Olivetti M20

dwight elvey dkelvey at hotmail.com
Wed Jan 10 00:49:27 CST 2007

>From: Gene Buckle <geneb at simpits.com>
>dwight elvey wrote:
>>For those that might be interested in an Olivetti M20, there is one
>>on eBay. This was one of the few machines made with a Z8000 processor.
>>I think the only personal computer.
>>If someone on this list gets one, contact me and I'll help you get
>>it up and running.
>That's cool.  I just posted PDFs of the CP/M for Z8000 manuals on the 
>Retroarchive site. :)

Hi Gene
I would suspect that the one on eBay isn't ready for CP/M8000 right
out of the box. I'm sure it'll handle PCOS, at least version 2.0f.
The problem is that most machines didn't come with enough memory
for CP/M8000. Although, the manual for CP/M8000 states that one
can run in a machine with 128K of memory, because of a number
of allocation issues in the M20, you really need 384K.
The M20's comes with 128K on the mother board and usually
have two 32K expansion memory cards. Even with three of these
32K cards, your only up to 224K. Still not enough.
When I first got CP/M8000 running on my machine, I modified
two of my memory cards to take 64K DRAM chips. This brings it
to 384K. I was always going to increase it to 512K, the most that
the M20 decodes, but I don't see the need any more.
A fellow in Italy had thought that it might be easier to take two
1 meg simm's and use those ( you need at least 2 because it
does access 8 bit or 16 bit and needs the separate selects ).
We talked it over and came up with a way to wire them up
such that they can provide 384K with a minimum of external
hardware( total of 512K with the mother boards memory).
The only painful issue is that the M20 normally has a select for
either 32K or 128K at each board position. The first solution
used three board edges to connect the two other connectors
selects. Another fellow built one that he made a header to
go under the NAND gate chip that normally sent out the
selects. This did require lifting one part and putting it on a socket.
It also seems that some of the memory boards came with sockets
and could be configured for either 32K or 128K by jumpers
but none of my board were that easy.
There are two main memory board types. One is for B/W monitors
while the other is for Color. The only difference in the systems
to run color is the expansion memory cards. The color cards had
a parallel to serial shift register added to be used for the
color bits rather then the memory mapped DMA that the B/W
monitor used.
For color, depending on how many memory expansion cards one
had, one could have 2,4 or 8 colors. The video takes 16K from
each card. It doesn't cause holes in the memory map because
the M20 had a PROM to map the physical memory into specific
locations in the logical memory.
They did one other thing that was different than done on other
machines. It decodes intruction memory different than data memory.
This is part of the reason it can't have the full memory that the
Z8000 would otherwise be able to address. It did have the advantage
that for the setup for some of the segments, one could address
128K at one time ( 64K instruction space and 64K of data space ).
This meant that this instruction space needed to be mapped into
some other segment as data memory or one couldn't write
to that space from disk. This causes confusion for those working with
the debugger or looking at how the images are loaded from disk
to memory for these dual mapped physical memory areas.
Who ever gets the unit on eBay, make sure to have them send the
monitor. It uses some unusual vertical and horizantal rates that may
not be easy to find in other monitors.

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