To be fair, we can’t expect everyone to know that 2 OPLL’s can coexist. It is not a common usecase and it was not very well documented (or even at all). In the late 80’s / early 90’s it was still possible to import brandnew FM-PAC’s or MSX-2+ machines with built in MSX-Music. But at some point that stopped and getting second hand stuff from Japan surely wasn’t that easy as it is now. There was a demand for (cheap) FM-PAC clones primarily to play games and listen to FM / Stereo songs and demos. And people filled that demand with clones that worked for what the people wanted.
I quickly traced the MSX-Music. At MSX start up nothing is executed.
When CALL MUSIC is executed, it depends on the configuration.
Internal MSX-music only: No access to 7FF6h.
Internal MSX-music + FM-PAC: No access to 7FF6h.
FM-PAC only: Select the FM-PAC ROM and set the bit 0 at 7FF6h.
Same with a call to 4113h.
This means that the MSX-Music cartridges that have the "APRLOPLL" ID (by mistake) have always its I/O ports enabled or at least by default. This kind of cartridge should not be used on a MSX with an internal MSX-Music. And Datapack is not accurate, even wrong on some details.
