FDS Audio Master I/O Enable ($4023)

For emulation purposes, can the Master I/O Enable ($4023) Register be ignored? Does anything write to the sound/disk registers prior to the registers being enabled for copy protection purposes? Will any games fail if the flags are completely ignored?

The BIOS initializes this at startup, so really nothing has a chance to write to anything before it's enabled.

The first 3 register writes by the BIOS are:


I have no idea if any games turn it off later, though. I wouldn't presume it's necessary to emulate for compatibility, but I am not sure why you'd want to leave it out either.

Since the topic refers to it as "audio" enable, if you're asking if an NSF player needs to implement it, there are no FDS NSFs that rely on it to my knowledge, and they all presume it has been enabled before the NSF code is run. (I think most NSF players do not implement it, but again, not sure why you'd intentionally leave it out.)

Even if you could confirm that no commercial game depends on it, there's always the possibility of a future software that depends on it. So yeah why would you like to leave something out if it's easy to implement?

I imagine that with the exception of FDS emulator tests, practically all "future software" will be in cartridge form because we can't make new discs, unlike new cartridges.

Hawken released an FDS disk soundtrack just a few days ago: thread

I don't know if anyone's still making quickdisks, but used third party FDS disks are relatively cheap and plentiful. Original and bootleg disks are easy to rewrite, much less difficult than using a donor cartridge.
http://www.famicomdisksystem.com/disks/

For not mentioning devices like FDSStick that makes it easy to run disk images without a disk or disk drive nowdays.