DOCUMENT:Q73752  04-MAY-2001  [masm]
TITLE   :FIX: MASM 6.0 Does Not Detect Overflow Initializing Structures
PRODUCT :Microsoft Macro Assembler
PROD/VER:MS-DOS:6.0
OPER/SYS:
KEYWORDS:

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

 - Microsoft Macro Assembler (MASM), version 6.0 
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SYMPTOMS
========

In certain situations, the Microsoft Macro Assembler (MASM) version 6.0 will not
detect an overflow condition when initializing structure elements. This will
most likely occur when the MASM 5.1 compatibility option is used (that is, /Zm
on the command line, OPTION M510 in the source code, or when using the MASM
driver) and the initialized type is of word size. In cases where the error is
not generated but should be, the specified initialization value, modulo the
maximum size for the type, is used as the value of the variable.

STATUS
======

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

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

In the sample code below, a structure is declared without initializing the
elements in the structure. Later, another structure is declared that contains
the first structure and it initializes the structure to a value that will create
an overflow condition. This error is not caught by the assembler.

In the second case, a data item is allocated for the first structure and
initialized to a value that would be an overflow. Again, the assembler does not
generate an error if assembled with MASM 5.1 compatibility.

In the third case, a structure is declared and initialized at the same time. In
this instance, MASM will generate the following error if an overflow condition
exists.

   error A2071: initializer magnitude too large for specified size

In this particular example, a number in the range of 65536 (010000h) to 65663
(01007fh) will not be caught by the assembler if decimal notation is used. If
hex notation is used, the assembler will catch the error.

To work around this, either avoid using the MASM 5.1 compatibility options or use
hex notation when initializing variables.

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

   ; Assemble options needed: /Zm /Fl

   .MODEL small
            OPTION M510
   Struc1   struc
            StrucWord   dw     ?
   Struc1   ends

   Struc2   struc
            mystruct    Struc1 {65536}      ; Should generate A2071 Error
   Struc2   ends

   Struc3   struc
            StrucWord2  dw     65536        ; Does generate A2071 Error
   Struc3   ends

   .DATA
   test1    Struc1  {65536}                 ; Should generate A2071 Error

   test2    Struc2  { }                     ; Should generate A2071 Error

   test3    Struc3  { }                     ; Does generate A2071 Error

   END

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

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600
Version           : MS-DOS:6.0
Solution Type     : kbfix

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

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