Subject: Re: Quick survey on equipment
From: ard at p850ug1.demon.co.uk (Tony Duell)
Date: Mon, 23 Apr 2007 23:30:04 +0100 (BST)
To: cctalk at
classiccmp.org
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.
Maybe because the IO instructions are slow compared to something like
(8080 neumonics) Mov M,A. Also since they used a Memory mapped video
that ate 1k of space along with the 12k of rom why not do the keyboard
that way.
Actually there are quite a few IO mapped ports on the M1 but the
keyboard and Video are in the memory map.
Allison
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