Try to avoid using ORG statements in your file UNLESS you really need to put something at a particular address - like interrupt and reset vectors. For the rest of your code, the linker does the job of locating the code & data and produces a link file so you know where it has put things. If you start putting code at hard locations by using ORG statements for each of the modules, if one block of code grows into the area of the other module(s) then the linker is going to complain as it won't be able to do anything about it.

A good read of the manual will help.