I'm not sure if this is the right place to put this observation, quite possibly not. But I discovered that the Apple II Emulation mode in the Apple /// isn't quite working. The symptom was that if I pre-boot with the Apple II Emulation disk, then boot a DOS 3.3 disk, then enter the monitor with CALL-151, my interaction with the monitor pretty much doesn't work. Commands don't do what they're supposed to, it doesn't seem like it is really "hearing" me. I can do 9D84G and get back to the Applesoft prompt. I expect this is easily reproducible. The Apple II Emulation disk is available, e.g., at apple3.org, and I've yet to find a DOS 3.3 disk that didn't show the behavior.

Upon investigation, I think the issue is that the special Apple /// VIA registers that live in FFDx and FFEx (and control various things like processor speed, RAM page, zero page) are not being properly "hidden" when in emulation mode. When emulation mode is on, I believe those addresses should no longer be active, but instead should be protected from writes and be read from RAM. This is presumably why once you get into emulation mode, you can't get out again. Those FFDx and FFEx addresses happen to be right where the monitor's command tables are, which would explain why the monitor can no longer understand me. Even if the addresses are not triggering state changes, they are not returning the bytes that should be there in FFDx, FFEx.

I somewhat verified this interpretation by writing a modified monitor ROM to the Apple II Emulation disk that relocated those lookup tables elsewhere (into the cassette read/write area) and once the monitor no longer relies on values in that area, it works fine.

I'm afraid I'm not savvy enough with the MAME codebase to know how to fix this, but I think essentially the case is when emulation mode has been turned on (by setting the $40 bit in $FFE3?), reading from FFD0-FFFF should return values from the system bank and ignore any contributions from the VIA.

Sorry if I invaded an unrelated thread, I was hypothesizing that this is kind of a grab bag thread of Apple II-and-friends-related issues.