C64 MERCENARY (1985): a novel exploit bug at the start of the game

This is a technical reverse-engineering tour of the Commodore 64 game Mercenary, explaining how the engine manages the city of Targ, the wider 256×256 world, and individual objects. The renderer keeps only one building model in memory at a time so buildings “pop in” when you cross square boundaries; roads and distant objects are drawn only above specific altitudes. The write-up catalogs how models, lifts and hangars are implemented as objects, where the game loads data, and how movement, motion records and draw-distance rules produce the visible world. It also documents exotic outlying objects (two outland lifts, a pyramid and a mast), model coordinate encodings, and which objects are fixed or mobile.

A prominent finding is a reproducible exploit at the starting square (08-08): a 1,024-unit invisible grass patch sits directly beneath the Colony Craft’s deck and triggers the E key because the elevator entry test ignores altitude. Pressing E on that patch transports the player into the craft’s hangar (room 8) and back onto the deck; the hangar doors are locked, so the usual escape is to walk off the deck. The analysis also describes autonomous craft behavior: a brother‑in‑law ship that continuously laps the city at a fixed X and ALT 32,769 (ALT 500) and cannot be boarded, and attacking ships that aim in discrete legs (overshooting then re-aiming) and fire only if within a defined proximity at the end of a leg; shooting the autonomous ship triggers scripts that mark the player as a traitor.