$2C:3 - Ch6 DAC ninth (least significant) bit. Yup, I remember reading about this in this thread, someone theorized this bit's existence, and here it is! It goes into the ninth bit of the DAC output register block, in an exactly corresponding place as where the eight signals from $2A go into the first eight bits. Even better, since it doesn't look like BUSY affects the chip-global registers ($2X) at all, you should be able to write the eight MSBs into $2A and write the LSB into $2C:3 as soon as the Z80 will let you. As long as you get them both in before the YM2612 gets around to the Ch6 cycle again, you should be able to get full-precision 9-bit output without missing samples.
jotego
commented
7 years ago
According to Sauraen