flipacholas
commented
3 years ago Thanks for this info, I'm a bit short on time right now so I'm focusing on recent issues of the just-released ps3 article. But I want to examine this with more patience in the following weeks, thanks again.
I worked at a company (Accolade) who didn't license with SEGA and had to reverse engineer the console in order to make games for it.
Your claim: The sound chips are only accessible from the Z80. Correction, you can access the FM chip from the 68000. Before we figured out how to control the audio with the Z80 our first games only used the 68000 to make sound and music. Bubsy maybe? Therefore it is possible to control the YM2612 FM sound chip via the 68000 but IIRC the SN764898 was only available for control from the Z80. I can maybe find some references to back up this correction.
Clarification: the VDP native mode itself, it can be noted, was a descendent of the TMS9918 API. The Mode IV command structure was evolved from the 9918 architecture. We used the publicly available Texas Instruments 9918 data sheets to aid in reverse engineering the native mode Mega Drive VDP.
I've brought up playing fast and loose with IP before with regard to the NES but the 9918 used in the Master System and evolved into the Mega Drive was a "A “Z80-like” register file compatible superset clone of the 9918". (9918 designer Karl Guttag's blog linked below). As a side note, in this blog post he even claims that the NES PPU was a derivative work of the 9918 but not so directly as in the Master System / Mega Drive. When you look into it, and given my own familiarity with reverse engineering the Mega Drive, it does hold up.
Mention of Sega's implementation being a clone https://kguttag.com/2017/06/28/texas-instruments-994a-and-tms9918-history/
And more specifically, see the comments here from Mickael Petit and admin (Guttag) response for much more insight. "Strangely in spite of all the patent litigation that TI was pursuing at the time, I don’t remember them ever litigating the 9918 Patent " https://kguttag.com/2013/08/10/if-you-havent-tested-it-it-doesnt-work/
"At the same time Yamaha did a register level clone and superset that was similar to the AVDP. Yamaha was apparently doing the chip for Nintendo and being in Japan they had the inside track on that design (I don’t know why TI never sued Yamaha based on our sprite patent, maybe we had a cross license, I don’t know). I’m that their chip was register level compatible with the 9918 and suspect that Nintendo, which developed Donkey Kong for Colecovision, use the 9918 in there early game system development." http://spatula-city.org/~im14u2c/vdp-99xx/e1/99-4_History_By_KG_in_answers_to_Matthews_Questions.doc
9918 presentation by Karl Guttag https://www.smspower.org/Development/TMS9918ArizonaTechnicalSymposiumDraft
Let me know if you need more details.