Differences between Mega Drive versions? (VDP in particular)

Nemesis · Post by **Nemesis** » Fri Sep 02, 2011 11:20 pm

I'm interested in particular cases anyone here has ever seen/heard of/knows of, where different revisions of the Mega Drive hardware behave differently. Specific cases people have seen in their own testing and experimentation, or while working on emulating various games, would be fantastic. Even if you've just heard about something or seen a small glitch in a particular game though, I'm interested in hearing about it.

I'm especially interested in the VDP, and if anyone knows of any specific differences between the 315-5313, 315-5313A, and the various ASIC versions, or if they all seem to be identical. I'm very interested in the 315-5313A, since I don't know of any difference with that chip at all, but they must have revised the chip for some reason...

I'll start off. Here are a few games I know of differences with:

-Hellfire: On Mega Drive 2 systems and up (without the discrete YM2612), the music plays at half speed. There are some theories as to why, but I don't think the reason has ever been confirmed.

-Gargoyles and Ex-Mutants: Reportedly, on the Mega Drive 3, these games don't work. They use the M68K TAS instruction, which on most versions of the Mega Drive only performs a read, with the write component being ignored. On the Mega Drive 3, the write works, which breaks these games.

Can anyone else add to this list?

Chilly Willy · Post by **Chilly Willy** » Sat Sep 03, 2011 4:20 am

Jorge and Oerg866's MDEM demo has one section that runs different on some MDs.

viewtopic.php?t=936

Stef · Post by **Stef** » Sat Sep 03, 2011 8:30 am

I heard they are also differences in game using 2 cells vertical scrolling without hiding first column. I'm thinking about Gynoug for instance, in the first level when screen swings... difference can be observed between early MD1 and MD2 as the first column "bug" is not handled the same way.

Nemesis · Post by **Nemesis** » Sat Sep 03, 2011 11:04 am

Thanks for these references. The reason I'm asking this question is because I plan to investigate and document any and all differences between the main systems. I have just about every version of the Mega Drive ever made (including TeraDrive, LaserActive, MultiMega, Nomad, Amstrad MegaPC, and so on), in just about every different region, so I can quickly and easily test differences on the hardware. I'm not fussed too much on the more crazy versions of the Mega Drive, but things that behave differently between the Mega Drive 1/2/3, and possibly the Nomad and MultiMega/CDX, are things I really want to understand and document. I just need a starting point of where to look for differences.

ElBarto · Post by **ElBarto** » Sat Sep 03, 2011 2:20 pm

There is also the VDP 315-5313A-01.

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Sep 03, 2011 5:36 pm

Hellfire music is too slow because the YM3438 has different wait times on YM register accesses.
I can verify it on my MD1 with YM3438 installed on it... I will flash the game and try it out and post my results
EDIT: That game makes no sound at all with YM3438