Nintendo was able to block unauthorised publishing by including a proprietary lockout chip called the Checking Integrated Circuit (CIC) . It is located on the console's motherboard and connected to the reset signal, making it difficult to remove.
The CIC runs 10NES, an internal program that verifies the presence of another lockout chip in the game cartridge. If the verification fails, the console enters an infinite reset loop.
Both lockout chips are in constant communication during the console's uptime. Nonetheless, this system can be bypassed by cutting one of the pins on the console's lockout chip, which places it in an idle state. Alternatively, sending a -5V signal can halt it.
The CIC exists in response to the video game crash of 1983. Nintendo's then-president, Hiroshi Yamauchi, decided that to ensure high-quality games, the company would take responsibility for approving every single title .
You'll notice that the Japanese model of the console, the Famicom, was released before the 1983 crash. That's why neither game cartridges nor the console includes CIC circuitry . Instead, the pins are used for optional sound expansion.