r/Commodore • u/a1r • 19d ago

I analyzed MERCENARY and discovered an exploit bug at the start of the game

MERCENARY is my favourite C64 game ever. I reverse engineered it and discovered a bug: right from the start of the game, if you take 100 paces north-west, there is an invisible patch of grass that bugs out and thinks it's an elevator inside the Colony Craft, high in the sky above you.

Check out the interactive site: https://gamesexplained.com/c64/mercenary/

I haven't seen this documented anywhere before, so it might be a new discovery! The bible for stuff like this is The Mercenary Site, http://mercenarysite.free.fr/ - given it's not listed there, I think it might be novel.

(If you're interested in breakdowns like this, I've open sourced everything - check the front page of the site.)

32 Upvotes

92% Upvoted

5 comments sorted by

•

u/AutoModerator 19d ago

"comment: "Thanks for your post! Please make sure you've read our rules post" "

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

5

u/a1r 18d ago

I added some nice interactive stuff today:
1. you can fly the whole city at https://gamesexplained.com/c64/mercenary/#city-view
2. view all the game objects in 3D at https://gamesexplained.com/c64/mercenary/#objects
3. and my favourite, see how BENSON talks to you at https://gamesexplained.com/c64/mercenary/#benson-window

2

u/a1r 18d ago

I added a full playable version of the game, https://gamesexplained.com/c64/mercenary/play.html

  • By default it's the exact C64 experience. You can try the bug and it works.
  • Quality of life improvements: turn on smooth motion (gives you modern FPS) and/or native resolution (no jagged lines).
  • I added a fun mod (Targ Traffic) and two cheats: give all keys, and summon the interstellar ship.

I never realized the interstellar ship actually gets summoned and flies in, from above the Colony Craft! If you jump in the Dart, get off the ground and look up, you can watch it come down. So cool.

2

u/[deleted] 16d ago

[deleted]

2

u/a1r 16d ago

Purely from looking at the code, but using Claude Opus to check that every single byte is understood. I wrote a skill to look for unexpected behaviors and it picked up on this one. It's very unlikely anyone would ever discover it by accident - you can try it yourself at https://gamesexplained.com/c64/mercenary/play.html - I added a coordinates readout under the game, walk (or drive) until both middle bytes are 72.