CompuServe Messages

I have been hearing some things about how the various models of the Amiga are not 100% compatable with each other, or how higher versions of the operating system are not downward compatible and that older software will cease functioning on them. I distinctly remember when the Amigas started using 68020's that one assembly language call that wasn't accepted by the 68000 and that I had to run a program called FixHunk (I think) to change newly created programs to work on my A1000. (Do you have a question Mike?) Yes, I do. I want to start writing code again, and after having a piping hot cup of Demming poured on my brain, I intend to write code in the Zero Fault Tolerance mode. So I want any program I create to work on any Amiga machine that is out there. So my questions are: 1) What are the cause of these incompatabilities I hear about? 2) What reference can I use to find out what the new features and the obsolete features are of any of the hardware or software upgrades? 3) How can I avoid writing code that won't transfer between systems? Do I use minimal library calls? Do I write code that operates differently on different machines? Do I compile a different program to work on specific machines? Do I write my own library functions? It really frustrates me when a program I've been using for years stops working because of an upgrade. I know this is mostly due to assumptions by the programmer and code that doesn't follow the guidelines, but that is exactly what I'm trying to learn. How to avoid those assumptions in my code. Thanks! Mike McCormick