When coding your program, you will be thinking, looking up documentation and editing files. Emacs already helps you with editing files, but Emacs can do much more. In particular, looking up documentation is one of its strong points. Besides the info system (which you are probably already using), mixal-mode defines commands for getting particular information about a MIX operation code.
With M-x mixal-describe-operation-code (or its keyboard shortcut
C-h o) you will get the documentation about a particular MIX
operation code. Keep in mind that these are not assembly (MIXAL)
pseudoinstructions. When the
point is around a MIXAL
pseudoinstruction in your source file, Emacs will recognize it and
will suggest the right MIX operation code.