes.

The sample code below demonstrates this behavior.

Sample Code
-----------

  ; Assemble options needed: none

  tst1 SEGMENT para public
  ASSUME cs:tst1
  start:
          jmp SHORT forward
  forward:
          mov ax, 4C00h
          int 21h
  tst1 ENDS

  count = offset forward - offset start

  tst2 SEGMENT para public
          DB count DUP (2)  ; This assembles as DUP 10 in MASM versions 6.0,
                            ; 6.0a, and 6.0b. It assembles as DUP 2 in MASM
                            ; versions 5.1 and 6.1.
  xxx     DB 1
  tst2 ENDS

  END start

Additional query words: 6.00 6.00a 6.00b buglist6.00 buglist6.00a buglist6.00b fixlist6.10

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM600a kbMASM600b
Version           : MS-DOS:6.0,6.0a,6.0b
Solution Type     : kbfix

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

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