With this 2-pass assembler you can code as well for the 8502 microprocessor as for the Z-80 microprocessor.  Both processors are present in that wonderful machine, the CBM-128.

This document describes how to use the Double-Ass assembler, but doesn't teach how to create correct machine language programs.  It assumes you have experience with the built-in Monitor of the 128-mode.  However, this document contains some clues on how to code in assembly language.