DECmate III and Rainbow 100+

From: Jeffrey Armstrong <>
Date: Wed Feb 23 07:51:18 2005

On Tue, 22 Feb 2005 wrote:

> On Tue, 22 Feb 2005, Geoff Reed wrote:
>> At 09:02 AM 2/22/2005, you wrote:
>>> The Rainbow is a 8086 with a Z80 coprocessor (it's not PC compatable),
>>> and will boot into either MS-DOS or CPM-86. CPM-86 cost about 10X
>>> that MS-DOS did, so wasn't used much.
>> I prefer to think of it as a Z-80 with an 8086 coprocessor :)
> In some ways that's more accurate :-) The Z80 owns all the
> peripherals. There's shared memory between the two schizophrenic
> halves. The Z80 owns the body, but the 8086 is the paranoid
> personality that runs the show from behind.

Actually, the Z80 in the Rainbow only owns the floppy controller. The
8088 bus (Rainbow's only have 8-bit data buses, hence no 8086) has the
hard disk controller, communications controllers, and all video, text and
graphics. The z80 bus shares 62KB of memory with the 8088 bus (it gets a
whole 2KB of its own). The 8088 runs the show, while the Z80, when not in
use, takes care of all floppy access. When an 8-bit CP/M program is
detected, the 8088 lets the Z80 take over.
SDF Public Access UNIX System -
Received on Wed Feb 23 2005 - 07:51:18 GMT

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