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From: knudsen@ihwpt.UUCP (mike knudsen)
Newsgroups: net.micro.6809,net.micro.trs-80
Subject: Tandy didn't warn us, Motorola did
Message-ID: <1201@ihwpt.UUCP>
Date: Mon, 3-Nov-86 13:30:50 EST
Article-I.D.: ihwpt.1201
Posted: Mon Nov  3 13:30:50 1986
Date-Received: Tue, 4-Nov-86 03:26:45 EST
Distribution: net
Organization: AT&T Bell Labs, Naperville, IL
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Xref: mnetor net.micro.6809:571 net.micro.trs-80:564

Because lots of Coco-I & II software and hardware aren't
compatible with the Coco III, some folks have wished out
loud that Radio Shack had warned us in advance, especially
about the FExx page.  Last nite I had to dig into my Motorola
manual for something else, and I noticed that under the SAM
chapter it described FE00-FEFF as "reserved for MPU enhancements"
(MPU is the 6809, not the SAM, weird) and that the portion of
the FFxx I/O page available for add-ons is much more restrictive
than even Tandy's official range, which I believe is FF60-FF7F.
You could interpret Motorola as saying that only FF40-FF5F
is free -- most of that is taken up by Tandy's disk controller
that ghost-addresses all over the place.

I didn't check my old Coco-I tech manual to see how much
of this had been reproduced there -- if it has, then Tandy had
indeed given a clear warning.  Of course like everyone else
I figured "enhancements -- sure, when I can ice skate in Hell"
and wrote code that lay all over FExx.  Lucky I didn't sell it.

PS: There is plenty of I/O space in FF04-FF1F and FF24-FF3F,
left over from the two PIA chips.  To use it, a peripheral
must decode all the bits itself and pull down the Phantom
line (I forget it's official name) to keep the ghosty PIAs
from fighting it.  That Phantom lead was a great addition to
the Coco cart slot (it disables all I/O stuff inside from responding
for a cycle) -- too bad no commercial outfits used it.
Don't know how much it works on the III, tho.
-- 
Mike J Knudsen    ...ihnp4!ihwpt!knudsen
Bell Labs (AT&T)   (312)-979-4132 (work)
Nobody pays for my opinions, which are mine alone.
"A mind is a terrible thing to waste, but the pay is good."