----- Original Message -----
From: "Chuck Guzis" <ccl...@sydex.com>
To: <gene...@classiccmp.org>;
"discuss...@classiccmp.org:On-Topic and Off-Topic
Posts" <cctalk@classiccmp.org>
Sent: Friday, July 17, 2015 12:21 PM
Subject: Re: Reproducing old machines with newer
technology
On 07/16/2015 11:45 AM, Mike Stein wrote:
Not the same thing of course but remotely
on-topic, and I never miss an
opportunity to put in a plug for Cromemco:
By comparison, Cromemco used semi-autonomous
4MHz Z80A SBCs for their
I/O processors, with 16KB of local RAM and up
to 32KB of ROM;
communication with peripheral cards is via a
separate 50-pin 'C-Bus'.
That wasn't all that uncommon in the
microprocessor world--once the price dropped
sufficiently, doing multiuser applications by
giving each user their own CPU was practical.
Molecular was another outfit that did
practically the same thing.
Dual-CPU setups, where the "weaker" of the two
CPUs was in control of the "stronger" one were
even more numerous--just consider the number of
"add in" processor cards for the PC archicture.
68K, NS32xxx...you name a CPU, it's probably
been on an ISA card.
And there's the veneered and generated Radio
Shack 68K series (16, 16B, 6000) where it's the
Z80 that starts things and controls the show
initially, even if you're running Xenix.
In pretty much all cases, the system is capable
of running without the "stronger" CPU.
--Chuck
----- Reply -----
AFAIK Cromemco never went the 'CPU for every user'
route; multi-user systems were implemented using
the multi-user Cromix OS, with a 64K memory card
for each user in a Z80 system and dynamically
allocated memory in a 680x0 system, but always
with only one CPU in control.
The early versions of Cromix ran on a Z80; when
they brought out the dual-CPU Z80/680x0 processor
cards you could still run the Z80 version or the
68K version which would use the Z80 as required
for Z80 software.
When the single 680x0 processor cards came out
along with the memory management hardware required
to run UNIX they needed a way to still run Z80
(i.e. CP/M or CDOS) software, and that's where the
ability to use a Z80 on an I/O controller was
handy.
I always wondered which was more efficient,
multiplexing among essentially complete 'computers
per user' sharing a common I/O 'channel' or
swapping processes and memory banks...
m
