The 10NES chip did not protect against copying. It protected against publishing without permission

Inside every NES sits a small four-bit microcontroller called the 10NES. An identical one sits in every licensed cartridge. At power-on both run the same program from the same seed and begin emitting a stream of numbers; the console compares its stream against what arrives from the cartridge. If the answers diverge, or there are none, the console pulls its reset line. And around again, several times a second.

It is worth naming this precisely. It is not copy protection: cartridge contents copy freely and the chip encrypts nothing. It is a compatibility lock. It stops a cartridge that has not been through Nintendo from running.

What was actually being protected

The licence. Nintendo's terms in the mid-eighties were strict: a publisher ships no more than five games a year on the platform, cartridges are manufactured only by Nintendo, runs are ordered in advance and paid for up front, and the seal of quality goes on the box.

This was a direct conclusion from the 1983 crash. Nintendo decided the market had been killed by uncontrolled supply — dozens of publishers burying shelves in indistinguishable cartridges — and built supply control directly into silicon. An economic rule cast as a chip.

The 1992 case

Atari Games, trading as Tengen, wanted to publish NES games without permission. Its engineers opened the chip, studied it under a microscope, traced the logic and worked out nearly all of it on their own.

Then they did something extra. Atari Games' lawyers requested a copy of the 10NES source code from the United States Copyright Office, stating that the company was a defendant in litigation and needed the code for its defence. At the time this was untrue. With the code in hand they produced their own chip, called Rabbit, and began selling unlicensed cartridges.

The Federal Circuit's decision in Atari Games Corp. v. Nintendo of America Inc. in 1992 is curiously built. The court affirmed the principle: reverse engineering to understand unprotected functional elements is fair use and a lawful activity. But Atari Games lost, because the copy it worked from had been obtained by deception, and unclean hands forfeit the right to invoke fair use.

The company won the principle and lost the case. Everyone since has used the principle except them.

The same year the Ninth Circuit reached the same conclusion in Sega v. Accolade, and in 2000 in Sony v. Connectix, extending the logic to emulation. Those three decisions are the legal ground on which writing an emulator is lawful in the United States.

Where the lock moved

From hardware into signatures. A modern console does not shake hands with a cartridge; it verifies a cryptographic signature on the executable. Breaking that is technically harder, but the decisive change is that since 1998 the United States has had section 1201 of the Digital Millennium Copyright Act, which prohibits circumventing technical protection measures as such, whether or not any copyright is actually infringed.

So the 1992 precedent has not gone anywhere, but the ground under it shifted. Taking a chip apart to understand a protocol is still permitted. Circumventing cryptography to run your own code is a separate offence.

The general conclusion is simple and useful: a compatibility lock is almost always an economic instrument in an engineering casing. In 1985 it collected dollars on every cartridge. Today it collects thirty per cent from a storefront.