Resource centre for ZX Spectrum games
using Manic Miner and Jet Set Willy game engines
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Manic Miner & Jet Set Willy Yahoo! Group
messages
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Message: 7133
Author: ian.rushforth
Date: 09/08/2017
Subject: Re: "MM [but not JSW64] Game-engine bug
Andrew wrote:
> I have yet to determine whether jumping up off the top, or falling off the bottom, of a JSW64:MM > game with my "Disabling the room-exits" patch from JSW64MM: James Bond applied, has any > similar harmful effects. That's why I designed James Bond v1.0 so that the tops and bottoms of the > screens are always blocked... except for the unintended loophole in "The World Is Not Enough", which I > intend to fix for v1.1. I might also > have to consider adapting my bug-fix from MM:N-AT for JSW64.
I don't believe it is the case that the bug will affect any JSW game engine, because the Screen Buffer Address Lookup Table (at either #8200 or #9E00) is immediately followed by the Rope Animation Table (at either #8300 or #9F00). Therefore all the values that the program might conceivably try to use as addresses to draw Willy's (or James's) pixels to, if his y-coordinate wraps round beyond the bottom of the appropriate range, have values of 00, 01, 02 or 03 - thereby pointing at the ROM, which is immune from over-writing!
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