DOCUMENT:Q81327  06-MAY-2001  [masm]
TITLE   :Positioning External Declarations in Assembly
PRODUCT :Microsoft Macro Assembler
PROD/VER::5.0,5.1,5.1a,6.0,6.0a,6.0b
OPER/SYS:
KEYWORDS:

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

 - Microsoft Macro Assembler (MASM), versions 5.0, 5.1, 5.1a, 6.0, 6.0a, 6.0b 
-------------------------------------------------------------------------------

SUMMARY
=======

Microsoft Macro Assembler (MASM) versions 5.1 and 6.0 and Microsoft
QuickAssembler versions 2.01 and 2.51 place several restrictions on the
placement of EXTRN and EXTERN statements. The assembler also makes assumptions
on where the declarations are placed.

Because of these assumptions, the code may assemble correctly, but cause the
following link errors:

   Warning: L4004 Possible fixup overflow at <addr> in segment
   <name>

   -or-

   Error L2002: fixup overflow at <addr> in segment <name>

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

Here are the rules you should follow when declaring externals:

1. If you know which segment defines an external symbol, put the EXTRN statement
   in that segment.

2. NEAR code labels (including procedures) must be declared in the code segment
   from which they are accessed.

3. If you know the group but not the segment, position the EXTRN statement
   outside any segment and reference the variable with the group name. For
   example, if var1 is in DGROUP, you would reference the variable as

            mov DGROUP:var1, 10

4. If you know nothing about the location of an external variable, put the EXTRN
   statement outside any segment. You can use the SEG directive to access the
   external variable like this:

            mov ax, SEG var1
            mov es, ax
            mov ax, es:var1

5. If the symbol is an absolute symbol or a far code label, you can declare it
   external anywhere in the source code.

While following these rules will help to avoid L2002 and L4004 fixup overflow
errors, the above rules are not absolute. Using groups, ASSUME statements, and
.MODELS you can often work around these errors. This error can also be generated
by incorrectly positioning external data.

EXTERN is the MASM 6.0 version of EXTRN. For more information about EXTERN see
pageS 222 to 224 of the MASM version 6.0 Programmer's Guide. For more
information on EXTRN see pageS 161 to 163 of the MASM version 5.1 Programmer's
Guide, or pages 169 to 171 of the Microsoft QuickAssembler's Programer's Guide.

Additional query words: kbinf 5.00 5.10 5.10a 6.00 6.00a 6.00b

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM500 kbMASM600a kbMASM510a kbMASM600b
Version           : :5.0,5.1,5.1a,6.0,6.0a,6.0b

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

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 2001.