Minimal CP-M SBC design

________________________________ Hi, all, I was staring at an SBC I have here with a 6MHz Z-80, some ROM, some RAM, and a 26-pin off-board bus for some Z80-PIO boards (this thing was built as a multi-parallel-printer switcher). I've been musing about what it would take to boot CP/M up on this. For user I/O, I was planning on a console serial port and a terminal/terminal emulator. I have IM6402s on hand, but I'd be interested in hearing if certain other chips are preferred, based on what BIOS code is floating around out there. I also have a 16550, but I don't think I have any Z80-SIO chips handy. For mass storage, I was planning on either Compact Flash or an SD card. I think I've seen both as I googled around for modern SBCs. Any of the media I have lying around is plenty large enough (I even have some 4MB CFs and a 2.5MB full-sized PCMCIA flash card on hand). I am a little unclear, though, about how traditional CP/M systems were set up for ROM and RAM. Was it common to use a "shadow ROM" in low mem at reset, then have the BIOS live at the top of memory? How did 64K RAM CP/M machines handle the BIOS? Did they temporarily ghost the ROM on top of RAM until some bit of code could read ROM and write RAM then bank out the ROM? Since I think I "need" at least 48K of RAM, I was planning on a pair of 62256s. I could easily do 56K of RAM low and 8K of ROM high, I think, unless there's some other arrangement that's obvious to try for a simple design. I've never tried writing a BIOS for a CP/M machine, but my understanding is that things are modular enough that once you know what I/O chips you have and at what I/O addresses, for a straightforward, non-clever design, the coding is equally straightforward and non-clever (but please feel free to enlighten me if otherwise). Thanks for any tips, especially from anyone on the list who has ever rolled their own CP/M machine. -ethan -- Ethan Dicks, A-333-S Current South Pole Weather at 4-May-2008 at 19:40 Z South Pole Station PSC 468 Box 400 Temp -74.2 F (-59.0 C) Windchill -105.4 F (-76.4 C) APO AP 96598 Wind 7.4 kts Grid 77 Barometer 691.6 mb (10194 ft) Ethan.Dicks at usap.gov <http://www.classiccmp.org/mailman/listinfo/cctalk> http://penguincentral.com/penguincentral.html -----REPLY----- Hi Ethan, Funny you should mention the subject, I am working on a very similar project. Last year, I made a completely home brew Z80 computer with prototype boards. It is a fun project and you can definitely build your own CP/M computer from scratch. It really is not that hard. This year, I am remaking the design using manufactured PCBs. I recently got my first batch of prototype PCBs and have been building up and testing them. So far, I have gotten the CPU, ROM, RAM, UART, and most of the glue logic working and tested. It is not complete yet as I still need to wring out much of the hardware and have not even started on the PPI or RTC. However, last year I was able to boot the previous system into its monitor and even boot CP/M. CP/M used a 32K ROM drive for drive A:, a 448K RAM drive for drive B:. I implemented an IDE port and a hard disk for drive C:. Those all worked pretty good. I built but never tested a floppy drive interface. The whole system is pretty simple, uses plain 74LSxxx chips, no programmable parts (except the EPROM) or hard to get parts. No SMT or funky technologies, just plain through hole DIP chips. I have all the software and am presently going through the library reinstalling it on the new system. So far, I have most of the easy stuff working. I do have the RAMless monitor working and next will be the regular monitor. Finally it will be the CP/M system and the RTC software. Anyway, if your interested, I am keeping a small Google group of files and photos at: http://groups.google.com/group/n8vem Thanks! Andrew Lynch