I notice whenever a auto variable or parameter are referenced on the stack in mixed assembly, the form is SP+@SP+X. That implies to me that X would be a negative offset if it is referencing an auto, and a positive offset when referencing a parameter. Is this a correct assumption?
I am no assembly expert but there are two great resources that I can recommend.
One is the instruction set manual itself http://ftp1.digi.com/support/documentation/0190098_l.pdf and the other is a book written by Larry Cicchinelli called Assembly Language Essentials.