Tearing it apart

Once again, the VDP does something that shouldn't be possible. In Competition mode, the screen is split down the middle and both halves scroll independently.

The VDP can only scroll entire rows or columns, so when the entire screen is split horizontally like this, it should be impossible for one half to scroll vertically without bringing the other half along with it.

Once again, much like we saw before, the software is cheating the hardware. Halfway through drawing the screen, the game disables the video output and quickly replaces all the background and sprite data. This takes some time, leaving a gap across the middle of the screen where nothing is drawn. Then, it enables the output again, and the VDP dutifully resumes drawing the screen, unaware that someone swapped the pictures while the lights were out.

Competition mode isn't the only place where this trick is used, though. The flying ships at the end of Angel Island Zone and Mushroom Hill Zone are actually rendered using the foreground layer, so if left unchecked, they would move the floor up and down as they bob in the air.

Unlike in Competition mode, players may cross the screen boundary when they jump, so the strategy here is to replace only the Plane A vertical scroll data, during the gap between the ship and the floor where Plane A has nothing to draw anyway. This allows sprites and the background to seamlessly extend across the entire screen, but as a consequence, the attached bomb and propeller sprites have to be manually moved up and down in order to keep them synchronized with the rest of the ship.