The first step to explore the ROM is to read it. As it turned out it was really easy and read just fine with the "NEC UPD27C4001 @ DIP32" chip selected in the software of my programmer.

Removed ROM:

ROM in programmer:

To find out if the data i read is good (because i dont know if the "user definable" pins are configured right on the D23C4001E) i just wrote a 1:1 copy of the data to a AM27C040 IC (same as 27C4001).

And yes. It works completely normal with a self written ROM. HUGE SUCCESS.

The next step is to have a closer look at the content of the ROM...