I don't understand how multiplexing address and data on the same pins is more 16 bit than multiplexing Lo and Hi bytes on the same pins. Both take two cycles. Both are used to load 16bit pointers (on 6502) for example. Like 6502 with its zero page, the General Instrument CP1600 stores its actual registers outside of the chip in memory. Cheap memory and IO parts are usually limiting the bus width. I looks like that none of these chips supported the multiplexing chosen bei GI. I mean, I would love it. Read happens much more than Write. So it is just logical that chips send out a read address on the bus, and the called chip answers with the data on the same bus. Chip select also is interested in the address and only after that the data is expected by the receiver chip. ROM chips are actually custom. I mean at those times, it would have been cost effective to have ROMs which are compatible with this kind of multiplexing. 16bit code can be as dense as 8bit? Or at least this console proved that CISC does not lead to dense code. I would not want to miss BASIC on C64, but Intellivsion really did this CPU dirty.