Q32003: Assigning Addresses to Overlay Segments

Article: Q32003
Product(s): See article
Version(s): 3.x 5.01.20 5.01.21
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_basic
Last Modified: 15-JUL-1988

The linker assigns only code segments (identified by class names
ending in "CODE") to overlays. Data segments go in the root. The
particular overlay the segment goes in is determined by the module
that first defines the segment.

   Note that the linker assigns segments, not object modules, to
overlays. Therefore, if you have the following command

    LINK A+(B+C)

where

    A   defines public SEG1
    B   defines public SEG2
    C   defines public SEG1

then C's contribution to SEG1 goes in the root, not in the first
overlay, because SEG1 was first defined in a root module.
   The linker orders all segments as it normally would, ignoring the
overlays and using the class and combine-type rules described
elsewhere. The linker then assigns addresses for every segment in the
root up to the first overlay segment. Before the first overlay
segment, it defines a special empty segment called OVERLAY_AREA. Then
for each overlay it assigns addresses for every segment in the overlay
so that the first segment in the overlay starts at OVERLAY_AREA. One
particular overlay will end at a higher address than all the others;
at this address, the linker defines a special empty segment called
OVERLAY_END. Finally, the linker assigns addresses for all remaining
segments in the root so that the first one starts at OVERLAY_END. The
load image looks like the following:

        ------------+-> 0000
        |           |
        |  root     |
        |           |
        +-----------+-> OVERLAY_AREA
        |  overlay  |
        |  area     |
        |           |
        +-----------+-> OVERLAY_END
        |  root     |
        +-----------+

   The resultant MAP file has some different contents than the
pre-overlay file. All items listed are declared as "res", meaning
resident. The items contained within the overlay modules are also
declared as "res".
   At the top of the MAP file, the segments are listed in the exact
order in which they will be in the load image, and each segment is
identified as being in a particular overlay or the root.
   Every symbol marked "res" is in a root segment and every symbol
marked "ovl" is in an overlay segment. If symbols are not where you
think they ought to be, remember the linker assigns segments, not
modules, to overlays.
   For more information on overlays, refer to the 1988 "MS-DOS
Encyclopedia."

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