Olivetti M20

dwight elvey dkelvey at hotmail.com
Tue Jan 16 21:03:34 CST 2007

>From: "Chuck Guzis" <cclist at sydex.com>
>For the M20's price point and market positioning, I really think that
>an MMU would have been a waste of resources--and could well have been
>a budget-buster.
For the purpose that I can see that Olivetti used a ROM, I think it
was the right choice. For the most part it was related to logical to
physical mapping. They used a portion of the physical memory for the
video memory. This was a 16K block for B/W but it required up to
3 blocks for color. They could configure for 2, 4 or 8 colors.
To do more than one color, required expansion memory cards with
parallel to serial registers to send the video information to the video
controller chip.
The ROM could be quickly setup with jumpers for what ever combination
of memory that was needed. This simplifies the need to have additional
software to configure the system.
They also use the ROM to create windows in memory that had the
data memory mapped different than the instruction memory. This
allowed the processor to access 128K while staying within a single
segment. For applications that ran in non-segmented mode, this
allows a better use of memory while taking advantage of the smaller
code size of non-segmented code.
While both of these could have been done with a MMU, it would have
been over kill for a desk top machine.
They did make some larger machines with Z8000's. I would guess that
because of the type of machine these were, they would be expected
to have MMU's. These were called the M30 and M40. I've only seen
a picture of the M30. It was a half height rack setup.

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