[SilasX]

That much makes sense, but the original framing of the opcode as illegal still comes off as a category error to me (though perhaps that phrasing is common and excepted). The opcode itself isn't illegal, as opcodes are a CPU-level concept, where nothing is illegal.

Rather, the opcode may bring the memory state to something that might violate some OS's security model. But the opcode still does something to the CPU (Boolean) circuit state.

[sand500]

You could think of it as a switch case statement where the valid opcodes are the cases and there is a default that catches all non defined opcodes and throws an error. Its illegal in the sense you should never expect to run an instruction with those opcodes.