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Q80116: FIX: Illegal Instruction from Structure Member Offset

Article: Q80116
Product(s): Microsoft Macro Assembler
Version(s): 5.1,5.1a
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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

- Microsoft Macro Assembler (MASM), versions 5.1, 5.1a 
-------------------------------------------------------------------------------

SYMPTOMS
========

In the Microsoft Macro Assembler (MASM) versions 5.1 and 5.1a, if the offset of
a structure template member is used as a 32-bit operand, the code generated may
be incorrect. This will cause the program to hang or generate an illegal
instruction message in CodeView.

CAUSE
=====

The problem occurs when the offset of a structure template member is being used
as a 32-bit immediate value, and the member's offset is greater than 100h. If
the offset is greater than 100h, the code generated will not contain the 66h
prefix byte, which makes the instruction a 32-bit operation.

RESOLUTION
==========

Embed the 66h byte by using the DB directive, as shown in the example below.

STATUS
======

Microsoft has confirmed this to be a problem in MASM version 5.1 and 5.1a. This
problem was corrected in MASM version 6.0.

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

The following sample code can be used to demonstrate the problem.

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

  ;  Assemble options needed: none

  .MODEL small
  .386

  strctr  STRUC
       data1 DB 100h DUP (?)
       data2 DB 100h DUP (?)
  strctr  ENDS

  .CODE
  start:
       mov esi, strctr.data1 ;This works since data1 < 100h.
       ;DB 66h               ;Uncomment to fix next instruction.
       mov esi, strctr.data2 ;This doesn't work since data2 > FFh.

       add esi, strctr.data1 ;This works since data1 < 100h.
       ;DB 66h               ;Uncomment to fix next instruction.
       add esi, strctr.data2 ;This doesn't work since data2 > FFh.

  END start

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

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

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

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