At Stiletto's suggestion, I contacted John Dyer, who is familiar with old NEC microcontrollers, to find out about the chip in the Epoch Game Pocket Computer. Here are some of his comments. Of particular interest, he confirms that the chip uses maskROM, and says he might be able to figure out how to trojan it if the game ROMs are dumped. I need to check with Guru to see if he's ever dumped those carts.
"I have experience working with NEC microprocessors as a software engineer and worked for the local representative of 2 japanese companies that used NEC microprocessors. The 78C06 is not a member of the uCOM87 family but is related to an earlier
family based on the uPD7801/uPD7800. The instruction set is similar but much simpler than the 78C10 family.
The 78C06 has 4k bytes of mask rom, the 78C05 is the romless part. As far as I am aware, there is no easy way to read the chips rom unless there is external eprom which would allow you to gain control of the processor. The main advantage of the 78C06 over the 7801 is that it is a low power
cmos version designed for use in portable equipment etc.
The information I have is taken from the 1983/84 NEC Microcomputers Databook. If you require further information, I am happy to oblige."
"There are no differences between 78C06 and 78C05, its just that the 78C05 has no internal rom. If the game box uses a rom cart then you have a means of gaining control of the processor.
I used exactly this technique in the early 1980's - by attaching a uart like the 8251 to the address and data bus, I was able to catch the cpu and execute a simple program (hand assembled) to
dump the internal rom out the serial port.
If you have difficulty identifying the code from the cart then I may be able to help, if you can email me the binary code from the cart, I can disassemble it to identify the entry points.
Although its quite a long time ago since I worked on the 7801 I still have all the tools I used back then including assemblers and disassemblers etc."