Positioning External Declarations in Assembly (81327)

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:

If you know which segment defines an external symbol, put the EXTRN statement in that segment.
NEAR code labels (including procedures) must be declared in the code segment from which they are accessed.
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
```
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
```
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.