Expresso running bug

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Expresso running bug

Postby assassin » February 25th, 2016, 4:21 pm

As with Donkey, Diddy, Rambi, and Winky, Expresso can be made to walk with the arrow buttons, and to run by holding them down with the Y button. However, due to a couple of cases that make him run all of the time, it's easy to think that walking isn't an option. Using Barrel Cannons (such as when emerging from the bonus area in Temple Tempest with Expresso) or jumping and flapping your wings will put the bird into an always-run state. As the ostrich is very fast, this can compromise control some, and increase your chances of moving into a ditch or an enemy.

To reliably disarm the bug, dismount, have your Kong move around until he stops auto-running, then remount. However, it'll be re-enabled every time you flap.

What's happening is that the game will continually set an "Auto move faster" flag (Bit 1 of 7E/169A for Donkey, $169C for Diddy) from the time you flap your wings in mid-air until you land. (This is done at BF/97B2 in USA Version 1.2.) Presumably, this is to let you fly faster and further without having to hold Y, as holding one button while pumping another with the same thumb is a pain in the ass (see Super Mario Bros. 3, Level 6-5). While the flag is no longer repeatedly set when Expresso is on the ground, it doesn't get cleared, either. It does with a Kong (albeit after a possible delay). That's why you'll need to dismount to allow walking.

The flag also gets set by jumping off of a rope, which is another kind of mid-air jump. (I don't yet grasp the reasoning behind this or the Barrel Cannons' setting. I'd have to study game behavior with it never being set.)

Regarding a fix, I'm too clueless of this game's internals to know where to put it. Because BF/97B2 sets the bit continually, I might be able to get away with continually clearing it shortly before the containing function is called, but that's cheap and risks affecting too much.

I discovered this bug circa July 4, 2006 or sooner, but inexplicably never typed up a post until this January. Google searches and looking at a few good webpages comes up empty, which makes me think that this one is so pervasive, players don't even realize they're being deprived of walking.
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Re: Expresso running bug

Postby OneOf99 » February 26th, 2016, 5:14 am

Er... welcome to the forums!

Nice job figuring out the actual explanation for the "bug"! (I wouldn't really call it a glitch/bug, more of a flawed mechanic) This might be useful if someone decides to change the code to stop Expresso from running, but as you said it seems a bit too integrated within the code to figure out. Maybe it is somehow tied to the normal running mechanics? I think I will look into this too because there might be something useful and interesting here.
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Re: Expresso running bug

Postby assassin » February 27th, 2016, 2:41 pm

thanks!

i definitely consider Expresso temporarily but frequently losing the ability to walk a bug, even if it wouldn't make for the eye-popping videos associated with other bugs posted here. but i won't sweat the terminology; "flawed mechanic" is fine. :)

it turns out Rambi is also affected by the Barrel Cannon form of the flaw. now, there aren't many Barrel Cannons in Rambi levels, and he obviously can't flap wings, so you'll encounter the issue less with him. it's still remedied the same way: dismount, have your Kong move around until he stops auto-running, then remount.
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Re: Expresso running bug

Postby Tompa » March 1st, 2016, 6:48 pm

This is present for Diddy and Donkey as well. After you've landed, if you hold right/left for 25 frames, without jumping, the effect will wear off. Though you can keep the speed by jumping.

I wouldn't consider it a bug though... It is however useful in speedruns and very handy when I work on my Tool Assisted Speedruns. Not by the fact that you keep running without holding Y, as holding Y will give you the same effect. Though each time you have to start a new roll, you need to release Y which would normally force you to lose speed, saving a slight amount of time. For TASing this makes it a lot easier, as the most optimal time to release Y for the new roll differs depending on your midair speed at the time, frame rules for the speed cycles as well as the upcoming obstacles or enemies. A bit of a side note :)
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Re: Expresso running bug

Postby assassin » March 1st, 2016, 10:53 pm

ah, wasn't aware of jumping keeping the speed. i did notice you could move back and forth in a fairly tight range and keep it for awhile in some circumstances, which is admittedly not very useful. yet other times, it'll be lost right after the Expresso dismount, or after a short wait without needing button presses. also, my brief testing with Diddy seemed to have it disappear more readily than with Donkey; was that just bad luck?

I wouldn't consider it a bug though...


well, the speed boost carrying over to land for a fraction of a second seems sensible, consistent with the concept of momentum. but when it lasts longer than that, as with Expresso and Rambi, being unable to walk normally is a bit illogical and prohibitive (as not everybody's speedrunning ;) ).

the game must've failed to extend the Kongs' checks to those two animal buddies. (Winky doesn't seem to have the problem, but i've only tested a single bonus area so far.)
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Re: Expresso running bug

Postby assassin » March 3rd, 2016, 5:38 am

haha; figures i'd test on the rare/unique Non-Blast Bonus Return:
http://www.dkc-atlas.com/dkc/levels/2-1

i since tried on Bouncy Bonanza, and Winky indeed has the flaw as well (triggered by Barrel Cannons). it's disarmed the same way as with the other two animal buddies.
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