10NES

10NES was the authentication code for the Nintendo Entertainment System (NES). The system consisted of two parts — a chip in the NES which would check the cartridge in the system for authentication, and a chip in the cartidge that would give the 10NES code upon demand. If the cartridge did not provide the authentication, then the system would not boot up.

Since 10NES was patented, only Nintendo could produce the authorization chips. Still, that did not stop other companies from finding ways to evade Nintendo and their policies regarding game production.

Some of their policies were that only Nintendo could produce the games, and that games made for the Nintendo would not be made for any other system.

The most common method to bypass the chip was to have on the unlicensed cartridge a dongle that a legitimate cartridge would be plugged into, allowing the authentication chip on it to be used. More advanced unlicensed cartridges actually used a circuit that would spike the voltage sent to the challenge chip in the NES, knocking it offline. Tengen, which had tried to reverse engineer the authentication chip unsuccessfully, actually stole the 10NES code from the U.S. Patent Office — they requested it as part of a copyright suit between themselves and Nintendo, but then proceeded to use it in the manufacture of NES cartridges. Needless to say, Nintendo was less than amused, and successfully sued Tengen over patent violation — it was this, not the huge fight over Tetris, that led to Tengen's demise.

Camerica managed to get away with reverse engineering the code to produce unlicensed games.