Sonic 2 input emulation error

[Former user]

Original [issue 208](https://code.google.com/p/genplus-gx/issues/detail?id=208) created by lexlexlex on 2012-03-09T15:36:47.000Z:

What steps will reproduce the problem?

1. Load "Sonic the Hedgehog 2 (JUE) [!].bin" (1048576 bytes) 2. Play the game until you get to the Aquatic Ruin boss or the Sky Chase zone. 3. Try to do the repeated spindash trick from the top of the Aquatic Ruin boss towers to beat him quickly, as one would do on the console, or try to spindash from the top of the airplane more than once in a row without walking anywhere between spindashes.

What is the expected output? What do you see instead?

I expect to see Sonic repeatedly spindash into the boss, defeating him quickly. Instead, after spindashing into the boss once, trying again results in moving nowhere. Any number of spindash attempts without moving between attempts continues to result in Sonic remaining stationary. Walking a little bit fixes it for the next spindash, but the problem is consistently reproducible when attempting to spindash into the boss multiple times.

Please provide any additional information below.

This Sonic 2 speed run from the 2012 Speed Demos Archive marathon shows the expected behavior on a real Sega Genesis starting at 10m28s into the video: http://www.archive.org/download/AwesomeGamesDoneQuick2012/AwesomeGamesDoneQuick2012\_part022\_SonictheHedgehog2.mp4

I have personally used this trick on my Sega Genesis many times while playing Sonic 2.

[Former user]

Comment # 2 originally posted by ekeeke31 on 2012-03-10T02:45:25.000Z:

I have no problem doing the repeated spindash (tried against aquatic ruin boss), it works as expected as long as you get the timing right (hold down, hold C, release C, etc), using either wiimote or genesis controller with wiimote adapter, so unless you are speaking about a different thing, the problem is probably with the controller you are using, not the emulator.

[Former user]

Comment # 3 originally posted by lexlexlex on 2012-03-10T05:02:29.000Z:

This video I just made using Genesis-Next (a port of Genesis Plus GX for libsnes) clearly demonstrates the bug. Notice how I can do the spindash whenever I want and I know how to do it. It's not hard at all. If I try to do it repeatedly without moving, it fails, but in the end, you see me move after each spindash and the next spindash works. This happened exactly how I expected it to happen, with knowledge of the bug. It's not a controller timing issue, since it's super easy to do.

You can play the video with MPC-HC and LAV Filters or just VLC 2.0.

http://dl.dropbox.com/u/35695760/sonic2bug-encoded.mkv

[Former user]

Comment # 4 originally posted by lexlexlex on 2012-03-10T05:21:17.000Z:

Hmm! The problem happens in Gens 2.14 also. Maybe it's the ROM. :O You can close this issue.

[Former user]

Comment # 5 originally posted by lexlexlex on 2012-03-10T05:40:23.000Z:

I tried a different ROM and I still see this behavior. I know it doesn't do this on the real system. Maybe this only happens on certain models of Genesis/Mega Drive, but not on the model I have or the model that guy in the SDA speed run has.

[Former user]

Comment # 6 originally posted by lexlexlex on 2012-03-10T05:49:33.000Z:

http://info.sonicretro.org/SCHG\_How-to:Fix\_screen\_boundary\_spindash\_bug

It seems this is the bug I'm seeing.

[Former user]

Comment # 7 originally posted by ekeeke31 on 2012-03-10T10:02:26.000Z:

I see, it's a bug in the original game indeed. When i tried the trick, i was on the left pillar but not completely stuck to the edge of the screen. As i said earlier, repeated spindash (as with only using DOWN and C buttons with good timing) worked perfectly, and exactly as in your initial video (Sonic comes back to its initial position after spindash,even when boss is moving to the middle of the two pillars...)

[Former user]

Comment # 8 originally posted by lexlexlex on 2012-03-10T10:09:14.000Z:

There's no "good timing" necessary. It simply works the first time or it doesn't once you've come back and not moved anywhere since then. What's done in the SDA marathon video can't be done on this ROM with these emulators. I can do it just fine on my Genesis with my real copy of Sonic 2. It may not be an emulation error, but it's definitely not something that you can do here, unless you have a different ROM in which the bug is fixed (which is what I suspect is the case with the Sonic 2 cart I own).

[Former user]

Comment # 9 originally posted by lexlexlex on 2012-03-10T10:10:45.000Z:

If I could dump my cart somehow, I could test this better. I don't have a ROM dumping tool though.

[Former user]

Comment # 10 originally posted by ekeeke31 on 2012-03-10T10:19:46.000Z:

I have tried with both revisions in no-intro romset and what is done in the video can be done in both. I am not sure if the position is \*EXACTLY\* the same but it works and the boss was quickly defeated without moving from my initial position on the pillar.

[Former user]

Comment # 11 originally posted by ekeeke31 on 2012-03-10T10:23:32.000Z:

Also, it could be that the speedrun was done with a flashcart and a fixed ROM but what is sure is that sonic retro clearly shows it is a bug in the game code, not an emulator bug.

[Former user]

Comment # 12 originally posted by lexlexlex on 2012-03-10T10:25:40.000Z:

My cart was bought back when I was a kid, and definitely isn't a flash cart with a fixed ROM.

[Former user]

Comment # 13 originally posted by lexlexlex on 2012-03-10T10:52:37.000Z:

Ohhhh! It only works from the left! Here's it working properly:

http://dl.dropbox.com/u/35695760/sonic2bug-part2-encoded.mkv

I must have always naturally done it from the left as a kid, and when I came back to playing it as an adult, I naturally tried from the right for some crazy psychological reason. I'm sorry for bothering you with all this! It's definitely my mistake, especially for not noticing he was doing it from the left in the SDA marathon video.

I can't apologize enough for wasting your time. I hope at least you get some entertainment out of this. Lol.

[Former user]

Comment # 14 originally posted by ekeeke31 on 2012-03-10T10:56:36.000Z:

No problem. I hate playing sonic 2 but it's always fun to learn new things about mega drive games.