Checking Integrated Circuit

The Checking Integrated Circuit, or CIC, in Nintendo 64 game cartridges prevented unlicensed and pirated game cartridges from running and allowed for regional lockout. During game execution, the PIF continuously communicates with the CIC to verify the game is valid.

Reference
REcon 2015 - Reversing the Nintendo 64 CIC

Open Source Implementations

 * https://github.com/mikeryan/UltraCIC
 * https://github.com/jago85/UltraCIC_C
 * https://github.com/perkinsb1024/UltraCIC-II
 * https://github.com/ManCloud/UltraCIC-III
 * https://github.com/hcs64/boot_stub