Renegade-1.19/OVRUMB.DOC

			    OvrUMB version 1.2
			  by Jean-Marc Lasgouttes
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Overview
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The purpose of this unit for Turbo Pascal 6/7 is to allow your programs
that use overlays to free more conventional memory.  This is made possible
by the relocation of the overlay buffer in upper memory on systems that
support that kind of memory.

This unit should be compatible with the use of OvrInitEMS, OvrInitXMS (from
the unit OverXMS - overxms.zip at garbo.uwasa.fi) or the procedures from the
Streams unit (streams15.zip by Duncan Murdoch).  The purpose of all these
procedures is to speed up the overlay reading by keeping the overlay file in
EMS or XMS memory.  It is recommended to use OvrInitEMS/XMS or the streams
unit in conjunction with OvrUMB.

This unit is released to the Public Domain.  Feel free to use it in your
applications and distribute it, as long as this documentation is included.
You can distribute a modified version of it, as long as credit is given to me
as the original author; in this case, I would appreciate to get a copy of
your modifications.  Although this unit has been tested on several systems,
there is no warranty that it will work for your application and I should
not be liable of any damage that it may cause.  However, I would be glad
to try to correct any problem that you may encounter (see my address at the
end of this document).



Why you should use OvrUMB
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If you use overlays in your applications, you are probably aware that
all the free memory that you can get will be welcome.  One way to gain
memory is to use the upper memory which is accessible for real-mode
programs on a wide range of systems (most 80386/486/... and some
8086/286).  With this unit, your applications will make use of upper
memory if it is available, with a slight modification of your code.

The idea of this unit is very simple: Borland Pascal overlay manager
uses a part of the heap as a buffer where it loads the overlaid code
that needs to be executed. OvrMovBufToUMB tries to allocate a buffer
of at least the same size in upper memory and to set the variables of
the overlay manager to point to this new buffer. The old block can
then be disposed of and is added to the heap. The gains in memory will
be equal to the size of the biggest overlaid unit (20-30k in my case)
if you use the default buffer.  But I find it more efficient to
overlay as many units as possible and to declare a buffer of 60-80k
with OvrSetBuf. This will free more memory and the slowdown will be
hardly noticeable. In this case, OvrUMB will free even more
conventional memory (as long as there is a 60-80k contiguous upper
memory block on the host computer).

There are other means to use upper memory in your Turbo Pascal
applications; one of them is the unit UMB_Heap published in PC
Magazine (vol. 11 no. 20).  This unit will map the existing upper
memory blocks in the regular Turbo Pascal heap.  Although this is the
most transparent way to use upper memory, this unit has some
shortcomings:

  - a program using this unit could have big problems when trying to
    execute child DOS processes, since Turbo Pascal does not expect its
    memory to be allocated as several DOS memory blocks;
  - the unit itself has some bugs concerning the restoration of the DOS
    memory allocation strategy and tests for the presence of an XMS driver,
    which cannot be installed on a 8086 based computer.



System Requirements
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  - TP6 or TP7/BP7
  - A program that uses overlays...
  - Some free UMBs in the system on which your program is to be executed.
    Note that your program will work without UMB, except that no memory
    will be gained.  Since the overhead of the unit is about 320 bytes,
    decrease in available memory will hardly noticeable.



Contents
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The complete package contains:

  - ovrumb.doc          This file

  - ovrumb.pas          The main unit source

  - ovrtest.pas         A test program that demonstrates the use of the
    ovr1.pas            OvrUMB unit.
    ovr2.pas



How to use this unit in your programs
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This is very easy : the modifications must be done in the main program
or in the initialization part of a unit that is declared before any
overlaid unit.

  - Add "OvrUMB" in your uses statement;
  - Instead of code like

      ...
      OvrInit('MYPROG.OVR');   {Opens the overlay files}
      OvrSetBuf(MyBufSize);    {Sets the overlay buffer size to MyBufSize,
				since the default value is often too small}
      ...

    use:

      ...
      OvrInit('MYPROG.OVR');   {Opens the overlay files}
      OvrSetBuf(MyBufSize);    {Sets the overlay buffer size to MyBufSize,
				since the default value is often too small}
      OvrMovBufToUMB;          {Tries to move the overlay buffer in upper
				memory.  It this is not possible,
				nothing happens}
      ...

  - That's all : OvrUMB will automatically free the UMB on exit.


You can also use the procedure OvrSetBufUMB(Size:longint) that allows
you to specify the size of the wanted buffer.  This function can be
useful if you want to change the buffer size when it is in upper
memory.

In order to help you to select the best block size, OvrUMB provides
the function umb_MaxAvail which returns the size of the biggest upper
memory block as a longint.

In fact, OvrMovBufToUMB is logically equivalent to:

  if umb_MaxAvail>OvrGetBuf then OvrSetBufUMB(umb_MaxAvail);

But of course it is more efficient and generate less code.

These two procedure will do nothing if they detect any problem and
return an error value in OvrResult.  I have tried to map the error
conditions to existing error codes.

The procedures return ovrError when:
  - the overlay file has not be opened (use OvrInit to open it)
  - there are some overlays loaded (use OvrClearBuf to unload them)
  - the heap is not empty
  - the buffer has already been reallocated somewhere
  - you tried to allocate a buffer smaller than the minimum allowed
    (only with OvrSetBufUMB).

The procedures return ovrNoMemory when:
  - There is no upper memory manager running
  - There is no big enough upper memory block to contain the overlay
    buffer.



Acknowledgments
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I would like to thank the testers without whom I could not have
written this unit, since I have no upper memory dispenser on my 286 :-( 
In particular, my thanks go to Mike McWhinney (elja.inc@mixcom.com),
Herbert Zarb (hzarb@unimt.mt) and Jack Nomssi
(Nomssi@Physik.TU-Chemnitz.DE).



Revision history
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  version 1.0 (27/10/93)
       First public version.
  version 1.1 (21/07/94)
       Added function umb_MaxAvail;
       OvrMovBufToUMB now tries to allocate the biggest possible block.
  version 1.2 (15/09/94)
       Fixed bug that could make program fail in some cases where there
         are no UMB available.
       Changed the compilation directives of the test program so that 
         they compile on Timo Salmi's PC :-)  



Known Bugs and limitations
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  - The system will crash if you try to call OvrSetBuf after reallocating
    the overlay buffer in upper memory (however, I don't know why you
    would want to do that...)
  - There might be a compatibility problem with ExecWithSwap (from
    TurboPower Software)

If you use this unit (or even if you don't like it), please send me a
message at the address below.  Any feedback that I can get is welcome.



Contacting me
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Report bugs and send comments and suggestions to:

    Jean-Marc Lasgouttes

         e-mail  Jean-Marc.Lasgouttes@inria.fr

         tel.    (33) 1 39 63 56 40

         postal address
                 118, rue Marcadet
                 75018 Paris
                 FRANCE