C64 Mercenary: a novel exploit bug in a 41-year-old game

A deep reverse-engineering of the Commodore 64 game Mercenary documents how the engine streams and renders the city of Targ one building at a time and uncovers a novel exploit: an invisible 1,024‑unit square of grass near the landing site answers the E key as if it were the Colony Craft’s lift because the lift test checks square and mid‑bytes of X/Y but never reads height. That logic lets a player press E on the patch to enter hangar room 8 and then return to the deck at altitude 64,997; the elevator routine adds a fixed height offset when it reaches surface. The writeup explains when models are loaded (only squares 0-15 on each axis), how buildings pop in when the high byte of X or Y changes, roads appear above height 2,048, and how underground rooms are separate boxes with locked doors, so the exploit is gated but usable.

Beyond the city, the world is a 256×256 grid of 65,536‑unit squares; a few objects sit in the wasteland (two external lifts, a pyramid, a mast). One NPC craft - the Palyar commander’s brother‑in‑law’s ship - flies continuously at fixed X (4,287 units west) and altitude 32,769 (ALT 500), advancing Y by 256 per pass so it laps the city forever yet cannot be boarded. Attack behavior and object mechanics are quantified: distant objects plot as a single pixel past 65,536 units, an attack ship adjusts velocity by 1/32 of the distance per pass and overshoots repeatedly, its missile uses 1/16 and is unsteered, and many behaviors are tied to specific object slots and memory routines.