[RetroSpark]

Throughout the entire NES game library, almost no games use these illegal opcodes. Apparently as part of the licensing process, Nintendo would verify that games only used the official instructions.

I wonder how they tested that, though? I don't think developers had to submit their source code to Nintendo, so they would have had to analyse the binaries in some way?

[selcuka]

> I wonder how they tested that, though?

The QA process might have been performed on an emulator that doesn't support illegal opcodes.

[RetroSpark]

I don't know if they even had an emulator at the time - I don't think a 1980s PC could run a NES emulator at a reasonable speed.

Another possibility is that they used a hardware device. Perhaps something that watches the 6502 `sync` pin to know when an opcode byte is being read, and verifies that the data bus contains a legal value.