Strange Contra glitch?

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ShinyStar
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Strange Contra glitch?

Post by ShinyStar » Mon Aug 31, 2020 1:35 pm

This video at 35.18 mark: https://youtube.com/watch?v=GgOE64kgjjo has an incredibly interesting glitch that has left me wondering. What is the cause of this? Is it possible to reproduce it in command? In the video, it explains that a guy known by the name of Alyosha was trying to reproduce this specific glitch by attempting all sorts of tests, alas, he came up with nothing.

However, the mystery behind this is rather upsetting. Was it really a hardware failure? Could there have been any other conditions for this to be reproduced? Is this one of those things that will never get an explanation? The video kinda cut it short but nonetheless, finding what's the condition for this to happen could be intriguing.

If anyone has any ideas to explain why did this happened and the many ways it could in theory be reproduced, that could merit an article of its own.

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Bregalad
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Re: Strange Contra glitch?

Post by Bregalad » Mon Aug 31, 2020 2:42 pm

I don't know but I think hardware failure can be exculded; if hardware fails then it's not just a glitch that happens like that.

Most likely it exploits some vulnerability to jump to the "victory" code by corrupting the stack or some pointer.
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Memblers
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Re: Strange Contra glitch?

Post by Memblers » Mon Aug 31, 2020 5:20 pm

Non-fatal CPU glitches can happen, I think I've seen a few of them, but it's really rare to do something other than immediately crash. Maybe take a look at Contra's IRQ/BRK vector? Capcom games like to point it to the reset, some stuff just points to an RTI. Tecmo Bowl has a debug screen on BRK that a friend and I have actually encountered once. And that's an extremely popular game (even is on the NES Classic), I never saw anyone ever mention it anywhere (until now, I checked on The Cutting Room Floor, and it is on there). I have a hard time believing that was a software bug in such a widely known game. (this happened on an NES with a very unreliable connector)

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Dwedit
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Re: Strange Contra glitch?

Post by Dwedit » Mon Aug 31, 2020 6:09 pm

Here come the fortune cookies! Here come the fortune cookies! They're wearing paper hats!

strat
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Re: Strange Contra glitch?

Post by strat » Mon Aug 31, 2020 7:38 pm

I had a hardware glitch happen with the PCE/TG-16 game Parasol Stars one time. Somehow I started a two-player game with only one controller plugged in (two controllers must be plugged in to start one normally). Years later I looked at the game in Mednafen to see if it was a software bug - since the PCE requires a multitap just to have two controllers, it strobes the controller port 5 times to see if extra controllers are plugged in. If the button presses are different, it assumes a second controller is present. Of course this isn't that exciting since it's easy to abuse with hacking - just let the jump to the two-player game happen and off you go.

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