I thought this was especially interesting as it's one of the earliest examples of PACE copy protection, and has remained uncracked for a good 35 years. The article author describes in some detail how the scheme works, and it's quite interesting to read. Very impressive efforts that protected the product for literally decades. PACE later developed the now-common iLok dongle used for copy protection still today.
https://paceap.com/about-us-basic.html