Q94941: BUG: EXTERNDEF ABS Fails with Span Dependent Value

Article: Q94941
Product(s): Microsoft Macro Assembler
Version(s): 6.0,6.0a,6.0b,6.1,6.11,6.1a
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, 6.11 
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SYMPTOMS
========

In an application developed with the Microsoft Macro Assembler (MASM), an
attempt to export a constant value with the EXTERNDEF directive fails.

CAUSE
=====

The exported value is defined in a macro as the difference between the values of
two labels (a span-dependent value).

RESOLUTION
==========

To work around this problem, perform one of the following two steps:

- Modify the source code to specify the PUBLIC directive instead of the
  EXTERNDEF directive

- Modify the source code of the macro to place a label before the value
  definition

STATUS
======

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, 6.0b,
6.1, 6.1a, and 6.11. We are researching this problem and will post new
information here in the Microsoft Knowledge Base as it becomes available.

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

The following information is part of the README.TXT file distributed with MASM
version 6.1.

  Span-Dependent Equates in Macros and EXTERNDEF ABS
  --------------------------------------------------

  The ABS operator causes an identifier to be exported as a relocatable
  unsized constant (see Programmer's Guide page 220). If ABS is used with
  EXTERNDEF within a macro, and the constant being exported depends on the
  difference between two addresses, the constant may not be exported
  correctly. In some cases, the listing file will show the correct value,
  but the value in the resulting .OBJ will be incorrect. For example, the
  following code will not evaluate correctly:

     EXTERNDEF TableSize:ABS  ; Will not be exported correctly

     MAKETABLE MACRO
     Table1 LABEL BYTE
        DB 0, 1, 2
     TableSize EQU $-Table1
     ENDM

     SEG1 SEGMENT
     MAKETABLE
     SEG1 ENDS

  To avoid this problem, either use the 'PUBLIC' directive in place of
  'EXTERNDEF', or put a label before the equate, within the macro.

The code example below demonstrates this behavior. To see the problem,
build both modules with CodeView information and link them together.
Stepping through the program in the debugger to see the incorrect
TableSize value.

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

  ; Assemble options needed: /Zi
  ; link part 1 and part 2 together with /CO link option

  EXTERNDEF TableSize:ABS

  _text SEGMENT para public 'CODE'
  ASSUME cs:_text
  start:
     mov ax, TableSize
     mov ax, 4C00h
     int 21h
  _text ENDS

  END start

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

  ; Assemble options needed: /Zi

  EXTERNDEF TableSize:ABS ; Will not export correctly

  MakeTable MACRO
     Table1 LABEL BYTE
        DB 0, 1
     ;Table2 LABEL BYTE  ; Remove comment character to work around problem
     TableSize EQU $-Table1
  ENDM

  _data SEGMENT para public 'DATA'
     MakeTable
  _data ENDS

  END

Additional query words: 6.00 6.00a 6.00b 6.10 6.10a buglist6.00a buglist6.00b buglist6.10 buglist6.10a buglist6.11

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM610 kbMASM611 kbMASM610a kbMASM600a kbMASM600b
Version           : :6.0,6.0a,6.0b,6.1,6.11,6.1a

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

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