Thanks for your information. Just did some further studying: the 0xff only happened in the front 2MBytes of the two 4MB rom data files, while the 2nd half data look good, but the graphics content are the same as the 1st halves(repeated?). However, some necessary in-game graphics can not be found, for example, song titles.
Looks like for these two 16-bit 4-MByte rom chips, their highest address pins(A20), and word/~byte mode pin are reversed?
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