Exolon Crashing Bug Fix (when you jump too high)

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Ast A. Moore
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Exolon Crashing Bug Fix (when you jump too high)

Post by Ast A. Moore » Thu Jan 31, 2019 2:55 pm

Woot! Finally managed to get to the bottom of this nasty bug. Visually, the solution is a tiny bit inelegant—the guy bounces slightly off the top of the screen—but, hey, the game no longer crashes! :D

Up for grabs at the usual place.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Alessandro
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Re: Exolon Crashing Bug Fix (when you jump too high)

Post by Alessandro » Sat Feb 02, 2019 10:06 am

I never stumbled into this bug actually - but thank you for fixing it :D
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Ast A. Moore
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Re: Exolon Crashing Bug Fix (when you jump too high)

Post by Ast A. Moore » Sat Feb 02, 2019 11:15 am

Yeah, I thought it was pretty rare myself, but then I noticed I could trigger it every time on a multitude of screens (including some of the very early ones):

Image

And my fixed version:

Image
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Pegaz
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Re: Exolon Crashing Bug Fix (when you jump too high)

Post by Pegaz » Sat Feb 02, 2019 11:41 am

This is great, I really like your fixes.
Did you possibly try to analyze Dynamite Dan II and missing ending issue?
We discussed this on wos long ago, but still i'm not sure why the game can not end in the way described in the manual.
After all, it might be good to make at least a congratulations message if there was any free space at all.
Would it be possible to do such a fix?
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Ast A. Moore
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Re: Exolon Crashing Bug Fix (when you jump too high)

Post by Ast A. Moore » Sat Feb 02, 2019 12:01 pm

Pegaz wrote:
Sat Feb 02, 2019 11:41 am
This is great, I really like your fixes.
Thank you!
Pegaz wrote:
Sat Feb 02, 2019 11:41 am
Did you possibly try to analyze Dynamite Dan II and missing ending issue?
We discussed this on wos long ago, but still i'm not sure why the game can not end in the way described in the manual.
After all, it might be good to make at least a congratulations message if there was any free space at all.
Would it be possible to do such a fix?
No, I haven’t looked at it. I didn’t play it much back in the day, so I’m not familiar with it.

I generally try to make minimal changes to the originals and—whenever possible—preserve the original loaders (which is particularly easy with some of the strongly encrypted ones). Often, there’s is barely enough room for even a simple fix. Exolon is a good example, actually. It’s very tight and I barely managed to find a few spare bytes.

But I’d be happy if someone pointed me in the direction of some compendium of known bugs in Spectrum games. The games that I provide fixes for are for the most part the ones I discovered personally.
0 x
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Pegaz
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Re: Exolon Crashing Bug Fix (when you jump too high)

Post by Pegaz » Sat Feb 02, 2019 3:02 pm

It took me a while, but I found an interview with the author (Rod Bowkett) in the RetroGamer magazine (pages 56-61):
https://computerarchive.org/files/compu ... ue_062.pdf

Basically, the problem is that after the last eight level, theres no secret Blitzen lab, where you need to set up explosives (time bomb), as it says in the instructions.
Instead, the game returns back to the first level, without any congrats message or anything else.
I can only assume that its a lack of memory, but Rod Bowkett says he is sure he wouldnt leave the game unfinished in such a way.
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Ast A. Moore
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Re: Exolon Crashing Bug Fix (when you jump too high)

Post by Ast A. Moore » Sat Feb 02, 2019 3:43 pm

Pegaz wrote:
Sat Feb 02, 2019 3:02 pm
I can only assume that its a lack of memory, but Rod Bowkett says he is sure he wouldnt leave the game unfinished in such a way.
Right, but the article also says:
Since speaking to Rod, a disassembly of his code has been posted on the World of Spectrum forums and it confirms that there is indeed no proper ending to the game.
Still, an interesting case. It’s possible that he accidentally sent in the wrong version (unfinished).

EDIT: Just took a very superficial look at the game, and boy is it packed! I don’t think there’s room for adding extra game logic.
0 x
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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