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[File] JSW - The Attic Bug

jsw attic bug crash

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#1 Spider


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Posted 07 July 2017 - 09:35 PM

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File Name: JSW - The Attic Bug

File Submitter: Spider

File Submitted: 07 Jul 2017

File Category: Jet Set Willy [Patched]

System: Sinclair
Original Author(s): Matthew Smith


The well known 'Attic Bug' , less commonly known as a "bad program bug" back in the day has been quite well documented on various sites over the years however apart from a small test file submitted here a while ago and a selection of screenshots not a great deal else was provided.


The cause is relatively simple: A misplaced arrow which then overwrites various sections of game code, it damages the guardian entity bytes for the following rooms:

  • Room 01: The Bridge
  • Room 06: Entrance to Hades
  • Room 08: Inside the MegaTrunk
  • Room 12: Tree Top
  • Room 13: Out on a limb
  • Room 14: Rescue Esmerelda
  • Room 15: I'm sure I've seen this before
  • Room 16: We must perform a Quirkafleeg
  • Room 23: The Kitchen
  • Room 24: West of Kitchen
  • Room 26: East Wall Base
  • Room 27: The Chapel
  • Room 38: Priest's Hole


One of the best detailed but 'easy to read' descriptions is the one provided by SkoolKid in his excellent disassembly here of the JSW game.


This submission contains a standard JSW game (with keypad bypass) although all the keypad routines, data and unused code is still present.


To assist in the exploration of the mansion, a few small 'helper' tweaks were necessary otherwise an instant IDS would occur upon entering certain screens, Kitchen and East Wall Base for example would trigger this due to the damaged guardians.

  • Immunity to arrows, guardians, fire cells and long falls
  • Infinite lives
  • 'Writetyper' enabled
The starting location has been set to The Attic to ensure the 'bug' is triggered more or less immediately. If you would like to reset this back to normal (after starting the game once) then its simply POKE 34795,33 applied via your emulator/multiface.


It is hoped this small submission will allow those still curious about the effects of this to be able to explore the mansion and see it in action as it progresses in relative safety, at least until the corruption becomes too heavy and/or the game itself crashes completely.


Interestingly the effects do seem to vary a little bit each time it is played although this is likely due to differing player routes and / or the amount of time spent on a room. You'll note various odd effects such as invalid characters appearing the description and clock, sometimes these are permanent and will not clear upon starting a fresh game. A reset would be needed in this case.


Although it is stating the obvious but please bear in mind this is merely a test file: I would strongly not recommend attempting to recycle it or use it for building a game from.


Enjoy the potential mayhem! :)


Click here to download this file

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Also tagged with one or more of these keywords: jsw, attic, bug, crash

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