Michael J. Mahon wrote:
True, but the decoding in the 6502 is handled by a kind of PLA, so it would likely not be very expensive (in real estate) to trap or NOP the invalid combinations.
The 6502 is a design to have as few transistors as possible. The whole CPU only has a few thousand of them. Handling the illegals would have increased this small amount a lot.
Admittedly, though, the microprocessor design culture at that time was not oriented toward "protecting" undefined space, as they all have been in the years since.
It's not about culture, it's about being able to do more with just a few transistors. What CPU would you buy? The one which can do less has NOPs instead of illegals, or the one which can do more?
Removing illegals is ok if you only waste 1000 transistors of 1000000, but if it's 1000 transistors of 6000, then it's a whole different matter.