Q82290: FIX: EXTRN ABS Doesn’t Work As Software Interrupt Number

Article: Q82290
Product(s): Microsoft Macro Assembler
Version(s): 5.1,5.1a
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 5.1, 5.1a 
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SYMPTOMS
========

In the Microsoft Macro Assembler (MASM) versions 5.1 and 5.1a or QuickAssembler
versions 2.01 and 2.51, using an absolute external symbol as a software
interrupt number fails to generate the correct number at link time. The module
accessing this PUBLIC symbol does not generate the proper number; instead, it
gets set to 0. This is a problem only when the symbol is used with the INT
instruction. In MASM version 5.1, using an absolute external symbol causes the
linker to generate:

  error L2029: Unresolved externals.

RESOLUTION
==========

You can work around this problem by using an equate declared in an include file
rather than relying on an absolute external. The include file that contains this
equate can be included in all the source files that need to access the equate.
This allows the assembler to determine the value of the symbol, instead of
relying on the linker to provide the value.

STATUS
======

Microsoft has confirmed this to be a problem in MASM versions 5.1 and 5.1a and
QuickAssembler versions 2.01 and 2.51 (buglist2.01 and buglist2.51). This
problem was corrected in MASM version 6.0.

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

The following code demonstrates the incorrect result. To reproduce this error,
assemble both HELLO.ASM and DISPLAY.ASM with MASM 5.1a, then link together using
LINK version 5.01.20.

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

  ; HELLO.ASM
  ; Assemble options needed: none

  .MODEL small, c
  DOSSEG

  PUBLIC mydata
  mydata EQU 21h

  .STACK 256

  .CODE
  EXTRN display:proc
  start:
      call display

      mov ax, 04C00h
      int 21h
  END start

  ; DISPLAY.ASM
  ; Assemble options needed: none

  .MODEL small, c
  DOSSEG

  EXTRN mydata:ABS

  .CODE
  display PROC
      mov ah,mydata ; Gets set to 21h by the linker
      int mydata    ; Doesn't get set to 21h by the linker
                    ; Results in Divide overflow
                    ; Results in Illegal Instruction under CodeView
      ret
  display endp
  END

Additional query words: 5.10 5.10a extern extrn buglist5.10 buglist5.10a fixlist6.00

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM510a
Version           : :5.1,5.1a
Solution Type     : kbfix

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

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