From:	"Andrew Betts" <andy AT jasp DOT com>
Newsgroups:	alt.msdos.programmer,comp.lang.c++,comp.arch.embedded,comp.os.msdos.djgpp
Subject:	Specifing the location of a function
Date:	Sun, 13 Feb 2000 12:07:17 -0000
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Specifing the location of a function / dynamic loading for .COM



This is my problem: I have 64K of program storage space (tiny memory model)
and vast quantities of disk space.  To increase program functionality I want
to be able to load/unload program code at runtime.

As tiny contains only fixed addresses/non relocatable....

The way I thought I could acheive this was by telling the compiler I have a
function at address XYZ and then loading from disk into this address the
code I want.  I'm assuming I can generate the code that is loaded by:

1) compiling a separate, complete program for each module I want to build


 eg
  If we represent each block of program as a letter

  prog 1 - XXXXXXXXX....AAAAAA....
  prog 2 - XXXXXXXXX....BBB.......
  prog 3 - XXXXXXXXX....CCCC......

  where X is the common code, '.' is free space and A, B, and C are
different functions.

 so now any reference A, B, and C make to global data/functions will be the
same for all three programs. (I hope)


2) Extracting and storing each section on disk

 ie
  file 1 - AAAAAA
  file 2 - BBB
  file 3 - CCCC


3) Load and run any one of the progs (doesn't matter which -say prog 1.  And
somewhere in the code do

 typedef void(*MYFUNCTION)();

 MYFUNCTION x=placeHolder; //  this is where I have fixed the function to be

 ...

 copyFromFileToRam(x,"file3");
    //  XXXXXXXXX....AAAAAA....  becomes
    //  XXXXXXXXX....CCCCAA....

 x(); // call function C

 ...

 copyFromFileToRam(x,"file2");
    //  XXXXXXXXX....CCCCAA....  becomes
    //  XXXXXXXXX....BBBCAA....

 x(); // call function B

  ...etc



---------

It this technique possible?

Specifically:
 a)  Can I tell the linker to put a function at a certain location
 b)  Is it safe to assume in the above 3 programs that the common code with
be in the same place in each.  (of course this relies on the fixed location
code (ABC) not getting in the way)


Alternatively, does anyone know a way to acheive the same thing?



Many thanks for your input.



Andrew Betts

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