Originally Posted by hap
Some of the Nelsonic G&W games have a Holtek MCU, not this one but I expect it's similar technology. And the ROM bits were not visible on the decap.
Does that mean it uses not mask rom (i.e. eprom or flash cells) or is there only a layer above to hide it from hardware pirates? Was it 1990th or later?

I ask because of long-term bitrot risk if the "rom" is actually something else. Many lofi music keyboards and soundtoys employ Holtek chips (likely of different architecture), but if I remember well, datasheets (or specs on Holtek website and forums?) claimed that prototyping MCU versions for such things were OTP (no erasable versions exist) but mass produced ones are rom, hence they have no reusable chip version for DIY projects.