missing cover art
DescriptionProhibition is a shoot'em up taking place in New York in 1930. The town is overrun by gangsters and you're hired by the police to rid the town of them.
The gameplay is simple: you're facing a row of buildings, a gangster pops up and you have to shoot him before the on-screen timer goes empty and he shoots you first. You can hide until the timer restarts but this is limited too. At the end of each stage is a bonus round where you have to shoot gangsters in a room, with no more ability to hide and a faster timer. As you progress through each stage, the game will become faster, and enemies will become harder to find and to hit. Of course, your goal is to reach the end of the game with the highest possible score.
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TriviaProhibition 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.