DLL's - HELP ME

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My PhD is in imaging sciences.

So let me see if I have this straint then? I put my DLL in the userlibs folder, create a function declaration in the same name containg the function call(s) starting the line ,lib "mydll.dll" and then follow that with the declarations of the functions in the DLL.
am I on track so far?
So then in my main Blitz program, how do I tell it what I want to call or will it just do the function if I call it and it finds it is a Decls file?
So for example if I tried to call the windows API of Setprocesspriority(processid#,value#)
and it should do it finne as long as I declared the Decls file with the .lib the DLL name in quotes and then a function declaration in it.
That right?
Its important - very omportant that I get this working. Does anyone have anouther simple example - I aleady have the DLL's that I need.
Please give me an exampe of how I would set this up:
My DLL name is say "convolve.dll"
and my function is filter$(a#,B#) where to long int's are expected as params and a string is returned. How do I set this sample up. I'm not sure I follow the need to create banks and peek or poke to them. Is the no "Pass by value"????
So the above example is just a made up one but if I have a refrence starting point, I can figure out why what is being done needs to be. Please tell me how, if I just get code thrown at me its the same as lookig at other programs. I need the "WHY" as much as the HOW"
Any suggestions??? Please????
I appreciate your example Fraggle, Thanks, Could you toss in some more whys in there?
Thanks in advance all.
-T