Downgrading a Apple IIe Enhanced/Platinum or //c's CPU

[Great Hierophant]

I know the Enhanced and Platinum Apple IIe and all Apple //c computers came with the 65C02 processor. The 65C02 adds new instructions, fixes bugs and eliminates unofficial and undefined instructions. However, there are Apple II programs that rely on those bugs or undefined instructions to work. What would happen if you inserted a plain 6502 in these machines? Would their firmware freak out and refuse to boot with an error message? I know that programs that are //c or Enhanced IIe aware may fail due to assuming that there is a 65C02 in the system, but the 6502 would only be for programs that are not aware of those machines.

 

[KC9UDX]

I can't speak for the //c, but I can speak for the //e and would hazard a guess that the c is the same.

The official answer is no, the Enhanced ROM runs just fine on a 6502. In my own experience, I have had several machines with original dealer installed "Enhanced" ROMs that would not. I think I sold all of them, so I can't provide the ROM code, or proof.

 

[David_M]

I've had boards on the test bench that ran 6502 with an enhanced ROM set, but I've never tested extensively.
But if you are downgrading the cpu then the ROMS are simple and cheap to downgrade with it.

 

[Great Hierophant]

I think that makes sense for a IIe, although pulling one chip is easier than pulling four As for a //c, I wonder if the use of 65C02 features varies by the ROM version (four versions before the //c+)

 

[KC9UDX]

I've run into software that requires Enhanced, and detects it on startup. If it's not found, it exits with an error message.

I'm not sure if it detects processor behaviour or ROM code. If it detects ROM code, you may end up with unpredictable results with a 6502 and Enhanced ROMs.

A processor test is easier, given the number of possible and unknown future ROMs. But the processor test relies on the behavior of unpredictable instructions. At this point in history, we know that there are only so many variations of 6502, and we know what all the possible results are. But when commercial software was being actively written for the Apple ][ series, this wasn't always the case. So neither test is more likely to be used than the other.