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Q31994: How Overlays Are Set Up By the Linker

Article: Q31994
Product(s): See article
Version(s): 3.x 4.06 4.07 5.01.20 5.01.21 5.03
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_basic
Last Modified: 26-JAN-1990

The overlay linker uses an interrupt to call in overlaid files. By
default, the interrupt number used for passing control to overlays is
63 (3f hex).

The linker links the Microsoft overlay manager from the standard
Microsoft language run-time libraries. The linker assigns segments to
overlays based on the grouping you indicate. It replaces
overlay-to-overlay far calls and root-to-overlay far calls with an
interrupt sequence that calls the overlay manager. The overlay manager
swaps overlays if necessary and returns control to the program.

The linker replaces root-to-overlay and overlay-to-overlay far calls
with the following sequence:

   INT     3Fh         ; can change # with /OVERLAYINTERRUPT:#
   DB      ?           ; target overlay segment number, where
                       ; every non-root code segment is numbered
                       ; starting at 1
   DW      ?           ; target offset within segment

The interrupt handler is set to the overlay manager code that swaps
out the resident overlay if necessary and swaps in the target overlay,
then jumps to the target address. The overlay manager does nothing
unusual in servicing the interrupts; it does not disable interrupts.
However, it may issue INT 21h calls to swap overlays.

The overlay manager assumes that once your initialization code has
been executed, DS and SS will always be the same. Furthermore, it
assumes you will initialize DS and SS to the value of DGROUP (the
default data segment defined by Microsoft languages). Note that your
program also must have a stack segment.

A more detailed description of the overlay manager can be found on
Page 715 of the "MS-DOS Encyclopedia," "Article 20: The Microsoft
Object Linker."

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