some guys from different sega Communities had create a sonic 1 cd version in 2011 which is compatible with the mega CD drive. That means if you have a MD1 + Sega CD, the old cartdridge works with a cd soundtrack via sega CD. If i understand it right this function was official included but never used in commercial games. Only the Game Pier Solar used the Mode1 function and some hacks like top gear.
In the last 2 years we create a german version of wonder boy 4 and in the last steps we saw the sonic 1 hack. I tried to understand the function and want trace it with a debugger like gens r57shell but via emulator the mode1 is only compatible with kega fusion. So its hard to understand how it works and how we could use it to built it in our mw4 version.
I already implemented MSU hacks in some snes games. The mode1 hack runs a subroutine and if the sega cd is available he check for a valid tracktable (TOC).
There is also a mode 1 player at the sega source but so far my knowledge about the MD cpu is not very good.
Did anyone a hack with mode1 for his games and when yes, how could you debug the source?!