>Tengen (Atari’s NES games subsidiary) took a different tactic: the corporation obtained a description of the code in the lockout chip from the United States Copyright Office by claiming that it was required to defend against present infringement claims in a legal case.[4] Tengen then used these documents to design their Rabbit chip, which duplicated the function of the 10NES.[4]
Oh it's a much better story than that! They failed at trying to reverse engineer it so they just straight up stole the source code instead
https://en.m.wikipedia.org/wiki/CIC_(Nintendo)
>Tengen (Atari’s NES games subsidiary) took a different tactic: the corporation obtained a description of the code in the lockout chip from the United States Copyright Office by claiming that it was required to defend against present infringement claims in a legal case.[4] Tengen then used these documents to design their Rabbit chip, which duplicated the function of the 10NES.[4]