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Prohibition (Commodore 64)

Prohibition Commodore 64 Loading screen

MISSING COVER

Genre
Perspective
Theme
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Trivia

Prohibition holds the distinction of being one of the only two games (Super Zaxxon being the other) to support four-way hardware scrolling in IBM CGA at 60Hz, which is notable because IBM CGA only had one video page that took up exactly the size of the screen, so scrolling (in any direction, let alone all of them) shouldn't have been possible.

The game achieves this trick by exploiting a quirk of how IBM implemented CGA in conjunction with the Motorola 6845 character generator chip. IBM only used 14 bits for addressing, so attempting to scroll in any direction using the MC6845 shows the screen "wrapping around" to the other side as addressing space is used up. Prohibition draws new information into the section that is about to wrap around, and then scrolls anyway, bringing that new information into place.

Because the program is literally "racing the beam" to get that information into place before the MC6845 starts drawing the screen, it took careful precomposition of the game's graphics in system RAM so that no calculation would be necessary other than moving memory to video RAM.

Contributed by Trixter (8740) on Apr 27, 2010. -- edit trivia



On August 19, 1987, Prohibition was put on the infamous German index by the BPjS. For more information about what this means and to see a list of games sharing the same fate, take a look here: BPjS/BPjM indexed games.

Contributed by Xoleras (66260) on Dec 11, 2005. -- edit trivia