The problem
was that CP/M needs RAM starting at location 0, the
TRS-80 has the BASIC ROMs there and starts RAM at, IIRC, 0x4000.
Hmm, same problem as the Model III then (ISTR that had its ROM located at
$0000 too, hence the reason for the Model IV).
Exactly. The Model 1 and Model 3 are actually quite similar machines
internally, to the extent that a Model 3 ROM set will run on a Model 1
(if sutiably addressed). Admittedly the Model 3 makes more use of the Z80
I/O ports (almost all I/O on the M1 is memory-mapped for some unknwon
reason), but the memory nap, video, etc, are the same.
The M4 has quite a few changes over the M3. It has the
software-selectable memory maps, allowing RAM at location 0 for CP/M. And
a much improvded video section with a 6845 CRTC chip (and not a
fixed-configuration row of TTL of the older 2 machines) thus giving am 80
column text display, etc.
....The second
solution was an add-on circuit board....
Hmm, sounds like these are probably going to be as easy to get hold of now
as hen's teeth...?
I don't recall there being any published schematics for them either, so
if you want to build one you'll have to design it yourself.
Think I'll just get a Z80 Second Processor for my Beeb instead (far less
hassle).
I didn't think those were _that_ common...
-tony