----- 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

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