Simtel MSDOS - Coast to Coast/simteldosarchivecoasttocoast2.iso/asmutil/ovl301.zip/OVL.DOC

Text File | 1989-02-05 | 87KB | 2,296 lines

                         ======================
                         OVL 3.01 DOCUMENTATION
                         ======================



     LICENSE AND WARRANTY
     --------------------

     OVL copyright 1988, 1989 by Michael E. Devore.  All rights
     reserved.

     The copyright holder licenses you to use, copy, and distribute
     OVL files for yourself and others subject to the following four
     restrictions:


     1.  The OVL package must be copied and distributed only in its
         original, unmodified form.  This includes the documentation,
         executable, source, object, and all example files.

     2.  No fee or other compensation can be charged for copying or
         distributing the program files.  The only exceptions are
         user groups and incorporated public domain or shareware
         software distributors who may charge a handling fee for its
         distribution NOT TO EXCEED $8.00 PER COPY.  ONLY ONE COPY
         PER COMPUTER DISK IS ALLOWED FOR SUCH DISTRIBUTION.

     3.  OVL files may NOT be distributed in combination with any
         other products, software or hardware, without the express
         written permission of Michael E. Devore.

     4.  OVL files are for your personal use only and may NOT be used
         for commercial applications.  See the PROVL program
         description in this documentation file if you need to use a
         version of the OVL overlay manager package commercially or
         need a custom version of OVL.


     There is no warranty of any kind with OVL and the copyright
     holder is not liable for any damages of any kind whatsoever,
     directly or indirectly, that may occur due to use of, or
     inability to use, this software.  By using or distributing OVL
     you agree to these terms.  If you do not agree to these terms,
     you may not use or distribute OVL.

     It is the copyright holder's intention to fix any errors in OVL
     and to allow both its noncommercial use and distribution free of
     charge.  The copyright holder, however, is NOT LEGALLY OBLIGATED
     TO DO SO.

     This document copyright 1988, 1989 by Michael E. Devore.  All
     rights reserved.



     License and Warranty                             page 1 of 1





     OVL FILES
     ---------

     Included in OVL301.ARC should be the following files:

     OVL.DOC        (OVL documentation file)
     LINKOVL.OBJ    (linkable OVL program object file)
     OVL.EXE        (OVL stand-alone executable)
     QB4MOD.OBJ     (Object module for use with OVL.EXE)

     SPLIT.EXE      (Separate overlay files utility, executable)
     SPLIT.C        (Turbo C source code for the SPLIT program)
     MAPOVL.EXE     (Overlay mapping utility, executable)
     MAPOVL.C       (Turbo C source code for the MAPOVL program)
     PRONK.EXE      (Utility to allow use of Clipper or indirect
                     calls to overlays with OVL)

     MASMTEST.ASM   (MASM example files)
     MASMTEST.OBJ
     OV1.ASM
     OV1.OBJ
     OV2.ASM
     OV2.OBJ
     OV3.ASM
     OV3.OBJ

     COW.C          (Turbo C example files)
     FARM.C
     HORSE.C
     PIG.C
     PIGLET.C
     SHEEP.C

     QBNEST.BAS     (QuickBASIC 4.x example files)
     BNEST1.BAS
     BNEST2.BAS
     BNEST3.BAS
     BNEST4.BAS

     PRTEST.ASM     (MASM example files for PRONK demonstration)
     PRTEST.OBJ
     PTEST2.ASM
     PTEST2.OBJ
     PTEST3.ASM
     PTEST3.OBJ

     MULTEST.ASM    (MASM source code to multiple overlay area
     MULTA.ASM       demonstration program)
     MULTB.ASM
     MULTC.ASM
     MULTD.ASM

     MULTEST.EXE    (Demonstration use of multiple overlay areas)



     OVL files                                        page 1 of 1





     What is OVL?
     ------------

     OVL is an overlay manager package containing two types of an
     overlay manager, a linkable and a stand-alone version.  So what
     is an overlay manager and what good are they?  To answer that
     question we first need a little background on what overlays are
     and why they are useful.

     Overlays are portions of program code which are loaded from disk
     into memory on an as-needed basis.

     Use of overlays allows an extremely large (potentially equal to
     hard disk capacity) program to run in a much smaller amount of
     memory.  Code from the program's .EXE file is brought in from
     disk (overlaid) only when the program needs it.  Since all
     overlays share the same memory space, the memory requirements
     can be much less than the total code size of the program.  Also,
     there is no longer a need for the messy chaining, spawning, or
     exec'ing to another program that some languages require.

     There is a tradeoff for these advantages.  The tradeoff is the
     time and code overhead necessary to read overlays in from disk.
     If you make the decision to use overlays with your program, any
     timing critical code should not be divided between overlays.
     For most applications, though, the relatively small amount of
     time it takes to read in an overlay from disk is not important
     if transfers to overlays are not overdone or poorly timed.
     Constant shuttling between overlays is an exercise guaranteed to
     give disappointing results.  A little bit of thought will go a
     long way towards efficient overlay operation.

     LINK.EXE, Microsoft's Overlay Linker, included with MS-DOS and
     Microsoft languages, versions 3.x and some 2.x versions, allow
     creation of overlaid programs.  However, LINK only inserts
     software interrupts and some overlay information in the program
     for an overlay manager to use.  LINK doesn't provide the actual
     code that loads the proper overlay from disk and passes control
     to it.  That's a job for an overlay manager, and that's where
     the OVL overlay managers come in.

     OVL "manages" overlays.  OVL loads overlays from disk at the
     appropriate time and jumps program execution to the overlay
     code.

     Beginning with OVL version 3.0, there are two different ways to
     use the OVL package.









     What is OVL?                                     page 1 of 2





     OVL.EXE and LINKOVL.OBJ are separate versions of the overlay
     manager in the OVL package and are not to be used together.
     Each version takes a different approach to managing overlays.
     OVL.EXE is a stand-alone overlay manager that loads an .EXE
     program containing overlays.  LINKOVL.OBJ is a linkable overlay
     manager that is linked in as an object module with the program
     containing overlays.  Most users of the OVL package will
     probably use the LINKOVL.OBJ overlay manager since it is cleaner
     and more powerful.  OVL.EXE is included for backward
     compatibility with previous versions of OVL and for
     circumstances where OVL.EXE is more appropriate or works better
     with a particular program.  LINKOVL.OBJ, for instance, will take
     longer to execute a program during startup since it actually
     loads an overlaid program twice -- once to get information and
     allocate extra overlay areas if necessary, and once to begin
     program execution.

     Beginning with version 2.2, OVL supports use of multiple overlay
     areas.  The overlay areas are set up via command line switches
     at runtime.  To see detailed examples of how to use this option
     and why it is useful, read the 'USING OVL (Advanced Operation --
     Multiple Overlay Areas)' section.

     Several other command line switches or options were added with
     version 3.0 of OVL, including a overlay log file for debugging
     purposes, support of separate overlay files, and a couple of
     other options.  Examples and a description of these options are
     covered in the 'USING OVL (Advanced Operation -- Command Line
     Option //x)' and 'USING OVL (Advanced Operation -- Other Command
     Line Options) sections.

     If you are or become completely overwhelmed at the size or
     complexity of OVL, be sure to read the 'TOO LARGE!  DOESN'T WORK
     WITH MY DEBUGGER!  TOO COMPLICATED!  PROVL TOO EXPENSIVE!'
     section for an alternative to the OVL overlay managers.





















     What is OVL?                                     page 2 of 2





     SYSTEM REQUIREMENTS
     -------------------

     The OVL package requires an IBM PC or PS/2 microcomputer or
     close compatible with MS-DOS or PC-DOS operating system version
     2.1 or later.  Microsoft's Overlay Linker version 3.06 or later,
     or some versions of Microsoft's Object Linker version 3.05 or
     earlier, must be used to link together the object modules of the
     overlaid program.

     In their standard configurations LINKOVL.OBJ requires 14K
     (kilobytes) RAM and OVL.EXE requires 12K RAM, in addition to the
     memory requirements of the overlaid program.  Use of multiple
     overlay areas or increasing the relocation table buffer size
     with OVL's /ZBn option will increase memory requirements above
     the standard configurations.








































     System requirements                              page 1 of 1





     LINKOVL SPECIFICATIONS AND REQUIREMENTS (Linkable OVL program)
     --------------------------------------------------------------

     *  LINKOVL.OBJ has been tested with MASM 4.0 and 5.0; Turbo C
        1.0, 1.5 and 2.0; QuickBASIC 4.0 and 4.5; Microsoft's BASCOM
        6.0; and Clipper (PRONK.EXE, an included support utility,
        must be used with Clipper programs).  Other languages that
        compile to standard Microsoft Overlay LINKable object modules
        MAY also work with LINKOVL.

     *  Use of LINKOVL with QuickBASIC 4.x and BASCOM 6.0 requires
        compiling the program with the /O option of BC.EXE.  Use of
        the file QB4MOD.OBJ is NOT required as it is for OVL.EXE.

     *  The object files created by versions of QuickBASIC earlier
        than 4.0 abort with an error if a link is attempted with
        overlays.  Therefore, LINKOVL will NOT work with versions of
        QuickBASIC earlier than 4.0.  This is a limitation of the
        earlier versions of QuickBASIC and not an error or
        shortcoming in LINKOVL.

     *  The Microsoft LINK object module preprocessor utility
        PRONK.EXE must be used if indirect calls to overlay code are
        required.  Otherwise, calls to a function or procedure FROM
        the root code TO overlay code, or from one overlay to another
        (INTERoverlay calls) MUST be direct, that is, not indirectly
        through a pointer.  This restriction is necessary because
        Microsoft LINK can only set up an interrupt invoking the
        overlay manager for direct calls to program code.  For
        assembly language programs this means that interoverlay calls
        to procedures must be coded as CALL ADDR, not CALL MEM/REG.
        For 'C', interoverlay function calls should be directly
        called, not called through a pointer to a function.  Calls
        within an overlay (INTRAoverlay calls), within the root code,
        or FROM an overlay TO the root can be made indirectly.
        QuickBASIC and BASCOM programs should not be affected by this
        restriction unless non-BASIC modules which use indirect calls
        to overlays are also linked in.  Clipper programmers must use
        PRONK whenever they use the LINKOVL.OBJ overlay manager.

     *  Microsoft's Overlay Linker requires the presence of at least
        one overlay when linking LINKOVL.OBJ with a program's object
        modules.  An attempt to link LINKOVL.OBJ with a program that
        doesn't have at least one overlay will cause LINK to abort
        with several "Unresolved external" errors.

     *  LINKOVL has a limit of eight overlays in a program.  More
        than eight overlays will cause LINKOVL to abort with a "Too
        many overlays." error upon startup.  LINKPRO, the enhanced
        version of LINKOVL, has a limit of 63 overlays -- LINK's
        current maximum.





     LINKOVL specifications and requirements          page 1 of 2





     *  LINKOVL supports calls from one overlay to another (nesting).
        Overlays can be nested up to four levels.  An attempt to nest
        deeper than four overlays will cause LINKOVL to abort with an
        "Overlays nested deeper than 4 levels." error.  LINKPRO
        supports a nesting depth of 32 levels.

     *  LINKOVL requires use of the Microsoft LINK default overlay
        interrupt 63 (3fh).  Do not use the /OVERLAYINTERRUPT or /O
        option of LINK to change the overlay manager interrupt.
        LINKPRO supports changing the default overlay manager
        interrupt via the /O option of LINK.

     *  LINKOVL WILL work with the /EXEPACK or /E option of LINK, if
        the program containing overlays is suitable for using the
        /EXEPACK option.  LINKOVL will work with the EXEPACK.EXE
        utility as well.  Note that EXEPACKed files are NOT supported
        with OVL.EXE.

     *  LINKOVL supports two overlay areas: the overlay area
        allocated by Microsoft LINK during the linking process and
        optionally one other overlay area.  If two overlay areas are
        used, memory allocation for the second overlay area is made
        in normal DOS memory.  As a result, the amount of memory
        available to the loaded program decreases.  This decrease,
        that is, the amount used by the second overlay area, is equal
        to the largest overlay file that will load into that overlay
        area rounded up to the next 512-byte block.  LINKPRO supports
        up to nine overlay areas.

     *  LINKOVL uses the PATH environment variable when searching for
        the program to execute or an overlay to load.  If LINKOVL
        attempts to load an overlay and the program containing that
        overlay is neither in the current directory nor in a
        directory in the PATH variable, LINKOVL will abort with a
        "File not found." error.

     *  LINKOVL uses a maximum of 24 bytes of the overlaid program's
        stack during its operation.


















     LINKOVL specifications and requirements          page 2 of 2





     OVL SPECIFICATIONS AND REQUIREMENTS (Stand-alone OVL program)
     -------------------------------------------------------------

     *  OVL.EXE has been tested with MASM 4.0 and 5.0; Turbo C 1.0,
        1.5, and 2.0; QuickBASIC 4.0 and 4.5; Microsoft's BASCOM 6.0;
        and Clipper (PRONK.EXE, an included support utility, must be
        used with Clipper programs).  Other languages that compile to
        standard Microsoft Overlay LINKable object modules MAY also
        work with OVL.

     *  Use of OVL with QuickBASIC 4.x and BASCOM 6.0 requires
        linking in the object module QB4MOD.OBJ as the first object
        file and compiling the program with the /O option of BC.EXE.
        The object files created by versions of QuickBASIC earlier
        than 4.0 abort with an error if a link is attempted with
        overlays.  Therefore, OVL will NOT work with versions of
        QuickBASIC earlier than 4.0.  This is a limitation of the
        earlier versions of QuickBASIC and not an error or
        shortcoming in OVL.

     *  Very recent versions of Microsoft LINK (3.69) require ALL
        languages to link in QB4MOD.OBJ as the first object file when
        using OVL.EXE.  Otherwise LINK will abort with a 'Microsoft
        Overlay Manager module not found' error.

     *  The Microsoft LINK object module preprocessor utility
        PRONK.EXE must be used if indirect calls to overlay code are
        required.  Otherwise, calls to a function or procedure FROM
        the root code TO overlay code, or from one overlay to another
        (INTERoverlay calls) MUST be direct, that is, not indirectly
        through a pointer.  This restriction is necessary because
        Microsoft LINK can only set up an interrupt invoking the
        overlay manager for direct calls in program code.  For
        assembly language programs this means that interoverlay calls
        to procedures must be coded as CALL ADDR, not CALL MEM/REG.
        For 'C', interoverlay function calls should be directly
        called, not called through a pointer to a function.  Calls
        within an overlay (INTRAoverlay calls), within the root code,
        or FROM an overlay TO the root can be made indirectly.
        QuickBASIC and BASCOM programs should not be affected by this
        restriction unless non-BASIC modules which use indirect calls
        to overlays are also linked in.  Clipper programmers must use
        PRONK whenever they use the OVL.EXE overlay manager.

     *  OVL requires the presence of at least one overlay in the
        program it loads.  It will abort with an "Invalid overlay
        information." error upon startup if none exist.

     *  OVL has a limit of eight overlays in a program.  More than
        eight overlays will cause OVL to abort with a "Too many
        overlays." error upon startup.  PROVL, the enhanced version
        of OVL, has a limit of 63 overlays -- Microsoft LINK's
        maximum.



     OVL specifications and requirements              page 1 of 2





     *  OVL supports calls from one overlay to another (nesting).
        Overlays can be nested up to four levels.  An attempt to nest
        deeper than four overlays will cause OVL to abort with an
        "Overlays nested deeper than 4 levels." error.  PROVL
        supports a nesting depth of 32 levels.

     *  OVL requires use of the Microsoft LINK default overlay
        interrupt 63 (3fh).  Do not use the /OVERLAYINTERRUPT or /O
        option of LINK to change the overlay manager interrupt.
        PROVL supports changing the default overlay manager interrupt
        via the /O option of LINK.

     *  Neither the /EXEPACK or /E option of Microsoft LINK nor the
        EXEPACK.EXE utility will work with OVL.  Note that EXEPACKed
        files ARE supported with LINKOVL.OBJ.

     *  OVL supports two overlay areas: the overlay area allocated by
        Microsoft LINK during the linking process and optionally one
        other overlay area.  If two overlay areas are used, memory
        allocation for the second overlay area is made in normal DOS
        memory.  As a result, the amount of memory available to the
        loaded program decreases.  This decrease, that is, the amount
        used by the second overlay area, is equal to the largest
        overlay file that will load into that overlay area rounded up
        to the next 512-byte block.  PROVL supports up to nine
        overlay areas.

     *  OVL uses the PATH environment variable when searching for the
        program to execute or an overlay to load.  If OVL attempts to
        load an overlay and the program containing that overlay is
        neither in the current directory nor in a directory in the
        PATH variable, OVL will abort with a "File not found." error.

     *  OVL uses a maximum of 20 bytes of the overlaid program's
        stack during its operation.

     *  OVL may operate improperly when executing memory resident
        programs -- also called TSR (Terminate and Stay Resident)
        programs.  When operating in such situations OVL may work
        correctly none, some, or all of the time.  Intermittent
        machine lockup or erratic program operation is possible
        depending on how the code for the TSR program is written.

     *  OVL versions 2.21 and higher support overlay files larger
        than 64K in size.

     *  OVL versions 3.0 and higher have no data declaration
        restrictions for multiple overlay areas.








     OVL specifications and requirements              page 2 of 2





     USING OVL (Basic Operation)
     ---------------------------

     To demonstrate how to use LINK to set up overlays with OVL.EXE
     and LINKOVL.OBJ there are three sets of sample programs included
     in the OVL package that show use of the overlay managers' basic
     operations.  One set is written using MASM 5.0, another using
     Turbo C, and the third using QuickBASIC 4.x.

     As stated in the Microsoft LINK documentation, you MUST
     compile using a large code model for for your programs to work
     correctly with overlays.  For MASM 5.0, this means you must
     compile using .MODEL MEDIUM (far-code, near-data), .MODEL LARGE
     (far-code, far-data), or .MODEL HUGE (far-code, far-data).  If
     you are using an earlier version of MASM you need to make sure
     that PUBLIC procedures in overlays are declared FAR if they
     called by the root code or another overlay.  For Turbo C
     versions 1.x and 2.0, compile your files using the medium,
     large, or huge model.  QuickBASIC and BASCOM users must compile
     their program with BC.EXE using the /O option.  Microsoft's
     runtime program for BASIC, BRUNxx.EXE, will not work with the
     OVL package.  Clipper programmers should carefully read the
     'USING PRONK.EXE' section before using OVL on their programs.

     The first example will use the assembly language .ASM files
     MASMTEST, OV1, OV2, and OV3, written using Microsoft's MASM 5.0.
     Although the .MODEL LARGE is used for these example files,
     .MODEL MEDIUM and .MODEL HUGE should work in exactly the same
     fashion for overlaying purposes.  Since MASM requires no
     additional support .OBJ files, you can test the overlay managers
     with the MASM .OBJ files even if you do not have access to an
     assembler such as MASM.

     Link together the MASM .OBJ files, enclosing in parentheses the
     modules you want to be overlays.  Make sure that you use at
     create at least one overlay or the overlay managers will not
     work with the file you create.  If you link the object files
     together like this:

          link linkovl+masmtest+(ov1+ov2)+(ov3),masmtest;

     using LINKOVL.OBJ or

          link masmtest+(ov1+ov2)+(ov3),masmtest;

     using OVL.EXE,

     a standard .EXE file called MASMTEST.EXE will be created.
     MASMTEST.EXE will include the root .EXE file MASMTEST comprised
     of the code of the MASMTEST (and LINKOVL if it is used) object
     module and will also include two overlay files appended to the
     root .EXE file.  One overlay file will contain the code of the
     OV1 and OV2 modules, the other will contain the code of the OV3
     module.


     OVL basic operation                              page 1 of 5





     When LINKOVL.OBJ is used in the previous example, typing
     MASMTEST at the command line causes the program to begin
     execution.  As program code is needed from an overlay, that
     overlay is loaded from disk, if necessary, and program execution
     continues.

     When OVL.EXE is used, the MASMTEST.EXE file created from the
     linking process is executed by typing either OVL MASMTEST or OVL
     MASMTEST.EXE -- typing the .EXE extension is optional.  OVL then
     begins to run, controlling the loading and overlay management of
     the MASMTEST program.

     IMPORTANT NOTE:  Throughout the rest of this document, both
     overlay managers will be generically referred to as OVL.
     Further examples in this manual will assume use of the
     LINKOVL.OBJ overlay manager unless otherwise noted.

     The amount of memory required to load the example file
     MASMTEST.EXE, or any file created by LINK using overlays, is
     equal to the size of the root file PLUS the size of the largest
     overlay file PLUS any program overhead and dynamic memory
     allocations made by the program plus OVL's overhead.  This
     brings up an important point.  There is no advantage to only
     using one overlay in a program since memory must be allocated
     for it during program execution anyway.  There is only the
     disadvantage of the overlay overhead.  In other words, while the
     link commands:

          link linkovl+masmtest+(ov1)+ov2+ov3,masmtest;

          link linkovl+masmtest+ov1+(ov2+ov3),masmtest;

          link linkovl+masmtest+(ov1+ov2+ov3),masmtest;

     are all valid, there is no useful purpose served by linking
     overlays in this fashion.  Use two or more overlay files in the
     link command.

     An example of linking with two overlays is:

          link linkovl+masmtest+(ov1)+(ov2)+ov3,masmtest;

     Here there will be two overlay files containing the code of,
     respectively, OV1 and OV2 and a root file containing the
     MASMTEST, OV3, and LINKOVL code.











     OVL basic operation                              page 2 of 5





     An example of linking with three overlay files is:

          link /exepack linkovl+masmtest+(ov1)+(ov2)+(ov3),masmtest;

     Three overlay files contain the code of OV1, OV2, and OV3.  The
     root file contains the MASMTEST and LINKOVL code.  The /EXEPACK
     or /E option of Microsoft's Overlay Linker shown in this example
     may successfully be used with those programs suitable for the
     option if LINKOVL.OBJ is used as the overlay manager.  OVL.EXE
     does not support the /EXEPACK option of Microsoft LINK and the
     '/exepack' command and 'linkovl' object module in the object
     file list for this example should be deleted if it is used.

     Another example of linking with two overlays is:

          link linkovl+masmtest+(ov1+ov3)+(ov2),masmtest;

     Two overlay files contain the code of OV1 and OV3, and OV2.  The
     root file contains the MASMTEST and LINKOVL code.

     The only overlay combination not allowed by OVL is one where the
     the initial entry code object module(s) -- in this case the
     MASMTEST.OBJ and LINKOVL.OBJ modules -- are linked as an
     overlay.  Such startup code must be loaded in memory during the
     beginning of the program's execution.

     LINKOVL.OBJ users with large root .EXE files, especially Clipper
     programmers, should explore using EXEPACKed files.  Using the
     /EXEPACK option of Microsoft LINK will almost always result in a
     significantly smaller .EXE file with large files than if it is
     not used.  Remember that the EXEPACKed files will not reduce the
     memory requirements of a program and are not compatible with the
     OVL.EXE version of the overlay managers.

     The second set of files are examples using Turbo C.  To use them
     you need to have a C compiler compatible with the Turbo C source
     files COW.C, FARM.C, HORSE.C, PIG.C, PIGLET.C, and SHEEP.C.  No
     .OBJ files are included due to the many different versions of C
     compilers available; object modules for C are usually compiler
     version specific.  Compile the source files with your own
     compiler.

     OVL is put a little more to the test here because the code is
     larger, parameters are passed to the overlays, a value is
     returned from a function, and a command line argument can be
     passed.  Further, in one module, PIG.C, overlays can be nested
     if the 'piglet' function that is called from it is located in a
     different overlay file.








     OVL basic operation                              page 3 of 5





     To create FARM.EXE with an overlay file containing the code of
     HORSE and PIGLET and an overlay file containing the code of
     COW, PIG, and SHEEP and a root containing the code of
     FARM and the start up code of C0M, you would link like this,
     assuming that the code has been compiled using the medium model:

          link linkovl+c0m+farm+(horse+piglet)+(cow+pig+sheep),farm,
          nul,emu+mathm+cm;

     After you link the program try running it by typing FARM and
     FARM MACDONALD to see the command line parameter in use.  All of
     the modules can be put in an overlay except for LINKOVL, C0M,
     and FARM which contain startup code.  Remember, you must use
     Microsoft's LINK when linking this or any other Turbo C program
     for use with OVL.  The copy of TLINK included with Turbo C does
     not support overlays.

     The final set of examples are QuickBASIC 4.x files.  Since
     QuickBASIC programs need a QuickBASIC library to link correctly,
     you must own or have access to QuickBASIC 4.x to try these
     example files.  Again, no .OBJ files have been included.  These
     QuickBASIC source files also demonstrate how to use COMMON to
     share variables between more than one module if one module is in
     an overlay.

     Using QuickBASIC 4.x and BASCOM 6.0 programs with OVL.EXE
     involves an additional step not required by the other tested
     languages.  An extra object module called QB4MOD.OBJ must be
     linked in as the first object file during the link process.  For
     example, to create a program called QBNEST.EXE with a root of
     QB4MOD and QBNEST, an overlay containing BNEST1 and BNEST2, an
     and overlay containing BNEST4 and BNEST3, you would link in this
     fashion:

          link qb4mod+qbnest+(bnest1+bnest2)+(bnest4+bnest3),qbnest;

     Typing OVL QBNEST at the DOS prompt will run the program.
     QB4MOD must be included in the root with the main program.  In
     this example neither QB4MOD nor QBNEST can be in an overlay.
     QB4MOD.OBJ is not required by the LINKOVL.OBJ overlay manager
     and should not be used with it.

     To really stretch OVL to its limits, link each of the BNEST
     modules in a separate overlay.  This forces OVL to nest overlays
     to its four level maximum.

     To use your own QuickBASIC 4.x programs with overlays you should
     compile the files using BC.EXE.  Overlays will not work with the
     runtime file BRUNxx.EXE so the /O option must be used to compile
     QuickBASIC 4.x programs.






     OVL basic operation                              page 4 of 5





     Feel free to try any permutation of overlays allowed by these
     examples.  OVL should work with any of them as long as you stay
     within the restrictions already outlined.  It is especially
     important to remember not to put the initial entry code in an
     overlay.  The start up code must be in memory when the program
     is first loaded, i.e. in the root code.

     You may have noticed that these example files are all small
     enough to fit into memory without overlays.  You may even be
     asking yourself how you really know that OVL is bringing the
     overlays in as needed instead of sneaking everything in at once?

     First, you can try running a file linked with overlays without
     LINKPRO.OBJ or OVL.EXE.  If you do this your computer will
     probably lock up and need to be turned off or reset before it
     can be used again.  This is because your program is using
     interrupt 63 which OVL hasn't initialized to point to code that
     manages the overlays.  Your program tries to transfer control to
     whatever random address happens to be at the memory location
     that corresponds to the address at interrupt 63, leading to
     unpredictable results.  This is also known as a "thin air"
     transfer.

     The second test you can do is transfer the overlaid program (and
     OVL.EXE if you are using it) to a floppy and disable any disk
     caching.  Now run the program.  You should see the floppy disk
     drive light come on during execution of the example program as
     overlays are loaded from disk.  For maximum effect you may want
     to link the example program using as many overlays as possible.

     It is important to understand how the use of overlays can affect
     a program's performance.  Time critical code should not be
     divided between two overlays.  Depending upon the speed of the
     disk drive used and the size of the overlay file, the time to
     load each overlay from disk can be several seconds.  Extremely
     time critical code should also not be divided the root code and
     an overlay.  The execution time overhead of the overlay manager
     code can range from several microseconds up to a few
     milliseconds depending upon the type of machine used to run the
     overlaid program.

     There is NO time penalty involved in calling routines FROM a
     loaded overlay TO the root code or from one code module to
     another WITHIN the same overlay file.  There IS a time penalty
     in calling routines FROM the root code TO an overlay file.  This
     time penalty will range from several microseconds with an
     already loaded overlay file on a fast computer to several
     seconds with an overlay file that needs to be loaded on a
     computer with a slow disk drive.







     OVL basic operation                              page 5 of 5





     USING OVL (Advanced Operation -- Multiple Overlay Areas)
     --------------------------------------------------------

     OVL supports two overlay areas.  This allows you to have two
     overlay files in memory at the same time, and for these overlay
     files to be swapped in and out without the other being affected.
     Why is this important?  Suppose you have two sections of
     initialization code in your program that operate independently
     of each other.  With only one overlay area, in order to reclaim
     the dead space after initialization is completed, you have to
     put both sections in one overlay file.  Even after one section
     is complete the other may still be in use, so each section must
     take up memory until both are finished.  With two overlay areas,
     you could overlay one section as soon as it is complete,
     allowing your program to run under smaller memory requirements.

     Such an example only scratches the surface of how two overlay
     areas can be used.  Another example is using separate overlay
     areas for code that runs two different peripherals.  For
     instance, one overlay area could be overlaid with a
     user-selected printer driver, and the other with a particular
     video driver.  Other examples will probably come to mind once
     you start thinking of ways to apply the idea.

     Information about which overlay file loads in which overlay area
     is passed via the command line.  There are two switches or
     options allowed, /A (or /a) and /B (or /b).  Do not intermix
     these switches with command line switches or options used by an
     overlaid program.

     When using the OVL.EXE overlay manager, a program's command line
     switches come AFTER the program name.  OVL.EXE's switches come
     BEFORE the program name.  When using LINKOVL.OBJ as the overlay
     manager, both LINKOVL's and the program's command line switches
     come AFTER the program name, with LINKOVL's switches coming
     first.  To differentiate between LINKOVL.OBJ and a program's
     command line switches you should use OVL's //x option described
     in the 'COMMAND LINE OPTION //X' section.

     Each switch should be followed by one or more numbers separated
     by a comma, or range of numbers separated by a dash.  These
     numbers correspond to an overlay file.  Overlay files are
     numbered starting at 1 in the order that they were linked.  The
     /A and /B switches do not represent a particular overlay area,
     but rather differentiate which overlay files are loaded in one
     or the other overlay areas.  The specific overlay area used to
     load an overlay file is transparent to the enduser of the
     program.








     OVL multiple overlay areas                       page 1 of 4





     All files in the switch containing the largest overlay file will
     load in the standard overlay area designated by Microsoft LINK
     during the linking process.  All files listed in the other
     switch load in the second overlay area.  Such switches may be
     either explicitly or implicitly given.  Switches are explicit if
     they are listed in the actual command line, e.g. /A1.  They are
     implicit if an explicit switch does not list all of the overlay
     files; the remaining overlay files are assigned to the unlisted
     (implicit) switch by default.

     To demonstrate, if you linked the .OBJ files of the example
     QBNEST files in this manner (OVL.EXE users should replace the
     'linkovl' object module with 'qb4mod'):

          link linkovl+qbnest+(bnest1)+(bnest2)+(bnest4+bnest3),
          qbnest;

     you would have three overlay files.  Code for BNEST1 would be in
     overlay file number 1, BNEST2 would be overlay file number 2,
     and code for BNEST3 and BNEST4 would be in overlay file number
     3.  To run this program via OVL with the code from BNEST1,
     BNEST3, and BNEST4 in one overlay area and BNEST2 in the other,
     you would type:

          QBNEST /A1,3 /B2  [for LINKOVL.OBJ users]

     or

          OVL /A1,3 /B2 QBNEST  [for OVL.EXE users]

     In this example, overlay files 1 and 3 load in the standard
     overlay area, since overlay file 3 is the largest, and overlay
     file 2 loads in the second overlay area.  Because the overlay
     switches do not correspond to an actual overlay area, the
     following command line would give exactly the same results, if
     you are using LINKOVL.OBJ as the overlay manager:

          QBNEST /B1,3 /A2


















     OVL multiple overlay areas                       page 2 of 4





     Further, since any overlay files not listed with a switch are
     assigned to the switch not used, this command line would work
     identically as well:

          QBNEST /A1,3

     The /B2 is implied by this command line.  This is why, if you
     use both the /A and /B switch of OVL, you MUST list all overlay
     files in the switches.  Otherwise, when OVL attempts to assign
     the overlay files not listed to an unused overlay area it finds
     both overlay areas used and returns an invalid overlay switch
     value error.  Using the same example:

          QBNEST /A1 /B2

     results in an invalid overlay switch value error because overlay
     file 3 is not listed and there are no unused overlay areas to
     assign it to.

     If all of the overlay files are listed with one switch the
     advanced option second overlay area is effectively defeated.  In
     other words, if you put all of your overlay files in one overlay
     area there isn't any reason to have the second overlay area.
     While this may sound trite, it is relatively easy to overlook if
     you are depending upon an implicit switch.  For instance, the
     command line:

          QBNEST /A1-3

     is identical to:

          QBNEST

     and the second overlay area is not used.  Remember that ranges
     can be represented by a dash, so that /A1-3 is the same as
     /A1,2,3 (and /A1,2-3 or /A1-2,3 as well).

     Included with the OVL files is an program called MULTEST.EXE,
     created from the MASM source files MULTEST.ASM, MULTA.ASM,
     MULTB.ASM, MULTC.ASM, and MULTD.ASM.  The MULTEST.EXE program
     can provide a graphic demonstration of the power of two overlay
     areas.  MULTEST.EXE has been padded with 160,000 extra bytes to
     increase load time to simulate operation under large file
     conditions.  The size of the resultant file also shows the
     overlay managers' ability to handle overlay files greater than
     64K.  MULTEST.EXE was created with two overlay files using the
     LINK command:

          link linkovl+multest+(multa+multb)+(multc+multd),multest;

     To best demonstrate the program, transfer a copy of MULTEST.EXE
     (and OVL.EXE if you are using it) to a floppy disk and disable
     any disk caching.



     OVL multiple overlay areas                       page 3 of 4





     First, try basic OVL operation, i.e. type MULTEST if using
     LINKOVL.OBJ, or OVL MULTEST if using OVL.EXE.  After the program
     loads and displays the prompt, when you press the A, B, C, or D
     keys the floppy disk drive light will come on as overlays are
     loaded.  For example, if you first press the 'A' key, overlay
     file 1 will load from disk into memory.  At that point neither
     an 'A' or 'B' keypress will required a disk access because the
     overlay containing the code for process 'A' and 'B' is in
     memory.  If you press 'C' or 'D', the disk drive will be
     accessed again to load in overlay file 2.  Then either 'C' or
     'D' can be pressed without accessing the drive, but an 'A' or
     'B' keypress will reload overlay file 1.

     In short, under this arrangement 'C' and 'D' keypresses will
     swap out the overlay file containing the code for processes 'A'
     and 'B', and 'A' and 'B' keypresses will swap out the overlay
     file containing the code for the 'C' and 'D' processes.

     Now try typing MULTEST /A1.  This will cause overlay file 1 to
     load in one overlay area when its code is needed and overlay
     file 2 to load in the other.  The disk drive will be accessed
     the first time you press 'A' or 'B' and the first time you press
     'C' or 'D', but once both overlay files are loaded there will be
     no more disk accesses -- no matter which of the four keys you
     press and no matter what the order.

     A nice feature of multiple overlay areas and other command line
     options is that their use is optionally under the control of the
     enduser.  The enduser may dynamically configure the program to
     best fit the memory and speed requirements of a specific
     environment without the need to relink the program, or access
     the object files.  In fact, when using multiple overlay areas --
     unlike when using one overlay area -- assigning only one overlay
     to a particular overlay area can be advantageous.  Endusers with
     large amounts of free memory will probably wish to use two
     overlay areas to keep as much code in memory as possible.  Those
     with less free memory may only be able to use one overlay area,
     paying for the lower memory requirement with more frequent disk
     accesses.  The identical version of a program can be used in
     both cases.  Only the command line arguments passed to it at
     runtime will differ.















     OVL multiple overlay areas                       page 4 of 4






     USING OVL (Advanced Operation -- Command Line Option //x)
     ------------------------------------------------------------

     When using LINKOVL.OBJ, programs that use command line options
     that begin with a '/' need a way to distinguish between the
     LINKOVL options and the program options.  This is handled by
     OVL's //x option, where 'x' can be any printable character
     except for the DOS special characters for input and output
     redirection and command piping, i.e. <, >, and |.

     The //x option changes the character recognized by OVL as the
     beginning of an option to a different character.  OVL stops
     scanning for options as soon as it encounters a character which
     is not one of its options, passing any remaining command line
     options to the overlaid program.

     For example, if you need to pass the command line options or
     switches /DEBUG and /X to a program named RUNME.EXE and the
     command line options /A3 and /B1-2 to OVL, you would use this
     command:

          RUNME /A3 /B1-2 //$ /DEBUG /X

     The //$ part of the command line directs OVL to look for the '$'
     character for any further command line options.  When OVL
     encounters the next '/' character, it stops looking for options
     and passes the remaining command line, /DEBUG /X, to the
     overlaid program.  The '$' character can be replaced in this
     example by any other character that would serve the purpose of
     forcing LINKOVL to stop scanning when it reaches a program
     option.

     Even if you only need to pass command line options beginning
     with a '/' to a program and not LINKOVL, you must still use the
     //x option.  Otherwise, LINKOVL has no way of determining that a
     particular option is not meant for its interpretation.  If you
     have a program that needs the command line option /BELL, but you
     wish to pass no options to LINKOVL, you would use this command:

          RUNME //x /BELL

     Again, the 'x' character can be replaced by other characters if
     it is necessary or desireable.












     Command line option //x                          page 1 of 2





     The //x option can be used more than once on a particular
     command line, changing the character OVL scans for as a
     beginning to an option each time it is encountered.  For
     example, if you wish to pass the command line options
     /ZDLOGFILE, /A1,2, and /ZS to OVL and the command line options
     /NON, /QS, and /LO to an overlaid program called RUNME, you
     could do it in this fashion:

          RUNME //* *ZDLOGFILE **& &A1,2 &&/ /ZS //# /NON /QS /LO

     Of course this is more complicated than necessary.  A more
     mundane approach would be:

          RUNME /ZDLOGFILE /A1,2 /ZS //# /NON /QS /LO

     the //# switch or option forcing OVL to stop scanning with the
     next '/' character.

     Although the PROVL.EXE overlay manager supports the //x option
     for consistency, there is little or no need to use it with
     PROVL.EXE.



































     Command line option //x                          page 2 of 2





     USING OVL (Advanced Operation -- Other Command Line Options)
     ------------------------------------------------------------

     There are three other command line options that the OVL overlay
     managers can use.  The three options are /ZD<filename>, /ZBn,
     and /ZS.  Following is a description of each.

     (1)  The /ZD<filename> option allows a user to specify a debug
     file that will timestamp and log the loading, access, or swap of
     each overlay.

     For example, to log overlays for a program called MYPROG.EXE
     with two overlay areas into a file called MYPROG.LOG, you could
     type MYPROG /A1 /B2-4 /ZDMYPROG.LOG at the DOS prompt.  A
     typical debugging log file such a situation might look like
     this:

          09:15:06  Overlay 01  Loaded
          09:15:35  Overlay 02  Loaded
          09:16:07  Overlay 03  Swapped out Overlay 02
          09:17:12  Overlay 04  Swapped out Overlay 03
          09:20:14  Overlay 03  Swapped out Overlay 04
          09:24:00  Overlay 02  Swapped out Overlay 03
          09:33:53  Overlay 01  Accessed
          09:39:32  Overlay 02  Accessed
          09:54:17  Overlay 04  Swapped out Overlay 02
          09:55:40  Overlay 04  Accessed

     The timestamp comes first, then the overlay number being
     manipulated and the number of the overlay being swapped out, if
     any.   An overlay may either LOAD into an empty overlay area and
     be accessed, SWAP out another overlay in the overlay area it
     loads into and be accessed, or simply be ACCESSED, having been
     loaded during a previous call to the overlay.

     Devices such as LPT1 or CON are valid filenames for a log file.
     Preexisting log files will be appended to, rather than
     overwritten.

     There is no space between /ZD and the filename when using
     this option.















     Other command line options                       page 1 of 2





     (2)  The /ZBn option shrinks or increases the size of the buffer
     OVL allocates for reading the relocation table items in an .EXE
     file header.

     If the buffer allocation is too small, overlays will take
     somewhat longer to load because OVL will need to access the disk
     more than once to read in all of a relocation table.  If the
     buffer is too large, memory is wasted.  The 'n' following /ZB is
     the number of kilobytes allocated to the buffer and can be any
     number from 4 to 8.  The default buffer size is 4K (/ZB4).  This
     is a good compromise value and probably should not be changed
     unless you understand what the relocation table in a
     .EXE file is.  Overlay files with a very large relocation table
     may load somewhat faster if the buffer size is set to its
     maximum size of 8K, i.e. /ZB8.  PROVL expands the allowed 'n'
     from 1 to 32.

     If you wish to use the /ZB option, the MAPOVL.EXE utility
     described in the 'Using MAPOVL.EXE' section may be helpful in
     selecting an appropriate buffer size for your application.

     Do NOT insert a space between /ZB and the number when specifying
     this option.


     (3)  The /ZS option directs OVL to recognize and load overlays
     from separate files.

     You must use the SPLIT.EXE utility to generate individual
     overlay files from an overlaid program created by Microsoft
     LINK.  See the 'USING THE SPLIT.EXE UTILITY' section in this
     manual for more information on separate overlay files and the
     SPLIT utility.

     NOTE:  All command line options are case insensitive.





















     Other command line options                       page 2 of 2





     USING SPLIT.EXE -- A Utility to Allow Separate Overlay Files
     ------------------------------------------------------------

     SPLIT.EXE is a utility that splits off the overlay .EXE files
     normally appended to the root .EXE file by Microsoft LINK.
     These overlay files have the same name as the root .EXE file
     with an extension of '001', '002', and so on up to the number of
     overlay files in the program.  You must use the /ZS option of
     OVL if you split separate overlay files off of the main .EXE
     file.  Otherwise OVL will abort with a "Bad EXE file signature."
     error when it searches for an overlay at the end of the root
     .EXE file.

     To use the SPLIT utility, type SPLIT and press return.  SPLIT
     prompts you for name of the file to split, then splits up the
     files, and terminates with a message showing how many overlay
     files were created.  You may bypass the filename question by
     typing SPLIT <program name>, e.g. SPLIT MYPROG, if MYPROG.EXE is
     the overlaid program to split up.  Accidentally running SPLIT on
     an already split up .EXE file is harmless, SPLIT will find no
     overlay files to split off and simply terminate.

     Separate overlay files are most useful when you need to avoid
     one large program .EXE file, e.g. due to available storage space
     on floppy disk.  In addition, since OVL automatically searches
     the entire PATH environment string for an overlay file if it
     doesn't find the file in the current directory, it is also
     possible to run overlays off of a different disk drive when the
     /ZS option is used.

     Because of the small increase in development time involved in
     running SPLIT, it is recommended that you not bother with
     splitting your program into separate overlay files until it is
     ready for enduser operation.

     The source to SPLIT.EXE has been included in the OVL package for
     your study or amusement.  The source file is SPLIT.C.  SPLIT was
     written using version 2.0 of Borland's Turbo C.


















     Using SPLIT.EXE                                  page 1 of 1





     USING MAPOVL.EXE
     ----------------

     The utility MAPOVL serves two purposes.  It reports on the
     actual code size of overlay files without their .EXE header to
     allow you to see what the memory requirements for particular
     overlay file and overlay area configurations are.  MAPOVL also
     displays the number of relocation table items in each overlay
     file so that you can choose the best /ZBn option for your
     particular situation.

     To use the MAPOVL utility, type MAPOVL and press return.
     MAPOVL prompts you for the name of the overlaid file to
     report on.  As with SPLIT.EXE, you may bypass the filename
     question by typing MAPOVL <program name>.  Do not use
     MAPOVL on a program which has already been had SPLIT.EXE
     run on it.  MAPOVL searches for overlay files at the end of
     the root .EXE file.

     Following is sample output from MAPOVL for a file called
     TEST.EXE with three overlays:


     Report on file test.exe
     =======================

     Root file:  File size  87552 (15600 hex) bytes
         Relocation table    19 (13 hex) items,     76 (4c hex) bytes
         Unaffected by /ZBn option

     Overlay file  1:  File size  40448 (9e00 hex) bytes
         Relocation table  5803 (16ab hex) items,  23212 (5aac hex) bytes
         /ZBn option requires 'n' of 5 for no table reload

     Overlay file  2:  File size  80384 (13a00 hex) bytes
         Relocation table     6 (6 hex) items,     24 (18 hex) bytes
         /ZBn option requires 'n' of 1 for no table reload

     Overlay file  3:  File size  40448 (9e00 hex) bytes
         Relocation table 10003 (2713 hex) items,  40012 (9c4c hex) bytes
         /ZBn option requires 'n' of 40 for no table reload

     Overlay file 2 always loads into the standard overlay area.

     Report complete: 3 overlay files


     After printing the report header, MAPOVL reports on the
     root file.  This provides no information about overlays,
     but is handy for comparison of root to overlay file size.






     Using MAPOVL.EXE                                 page 1 of 2





     Following the root file report, MAPOVL displays information
     about each overlay file that it finds.  First it reports on
     the true file size of the overlay file rounded up to the
     next 512 byte page.  This is the amount of memory that an
     overlay file will take up when loaded into an overlay area.
     Each overlay area will be large enough to accomodate the
     largest overlay file that loads there.

     After the file size information, MAPOVL displays the number
     of relocation table items and the byte size of the
     relocation table.  The byte size will always be four times
     the count of items.  MAPOVL uses this information to
     calculate the minimum /ZBn size that OVL needs to avoid
     having to reload the relocation table, slowing down the
     speed at which an overlay loads from disk.  If you have the
     memory to spare, you should use the /ZBn listed for best
     program performance.  If the recommended /ZBn is higher or
     lower than the allowed range, use the highest or lowest
     value allowed.

     Unless an overlay file is very large or has an
     extraordinary number of relocation table items, you will
     generally find that the default /ZB4 is sufficient to avoid
     a table reload for most situations.  Even if a reload is
     necessary, the extra delay is usually less than a second and
     often not noticeable to the program operator.

     The largest overlay always loads into the standard overlay
     area allocated by LINK.  MAPOVL shows which overlay loads
     into the standard overlay area.  No additional memory
     allocations are needed for this overlay area, so the worst
     case memory allocation for the second overlay area will
     never be more than the size of the second largest overlay
     file, and then only if it loads in a different overlay area
     from the largest overlay file.

     The source to MAPOVL.EXE has also been included in the OVL
     package for those interested.  The source file is MAPOVL.C.
     MAPOVL was written using version 2.0 of Borland's Turbo C.

















     Using MAPOVL.EXE                                 page 2 of 2





     USING PRONK.EXE
     ---------------

     PRONK is an object module preprocessor for Microsoft's Overlay
     Linker that allows LINK to properly set up indirect calls to
     overlay files.  Clipper programmers MUST use PRONK.EXE in
     conjunction with OVL and any overlaid program.  It is unknown
     whether PRONK.EXE works other overlay managers, such as the one
     that comes with Microsoft C.

     PRONK directly reads and writes to object modules, creating
     an object module called $$MD.OBJ that is linked in with the
     other object modules in the link command.  Pass the same
     command line to PRONK that you normally pass to Microsoft
     LINK.  After PRONK has altered and created the necessary
     object modules, it automatically invokes LINK with the same
     command line passed to it except for '+$$md' added to the
     object file list.  The '+$$md' will be inserted as the
     second file in the object file list.

     The example PRTEST.ASM, PTEST2.ASM, and PTEST3.ASM source files
     show a variety of indirect calls to procedures.  If there were
     no indirect calls to procedures and you wanted to put PTEST2.OBJ
     and PTEST3.OBJ in an overlay, you could link in this fashion:

          link linkovl+prtest+(ptest2)+(ptest3),prtest;

     However since the object modules in this example contain
     indirect calls to overlay procedures, linking in this manner
     with Microsoft's Overlay Linker will not work.  Running PRTEST
     after linking like this will cause improper behavior when an
     indirect call to an overlay is made, and possibly lock up the
     computer.

     PRONK may be used in such cases to allow proper program
     operation.  This command line should be used for the previous
     example:

          pronk linkovl+prtest+(ptest2)+(ptest3),prtest;

     While PRONK is running it displays the names of the object
     modules that it has modified.  Not all object modules may need
     to be modified by PRONK, often it is necessary to modify only
     one or two.  After PRONK has finished, it automatically loads
     and runs LINK, displaying the command line it passes to LINK.
     The command line in this example is:

          link linkovl+$$md+prtest+(ptest2)+(ptest3),prtest;








     Using PRONK.EXE                                  page 1 of 3





     The PRTEST.EXE program created by PRONK should overlay
     correctly.  Notice that PRONK has created and added the file
     $$MD to the object file list passed to LINK.  This object file
     contains direct calls to publicly defined overlay procedures or
     functions, allowing LINK to properly set up the overlay manager
     interrupt.  Any preexisting object file called $$MD.OBJ in your
     current directory will be overwritten.

     PRONK also works correctly with response files or options in the
     LINK command line.  The following command is valid syntax for
     PRONK assuming that the PRRSP.LNK file exists and is in the
     current directory:

          pronk /exepack /nod [e-mail];

     Note that if you use a response file that contains more than the
     object file list, i.e. a map file, program name, or library
     list, you MUST place at least one object module outside of the
     response file.  PRONK always tries to place $$md.obj second in
     the object file list and such an arrangement will not work
     correctly if the first entry in the list is a response file
     which completes the object file list.

     PRONK correctly handles output file redirection for Microsoft
     LINK, although PRONK's copyright message, object modules
     modified message, and the command line passed to MS LINK will
     appear first in the redirected file.  PRONK will return an
     errorlevel of 1 to a batch file if it aborts with an error.

     PRONK reads each object file into memory and then rewrites it if
     any changes are necessary.  If PRONK is interrupted in the
     middle of its operation or aborts with an error, the possibility
     exists that some object modules have been corrupted.  PRONK
     attempts to warn you of circumstances when object modules may
     have been corrupted, but such warning should not be depended
     upon as a surety.  If PRONK abnormally aborts, check the object
     modules modified list to see what files were modified and
     recompile those that were.  IF YOU HAVE ANY IRREPLACEABLE OBJECT
     MODULES, MAKE SURE THAT YOU HAVE ONE OR MORE BACKUP COPIES OF
     THOSE MODULES BESIDES THE COPY PRONK IS WORKING WITH.  This is
     EXTREMELY important.

     Do not rerun PRONK with the same program object modules that
     have been modified on a previous PRONK pass.  You may want to
     set up a batch file to automate a save and restore (to pre-PRONK
     state) of all PRONK-modified object modules that are not being
     recompiled IF you need to rerun PRONK with a recompiled object
     module that PRONK normally modifies.








     Using PRONK.EXE                                  page 2 of 3





     For example, assume that you have three object modules that
     PRONK modifies, AMOD.OBJ, BMOD.OBJ, and CMOD.OBJ.  Further
     assume that you have finished development of the AMOD and BMOD
     code, but are still working on the CMOD code.  To avoid having
     to recompile the AMOD and BMOD source files to return the .OBJ
     files to their original state, you can set a batch file that
     saves the original AMOD.OBJ and BMOD.OBJ object modules under
     the names AMOD.TMP and BMOD.TMP before running PRONK.  Later,
     when you need to rerun PRONK again with a newly recompiled CMOD
     module, another batch file can copy the AMOD.TMP and BMOD.TMP
     files back to AMOD.OBJ and BMOD.OBJ.

     You need not rerun PRONK if you change an object module
     unmodified by PRONK in a previous pass UNLESS the object module
     has been changed to call a procedure indirectly (always indirect
     with Clipper) in an overlay object module.  Be sure to link in
     the last created version of $$MD.OBJ, however.

     Be aware that PRONK changes code labels in modified object
     modules.  If PRONK or LINK aborts with an error, you may see
     error messages containing code labels or procedure names that
     were not used in the overlaid program's source code.  Avoid
     using the global labels or names '___AAA' through '___BMI' in
     your program.  Only the leading three underscores and letters
     are used by the changed labels.  Three underscore characters
     before numbers and other legal name characters can be safely
     used in a program's source code.

     The code overhead when using PRONK amounts to 6 bytes per public
     name in the overlay modules, with a maximum capability of
     handling 1023 public names.  At worst, this will add 6138 bytes
     to your root code size.

     The source code to PRONK is available for a fee.  PRONK was
     written in Turbo C, version 2.0.  To order the source code, send
     $20.00 to the address in the PROVL section immediately
     following.  The ordering instructions for the PROVL package and
     PRONK's source code are the same.  The PROVL commercial overlay
     manager package does not include the PRONK source code and the
     $5.00 discount can only be applied once, even if both are
     ordered.















     Using PRONK.EXE                                  page 3 of 3





     PROVL -- An Enhanced Version of OVL
     -----------------------------------

     If you wish to sell a program that uses OVL to operate, or if
     you just want a more powerful version of OVL, an enhanced
     version of OVL called PROVL can be purchased for $80.  You may
     reduce this amount by five dollars (to $75.00) if you send me
     the name and location of the bulletin board, software
     distributor, or place where you received your copy of OVL.  If
     someone else gave you the copy of OVL, ask him or her where they
     got it from.  The reason for the $5.00 discount is that this
     information is valuable to me in determining distribution
     strategies.

     PROVL has the following enhancements to OVL:


          1.  PROVL supports up to nine overlay areas.

          2.  PROVL allows up to the Microsoft Overlay Linker maximum
              of 63 overlays.

          3.  The depth that overlays can be nested to is increased
              to 32 levels with PROVL.

          4.  PROVL works with any overlay interrupt specified by the
              /O option of LINK.

          5.  PROVL expands the allowed buffer size of the /ZBn
              option to 1K through 32K.  For some overlay files this
              can reduce the overlay manager's overhead by up to 3K
              with no side effects.


     PROVL also dispenses with the runtime copyright notice.  This
     removal of the runtime copyright notice does not suggest, and
     should not be taken to suggest, that PROVL is not copyrighted.
     A potentially distracting message is simply not displayed.

     A bound manual containing an index and quick reference page is
     provided with PROVL.

     For those with particular needs, there are several customized or
     special versions of PROVL available.  One version of PROVL
     supports options passed via a configuration file instead of
     through command line options.  Another version allows a user to
     link in his or her own custom error-handler if a DOS error
     occurs inside of the overlay manager.  There is no extra charge
     for a customized change to the standard version of PROVL IF the
     change is deemed to have potential use for other PROVL users and
     is not too major.  Very specialized or large changes will cost
     more than the standard PROVL version.  Please contact me if you
     require a special version of PROVL.



     PROVL -- An enhanced version of OVL              page 1 of 2





     No royalties are charged for use of PROVL.  This means that you
     need only purchase a single version of PROVL for use with your
     program no matter how many copies of your program you
     distribute.  Purchase of PROVL entitles the purchaser to
     telephone support.  I reserve the right to require that all
     calls be at the purchaser's expense.

     Purchase of PROVL is MANDATORY if you wish to use a version of
     OVL for distribution with a commercial or potentially commercial
     application -- including programs distributed as shareware.
     PROVL is the only version of the overlay manager permitted
     for use with such applications and programs.

     To purchase PROVL, send $80.00 in US dollars -- $75.00 if you
     include a description of where you heard of PROVL -- by check or
     money order with your shipping address to:

         Michael E. Devore
         Devore Software & Consulting
         403 West Charles
         Champaign, IL  61820

     Please state your preference for either 360K 5 1/4 or 720K
     3 1/2 inch diskettes.  Thank you.

     All information contained herein, including the cost of PROVL,
     is subject to change without notice.

     PROVL copyright 1988, 1989 by Michael E. Devore



























     PROVL -- An enhanced version of OVL              page 2 of 2





     TOO LARGE!  DOESN'T WORK WITH MY DEBUGGER!
     TOO COMPLICATED!  PROVL TOO EXPENSIVE!
     ------------------------------------------

     There exists a cousin program to OVL, called LOVR, current
     version 1.14.  LOVR is strictly a linkable overlay manager with
     no command line options, e.g. it supports neither multiple
     overlay areas nor separate overlay files.  As a consequence,
     LOVR takes up less memory than LINKOVL, only 9K.

     LOVR uses less complicated techniques than the overlay managers
     in the OVL package.  For example, it simply integrates into the
     program's code without reloading the overlaid program as
     LINKOVL.OBJ does.  Such an approach may work better with the
     particular debugger you use on your programs.

     Since LOVR supports no extra options, it is most useful for
     those with more modest overlay manager needs who do not require
     the extra power of OVL.  After you decide on the way you wish to
     set up overlaid object modules, you can link in LOVR and never
     concern yourself with its operation again.

     If you wish a copy of LOVR and you have been unsuccessful in
     obtaining a copy, you may send $6.00 to cover shipping and
     handling for the latest version to the shipping address in the
     PROVL section.  No $5.00 discount applies.

     LOVR also has an enhanced commercial version, called PLOVR.
     PLOVR adds similar non-command line option enhancements to LOVR
     that PROVL does to OVL.  A small manual is provided with PLOVR
     as well.  Additionally, the PLOVR package includes a smaller
     copy of PLOVR.OBJ called SPLOVR.OBJ, which takes up 5K of
     program space in the root code, instead of 9K.  The trade-off
     for the decrease in space is an occasional increase in time to
     load an overlay from disk in some circumstances, but SPLOVR can
     be useful for those running close to the memory space limit even
     with overlays.  PLOVR costs $50, less a $5 discount if you
     include a description of where you heard of PLOVR (i.e. where
     you got this OVL package) when you order.  The ordering
     instructions for the PLOVR package and the PROVL package are the
     same and can be found in the PROVL section immediately
     preceding.














     Too large!  Too complicated!                     page 1 of 1





     TROUBLE-SHOOTING
     ----------------

     Check to make sure that you have compiled your program properly.
     Turbo C and MASM users must compile their source files using a
     large or far code memory model.  OVL will not work under a small
     or near code memory model.

     QuickBASIC 4.x, BASCOM, and Microsoft LINK 3.69 users using the
     OVL.EXE overlay manager should check to make sure that they are
     linking in QB4MOD.OBJ as the first object module.

     LINKOVL users should link LINKOVL.OBJ in as the first object
     module of Microsoft LINK's object file list.  Otherwise, memory
     requirements for the overlay manager will increase, possibly
     enough to exceed available memory.

     Use a recent version of LINK when linking overlays for OVL.  OVL
     was tested with LINK versions 3.05, 3.06, 3.60, 3.61, 3.64, and
     3.69.  2.x versions of LINK may not work properly.  For best
     results, the copyright notice of LINK.EXE should say Microsoft
     Overlay Linker, although some early versions that say Microsoft
     Object Linker will work as well.  TLINK, PLINK, or other
     proprietary linkers will not work with OVL; you must use
     Microsoft's linker.  In addition, if you experience problems,
     try linking with a different version of LINK.EXE -- at least one
     version has errors that causes improper operation when programs
     are linked with overlays.

     Check your link syntax, making sure that neither LINKOVL.OBJ nor
     the initial entry or main program module is in an overlay, and
     that you are not exceeding 8 overlay files or nesting overlays
     deeper than 4 levels.

     Carefully read the SPECIFICATIONS AND REQUIREMENTS sections to
     make sure that you are using the OVL programs in accordance with
     their operating requirements.  One possible mistake for those
     using OVL with 'C' or assembly language is to indirectly call a
     function or procedure in one overlay from another or from the
     root to an overlay without using PRONK on the object modules.
     Clipper users must always use PRONK, since all of Clipper's
     calls to overlay procedures are made indirectly.  Microsoft LINK
     requires direct calls to overlay procedures for it to correctly
     set up the overlay manager interrupts.

     Use of OVL's advanced options and support utilities brings with
     it more possibilities of operator error.  If you are using
     multiple overlay areas, it is important to understand that all
     overlay files not listed in an OVL command line switch are
     assigned to a different overlay area from the files listed in
     the switch.  Check that your switches and switch values match
     the rules outlined in the 'USING MULTIPLE OVERLAY AREAS' section
     of this manual.



     Trouble-shooting                                 page 1 of 3





     LINKOVL.OBJ users who wish to pass command line options to an
     overlaid program via the '/' character must use the '//x' option
     of OVL to avoid confusing LINKOVL into an attempt to take the
     overlaid program's command line options as its own.  All command
     line options for an overlaid program must come AFTER LINKOVL's
     own command line options.

     Since OVL will search all directories in the PATH environment
     variable when attempting to find a program containing an overlay
     file, it is important not to have another file with the same
     name as the overlay file in a directory earlier in the PATH
     variable, if the overlaid file is not in the current directory.

     If you use the utility SPLIT.EXE, remember to use the /ZS option
     of OVL.  Do not use the /ZS option if the most recent version of
     the overlaid program has not been run through SPLIT.  Old
     versions of separate overlay files will cause problems with a
     non-split program that inadvertently has the /ZS switch in the
     command line.

     PRONK users should pay careful attention to the manner of its
     operation.  Remember that PRONK can change some object modules
     on each pass and it should not be rerun on object modules
     changed by a previous pass of PRONK.

     If you are sure that you are compiling and linking properly and
     operating OVL within its specifications and requirements, but
     you still experience problems, make sure that the error is not
     in your program.  If memory size allows, compile your program
     without overlays and test it.  If the size of your program is
     too large for all of it to fit in memory, try to compile only
     the code that is giving you problems and see if it works
     correctly without overlays.

     If your program still fails after you have eliminated all
     suspects except for OVL, please contact me so I can fix the
     problem as soon as possible.  However, I will need to know the
     circumstances.  What error message, if any, does OVL display?
     What version and options of OVL were used?  What version and
     options of LINK were used?  What hardware and software
     configuration was your machine running under when the error
     occurred?  What language was the program written in?  If I deem
     it necessary to locate the error, I may request a copy of the
     program that causes OVL to operate incorrectly.  This copy will
     be held in the strictest confidence and either returned or
     destroyed after I am through testing.  You, of course, will have
     the option of refusing the request, although this will probably
     mean a delayed, or no, fix for the problem.








     Trouble-shooting                                 page 2 of 3





     If an error is in OVL and you are the first person to find the
     error, I will send you a free corrected version as soon as it is
     available.  The corrected version will either be sent by e-mail
     if possible, or regular USPS mail if not.  If you are not the
     first person to find the error and do not wish to wait for the
     corrected version to reach general distribution channels, you
     may send $6.00 to cover shipping and handling for the latest
     version to the address below.

     If you subscribe to either the CompuServe Information Service,
     the GEnie Information Service, or the Byte Information Exchange,
     you can send me electronic mail at the appropriate e-mail
     address below.  Barring vacation or unusual circumstances, I
     check in to the CIS and GEnie services at least once a week --
     usually more frequently -- and BIX at least twice a month.  Your
     e-mail will be acknowledged as soon as I receive it.

     If you do not subscribe to either CompuServe, GEnie, or BIX you
     should contact me through USPS mail at the address given at the
     end of this section.

     The realities of life and need for income being what they are,
     my billable work and support of my commercial software has
     priority over fixes to and questions concerning OVL or LOVR.  If
     I have a full work schedule, a fix or question may take more
     time to respond to than either you or I would prefer.  This is
     unfortunate, but unavoidable.

     MAILING ADDRESS:
          Michael E. Devore
          Devore Software & Consulting
          403 West Charles
          Champaign, IL  61820

     E-MAIL ADDRESSES:
          CompuServe      71540,62
          GEnie           MDEVORE
          BIX             mdevore


















     Trouble-shooting                                 page 3 of 3





     OVERLAY TECHNICAL DOCUMENTATION
     -------------------------------

     It is not necessary to read or understand this part of the OVL
     documentation to use OVL.  This technical documentation is
     included for those who wish to know more information about how
     overlay calls are set up by Microsoft LINK.

     When you link a program with overlays, LINK sets up direct calls
     to code in the overlays in a special way.  All of these calls
     are replaced with an interrupt.  By LINK default the interrupt
     is 63 decimal or 3f hexadecimal, although the interrupt can be
     changed using the /O or /OVERLAYINTERRUPT LINK option.  This
     option is not supported by OVL, although PROVL does support it.

     Immediately following the interrupt inserted by LINK are three
     bytes of data.  The first byte is used by the overlay manager as
     an offset into an table found in the root .EXE code (usually
     located at the end of the root) that determines which overlay
     should be loaded into memory.  The second two bytes give the
     value that the Instruction Pointer (IP) should have when a jump
     to the overlay code is executed.  When the overlay manager is
     called by the interrupt, it needs to adjust the return address
     to point past these three bytes.

     The remaining information that the overlay manager needs can
     usually be found at the end of the root .EXE file.  Note that an
     overlaid .EXE program has overlay .EXE files appended to the
     root .EXE file.  Also, at least one non-default option of MS
     LINK, the /EXEPACK option places additional information at the
     end of the root .EXE file.  This is why the nonlinkable version
     of OVL, OVL.EXE, will not work with EXEPACK'ed files.  OVL.EXE
     cannot locate the information it needs in this situation.

     Each overlay file has its own header that follows the standard
     format of normal .EXE file headers, including items in a
     relocation table that need to have fixups performed on them.

     The overlay information in the root file corresponds to data in
     the OVERLAY_DATA segment created by MS LINK.  All of the data
     has a MS LINK internal variable name, although these names are
     documented nowhere by Microsoft to the best of my knowledge.
     The names are prefixed with a '$$' and can be seen if you look
     at a binary dump of LINK.EXE.  The information that the root
     .EXE has is as follows:


          1 word containing the count of all segments in overlay
          files plus one.

          1 word containing the total .EXE files in the main .EXE
          file.  This will equal the number of overlay files plus one
          for the root.



     Overlay Technical Documentation                  page 1 of 2





          1 zero word.  This word is fixed up to the program load
          segment when the program is loaded into memory.

          A table with entries for each overlay that, when added to
          the root program load segment, provide the value the Code
          Segment (CS) should have when a jump to the overlay code is
          executed.  MS LINK fixes up the values in this table as
          well.  If using LINK's internal '$$' variables during
          runtime, the need to add in the root program load segment
          value is negated.

          1 zero word.

          A table that uses the first byte following the interrupt in
          the calling code for an offset.  The entries in the table
          determine which overlay has the necessary code and should
          be loaded in memory, if it isn't loaded already.

          Variable number of zero bytes.

          The name of the main .EXE file that normally has all of the
          overlay files appended to it.  This will not be true if you
          have used the SPLIT.EXE program to create separate overlay
          files.

          Variable number of zero bytes.  These bytes have a
          corresponding MS LINK internal variable name, but their
          use, if any, is unknown to me.

          The number of the interrupt LINK inserts in the program
          code that is vectored to the overlay manager code.


     This information has remained stable for Microsoft LINK
     versions 3.05 through 3.69.  It may change for future versions
     of Microsoft LINK.

     The information presented here is solely for your interest and
     entertainment.  Michael E. Devore makes no claims or guarantees
     of the accuracy of this information.  Use of this information is
     at your own risk.















     Overlay Technical Documentation                  page 2 of 2





     COMMENTS and MISCELLANY
     -----------------------

     If you have an idea for an improvement to OVL, a comment or
     question about OVL, or even a complaint about OVL, please send
     it to either my USPS mailing address or one of the e-mail
     addresses below.  I am receptive to new ideas and constructive
     criticism.  Your comments DO make a difference.  Several
     enhancements to OVL have come about due to user requests so
     don't be shy about dropping me a postcard or sending
     e-mail.  I am also interested in what applications or other
     languages you use OVL with.

     OVL is written entirely in 8088, also known as 8086, assembly
     language.  I feel that this is necessary to achieve acceptable
     speed and size overhead, especially with older computers, while
     maintaining maximum compatibility.  While some optimizations to
     OVL's code can undoubtably be made, I am skeptical that a
     significant decrease in code size is possible while retaining
     all of its functionality.

     There has been some confusion over the type of software OVL is
     and who may use it.  OVL IS A FREE, COPYRIGHTED PROGRAM WITH
     SOME USE RESTRICTIONS, MOST NOTABLY THAT IT IS ONLY FOR
     NONCOMMERCIAL USE.  OVL should be powerful enough for most
     casual or part-time programmers needing a solution to the
     problem of program that has gotten too large to fit in available
     memory.  OVL is also for professional programmers who have the
     same problem with a personal application.  Those who need a more
     powerful solution or who wish to use this type of overlay
     manager for a commercial application should be able to afford
     the enhanced PROVL version.  Such an approach is in keeping with
     my philosophy of allowing those chronically short on cash to
     still have the benefits of some programming tools.  I have great
     sympathy for persons with little money, having been in that
     position myself on more than one occasion.  This philosophy
     should not be interpreted as support for piracy, nonpayment of
     shareware fees, or other unethical acts.  I just think it is
     pleasant that not all programs have a price tag attached.  OVL
     is one of many programs in this category.  Look around, despite
     what cynics (and purveyors of high-priced software) may say,
     programs don't have to be expensive to be good.

     OVL began as a grass-roots solution to the problem of a low-cost
     overlay manager.  I hope to maintain that attitude and never
     acquire the arrogant traits exhibited by some software
     developers and corporations.  With your help, I plan to keep OVL
     a viable, useful, low- or no-cost alternative to overpriced
     software.

          Michael E. Devore                |   CompuServe   71540,62
          Devore Software & Consulting     |   GEnie        MDEVORE
          403 West Charles                 |   BIX          mdevore
          Champaign, IL  61820             |


     Comments and Miscellany                          page 1 of 1