From the neighbouring: https://mtsi.substack.com/p/we-picked-apart-the-milestone-hp https://mtsi.substack.com/p/we-picked-apart-the-milestone-hp > I found out

From the neighbouring: https://mtsi.substack.com/p/we-picked-apart-the-milestone-hp https://mtsi.substack.com/p/we-picked-apart-the-milestone-hp > I found out many years later that Mostek had put in special mask “cheats” to prevent people from copying our chip. Mostek co-founder and chief designer Robert Proebsting had discovered a way to put a pad on top of a silicon hill, but without connecting it to the obvious trace below. A copier would be unable to see this hill that served as an insulator unless they used a side-viewing microscope, which was rarely used. Copying was almost always done with a top-illuminated microscope. > That would throw off a copier who’d think it was just an ordinary via, and connect the trace to the layer below. This chip had a circuit to signal the specific 2+3 operation, and this would then be connected by the wrongly copied pad to the reset pin of the chip.