Questions on the days finds

I'm IIRC, the only 6800 instruction not present on the 4052/54/52A/54A was DAA - decimal addition adjustment. Since the 6800 has no decimal subtraction adjustment, it was pretty useless anyway. The main difference is the 128K byte address space. Two bits in the processor status register were used for the 17th address bit (most significant) - one for instruction fetches (default 1) and one for data load/store (default 0). Most of the additional instructions manipulated these. Nice features included: if you changed the bit for the instruction fetches, it didn't actually change until the next ALUOUT -> PC microcode instruction, i.e. branch or jump. Almost exactly. And it's not just separate odd and even ROMs - it has separate odd and even address buses! These hardware features do speed it up, though. The other main application of the 16 bit data path was that 2-byte instructions were fetched with a single memory cycle, and this happens quite often. The odd/even address logic meant this could be done at an odd or an even address and still work. My guess is that the hardware guys put a lot of work into that architecture, wanting to make it both nice and 16bit and like the 6800. Unfortunately, this took rather a long time, so the software guys just wrote a 6800 near-emulator in microcode so as to speed the software migration process. But this is just a guess. Philip.