Expression has 8 elements, but must have 9 elements, and also, else null

Thanks (again) for your comprehensive answer, Tricky. I did used the unsigned library, and I will try what you suggested.

As for the second question... do you have a recommendation on a good book/website on proper writing of VHDL code? I want to be able to write efficiently and understand the logic, like you said. A lot of times I run into logical errors and would love to really understand them and understand what I'm doing, because currently it seems like I'm just writing a piece of code and hoping to get it right.