Over the last week I have been working on Genesis/MegaDrive verifications; particularly trying to add decent support to my replay device and writing dump scripts.
The way Genesis games read controller inputs is rather quirky, and existing emulators don't provide great support for capturing those inputs with much precision. Neither BizHawk nor Gens gives you any way to see what values the game writes to or reads from the relevant memory mapped registers for the controller ports. BizHawk has a memory callback API, but as far as I can tell, it
always returns 0x00 as the value (indicating the core implementation doesn't support it).
Gens' callback API doesn't even provide a value parameter, even though the internal function that handles the callbacks, is in fact provided with the read/written value! In the case of Gens, I was able to write a very simple code patch that passes the value to the lua callback which I'll get posted on github soon. Along with some build instructions, because the repo doesn't offer much help, and it appears you are required to use VS2010 which Microsoft doesn't provide a download for anymore.
The callback value is important because it is required in order to understand how the game is polling a controller. Different games poll in different ways. There's no special "latch" signal like on the NES. There is something similar, but it can be in two states, and the input values change depending on how many times the state is changed, what the state is, and the delay between each state change. The game is free to write the same state multiple times, or to read multiple times without changing the state.
It also appears that for some games, such as Sonic Spinball, Gens may be misinterpreting which frame the polls actually belong to, which further complicates dumping and replay. The current publication of Sonic Spinball in particular may be impossible to replay on hardware; I kept running into weird problems like the above, or the start button not registering inputs properly for no measurable reason.
Additionally, there aren't many games I can't test, because I don't have a flashcart for this system, and very few carts exist at all. Everdrive is the only decent one but that costs either $100 or $260 depending on the version. But I'm concerned it may modify the state of the console (similar to the NES one does). I've made my own flashcart design, but I haven't ordered it (or thus tested it) yet.
However! There is one bit of good news. I did manage to verify
[570] Genesis Wonder Boy in Monster World by Aqfaq in 42:10.45! My recording setup is a mess right now due to Genesis research, and I'd like to get some input displays. But I should get a recording up for that within a couple weeks; or maybe I'll record a basic one in the meantime just to show it works.
