Q81327: Positioning External Declarations in Assembly

Article: Q81327
Product(s): Microsoft Macro Assembler
Version(s): 5.0,5.1,5.1a,6.0,6.0a,6.0b
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 5.0, 5.1, 5.1a, 6.0, 6.0a, 6.0b 
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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

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

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