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DOS Overlay file (.ovl) in C++ and DOS relocation table

Posted on 1998-03-21
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Last Modified: 2010-04-10
Hi all,
1)
    I would like to create .ovl files for my C/C++ programs like .ovr's in PASCAL because I would like to seperate the main code with the subcode such as the setup program. Do you know how to do?

2)
Also, i'm checking out the usage of the relocation table in dos header and i'm not quite sure why the first relocation table item (offset) contains 01 00 and segment contains 00 00, my program actually starts at 200h, and the second relocation item is also set to 8c 00 00 00. does it mean that thesecond segment starts at 00 8c (offset) with a displacement of 8c hex from the start of the file?
here is a dump of my program:
4D 5A 5E 00 88 00 38 00 20 00 C8 05 FF FF 8D 14
00 20 00 00 00 00 00 00 3E 00 00 00 01 00 FB 30
6A 72 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 00
00 00 8C 00 00 00 7A 01 00 00 29 5D 00 00 30 5D
00 00 37 5D 00 00 3E 5D 00 00 5D 5D 00 00 6F 5D
00 00 8F 5D 00 00 A2 5D 00 00 BE 5D 00 00 C5 5D
00 00 CE 5D 00 00 FA 5D 00 00 09 5E 00 00 35 5E
00 00 3C 5E 00 00 43 5E 00 00 5E 5E 00 00 7D 5E
00 00 8D 5E 00 00 A2 94 00 00 A4 99 00 00 C1 99
00 00 40 00 37 0D 5B 00 37 0D 75 00 37 0D 7C 00
37 0D F7 00 3F 0D 0B 01 3F 0D 16 01 3F 0D B0 00
51 0D BB 00 51 0D 4F 01 51 0D 5D 01 51 0D C0 00
6E 0D C8 00 6E 0D D7 00 6E 0D E4 00 6E 0D 1C 02
6E 0D 5F 09 6E 0D 1B A6 00 00 20 A6 00 00 3D A6
00 00 42 A6 00 00 51 A6 00 00 89 A6 00 00 53 AB
00 00 60 AB 00 00 65 AB 00 00 AF AB 00 00 F6 AB
00 00 EA AC 00 00 77 B5 00 00 2A B7 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
B8 6B 0F 8E D8 FA 8E D0 BC 20 72 FB FC 8C 06 AE
15 E8 E3 06 BF B4 15 B9 07 02 33 C0 1E 07 F3 AB
1E BA 79 01 0E 1F B8 23 25 CD 21 FC 1F BB 00 00
E8 88 06 8E 06 AE 15 26 8A 1E 80 00 32 FF 26 C6




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Question by:whluk
2 Comments
 
LVL 22

Expert Comment

by:nietod
ID: 1183681
It is up to your C++ compiler and linker as to whether or not you can have overlays.  I've never seen a C++ compiler that supports overlays.  The other option is to do it yourself, but that is a bit of work (I've done it).  However, you seem to like to look into the nasty stuff.
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Accepted Solution

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Tommy Hui earned 100 total points
ID: 1183682
Borland C++ supports overlay for DOS.
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