[peterevans]

My understanding is that in the 6502 (NMOS) chip, those other opcodes had undefined behavior, and you might crash if you executed one. So you'd need to know who was the manufacturer to be certain that opcode X was available.

In the CMOS version (65C02), illegal opcodes are treated similarly to NOPs--they don't crash, and don't do anything other than spend cycles--with the caveat that certain of these "illegal" opcodes have specific numbers of cycles and bytes that they consume which are different from other illegal opcodes. More confusingly, there is an actual proper NOP instruction (0xEA), which consumes the one opcode byte and two cycles, which exists in both the CMOS and NMOS version of the chip.

So some of the illegal opcodes are just like NOP, and eat one byte and two cycles; some eat two bytes, and 3 or more cycles; and there's at least one which not only eats 3 bytes, but eight (GASP!) cycles.

Source: if it's possible for one to consider MOS emulation a hobby, then let's say it's a hobby of mine.

[to3m]

I copied the 6510 illegal opcode behaviour for the SY6502A emulator for my BBC Micro, and I've yet to see a problem from it. If they didn't all have the same PLA, then perhaps the differences didn't cause any useful effects...

(Some BBC Micro games definitely do use illegal opcodes, but I didn't take very careful notes when I was writing ver 1, rather a long time ago. For the current version, I just made sure the Lorenz 6502 test suite ran to completion.)

[duskwuff]

Yep. And some implementations -- like the Rockwell 65C02, or the later WDC 65C816 -- added new instructions using the previously undefined opcodes, so it wasn't even safe to assume they were all no-ops.

[ci5er]

Can that be true? (I'm channelling 1985 here, so I could be well off, but...)

IIRC, some of the janky stuff I did on the Apple //e 6502 did run on the Apple IIc 65C02. I'm pretty sure they didn't fall of into NOP-land. (Again, I could be mis-representing this, but I was taking some shortcuts at one point to accelerate some graphics that ended up in a Broderbund game, so I don't think I am? (Time wastes all memories - I could well be wrong))

[peterevans]

Ah-ha, well! There are actually two Apple IIe models. (Well, there was a third "platinum" one too, but let's not talk about that.) One would have shown "Apple ][" at boot time, and this was the original model with a 6502 NMOS-based chip. But they later released an "enhanced" model, which had a 65C02 CMOS chip. You could tell the difference because the later model would have instead shown "Apple //e" when booting.

It's quite possible (perhaps because you typed it as Apple //e!) that you were actually working with that 65C02 variant, and thus, your janky stuff was forgiven by the Gods of Early Personal Computing (blessed be).

(See: https://en.wikipedia.org/wiki/Apple_IIe)