several games reads from the low bank of 0xc***** area
Interesting. I assume byuu will tell us how that actually works
src/memory/smemory/generic.cpp has your answers. You should just copy that part. There's some trickery involving SRAM sizes that will cause you trouble saving in Ys 3 if you try and do it yourselves.
There are about four dozen PCBs, and each one maps different data to different areas, and many have large open bus regions. Download Super Sleuth and look at pcbs.pdf for examples. Since the PCB ID isn't dumped with games, and the ZSNES team isn't interested in them, we get LoROM and HiROM, and have to make every game work. It's also the same reason we're stuck with copier headers and 20+ SNES ROM extensions in 2009.
Nach did an outstanding job and created generic layouts that work for all known games. Not the solution I'd personally want (I want the PCB IDs (or an exact list of mapping info for said PCB), as well as ROM, save RAM and RTC data all stored in the same file), but it works.
Anyway, I never map to $[c0-ef]:[0000-7fff] in LoROM games. That region falls through to open bus. If games are being fixed by that, you have other problems I'm afraid.
