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====================== 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 linkovl+@prrsp.lnk; 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