Call for final Elf99 design input

Since I never used it (an 82S123), I'm not certain. It seems to map into $0000 - $001F on the original Elf when the PROM switch is set to enable. I don't get this. Wait a minute. We have been having an extensive offline conversation and you said that 32K of RAM and 32K of ROM is enough. It can be more or less of either, at a cost of gates and complexity. Going over the Quest design, I managed to eliminate a 4049 by redesigning the PROM area - the original used several inverters and NANDs to decode A6-A8. If I did not convey that properly, I'm sorry. Yes, the boot option is always there because it's software. The hardware echoes the ROM at $0000 and $8000 when the PROM enable switch is on, and the act of reading a valid ROM address (high memory) resets the flip flop, restoring the more normal RAM at $0000 and ROM at $8000. If you make the first two instructions INP 4 and PHI 0 (your suggestion), you get the ROM bank select feature that you originally proposed. One vote for, several against or abstaining. I'm not inclined to do dual 1852's. We you and I have discussed, the 1851 uses three ports, the switch/display is another port, leaving three left over. While ports are scarce, they are no so scarce as to overshadow the benefits of a bit-programmable I/O port. The space taken up by two 40 pin chips is a bit much when I can implement two cheap ports (one in and one out) with octal latches. I have no problems in principle with a software UART, but I do have something to say about the speed. 2400 baud is *not* a ridiculous speed, especially if you want to talk to an external device that has a fixed clock, say, a serial-to-LCD board. The other issue with a software UART is timing. If you use a hardware UART, it needs a crystal of a particular frequency, but the CPU does not. You can then "clock chip" the 1802 up (to use a Mac term) and not recode your serial routines. The 1883 is a *7* bit latch with built-in decoders. We need no decoder because the low 15 bits of the full address bus are going to each the ROM and RAM sockets. There are no individual memory chips to select. The 1883 is a great chip if you want to use several 6116's, for example. I'm willing to consider 18xx chips, but they are each several dollars, while CMOS 4xxx chips are under a buck each, even in small quantities. I still have yet to hear why the 1855 Multiply/Divide Unit is worth the real estate. Sure, it's a neat chip, but unless it has a purpose, I can't see including it. The 1879 Real Time Clock might be neat, but certainly an optional part. I have several. BG Micro and other places sell used 20x4 and 16x2 modules for $2 to $7. I must say that I hadn't considered that as an optional peripheral. It might be worth laying out a 14 pin connector off of the output port for that purpose. Well... I wouldn't call a 6504/6532 board "relevant". Fun, probably, but not current stuff. Fun this is, but if it isn't at least passingly practical, nobody can do anything with it. I never really used my Elf because I had no I/O beyond the Q LED and the switches. The main reason for designing the Elf99 is to satisfy a 20-year-old desire to have a "useful" Elf, and to capitalize on the present wave of nostalgia to make a PCB cost-effective. I could always just bang out a single board for myself with the iron-on transfers, but that's so much work that I'd rather make something that I can share with a wider audience. It's risky to consider putting up a few thousand dollars on a replica of a classic, but if I maximize the appeal of the standard I/O ports, I'm likely to sell a few dozen. If I can't sell 60, I can't afford to invest in 100 PCBs.