Q30398: EXTRN Data Items Placed Outside Segment

Article: Q30398
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist5.10 | mspl13_masm
Last Modified: 23-MAY-1988

The EXTRN statement can be placed outside of a segment declaration.
When this is done, the item has no segment attributes, so it
automatically matches the DS register with the item regardless of the
active ASSUME statements.
   When the EXTRN statement is contained within a segment, the active
ASSUME statement applies to the data item.
   To avoid this problem, you should define all EXTRN data items
inside a segment.
   Microsoft is researching this problem and will post new information
as it becomes available.

   The following example demonstrates the problem caused by the
current behavior of EXTRN data items:

   If you write a .COM program that assumes DS:NOTHING and ES:NOTHING
and declare all data items inside segments, MASM will put CS:
overrides on all data items being accessed.
   If, to correct the problem, you decide to split the file into two
modules and declare the data items shared with the EXTRN statement at
the top of each module outside segments (this will not change the
ASSUME statements), the DS register will be associated with all data
items by default because you declared all EXTRN items outside
segments.

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