Q63161: Overlaid Modules Loaded into EM Only When 1st Overlay Called

Article: Q63161
Product(s): See article
Version(s): 7.00 7.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900608-158 | mspl13_basic
Last Modified: 5-SEP-1990

When using expanded memory with an overlaid program, the overlaid
modules are not loaded when the EXE file is invoked. They remain on
disk until the first overlay is called. When this occurs, all the
overlaid modules are loaded from disk into expanded memory. From then
on, the overlays are swapped to and from expanded memory and the disk
is no longer needed for that purpose.

This information applies to Microsoft BASIC Professional Development
System (PDS) versions 7.00 and 7.10.

For more information about using linker overlays in BASIC PDS 7.00 and
7.10, search for a separate article in this Knowledge Base with the
following words:

   LINK and overlays and expanded and memory and BASIC

To demonstrate this characteristic of BASIC overlays, boot up with an
Lotus/Intel/Microsoft (LIM) version 4.0 Expanded Memory Specification
(EMS) device driver and compile and link the three modules below as
follows:

   bc main;
   bc overlay1;
   bc overlay2;

   link main+(overlay1)+(overlay2);

MAIN.BAS
--------

   '* Note: The disk activity will be most obvious if this test
   '        is run on a floppy drive.
   PRINT "MAIN"
   PRINT "HIT ANY KEY TO LOAD OVERLAYS INTO EMS"
   SLEEP
   CALL ovl1
   PRINT "OVERLAYS LOADED"
   PRINT "HIT ANY KEY TO CALL SECOND OVERLAY"
   PRINT "THERE SHOULD BE NO DISK ACTIVITY IF YOU HAVE EMS"
   SLEEP
   CALL ovl2
   PRINT "HIT ANY KEY TO END THE PROGRAM"
   SLEEP
   END

OVERLAY1.BAS
------------

   SUB ovl1
      PRINT "OVERLAY1"
   END SUB

OVERLAY2.BAS
------------

   SUB ovl2
      PRINT "OVERLAY2"
   END SUB

When run, the resulting EXE file (MAIN.EXE) produces the following
output:

   MAIN
   OVERLAY1
   OVERLAY2

However, before "OVERLAY1" is displayed, there will be disk activity
while the code for overlay1 is loaded into the overlay area of
conventional memory (for execution) and overlay2 is loaded into
expanded memory. To see this clearly, run MAIN.EXE from a floppy
drive. When overlay2 is called, there will be no disk activity because
it will be swapped in from expanded memory.

Note that this is not a problem with BASIC PDS 7.00 or 7.10, but a
feature of the overlay manager. However, it can present a speed
problem for applications that rely on the quickness of swapping from
expanded memory for the first-called overlay. To work around this,
make the first executable statement in your program a CALL to an
additional overlay with no code in it. When it is called, all the
other overlays will be loaded into expanded memory. The functionality
and speed of the application will remain intact while the difference
in EXE size and load time will be minimal.

For example, the module MAIN.BAS above would be modified as follows:

   CALL loadovls
   PRINT "MAIN"
   CALL ovl1
   CALL ovl2
   END

The subprogram "loadovls" (meaning "load overlays") would be coded as
the following:

   SUB loadovls
   END SUB

When loadovls is called, the code for overlay1 and overlay2 will be
loaded into expanded memory. This eliminates the disk activity between
the display of "MAIN" and "OVERLAY1".

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