As far as I remember you need to return 1 when read from address 0x320000. I think thats how removed the "Z80 Error" message.
> To answer my own question for anyone who's curious:
> The bios will be happy with just the Z80. No ym2610 is needed.
> > The neogeo bios does a test of the Z80, and hangs with an error message if the
> > Z80 behaves improperly.
> > How much of the Z80 needs to be emulated to pass this test? Will the Z80 pass
> > on its own (with the ym2610 stubbed out), or does it need parts of the ym2610
> > well?