Minimal CP/M SBC design?

[Roy J. Tellason]

On Sunday 04 May 2008 15:57, Ethan Dicks wrote:
> 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 have some amount of docs on that stuff scattered here and there,  and am not 
real clear at the moment just where it's all located.  I do recall some 
stuff,  though.  One that comes to mind had the ROM (or EPROM?) mapped in at 
reset,  and one of the coldstart initialization routines copied some small 
portion of that up into high RAM.  I know that the Osborne Executive had 
3 "banks" one of which had the eprom and video RAM mapped into it.  I think 
that some of the TRS-80 machines needed a special version of CP/M because 
they had ROM in low memory and no way to map it out.  There were probably 
other variations I'm not thinking of besides.

> 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).

That's my understanding of it too,  though the mechanics of actually putting 
together a bootable CP/M system are something I'm still a bit fuzzy on.  I 
probably oughta do that with my BBII one of these days.

I have a fair pile of downloaded files pertaining to this and other 
CP/M-related stuff,  what used to be that portion of the files section on my 
old BBS on hand here,  I can send you a list of what's there off-list if you 
like.

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