I analyzed the Rockman code a bit, and I found out why the "grabbing ladder top too high" trick works in the way it does.
When the game thinks that Rockman has cleared the ladder (by climbing up) (that is, he's climbing up but the tile his upper portion lies on is not a ladder), it resets rockman's position with this formula:
Y = (Y & 0xF0) + 0x0C
The idea of that formula is to ensure that when he clears the ladder, he's properly positioned right on top of the ladder, and will not start falling.
But if his climbing position was a little too high, that formula will actually place him 16 pixels above the ladder, as demonstrated in this image:
https://files.tasvideos.org/gameresources/nes/rockman/updowntrick2.gif
The same code is also probably responsible of this effect:
https://files.tasvideos.org/gameresources/nes/rockman/vertgrab.gif
I also found out, how climbing down works.
When Rockman is trying to climb down, the game first checks whether Rockman's body is on a ladder. If that's not the case, it will
drop Rockman by 12 pixels. Then it will actually climb him. (Wow, that sounds quite obvious and bland when I think of it.)
So why does pressing "up+down" over a ladder work like it does?
At first, I had no idea. According to the code, the game should ignore "down" if "up" is pressed. But we already know that's not true.
Then I found that there's more to it.
The game completely ignores up/down buttons if there is no ladder anywhere near Rockman. By ignoring, I mean it won't even check if they're pressed. Nice.
When there
is a ladder in sight, it checks for two things:
* If Up (but not Down) is pressed, but the ladder in sight is
below Rockman, the key is ignored.
* If Down (but not Up) is pressed, but the ladder in sight covers
whole Rockman (both the upper and lower part of his torso) but
not below him, the key is ignored.
These tests are rather dubious. Here's where up+down steps in: When you hold up+down, neither of those tests match, so it will go straight into Ladder_Handler, assuming that Rockman is now climbing.
After that, it will hold priority to the Up key (Down is ignored if Up is pressed), and if Up was pressed and there's no ladder behind Rockman's upper torso, he'll perform an exit from the ladder,
finally explaining the up+down trick.
According to this analysis, the "down" key is also ignored when Rockman is walking past a ladder, but
not if the ladder is shorter than 2 blocks.
Edit: Analysis proven correct. See
http://bisqwit.iki.fi/kala/tinyladder.zip (Cutman level beginning). Press or hold down when you pass the ladder.
This quirk is responsible for this behavior (note closely what happens when Rockman passes below the ladder):
I can't think of ways to abuse the "drop Rockman by 12 pixels" feature. To do it, you would need to find a way to put Rockman so that the game thinks he's now located near a ladder (but not over a ladder), but so that when he drops by 12 pixels, he's still not over the ladder.
I'll test if this is the same for jumping later. I only very briefly looked at it with frame advance, but I think that it's slightly different (I'm thinking 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, from what I could see without taking screenshots). I'll do this more accurately later. Looks like you might need to make a bot to calculate when the best time to jump is if you're ever gonna remake Rockman, Bisqwit, since I know I wouldn't want to have to think so much about every single jump. :)
