Reverse-engineering Roadsearch Plus, or, roadgeeking with an 8-bit CPU

Summary

Sorry, Doc Brown: we still needed roads in 1985. That meant paper atlases and misfolded roadmaps and a lot of stereotypical male anxiety asking for directions. Fortunately, in 1985, this problem also had a solution. Yes, if your car inverter could handle a 45-ish watt load — and your wife doesn't want her seat back right away — you could navigate major routes across America on your home computer like this portable Commodore SX-64. I particularly enjoyed writing this article because my other irredeemably nerdy habit is roadgeeking, exploring and mapping highways both old and new, and it turns out that 8-bit roadgeeking on ordinary home computers was absolutely possible. For computers of this class, devising an optimal highway route becomes an exercise not only in how to encode sufficient map data to a floppy disk, but also performing efficient graph traversal with limited hardware. Today we'll explore Roadsearch-Plus, one of the (if not the) earliest such software — primarily on the Commodore 64, but originating on the Apple II — and at the end "drive" all the way from southern California to British Columbia along US Highway 395, my first long haul expedition, but as it was in 1985. Buckle up while we crack the program's runtime library, extract its database, and (working code included) dive deeply into the quickest ways to go from A to B using a contemporary home computer. Although this article assumes a little bit of familiarity with the United States highway system, I'll provide a 30-second version. The top-tier national highway network is the 1956 Eisenhower Interstate System (abbreviated I-, such as I-95), named for president Dwight D. Eisenhower who promulgated it, signed with red, white and blue shields. Nearly all of its alignments, which is to say the physical roads composing it, are grade-separated full freeway. It has come to eclipse the 1926 United States Numbered Highway System (abbreviated US, such as US 395), a nationally-numbered grid system of highwa...