BUG: EXTERNDEF ABS Fails with Span Dependent Value

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

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.

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

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

_data SEGMENT para public 'DATA'

   MakeTable

END