zlacker

>>embedd+(OP)
Some of those crazy instructions were used for copy protection, back in the day. Those mystery page boundary overflows were entertaining.

>>rossju+27
Aah, that's much better and more realistic than my previous assumption that they were "government instructions", something used in the military and similar more secretive contexts, but I suppose they didn't use off-the-shelves components perhaps like today.

>>embedd+C7
I don't think they were "intended" for anything - it's just that was the state of the control lines after it decoded that instruction byte, and combination might do something somewhat sane.

Wiring all the "illegal" instructions to a NOP would have taken a fair bit of extra logic, and that would have been a noticeable chunk of the transistor budget at the time.