http://stech68k.net/index.php?topic=45&page= The YM2612 is basically two YM2203s shoved on the same die (The second one is slightly more special than that because of the DAC channel). So I wondered, would the datasheet for the YM2203 be sufficiently compatable with the YM2612? And the answer is a resounding "yes". The register listing on the datasheet is almost precicely the same as the one in sega2.doc. The only major differences I found were below $30: registers $00-$09 (appear to be for controlling a noise generator - these may or may not exist in the 2612), register $21 (TEST in the datasheet but LFO control in sega2), $2A and $2B (DAC control), and $2D-$2F (clock divisors in the 2203, again, may or may not exist in the 2612). Now what we need to do is hack something up to test out these new registers on real hardware.