This is a copy protection scheme for arcade video games. I heard about it the summer I worked at General Computer. I don’t know if it is true, but it sounds good.
Before getting to the copy protection scheme, I have to explain a few things.
Arcade video games are large machines that are often placed in hostile environments, such as bars. Because the people who play video games in bars can become rowdy and abuse the machines, arcade games have a mechanical sensor called a “kick switch”. Like the tilt switch in pinball games, its job is to detect major movement of the game itself. The software monitored the kick switch, and if triggered, would end the game. This was meant to discourage patrons from beating up the games, extending their lives.
Even with arcade games, piracy was a problem. The hardware the games ran on was somewhat standardized, so the ROMs could be swapped around. Pirates would copy ROMs and put them into older machines, giving them an upgrade to a newer more profitable game. Apparently, though, the pirates would make one minor change: they would remove the copyright notice from the first screen. I guess there was at least a superstition that the copyright notice was the thing that would get you in trouble.
Here’s the scheme:
- When first started, the game behaves as usual, displaying its copyright notice, playing games, and so on.
- After a hundred games, the machine behaves a little differently.
- After displaying its copyright notice, a separate chunk of code examines the video memory to determine if the notice is really being displayed.
- If the notice is not on the screen, a message is displayed: “Kick me for a free game”.
- Whenever the kick switch trips, a free game a credited.
The clever thing about this scheme is that it doesn’t prevent piracy, it just prevents pirates from making money. How? Imagine the life of that game. The pirates copy the ROM, modify the copyright string so that it doesn’t display, and fire up the game. It works fine, they play a few games, everything checks out. They deliver the game to a bar someplace, where everything is normal for the first hundred games. After that, the patrons pound it mercilessly and never put in a another quarter.
A week later, the game has been physically destroyed, having collected about $25.