Reproducing old machines with newer technology

Mike Stein mhs.stein at
Sat Jul 18 23:06:45 CDT 2015

----- Original Message ----- 
From: "Chuck Guzis" <cclist at>
To: <General at>; 
"Discussion at and Off-Topic 
Posts" <cctalk at>
Sent: Friday, July 17, 2015 12:21 PM
Subject: Re: Reproducing old machines with newer 

> 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, 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 

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...


More information about the cctech mailing list