M-x hexl-find-file → visit a file in Hexl mode.

[printable-chars] → added as is

c-q Key → added char of Key

c-M-x XX inserts char 0xXX

So for c-M- with o, d

… (other keys are rebinded to fit the semantic hex-editor)

c-c c-c → exit hexl mode (and return back to previous mode)

c-h b for detailed document