Minimal CP/M SBC design?
ethan.dicks at usap.gov
Mon May 5 16:32:12 CDT 2008
On Mon, May 05, 2008 at 09:19:35AM -0400, Tim Shoppa wrote:
> Ethan asks:
> > 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?
> The most traditional CP/M system has a toggle-in front panel and
> 16K of RAM, Ethan :-).
Ah, yes... I was a bit imprecise in what I was asking about...
I was a bit young for the earliest days of CP/M, so in my mind,
"traditional" was the era of packaged systems like Osbournes
and Kaypros. My mistake. I even have a XOR 5-slot S-100 box
at home, a gift from an older gentleman who happened to mention
something to a relative of mine who works at the Post Office, but
that's a long story - the short of it is that I have enough hardware
to get it working, but no 8" CP/M disks, let alone the right
one for that machine.
> The much later CP/M systems that had boot ROM's either had them
> consuming a small amount of high memory permanently with some
> hardware hackery to jump there on a cold reset, or lived in
> low mem at reset and had a later stage of the boot process map
> them out.
That's the style I'm thinking of.
> And I am perplexed by your BIOS question. I think you are using
> that acronym in a non-CP/M context. If you want to read the
> "CP/M 2.0 System Alteration Guide" and rephrase your question I
> think we could make some progress.
I'll go hunting for that guide (it sounds exactly like what I was
looking for) when the 'net comes up.
I was originally thinking that even after boot-time, there were
still active routines in ROM that the OS was pointed at, but I'm
sure that perception was colored by how a standard '80s micro'
does things. If, as I now suspect is the case, there's zero need
for ROM code after the initial read from the block storage device
happens, then it seems easy to implement a 64KB RAM system after
>From some recent reading of various materials I've been finding
as I do my research, it does seem that a lot of post-S-100 systems
had 56K or 62K of RAM and 2K or 4K of ROM, still. Since I don't
know specifics, let me ask a general question in the hopes that
someone can figure out what machines and answer accordingly... for
those 2K/4K ROM systems, did any portion of those ROMs get accessed
once CP/M was up, or were they vestigal at that point? I can imagine
some sort of monitor program or such being mapped in for when the
OS got lost in the weeds, but what was that ROM space still good for?
Thanks for helping untangle a conceptual and linguistic lack
Ethan Dicks, A-333-S Current South Pole Weather at 5-May-2008 at 21:20 Z
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Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html
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